Contrib Mineral Petrol (1996) 122:387—405 ( Springer-Verlag 1996

Dirk Van der Wal · Jean-Louis Bodinier

Origin of the recrystallisation front in the Ronda peridotiteby km-scale pervasive porous melt flow

Received: 10 January 1995/Accepted: 1 September 1995

Abstract It is well established that porous melt flow inthe upper mantle may significantly affect partial mantlemelt compositions. Less well established are the length-scale of porous flow and whether porous melt flow canbe a volumetrically important magmatic process. Theonly source for observations concerning the length-scale and nature of pervasive porous melt flow areperidotite massifs. Here we present such observationsin the form of structural, and major and trace elementdata from peridotites of the Ronda massif, southernSpain. Trace element concentrations were obtainedwith high analytical precision (ICP-MS) and includetrace elements rarely analysed in peridotites, such asRb, Th, Nb and Ta. The western portion of the Rondamassif can be divided into two structural facies. Thefirst and oldest is composed of deformed, porphyro-clastic spinel peridotites, the second of virtually unde-formed granular spinel peridotites. They are separatedby a recrystallisation front across which grain growthof all phases occurred. The granular domain can befurther subdivided into three subfacies: coarse-granu-lar, fine-granular, and layered-granular peridotites. Ac-cording to structural facies, km-scale spatial variationsunrelated to Ca and Al abundances have been recog-nised for mg-numbers [atomic Mg/(Mg$Fe)] andincompatible elements such as rare earth elements(REE), Th and high-field-strength elements (HFSE; in-cluding Ti). Such variations are reminiscent of thosecommonly ascribed to mantle metasomatism, but havenever been documented on the km-scale. The origin of

D. Van der Wal1 · J.L. Bodinier ( )Unite de Recherche Geofluides-Bassins-Eau, Institut des Sciences,de la Terre, de l’Eau et de l’Espace de Montpellier, Universitede Montpellier II, Place Eugene Bataillon, F-34095 Montpellier,France

1 Present address: Philips Electron Optics BV, Building AAE,P.O.Box 218, 5600 MD Eindhoven, The Netherlands

Editorial responsibility: V. Trommsdorff

the recrystallisation front is related to km-scale perva-sive melt percolation. Feed-back processes betweengrain growth and melt fraction could have led to im-portant accumulation of melt at the recrystallisationfront, accomplished mainly by melting/dissolution.Variation in melt fraction across the front explains thespatial variation in the degree of recrystallisation, mg-numbers, REE fractionation, and HFSE abundances,and could account for many of the classical differencesbetween basalts from convergent and extensional tec-tonic settings. Whereas the coarse-granular peridotitesreflect a stage of steady-state pervasive porous meltflow, the fine- and layered-granular facies probablyreflect the terminate stages of porous melt flow. Pro-cesses associated with both domains are pyroxene-forming freezing reactions at decreasing melt volumes,and progressive channelling of melt flow associatedwith olivine-producing reactions. Both processes showcomplex overprinting relationships in both time andspace.

Introduction

The segregation of basaltic melts from the Earth‘smantle into the crust involves migration of mantlemelts through a km-scale upper mantle column. Modaland geochemical heterogeneities observed in mantlerocks are generally considered to be the result of inter-action with such ascending melts (Frey and Green1974; Wilshire et al. 1980; Stosch and Seck 1980; Harte1983; Menzies and Hawkesworth 1987; Navon andStolper 1987; Bodinier et al. 1990; Kelemen 1990; Kele-men et al. 1992). The extent to which melts can interactwith upper mantle rocks, however, depends principallyon the melt transport mechanism.

Two end-member transport mechanisms proposedare porous-medium flow and melt flow localised infractures. Melt transport in fractures is known to be an

important melt transport mechanism in the uppermantle (Spera 1987; Sleep 1988; Nicolas 1986, 1989;Spence and Turcotte 1990; Ceuleneer and Rabinowicz1992). When melt flows dominantly in fractures, thereaction surface between melt and host peridotite isvolumetrically limited, and the melt composition maynot significantly be affected by melt-rock interaction.The wall-rock may show important, but spatially lim-ited compositional and textural changes (meta-somatism) due to interaction with melts from the frac-ture (Wilshire et al. 1980; Menzies and Hawkesworth1987; Bodinier et al. 1990; Nielson and Wilshire1993).

On the other hand, if melt flows pervasivelythrough porous peridotite, the reactional surface isnearly unlimited, and large volumes of melt and hostperidotite are likely to interact (Navon and Stolper1987; Kelemen 1990). Porous flow in the upper mantlemay thus significantly change the composition of par-tial mantle melts. The extent to which this occurs de-pends on two factors which are relatively uncon-strained: (1) Although several authors have speculatedon the existence of km-scale pervasive porous flow inthe upper mantle (Kelemen 1990; Kelemen et al. 1992,1995), it remains to be determined from peridotitemassifs whether porous melt flow can be a volumetri-cally important process in the upper mantle. Porousflow has been demonstrated only in ‘‘channels’’ madeup of peridotites depleted in basaltic components (re-fractory peridotites) (Bodinier et al. 1991; Takazawaet al. 1992; Kelemen et al. 1992), with a maximumlength-scale (measured perpendicular to the inferredmelt migration direction) restricted to several tens ofa metre at the most (Takazawa et al. 1992; Kelemen etal. 1992). (2) Modelling studies have shown that min-eralogical reactions and melt-fraction (or porosity)variations associated with porous melt flow stronglyinfluence the geochemistry of mantle melts (Kelemen etal. 1990; Godard et al. 1995). It is important, therefore,to characterise porous flow in terms of melt fractionand melt-rock reaction.

It has been suggested that favourable conditions forporous melt flow could exist in the upper mantle hang-ing wall of subduction zones, due to the likely presenceof volatiles in the melt migration system, as well asinverse geothermal gradients (Kelemen 1986, 1990).Van der Wal and Vissers (1993) recently identified theRonda peridotite of southern Spain as a remnant ofa subduction zone hanging wall, emplaced during theNeogene by detachment of the subducted slab. Ourfield-oriented, combined structural and geochemicalstudy of the Ronda massif allows us to address theabove issues concerning porous melt flow. We willshow that the Ronda peridotite shows structural andgeochemical features consistent with km-scale perva-sive porous melt flow.

Geological setting

The Ronda peridotite is the largest (300 km2) coherent peridotitemassif exposed in the Alpine chain of southern Europe. In addition,it is the only European peridotite known to have preserved allperidotite metamorphic facies, i.e. garnet peridotites, spinel perido-tites and plagioclase peridotites (Obata 1980; Van der Wal 1993;Garrido 1995). The peridotites are underlain by high-grade gneisses(Lundeen 1978) and migmatites beneath a faulted contact, markedby extensive brecciation. Likewise, a zone of fault gouge separatesthe NW peridotite from an overlying sequence of high-gradegneisses that rapidly pass into lower grade pelites (Loomis 1972).

The Ronda peridotite can be subdivided into three km-scalestructural and metamorphic domains (Obata 1980; Van der Wal andVissers 1993). A first domain exposed in the NW part of the massif iscomposed of deformed garnet and spinel peridotites with locallyabundant garnet pyroxenite layers (garnet facies and Ariegite subfa-cies of Obata, see inset Fig. 1). Extremely depleted Sr—Nd isotopiccompositions suggest that this domain represents subcontinentallithosphere (Reisberg et al. 1989). This domain is overprinted byvirtually undeformed granular spinel peridotites, with locally abun-dant spinel pyroxenites (Seiland subfacies of Obata). This seconddomain, referred to as the granular domain, occupies most of theW part of the Ronda peridotite around Mt. Reales. Towards theS and E, deformed plagioclase-bearing peridotites (plagioclase faciesof Obata) developed at the expense of granular peridotites. Thisevent was ascribed to km-scale shear localisation, associated withthe young (22 Ma, Priem et al. 1979) emplacement of the massif (Vander Wal and Vissers 1993). This study will focus mainly on the originof the granular domain.

A geochemical study of Ronda peridotites and pyroxenites (Ger-villa and Remaıdi 1993) has established that melt migration occur-red in at least a small area of the granular domain, but possibly alsoin the plagioclase peridotites characterised by enriched Sr—Nd iso-topic compositions (Reisberg et al. 1989). This area, known asArroyo la Cala area (locality outlined in Fig. 1), is characterised bya strong compositional heterogeneity from fertile lherzolites todunites, arranged in 1—100 m-scale bands or layers. This heterogen-eous appearance is similar to peridotite sequences known from otherperidotites such as Trinity (Quick 1981), Josephine (Kelemen andDick 1995) or Horoman (Takazawa et al. 1992). The origin of theharzburgites and dunites is related to an olivine-producing melt-rock reaction, believed to occur in porous flow channels (Kelemen1990; Bodinier et al. 1991; Takazawa et al. 1992; Remaıdi 1993;Kelemen et al. 1995).

Further into the granular domain, the heterogeneous appear-ance is largely lost, and grades into homogeneous granular spinelperidotite. Van der Wal (1993) and Van der Wal and Vissers (1995)showed that these granular peridotites are bounded to the N, NWand W by porphyroclastic spinel peridotites, separated by a recrys-tallisation front (Fig. 1). The recrystallisation front is roughly per-pendicular to the inferred melt channels in the Arroyo la Cala area,which suggests that the origin of the recrystallisation front could berelated to melt migration.

The purpose of this work is to elaborate on this hypothesis bymeans of a combined structural and geochemical study of the granu-lar domain in the western Ronda peridotite.

Structure of the W Ronda peridotite

The W portion of the Ronda peridotite comprises two main struc-tural and metamorphic domains (after Van der Wal and Vissers1995): deformed spinel$garnet peridotites (domain I) and granularspinel peridotites (domain II ). Our mapping of the granular domainshows that the granular domain can be further subdivided, onthe basis of structural criteria, into three subdomains. These are,in order of decreasing relative age: coarse-granular peridotites,

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Fig. 1 Structural map of the western Ronda peridotite. Inset inlower right corner: metamorphic facies map of entire Ronda massif(after Obata 1980), with location of mapped area indicated. (GR¹

garnet peridotite facies, AR Ariegite peridotite subfacies, SE Seilandperidotite subfacies, P¸AG plagioclase peridotite facies)

fine-granular peridotites and layered-granular peridotites (indicateddomain IIa, b and c in Fig. 1).

Spinel tectonites

The N, NW, and W part of the area shown in Fig. 1 consists mainlyof porphyroclastic spinel peridotites (spinel tectonites). Myloniticspinel and garnet peridotites occur in a &500 m wide zone at theperiphery of the massif (Darot 1974, Obata 1980, Van der Wal andVissers 1995), but these will not be considered in this study. In themapped area, the spinel tectonites are mainly harzburgites withlocally abundant garnet pyroxenite layers. They are moderatelyfoliated with pyroxenite layers (sub)parallel to the foliation, fre-quently folded isoclinally. Cross-cutting dikes are extremely rare inthis domain, as in the entire Ronda massif (Obata 1980). Thefoliation is mostly steep, dipping to NW or SE. Orthopyroxenestretching lineations are subhorizontal. The intensity of the foliationis rather homogeneous, although locally regions of increased ordecreased intensity of the foliation may be recognised. On themicro-scale, the foliation is defined by elongate olivine and ortho-pyroxene, whereas clinopyroxene mostly occurs in equi-dimensionalclusters. All phases show undulatory extinction. The olivine grainsize is bimodal, with large porphyroclasts (1—2 mm) surrounded bya finer-grained matrix of neoblasts (&200 lm). The microstructureresembles those of porphyroclastic peridotites frequently sampled byalkali basalts (Mercier and Nicolas 1975). More details on the spineltectonites can be found in Van der Wal and Vissers (1995).

The granular domain

Coarse-granular peridotites

In a form-surface map of the orientation of the spineltectonite foliation it can be seen that the spinel tecton-ites are overprinted, towards the S, SE and E, bya dome-shaped domain of virtually undeformed granu-lar spinel peridotites (domain IIa, Fig. 1). The twoperidotite lithologies are separated by a transitionzone, not more than 200 metres wide, in which micro-structures suggest primary (annealing recrystallisation)and secondary recrystallisation (grain growth) of a for-mer deformation microstructure (Van der Wal 1993,Van der Wal and Vissers 1993, 1995). Recognition ofthese overprinting relationships on both map- andmicro-scale provides conclusive evidence that thegranular peridotites developed at the expense of thespinel tectonites. The transition zone will be referred toas the recrystallisation front. The orientation of therecrystallisation front is roughly vertical and curvesaround from a SSE-NNW orientation in the W part ofthe mapped area, to a NE-SW orientation towards theE part. The recrystallisation front can be followed fur-ther towards the E and is exposed over a distance '20km (Van der Wal 1993).

The granular peridotites SE of the recrystallisationfront are mainly harzburgites with a coarse-grainedgranular structure. They will be referred to as coarse-grained granular peridotites. Rounded orthopyroxenes

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recognised in the field have diameters generally'5 mm (see also Fig. 2a), and can be up to 1.5 cm.Apart from a few irregular m-scale dunites, the com-position and structure of the coarse-granular perido-tites is homogeneous. In the NE and SW part of themapped area the coarse-grained granular peridotitesreach a thickness of 1—2 km, whereas in the NW partthe thickness is limited to 200 metres at the most(Fig. 1). Pyroxenite layers are mostly spinel pyroxenitesand clinopyroxenites (Cr-pyroxenites of Obata, 1980).The microstructure is characterised by coarse olivinegrains with slightly curved grain boundaries and abun-dant triple junctions. The average olivine diameter in-creased by up to a factor of five relative to the spineltectonites (Fig. 2b) but left intact the pre-existing ol-ivine lattice fabric of the spinel tectonites ([100](010)type) (Van der Wal 1993, Van der Wal and Vissers1995). Spinel is often pinning olivine grain boundaries,and spinel inclusions in olivine are rare. The micro-structure can be entirely strain free, but olivine (100)subgrains occur. The microstructure can be classifiedbest as secondary recrystallised (Downes 1987).

Fine- and layered-granular peridotites

Towards the S and E, the coarse-granular peridotitesare bound by granular peridotites which are muchfiner grained (domain IIb). The orthopyroxene dia-meter recognized in the field is commonly (5 mm (seealso Fig. 2). They will be referred to as fine-granularperidotites. Two observations are tentatively inter-preted to indicate that the fine-granular peridotitesdeveloped at the expense of the coarse-granular perido-tites: (1) the evolution from coarse-grained to fine-grained is generally apparent in the field, where ortho-pyroxenes surrounded by an olivine matrix frequentlydisplay corroded grain-boundaries. (2) 100 m-scalelenses of coarse-granular peridotites are surrounded byfiner-grained granular peridotites (Fig. 1). The fine-granular peridotites are mainly lherzolites, which canbe rich in bright-green clinopyroxene (cpx) and spinel.Pyroxenite layers in this facies have mostly been re-placed by bright-green clinopyroxenites (Cr-pyro-xenites, Obata 1980, Garrido et al. 1993). Irregulardunite and harzburgite bodies locally impose a m-scalebanding, which is parallel to the pre-existing pyroxenitelayering near the recrystallisation front, but is discor-dant to the pre-existing layering towards the southernpart of the mapped area. The olivine microstructuresfound in this domain are identical to those of thecoarse-granular peridotites.

The layered-granular peridotites are confined toa small area in the SE part of the mapped area (domainIIc of Fig. 1), including the Arroyo la Cala area. Theyare characterised by a 1—100m-scale layering of har-zburgites and dunites in a matrix of relatively fertilespinel $ plagioclase-bearing lherzolites. Generally,

the orthopyroxene (opx) diameter is very small, on theorder of 2 mm. High cpx/opx ratios, low Mg(Mg#Fe)ratios and incompatible-element (LREE, Th, Nb, Ta)enriched patterns in the harzburgites and dunites ledRemaıdi (1993) to conclude that the layering of thisfacies formed by a melt-rock reaction process, witha melt flowing preferentially in the refractory rocktypes. The layering dips steeply towards the NE or E,and is oriented roughly perpendicular to the averageorientation of the recrystallisation front.

Although samples from the layered-granular facieswere included in our study, we will focus principally onthe formerly described spinel tectonites and coarse- andfine-granular peridotites.

Pyroxenites

Special attention is drawn to the orientation of thepyroxenite layering in the granular domain (Fig. 1).Although most of this layering has changed its mineralcomposition from garnet pyroxenite to spinel pyro-xenite and Cr-pyroxenite during the development ofthe granular domain (Garrido et al. 1993; Garrido1995), the orientation of these layers is still continuouswith the orientation of the pyroxenite layering in thespinel tectonites. In addition, pyroxenite fold shapestypical of the spinel tectonites are preserved in thegranular domain, but in the granular domain theyoccur in a homogeneous granular matrix (i.e. withoutan axial-plane foliation). It follows that the granulardomain replaced a km-scale deformed domain, witha structure probably similar to the spinel tectonites.

Geochemistry

Sampling procedure

Forty-one samples were collected from the mapped area, selected onthe basis of the above mentioned structural criteria, with the lowestpossible degree of serpentinisation. Serpentinisation can be con-siderable, but all geochemical trends discussed below are unrelatedto the degree of serpentinisation. Banding or layering discernible onthe specimen scale was avoided. Specimen size was in the order of1 kg. Half of the sample was crushed to powder, the other half wasprepared for sectioning.

Figure 2 shows that the field-based structural classification of thegranular peridotites (based on opx diameter) is reproduced down tothe scale of a thin section, although irregularities occur. The recrys-tallisation front is associated with a ‘‘jump’’ in opx diameter from(3 mm in the spinel tectonites, up to 7 mm in the coarse-granularperidotites. The fine-granular peridotites generally show opx dia-meters (4 mm. These data indicate that our sample collection isrepresentative for the studied area.

The powders were analysed for major elements and transitionmetals (Sc, Ti, Co, Ni, Cu, Zn) by XRF, and for rare earth elements(REE), and some other incompatible elements (Rb, Sr, Zr, Nb, Ta,Th) by ICP-MS (inductively coupled plasma-mass spectrometry).Titanium was also analysed by ICP-MS for better precision at lowconcentration levels. Outlines of the analytical procedure can be

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Fig. 2a Plot of maximum orthopyroxene diameter against the dis-tance from the recrystallisation front (tectonites negative x-values,granular peridotites positive x-values). Data measured by mean-linear intercept method on thin sections of dimension 6]8 cm. Noterecrystallisation front corresponds to increase in diameter forcoarse-granular peridotites by at least a factor of two. Note decreasein diameter for fine-granular peridotites. b Plot of average olivinediameter against distance from front. Note increase by up to a factor5 in granular peridotites

Fig. 3a, b Variation of orthopyroxene and clinopyroxene contentwith olivine content for a spinel tectonites and coarse-granularperidotites, b fine-granular peridotites and layered-granular perido-tites. Symbols a for Fig. 2. Also shown is compositional trend offertile spinel lherzolite (Kostopoulos 1991) after 0—40% partialmelting, in steps of 1% (stippled line). Modal compositions werecalculated with the inverse method of Tarantola and Valette (1982)applied to a mass-balance equation relating whole-rock and mineralcompositions (corrected for exsolution) for elements in the Cr-CFMAS system. Accuracy of the data is $1%

found in Ionov et al. (1992) and Remaıdi (1993). Representative dataare given in Table 12

Modal compositions

The spinel tectonites are mainly harzburgites with21—26% opx and 1—6% cpx (Fig. 3a). Three samples arelherzolites with &32% opx and &15% cpx. Harzbur-gites from the coarse-granular domain are composi-tionally very similar to the harzburgites from the spineltectonite domain (20—29% opx and 0—6% cpx), but thecoarse-grained lherzolites have lower cpx contents(11—14%) compared to the lherzolites from the spineltectonite domain (see also Fig. 7a). The range in modalcompositions for both the spinel tectonites and coarse-granular peridotites is consistent with an origin bypartial melting of a fertile spinel lherzolite host (Fig. 3a,Kostopoulos 1991), although similar modal variationscould be produced by an olivine-forming melt-rockreaction (Berger and Vannier 1984; Kelemen 1990).Three exceptions are the lherzolites from the spineltectonite domain which are too rich in opx. Similaropx-rich lherzolites have not been found in the granu-lar domain.

The fine-granular peridotites are mostly lherzolites,and show a continuous range from 22—30% opx and3—13% cpx. Deviations from the melting trend forfertile spinel lherzolite are more pronounced than forthe tectonites and coarse-granular peridotites, in par-ticular for opx at (60% olivine, and for cpx at65—70% olivine (Fig. 3b). Compared with the fine-granular peridotites, the analysed layered-granularperidotites show a similar range in modal compositions(Fig. 3b).

Major elements and transition metals

Major element concentrations show large variationswith several systematic trends. For example, CaO va-ries from 0.42 to 3.72 wt%, and increases with Al2O3content (Fig. 4a). The Al2O3 versus CaO trend is identi-cal for all structural facies and comparable to thatobserved in other parts of the Ronda peridotite (Freyet al. 1985), as well as in other lherzolite massifs(McDonough and Frey 1989). In contrast, mg-numbers[cationic ratio Mg/(Mg#Fe)] are slightly higher inthe coarse-granular peridotites compared to the otherfacies with similar CaO values (Fig. 4b). In addition,fertile peridotites (lherzolites with CaO'2.5 wt%) dis-play a wide range of mg-numbers unrelated to CaOvariations, with the highest values for the coarse-granu-lar peridotites and the lowest for the spinel tectonites.

2All data, including a sample locality map, are available from theauthors on request.

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Table 1 Modal compositionsand geochemistry ofrepresentative spinel tectonitesand granular peridotites fromthe W Ronda massif. (¸OI losson ignition, ¸hz lherzolite, Hzharzburgite)

Sample Spinel tectonites Coarse granularidentity

7 22 28 39 5 6 8 11

Modal compositions (wt%):OL 51 72 73 72 61 71 73 74CPX 32 21 22 26 28 23 21 22CPX 15 6 4 1 10 5 5 3

Lhz Lhz Hz Hz Lhz Lhz Lhz Hz

Major elements (wt%):SiO

242.4 39.8 39.6 39.7 41.5 41.0 40.8 39.5

TiO2

0.122 0.002 0.008 0.011 0.058 0.027 0.052 0.024Al

2O

33.18 1.28 1.12 0.76 2.84 1.62 1.32 1.12

Cr2O

30.36 0.36 0.37 0.47 0.38 0.43 0.43 0.40

Fe2O

38.85 8.22 8.04 8.20 8.31 8.33 8.21 7.68

MnO 0.15 0.14 0.14 0.14 0.14 0.14 0.13 0.14MgO 35.2 39.8 39.7 39.7 37.7 40.9 41.2 40.9CaO 2.71 1.36 1.08 0.62 2.17 1.40 1.22 0.88Na

2O 0.35 0.15 0.14 0.13 0.26 0.24 0.23 0.19

LOl 5.55 7.9 8.6 9.35 6.8 5.45 6.05 8.05& 98.9 99.0 98.8 99.1 100.2 99.5 99.7 98.9mg-number 0.887 0.906 0.907 0.906 0.900 0.907 0.909 0.913

Transition metals (ppm)Sc 14.7 9.8 9.4 8.0 13.3 9.9 8.3 7.7Ti 820 29 76 81 451 220 362 173Co 98 100 104 110 96 105 106 105Ni 1964 2172 2334 2456 2106 2289 2317 2396Cu 24.1 9.4 8.5 14.6 18.0 11.3 12.2 6.7Zn 45.6 36.3 37.1 36.8 36.7 39.9 37.5 34.9

Trace elements (ppm)Pb 0.281 0.087 0.142 0.203 0.512 0.099 0.134 0.258Sr 11.21 0.15 1.09 2.63 5.07 3.59 3.38 5.77Zr 5.24 (d.l. 0.36 0.27 2.15 1.20 1.91 0.71Nb 0.077 0.101 0.102 0.094 0.103 0.051 0.020 0.027La 0.005 0.062 0.226 0.034 0.023 0.023Ce 0.618 (d.l. 0.116 0.487 0.145 0.151 0.119 0.074Pr 0.117 (d.l. 0.016 0.049 0.0345 0.030 0.027 0.016Nd 0.684 0.007 0.071 0.144 0.252 0.181 0.194 0.105Sm 0.256 (d.l. 0.019 0.019 0.135 0.075 0.089 0.045Eu 0.1078 0.0011 0.0064 0.0065 0.0593 0.0315 0.0359 0.0204Gd 0.419 0.010 0.029 0.023 0.258 0.132 0.141 0.065Tb 0.0768 0.0029 0.0055 0.0044 0.0511 0.0235 0.0240 0.0131Dy 0.564 0.033 0.043 0.032 0.386 0.180 0.173 0.085Ho 0.1251 0.0108 0.0113 0.0081 0.0878 0.0407 0.0378 0.0199Er 0.396 0.042 0.041 0.029 0.275 0.124 0.115 0.060Tm 0.0547 0.0081 0.0077 0.0057 0.0435 0.0198 0.0175 0.0100Yb 0.369 0.066 0.060 0.040 0.291 0.133 0.115 0.066Lu 0.062 0.013 0.012 0.009 0.050 0.024 0.020 0.013Hf 0.2037 (d.l. 0.0139 0.0092 0.0895 0.0486 0.0690 0.0306Ta 0.0061 (d.l. 0.0064 0.0050 0.0009 0.0035 0.0019 0.0020Th 0.0065 (d.l. 0.0106 0.0250 0.0020 0.0021 0.0009 0.0013

Scandium is positively correlated with CaO, withno variations according to structural facies. The Tivaries in the range 20—1000 ppm and is positively corre-lated with CaO (Fig. 4c). However, in the refractoryperidotites (CaO(2.5 wt%) this correlation is ob-served only for the coarse-granular peridotites. Allother structural facies show substantial scatter irre-spective of CaO variation. The lowest Ti-values((200 ppm) are found in the spinel tectonites, whilethe fine-granular peridotites have Ti-concentrationsboth higher and lower than the coarse-granular perido-

tites. Copper and Zn are also positively correlated withCaO, but the scatter of the data is significant such thatno variations according to structural facies can berecognised.

Nickel varies in the range 1900—2500 ppm and isinversely correlated with CaO (Fig. 4d). No major vari-ations occur according to structural facies, except thatmany fine-granular peridotites are enriched in Ni irre-spective of CaO variation. The Co and Cr are alsoinversely correlated with CaO, but with significantscatter.

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Fine granular Layered

3 13 21 32 34 2 31

62 73 71 59 70 75 5828 23 23 28 22 22 299 3 5 12 7 2 12Lhz Hz Lhz Lhz Lhz Hz Lhz

42.0 40.3 40.3 41.9 41.1 40.9 42.30.041 0.005 0.058 0.019 0.02 0.012 0.0832.22 1.16 1.48 2.95 1.53 0.78 2.910.38 0.40 0.37 0.37 0.37 0.46 0.368.28 8.30 8.45 9.07 8.56 8.25 8.880.15 0.14 0.14 0.15 0.15 0.14 0.15

39.2 41.2 39.7 36.0 39.8 42.0 37.42.02 1.00 1.18 2.50 1.61 0.70 2.720.25 0.19 0.20 0.32 0.20 0.17 0.355.6 6.55 6.9 5.55 5.85 6.7 5.05

100.1 99.3 98.8 98.9 99.2 100.1 100.20.904 0.908 0.903 0.887 0.902 0.910 0.893

12.9 8.3 9.1 14.0 10.3 9.5 14.1240 67 412 775 150 99 529101.1 102.6 107.6 100.9 105.0 110.4 99.6

2131 2239 2291 2247 2255 2504 201116.0 8.4 14.5 23.0 12.2 8.8 20.849.8 42.0 38.3 39.9 43.4 38.6 45.8

0.220 0.165 0.129 0.524 0.169 0.196 0.3631.63 0.86 5.17 9.52 4.16 2.79 4.020.49 0.26 2.17 4.77 0.71 0.85 1.910.098 0.052 0.073 0.043 0.083 0.086 0.059

0.053 0.136 0.070 0.0320.188 0.077 0.197 0.492 0.377 0.181 0.1390.026 0.012 0.037 0.098 0.055 0.026 0.0390.123 0.060 0.226 0.583 0.248 0.118 0.3010.070 0.020 0.088 0.239 0.055 0.031 0.1630.0316 0.0065 0.0363 0.0973 0.0192 0.0121 0.07070.154 0.027 0.136 0.386 0.071 0.040 0.3030.0308 0.0046 0.0242 0.0713 0.0129 0.0064 0.06050.256 0.037 0.169 0.512 0.102 0.047 0.4670.0616 0.0088 0.0376 0.1156 0.0266 0.0114 0.10590.193 0.030 0.116 0.350 0.085 0.036 0.3300.0313 0.0055 0.0177 0.0539 0.0150 0.0062 0.04830.210 0.045 0.115 0.356 0.106 0.046 0.3230.0362 0.0088 0.0205 0.0603 0.0202 0.0098 0.05620.0321 0.0155 0.0761 0.1723 0.0249 0.0271 0.09940.0059 0.0018 0.0044 0.0050 0.0025 0.0047 0.00290.0051 0.0036 0.0024 0.0026 0.0113 0.0060 0.0030

In conclusion, mg-numbers and Ti$Ni variationsare decoupled to various degrees from elemental vari-ations correlated with CaO, and vary according tostructural facies. Elemental variations correlated withCaO or Al

2O

3have been observed in all massifs and

exist at all scales. Several studies have focussed on largerange geochemical variations occurring at 1—100 m-scale in layered sequences (e.g. Frey et al. 1991), butsmaller range variations occur at km-scale (Ronda:Frey et al. 1985; Lanzo: Bodinier 1988). Kilometre scalevariations of this type have been ascribed to variabledegrees of melt extraction. To our knowledge, elemen-tal variations unrelated to CaO or Al

2O

3concentra-

tions have been observed exclusively at cm—10 m-scale

(Wilshire et al. 1980; Fabries et al. 1989; Bodinier et al.1990; Takazawa et al. 1992; Kelemen et al. 1992). Suchsmall-scale variations are ascribed to (wall-rock) meta-somatic effects. Our data demonstrate the existence ofa third type of elemental variations, uncorrelated withCaO or Al

2O

3and occurring at km-scale.

Rare earth elements

Heavy rare earth elements (HREE), such as Yb, varyfrom 0.1 to 2.5 times chondrite, and are mostly posit-ively correlated with CaO (Fig. 4f ). As observed forTi, however, lower HREE abundances unrelated to

393

Fig. 4 CaO variation diagrams for Al2O

3, mg-number [Mg/

(Mg#Fe) cationic ratio], Ti, Ni, Ce/and Yb

n; n refers to chondrite-

normalised value preferred by Sun and McDonough (1989). Sym-bols as for Fig. 2. Note large range of data for all different structuralfacies distinguished in this study. Major elements and Ni obtainedby XRF (XRAL Activation Services Inc.). Titanium and REE ob-tained by ICP-MS (Montpellier). Precision generally better than10%

CaO-values occur in the refractory spinel tectonites, inparticular when compared with the coarse-granularperidotites. Light rare earth elements (LREE), such asCe, vary from 0.02 to 1.5 times chondrite and are also

positively correlated with CaO, but the scatter is signifi-cant, in particular for the spinel tectonites (Fig. 4e).

Chondrite-normalised diagrams of REE abund-ances are shown in Fig. 5a—c. The REE spectra for thestructural facies distinguished in this study show somesystematic differences as follows.

Fig. 5a–c Chondrite-normalised REE diagrams: a for spinel tecton-ites, b coarse-granular peridotites; c fine-granular peridotites. Nor-malising values from Sun and McDonough (1989). (Open symbols:lherzolites, closed symbols: harzburgites). Distance of sample fromrecrystallisation front in spinel tectonite domain indicated in a

394

Fig. 6a,b Primitive mantle-normalised diagrams for Rb, Th, Nb,Ta, Sr, Zr, Hf, Ti, Sc and most REE: a spinel tectonites; b coarse-granular peridotites. Element arrangement according to increasingperidotite-magma partition coefficient (Sun and McDonough 1989;McDonough and Frey 1990) Normalising values as preferred by Sunand McDonough (1989)

The spinel tectonites show different spectra for theharzburgites and lherzolites (Fig. 5a). The three lher-zolite samples have N-MORB (‘‘normal’’ mid oceanridge basalt) type REE patterns characterised by slightLREE depletion (0.50(Ce

//Sm

/(0.67), no signifi-

cant HREE fractionation (0.86(Eu//Yb

/(0.94),

with no significant variation according to the distancefrom the recrystallisation front (Fig. 7b). In contrast,the harzburgites have U-shaped REE patterns withvariable degrees of LREE enrichment (0.90(Ce

//

Sm/(6.40), which increase with increasing distance

from the recrystallisation front (Fig. 5a, 7b). The HREEare also significantly fractionated in the harzburgites(0.05(Eu

//Yb

/(0.70), and the degree of fractiona-

tion decreases with increasing distance from the recrys-tallisation front (Fig. 7c). The coexistence of N-MORBtype patterns in lherzolites and U-shaped patterns inrefractory peridotites have been observed previously inorogenic and ophiolitic peridotites (Prinzhofer and Al-legre 1985; McDonough and Frey 1989; Downes et al.1991; Frey et al. 1991). Note the striking ‘‘gap’’ betweenREE abundances, resulting from the contrasting modalcompositions and REE patterns [especially middle (M)REE] in lherzolites and harzburgites.

Compared with the spinel tectonites, the coarse-granular peridotites show less contrasted REE spectrabetween lherzolites and harzburgites. Both rock typeshave LREE-depleted (N-MORB type) patterns(0.27(Ce

//Sm

/(0.77), with relatively flat segments

for the HREE (0.57(Eu//Yb

/(0.91). Hence the

‘‘gap’’ observed for the spinel tectonites is strongly

reduced (Fig. 5b), mainly because the MREE andHREE in coarse-granular harzburgites have higherabundance relative to equivalent rock types in thespinel tectonites. The degree of fractionation of bothLREE and HREE is unrelated to the distance from therecrystallisation front, and about at the same level asmeasured for the lherzolites from the spinel tectonites(Fig. 7b, c). Such LREE-depleted patterns are rarelyobserved for refractory peridotites from peridotitemassifs (mostly U-shaped or flat patterns), except forrefractory peridotites from Lanzo (Bodinier 1988).Note that two rare samples in the coarse-granulardomain have trace element patterns similar to thespinel tectonites (samples 93.19 and 93.38, Fig. 5b,bracketed in Figs. 7b, c).

The fine-granular peridotites show a continuousevolution from LREE-depleted patterns in lherzolites(Ce

//Sm

/'0.19) to U-shaped patterns in the more

refractory peridotites (Ce//Sm

/(1.89) (Fig. 5c). How-

ever, the U-shaped patterns are less pronounced thanin the spinel tectonites and are observed mostly forolivine-rich lherzolites. In contrast to the coarse-granu-lar harzburgites, the HREE segments of the refractorylherzolites are not flat but show variable degrees offractionation (0.22(Eu

//Yb

n(0.94). The spectra for

the layered-granular peridotites included in this studyenclose the range defined by the fine-granular perido-tites.

Other incompatible elements (Rb, Sr, Th, Nb,Ta, Zr, Hf )

Primitive-mantle (PM) normalised diagrams for in-compatible element concentrations in representativespinel tectonites and coarse-granular peridotites areshown in Fig. 6. In both domains incompatible element

395

abundances range from .01 to 1 times PM. In addition,several different trends according to structural faciescan be recognised, summarised below in order of ele-ment incompatibility. We emphasize that these differ-ing trends are mostly unrelated to modal composi-tional changes, as refractory spinel tectonites andcoarse-granular peridotites have similar modal com-positions (Figs. 3a, 7a). For this reason we focus onspinel tectonites and coarse-granular peridotites only.

Fig. 7 Spatial variation of geochemical data for spinel tectonitesand coarse-granular peridotites across the recrystallisation front.For explanation see text. (Solid lines are regression of selecteddatapoints, n refers to primitive mantle-normalised values, recrystal-lisation front indicated by stippled line). Note fractionation ofCe/Sm, Eu/Yb and Ti/Sc in spinel tectonite domain

Rubidium: PM-normalised values (Rb/) for spinel

tectonites and coarse-granular peridotites fall in a lim-ited range (0.1—1.0 times PM) and, contrary to the REE,are not correlated with modal compositions or struc-tural facies. Rubidium is classically seen as a verymobile element, whose concentration is highly sensitiveto late alteration processes. The peculiar distribution ofthis element, however, does not result from re-mobilisa-tion associated with serpentinisation. Recent data onacid-leached minerals from Ronda (J-L. Bodinier et al.submitted) indicate that Rb is not concentrated in sec-ondary minerals, but in phlogopite enclosed in spinel(40—50% of whole-rock budget), probably derived fromformer fluid inclusions. Because of the very low par-titioning of Rb in the host minerals of phlogopite in-clusions, this element is ‘‘protected’’ from diffusionalre-equilibration processes. It thus shows variations un-related to those of other incompatible elements.

Thorium: Unlike Rb, Th shows concentration vari-ations consistent with REE trends. For example: Th inthe LREE-enriched harzburgites of the spinel tectonitedomain is ten times more concentrated (0.1—0.3 timesPM) than in the LREE-depleted harzburgites of thecoarse-granular domain (0.01—0.025 times PM). Thedifferent behaviour of Rb and Th according to struc-tural facies results in significant variation of theRb

//Th

/ratio across the recrystallisation front

(Fig. 7d). This ratio is (10 times PM in the spineltectonites, whereas the majority of coarse-granularperidotites shows much higher values, up to &50times PM.

Niobium and Ta: The lherzolites and harzburgitesof the spinel tectonite domain span a remarkably nar-row range between 0.1 and 0.15 times PM (Fig. 7e),although two samples close to the recrystallisationfront have lower values. Despite these convergingvalues, a difference between the lherzolites and harz-burgites should be noted. The values for the lherzolitesare consistent with other highly incompatible elements(Th and LREE: Fig. 6a), whereas the values for theharzburgites are not correlated with other incompati-ble element variations. Niobium and Ta in the coarse-granular peridotites show a much wider range from0.02 to 0.2 times PM and, more importantly, tend to becorrelated with other highly incompatible elements(Fig. 6b). The Nb—Ta concentration in Ti-oxidesaround spinel (J-L. Bodinier et al. submitted) can ex-plain the narrow range of these elements in spineltectonites, but these elements were probably re-mobil-ised during recrystallisation of the coarse-granular fa-cies, which involved spinel.

Strontium: The values in the spinel tectonites arewell correlated with other elements with similar degreeof compatibility (LREE), except for subtle positiveanomalies in the lherzolites. However, in the coarse-granular peridotites, Sr tends to be less variable andmore concentrated than LREE, which results in occa-sionally strong positive anomalies. In these rocks, Sr

396

could be somewhat concentrated in amphiboles, whichare generally present in minor amounts. However, sim-ilar Sr-anomalies were observed by Remaıdi (1993) inamphibole-free refractory peridotites from Arroyo laCala area, and were tentatively considered to representthe signature of a percolating boninitic melt (Gervillaand Remaıdi 1993).

Zirconium, Hf, Ti: These high-field-strength ele-ments (HFSE) are generally well correlated with REEof similar degree of compatibility, particularly in thecoarse-granular peridotites, except for subtle negative(Zr) or positive (Hf, Ti) anomalies in the spinel tecton-ites. Hence, they show the same variation as the MREEaccording to modal compositions and structural facies.It is mentioned here that the refractory fine- andlayered-granular peridotites (Remaıdi 1993) frequentlyshow marked negative Zr-anomalies, fractionated byminerals precipitated during melt-rock reaction (seealso Kelemen et al. 1990).

Scandium: Variations of this weakly incompatibleelement are well correlated with CaO unrelated tostructural facies. The Ti/Sc ratio (Fig. 7f), therefore,shows the same variation across the recrystallisationfront as previously noticed for Eu

n/Yb

n(Fig. 7c). This

value is high but almost constant in lherzolites at bothsides of the recrystallisation front (&30—50), but de-creases in the harzburgites from the spinel tectonitesdomain (down to &2) with decreasing distance fromthe recrystallisation front, before taking up renewedhigh values in the harzburgites from the coarse-granu-lar domain (5—40).

Discussion

Combining the above structural and geochemical dataallows us to separate them in both time (through thedifferent structural overprinting relations) and space(by, for example, relating REE data to distance fromthe recrystallisation front) to address the origin of thegranular domain, and the recrystallisation front in par-ticular. Major elements indicative of peridotite fertility(Al

2O

3, CaO), as well as compatible and only slightly

incompatible trace elements (transition metals such asSc and Ni) are highly variable in the studied area, butshow no significant variations across the recrystallisa-tion front. On the other hand, mg-numbers and mostincompatible elements such as REE, Th and HFSE(Nb, Ta, Zr, Hf and Ti) are unrelated to the formerelemental variations, and show systematic variations atthe recrystallisation front. Hence, the latter data mayconstrain the origin of the recrystallisation front andthe granular domain in general.

Below, the origin of the granular domain will bediscussed in order of development, i.e. from the possibleprotolith composition and structure, via a recrystal-lisation event and the origin of the coarse-granular

peridotites, to a mineral reaction event and the originof the fine-granular and layered-granular domain.

Spinel tectonites: protolith of granular domain

A protolith for the granular domain may be recognisedas follows. Within our sample collection, this protolithmay best be represented by the spinel tectonites col-lected at distance from the recrystallisation front. Inaddition, Frey et al. (1985) analysed other samples fromthe spinel tectonite domain collected at considerabledistances from the recrystallisation front. This group ofrocks is bimodal in composition (lherzolites rich in opxor harzburgites), shows variations in peridotite fertilityand compatible and slightly incompatible element con-centrations, with N-MORB REE patterns in the lher-zolites. We found no reliable indication that samplescollected in close proximity to the recrystallisationfront are more refractory than in any other area in thespinel tectonite domain, and we consider them to berepresentative of the entire tectonite domain.

Some differences are nevertheless observed betweenthe harzburgitic spinel tectonites analysed by Frey et al.and those analysed in this study, namely contrastingREE-patterns. The harzburgites analysed by Frey et al.at considerable distance from the front are character-ised by LREE-depleted REE-patterns. Harzburgitesin the vicinity of the recrystallisation front showU-shaped patterns. It will be shown below that thisfeature, in contrast with modal and major elementcompositions, is probably associated with the develop-ment of the recrystallisation front. As a consequence,the ‘‘protolith’’ REE composition is not represented inour samples, but most likely resembles the LREE-de-pleted harzburgitic spinel tectonites analysed by Freyet al. (1985).

Compared to other spinel lherzolite massifs, theprotolith is abnormal in the sense that refractoryperidotites are predominant. This probably indicatesthat the protolith has been affected either by localisedpartial melting (Frey et al. 1985) or percolation-reac-tion (e.g. Kelemen et al. 1992; Takazawa et al. 1992).This created variations, on a regional scale, in modalcompositions, peridotite fertility and compatible andslightly incompatible elements in a stage preceding (intime or space) the development of the granular domain.It is tempting to relate the development of the harzbur-gites in the spinel tectonites ahead of, but associatedwith, the granular domain. However, this is hamperedby the following observations: (1) all assemblages in thespinel tectonite domain are deformed to a similar ex-tent, which precludes an olivine-producing melt-rockreaction at a stage later than the deformation. (2) Theharzburgites have low mg-numbers, which are incon-sistent with partial melting of fertile spinel tectonites ata stage later than the deformation. Hence, the processesresponsible for the compositional heterogeneity of the

397

entire spinel tectonite domain and the processes alliedwith recrystallisation and development of the granulardomain are separated in time by an episode of deforma-tion, probably at low-temperature conditions (Van derWal and Vissers 1993).

Coarse-granular peridotites: recrystallisation athigh melt fractions

Relative to the protolith defined above, the coarse-granular peridotites show slightly higher mg-numbersand contrasted incompatible element patterns, de-coupled from variations in modal composition andperidotite fertility. It follows that recrystallisation musthave been a non-isochemical process. In addition,spinel tectonites near the recrystallisation front alsoshow an evolution of incompatible element concentra-tions spatially related to the coarse-granular perido-tites. Such geochemical variations necessitate the pres-ence of a fluid. Since the upper mantle is considered tobe non-permeable for C—O—H fluids (Watson andBrenan 1987; Brenan and Watson 1988), attention isfocused on a silicate melt.

High versus low melt fraction

At first sight, the hypothesis of porous flow of meltthrough the recrystallised domain appears paradoxical,since melt migration is generally thought to be asso-ciated with LREE enrichment (U-shaped REE pat-terns; Navon and Stolper 1987; Bodinier et al. 1990;Takazawa et al. 1992) which, in the area studied, isobserved principally in the non-recrystallised domain(the spinel tectonites).

This paradox is only apparent for the followingreason: U-shaped REE patterns are attributed tochromatographic effects associated with low melt frac-tions interacting with percolated peridotites. All stan-dard chromatographic models (Navon and Stolper1987; Bodinier et al. 1990; Vasseur et al. 1991) showthat for high melt fractions the chromatographic effectis strongly or completely reduced. This is expected tooccur in high-porosity systems and/or when the dura-tion of the percolation process is sufficient to achieveequilibrium between melt and peridotite. In this case,depending on mineralogy and melt composition, theperidotites are expected to show LREE-depleted pat-terns when the infiltrated melt is LREE depleted, orslightly enriched LREE patterns when the melt isLREE enriched. Thus, the recrystallised peridotitesmay very well have equilibrated with a LREE-depletedpercolating melt. Compared to ‘‘channelled’’ melt per-colation processes invoked for refractory peridotites(Bodinier et al. 1990; Takazawa et al. 1992; Kelemen etal 1992, 1995; Remaıdi 1993), the percolation processinferred for the recrystallised peridotites would thus

have been longer (time-integrated high melt fraction)and/or would have occurred at significantly higherporosity (‘‘instantaneous’’ higher melt fractions).

Instantaneous high melt fractions are preferred overtime-integrated high melt fractions for the followingreasons: (1) The size of individual, geometrically stablemelt pores must increase with increasing grain size, andthe melt fraction that can be contained in these poresincreases with the square of the grain size (see e.g. Rileyand Kohlstedt 1991). Thus, an increase of the averageolivine diameter, across the recrystallisation front, bya factor of 4 could have been associated with an in-crease in melt fraction by a factor of 16. In addition,pore size growth leads to an increasing fraction of grainboundaries wetted by a melt. Grain boundary diffusionkinetics are likely to be enhanced when a considerablepart of all grain boundaries are wetted. Consequently,the presence of a melt film at grain boundaries wouldaccelerate grain boundary migration and grain growthby providing a fast diffusion path between adjacentgrains, but also by dissolving small particles that poten-tially slow down grain boundary migration (Urai et al.1986). Feed-back processes are thus likely to existbetween increasing melt fraction and grain growth. (2)Recrystallisation and grain growth in the coarse-granu-lar peridotites are seen in both fertile and refractoryperidotites, which is consistent with pervasive percola-tion of high melt fractions, as opposed to ‘‘channelled’’flow of low melt fractions in refractory peridotites(Toramaru and Fujii 1986; Bodinier et al. 1991;Takazawa et al. 1992; Kelemen and Dick 1995, Kele-men et al. 1995). The similarity of geochemical vari-ations in coarse-granular harzburgites and lherzolites(e.g. REE) is consistent with this process.

An important implication of the above is that therecrystallisation front can be considered as a melt-accu-mulation front. Porosity cannot increase unless rock-volume increases or solid phases melt and/or dissolve.Partial melting and/or dissolution is preferred becauseit is consistent with changing modal compositions oflherzolites across the recrystallisation front (Figs 3a,7a) and higher mg-numbers in coarse-granular perido-tites. This melt was captured at the recrystallisationfront, because the fine-grained spinel tectonite micro-structure probably had a permeability much lower thanthe coarse-grained granular peridotites due to the com-bination of lower porosity and lower melt fraction. Therecrystallisation front may thus be seen as a permeabi-lity barrier, as well as a melt-accumulation front. Theaccumulated melt was probably a mix of the originalmelt infiltrated in the lithospheric peridotites with par-tial melt extracted from partially molten fertile lher-zolites, re-equilibrated with the percolated mantlerocks. Thus, the entire process may be viewed as partialmelting in an open system, associated with infiltrationand mixing with a deep-seated melt component.

Systematic variations among incompatible elementratios (Ce

//Sm

/, Eu

//Yb

/, Ti/Sc in Fig. 7) in spinel

398

tectonites as a function of distance from the recrystal-lisation front clearly indicate that these elements weremobilised by a percolating melt associated with recrys-tallisation in the granular domain. Thus, melt was infact present at both sides of the recrystallisation front.In contrast with the REE patterns of the coarse-granu-lar peridotites, most REE patterns in the tectonites arestrongly fractionated. This is consistent with a lowmelt-fraction percolation process as opposed to highmelt-fraction percolation in the coarse-granularperidotites (Navon and Stolper 1987). This would notonly explain the lack of recrystallisation in the spineltectonites (no enhanced grain boundary diffusion dueto lack of melt at grain boundaries), but also the factthat melt percolation was concentrated in the pre-existing refractory rocks (which are more permeablethan fertile peridotites, Toramaru and Fujii 1986) andnot in the lherzolites which show only minor REEfractionation.

All highly incompatible elements (volatiles, K, andhighly incompatible trace elements) are likely to beconcentrated to an unknown extent in these small meltfractions upstream from the recrystallisation front, dueto chromatographic effects (Navon and Stolper 1987).This is consistent with porous flow of low melt frac-tions in peridotites with a lower permeability than thegranular peridotites downstream from the recrystallisa-tion front, due to the low viscosity of the melt (McKen-zie 1989). Progressive solidification of this melt ac-counts for the LREE-enriched patterns observed atincreasing distance from the recrystallisation front.

REE patterns: source/sink effects

Another paradox arises from the statement in theabove paragraph that low melt fractions percolatedthrough the harzburgites in the spinel tectonite do-main. If we assume that the recrystallisation front rep-resents the base of an infiltrated column composed ofspinel tectonites, chromatographic models (Navon andStolper 1987; Vasseur et al. 1991) predict positiveanomalies of incompatible element ratios at increasingdistances from the front, arising from chromatographiceffects. While the evolution of Ce/Sm is consistent withthe models, those involving less incompatible elements(Eu/Yb, Ti/Sc) are not consistent. The latter ratiosshow an abrupt variation at the recrystallisation frontfrom elevated values in the granular peridotites to verylow values in the tectonites. With increasing distancefrom the recrystallisation front, these ratios increasetowards values more similar to those of the coarse-granular peridotites. This depletion of incompatibleelements in the spinel tectonites towards the recrystal-lisation front in particular affected the MREE (respon-sible for the ‘‘gap’’ in the REE patterns), but also HFSEelements with similar degrees of incompatibility as theMREE (Zr, Hf, Ti), which show similar depletions

without significant fractionation. The HREE and alsoLREE are affected to a lesser extent. This is particularlytrue for the highly incompatible element Th (Fig. 7d).

Such variations may be understood with the help ofa numerical model recently proposed by Godard et al.(1995), which aims to predict trace element evolutionsassociated with the migration of a melt-rock reactionfront through a percolation domain. A brief descriptionof this model, applied to the case of a reaction markedby increasing melt fraction without modal change, isgiven in the appendix, and the results outlined in thenext paragraph. Our combined structural and geo-chemical data indicate that the percolation-reactionmodel proposed by Godard et al. may be applied to thestudied area, with three important constraints: (1) Aswill be discussed further on, the percolation domainupstream from the recrystallisation front probably em-bodied all of the granular and plagioclase tectonitedomains as mapped by Van der Wal and Vissers (1995).In addition, the metamorphic zoning which is so typi-cal of the studied area, is also observed in other perido-tite massifs in southern Spain (F. Gervilla, personalcommunication). The recrystallisation front, therefore,must have been a regional feature extending over atleast tens of kilometres in the western Mediterraneanupper mantle. Compared to the lateral dimensions, thewidth of 200 metres may be viewed as very narrow.Thus the ‘‘instantaneous’’ reaction model developed byGodard et al. can be applied, which is a simplifiedversion of the general model. (2) Although a significantincrease of melt fraction associated with partial meltingcannot occur without some changes in peridotite mo-dal compositions (modal compositional changes areindeed observed for the lherzolites across the front),the reaction occurring at the front is mainly amelting/dissolution process in the sense that there isno evidence for minerals precipitated at this stage (incontrast to the evolution proposed for the fine-granularand layered-granular domains, see below). This is con-sistent with the lack of significant fractionation ofHFSE such as Zr, Hf and Ti relative to REE, as well asCr relative to Ni, in both spinel tectonites and coarse-granular peridotites. Significant fractionation of theseelements is predicted during melt-rock reactionsmarked with dissolution/precipitation of pyroxenes(Kelemen et al. 1990; Remaıdi 1993; Godard et al.1995). Indeed, such anomalies are typical of the fine-granular and layered-granular domain (Remaıdi 1993).A major consequence is that only source/sink effects(see appendix) of melt formation are to be considered.

Modelling results

According to the above constraints, the ‘‘instan-taneous’’ reaction model of Godard et al. (see appendix)allows us to predict that the migrating high melt-frac-tion front will be advanced by a domain depleted in

399

incompatible elements as follows. When porosities andmelt velocities in the unreacted and reacted domains('

0and '

1, and »

M,0and »

M,1, respectively), as well as

reaction front velocity (VR), are fixed (Fig. 8), the com-

position of peridotites located downstream from thereaction front critically depends on D*, which corres-ponds to the highest bulk peridotite/melt partition co-efficient value for which trace elements have theirchromatographic front downstream from the reactionfront (see appendix).

Fig. 8a Trace element concentration of two harzburgites of thespinel tectonite domain, collected close to the recrystallisation front(sample 93.22 at 150 metres and sample 93.23 at 200 metres), nor-malized to composition of a coarse-granular harzburgite with sim-ilar modal composition (sample 93.6) and plotted against bulksolid/liquid partition coefficients. Bulk partition coefficients werefixed according to the average modal composition of the threesamples considered (73% olivine, 22% opx and 5% cpx, spinelneglected). Mineral/melt partition coefficients for Th, Zr, Sc andREE as proposed by Remadi (1993). For Ti, Co and Ni we usedpublished values for cpx, and inter-mineral data for opx and olivineare used (Bodinier et al. 1987). On the logarithmic scale used here,variations in literature values of partition coefficients do not signifi-cantly affect the shape of the observed patterns. b Same diagram forthe trace-element concentrations of peridotites calculated with the‘‘instantaneous’’ percolation-reaction model of Godard et al. (1995).Compositions of peridotites located immediately ahead of the reactionfront are normalised to the composition of peridotites re-equilibratedwith the infiltrated melt. Curves labelled 1 to 5 indicate calculationsperformed with '

1"0.1 and five sets of »

M,1/»

Rand '

0values: (1)

»M,1

/»R"1.1, '

0"0.01, (2) »

M,1/»

R"1.3, '

0"0.05, (3)

»M,1

/»R"1.3, '

0"0.01, (4) »

M,1/»

R"1.3, '

0"0.001, (5)

»M,

/»R"2, '

0"0.01. Further explanation is provided in the text

For D(D*, the peridotites located immediatelydownstream from the reaction front are also locatedupstream from the chromatographic fronts of the cor-responding trace elements. Their composition (C

S{) is

related to that of peridotites equilibrated with the infil-trated melt (C

S,1) by Eq. (2) of the appendix. Curves

labelled 1 to 5 in Fig. 8b indicate calculations per-formed with '

1"0.1 and five sets of V

M,1/V

Rand

'0

values. For these parameter values, D* is systemati-cally lower than one; thus only incompatible elementshave their chromatographic fronts ahead of the reac-tion front. As predicted by Eq. (2), these elements aredepleted between the two fronts, compared to the com-position of peridotites equilibrated with infiltrated melt(C

S{/C

S,1(1). Note from Fig. 8b that the depletion is

much more pronounced for D values close to D* (e.g.,MREE and Ti) than for low D values (e.g. LREE andTh). Such depletion of incompatible elements is indeedobserved in the harzburgites of the spinel tectonitedomain (Fig. 8a), and may lead to very unusual REEpatterns in samples close to the recrystallisation front,which show extreme depletions of MREE relative toHREE (cf. sample 93.22, Fig. 5a). Sink effects asso-ciated with increasing melt fraction at the front andsource effects associated with progressive melt solidifi-cation downstream from the front accounts for thepronounced U-shaped REE patterns observed in therefractory peridotites from the spinel tectonite domain,and for the systematic variation of trace elements asa function of distance from the front.

For D'D*, trace elements have their chromato-graphic fronts located upstream from the reaction front(or at the front). For these elements, all peridotiteslocated downstream from the reaction front have thecomposition of the protolith, except for a minor devi-ation due to the formation of a low melt fraction.Therefore, concentration ratios of these peridotites tothe ‘‘re-equilibrated’’ ones are independent of thechromatographic and sink/source effects. Since modalcompositions are assumed to be constant, these ratiosare identical to those of the low melt fraction in equilib-rium with the protolith (C

M,0) to the infiltrated melt

(CM,1

). Though undetermined, the CM,0

/CM,1

, ratiosare assumed to plot within the variation range ofmantle derived volcanic rocks (represented by theshaded area in Fig. 8b). For D values greater than 0.1(e.g., Sc, Co and Ni), C

M,0/C

M,1displays only minor

deviations from unity.A model accounting reasonably well for the pat-

terns of the studied samples is shown by the heavycontinuous line in Fig. 8b. The low MREE values andthe abrupt compositional change from MREE toHREE are explained by D* values in the range 0.03 to0.05, which implies '

1(»

M,1/»

R) values in the range

0.05 to 0.2. For D values greater than D*, the analyticaldata are best fitted by assuming that the infiltrated meltis more enriched in incompatible elements than a lowmelt fraction in equilibrium with the protolith. This is

400

consistent with the overall refractory character of theprotolith in the studied area, as represented by thespinel tectonites.

Diffusional effects and buffering of Rb—Nb—¹a bymicrophases

The observed REE patterns (Fig. 8a) would be fittedeven better by the modelled patterns (Fig. 8b) by takinginto account the effects of diffusion. Although complex,these effects may be qualitatively predicted with thehelp of percolation-diffusion models (Navon and Stol-per 1987; Vasseur et al. 1991). Diffusion would mainlyreduce the abrupt compositional changes observed inthe vicinity of D*, by smoothing the chromatographicfronts of the elements. In particular, the unrealisticfeature of C@

S/C

S,1P0 when DPD* (see appendix)

would disappear if diffusion was considered (Godardet al. 1995). Moreover, diffusion in minerals is expectedto delay the re-equilibration of peridotites locateddownstream from the reaction front (see Fig. 9). There-fore, the actual concentration ratios can be significantlyhigher than the calculated C@

S/C

S,1values, especially

when CS,0

is much higher than C@S. This might explain

the relatively high value observed for Th in sample93.23.

Rubidium, Nb and Ta in the spinel tectonites arenot sensitive to the above processes because they arebuffered by phlogopite (Rb) and titanate (Nb and Ta)rims around spinel as follows. Volatile- (and K-) richmelts, inferred to have percolated through the spineltectonite domain, are likely to be saturated in Ti-oxides(J-L. Bodinier et al. submitted and references therein).Thus, small fractions of phlogopite and Ti-oxides afterspinel probably precipitated downstream from the re-crystallisation front, explaining the buffering of theabove incompatible elements (J-L. Bodinier et al. sub-mitted). In contrast, large fractions of a basaltic melt,inferred to have percolated through the granular do-main, are likely to be undersaturated in Ti-oxides(Ryerson and Watson 1987), affecting Rb, Nb and Ta toa similar extent as other incompatible elements withsimilar degree of incompatibility.

Fine- and layered-granular peridotites: freezingreactions

The transition from coarse-granular peridotites to fine-granular peridotites is accompanied by mineralogicalreactions, namely replacement of orthopyroxene byolivine resulting in typical corroded orthopyroxenegrain boundaries observed in the field, and the increasein clinopyroxene $ spinel abundance throughout theentire fine-granular domain. Remaıdi (1993) showedthat the olivine-producing reaction in the Arroyo laCala area produced U-shaped REE patterns, whereas

Fig. 9a—c Synoptic diagram explaining REE variation across therecrystallisation front. a Microstructural and melt-fraction vari-ations across the front. (M.F. melt percolation front, C.F. chromato-graphic front for a particular incompatible element with D(D*, e.g.LREE and MREE, see Fig. 8, R.F recrystallisation front, ' denotesporosity). Note the increase of porosity in coarse-granular perido-tites due to grain growth. b Hypothetical composition of peridotitesacross the recrystallisation front. (C

S,0composition of protolith,

CS,1

composition of peridotites fully re-equilibrated with infiltratedmelt, C@

Scomposition of peridotites instantaneously re-equilibrated

with percolating melt depleted in incompatible elements due to thesink effect of the recrystallisation front, see Figure 8 for explanation,C

SA composition of peridotites taking into account the effect ofdiffusion, see text for explanation). c Comparison with REE pat-terns found in peridotites at either side of the recrystallisation front.From left to right: protolith, spinel tectonite affected by a low meltfraction, and fully recrystallised granular peridotites equilibratedwith a high melt fraction. The strongly LREE- and MREE-depletedpatterns of spinel tectonites result from the sink effect of the recrys-tallisation front. Possible protolith REE composition: sample 856 ofFrey et al. (1985)

clinopyroxene-producing reactions are characterisedby convex-upward REE patterns. Both patterns areobserved in the fine-granular domain, in agreementwith the observed type of mineralogical reactions.

Whereas olivine-producing reactions are thought tooccur due to reaction with increasing melt fraction

401

(Kelemen 1990; Berger and Vannier 1984), clino-pyroxene-producing reactions are believed to occur atdecreasing melt fraction (Bodinier 1988). The fine-granular peridotites are enriched in clinopyroxene, butnot significantly depleted in orthopyroxene comparedwith the spinel tectonites and coarse-granular perido-tites. This indicates that melt-rock reactions at decreas-ing melt fraction were dominantly responsible for thedevelopment of the fine-granular domain. In addition,the high HREE abundances relative to the coarse-granular peridotites also point to mineralogical reac-tions at decreasing melt volume (Kelemen et al. 1992).

Our interpretation of the fine-granular peridotitesis that they developed at the expense of the coarse-granular peridotites, which reacted with melts that pro-gressively ‘‘drained’’ from the granular domain duringcooling of the massif. This would explain the increaseddegree of localisation of the fine-granular domain to-wards the SE (Fig. 1). Further ‘‘draining’’ and localisa-tion of melt flow could have happened in the layeredgranular peridotites, leading to the strongly layeredappearance that is so typical for many peridotitemassifs, but further interpretation of this issue is ham-pered by poorly constrained overprinting relationshipsbetween the fine- and layered-granular peridotites.

Comparison with previous work

Our proposed model explaining the structure and geo-chemical properties of the western Ronda peridotitesinvolves both percolation of infiltrated melt andmelting of percolated peridotites. Therefore, it permitsreconciliation of the model proposed by Frey et al.(1985) and the constraints brought by Reisberg et al.(1989) concerning the magmatic evolution of the Rondamassif. The former model, based on major and traceelement geochemistry, suggested that the Ronda massifsuffered a partial melting event of increasing degreefrom garnet peridotites to plagioclase peridotites. Thisprocess is suggested by more depleted LREE contentsin the latter, which is exactly what we observed at therecrystallisation front when comparing the coarse-granular peridotites to the spinel tectonites. Structuralevidence shows that plagioclase lherzolites were formedin a very late evolutionary stage (emplacement of themassif at subsolidus conditions), at the expense ofgranular peridotites (Van der Wal and Vissers 1993).Thus, their geochemical composition is probably theresult of the melting, percolation and melt-rock reac-tion evidenced in the granular peridotites. In particular,the melting event proposed by Frey et al. is probablythe same as the one associated with the migration of therecrystallisation front. On the other hand, Reisberget al. (1989) attributed the more enriched Nd-Sm iso-topic composition of the plagioclase lherzolites (andprobably also granular peridotites) to a recent event ofmelt infiltration through the massif. This is in agree-

ment with our model where melting is coupled witha major event of magma percolation. This model couldtherefore account for km-scale geochemical variations(such as are also observed in other massifs, e.g. Lanzo,Bodinier et al. 1991) characterised by an evolutionfrom depleted ‘‘lithospheric’’ isotopic signature (e.g.eN$

'MORB) to more enriched compositions.

Concluding remarks

Several conclusions can be put forward, summarisedschematically in Fig. 9. The first and most important isthat km-scale pervasive porous flow occurred in therefractory portion of the spinel tectonites and the entirecoarse-granular domain of the W Ronda peridotite.Two domains recognised on the basis of structuralcriteria correspond to two different modes of porousflow: low melt-fraction porous flow and high melt-frac-tion porous flow (Fig. 9a). The transition between thesetwo domains, recognised as a recrystallisation front, isinterpreted as a melting front and a permeability bar-rier, arising from feed-back processes between graingrowth, melting, permeability and melt flow. Lateevolutionary stages of porous melt flow in the Rondaperidotite led to mineralogical reactions (cpx- or ol-ivine-producing melt-rock reaction), characterised byincreased degrees of flow localisation.

The recrystallisation front is spatially associatedwith L#MREE-depleted REE patterns in the non-recrystallised peridotites located just ahead of the front.We propose that this depletion is due to ‘‘sink’’ effectsof increasing melt volumes at the recrystallisation front(Fig. 9b). Spatial variations of elements with variabledegrees of incompatibility ahead of the recrystallisationfront are principally controlled by diffusion kinetics ofthese elements in minerals, ‘‘diluting’’ the above ‘‘sink’’effects. In addition, chromatographic effects and solidi-fication of the small melt fractions at distance from thefront leed to selective LREE enrichment. The net effectof both is to create moderate to very strong U-shapedREE patterns in residual peridotites ahead of the re-crystallisation front (Fig. 9c).

Whereas low melt-fraction porous flow associatedwith melt-rock reactions may account for subduction-related trace element signatures such as HREE de-pletion, HFSE depletion (Zr, Ti: Kelemen et al. 1990)and Nb-Ta anomalies (J-L. Bodinier et al. submitted,this study), high melt-fraction porous flow such asis envisaged for the coarse-granular peridotites ac-counts for more ‘‘asthenospheric’’-type trace elementsignatures with no significant fractionation of HFSE. Itfollows that porous flow is probably not more likely tooccur in convergent tectonic settings than in any othertectonic setting (see also Kelemen et al. 1995).

It is likely that the recrystallisation front extendedover a lateral dimension exceeding tens of kilometresin the W Mediterranean upper mantle. This transition,

402

in time and space, between low to high melt-fractionporous melt flow could correspond to a magmaticevolution in time from a convergent tectonic settingtowards a spreading environment (cf. Fig. 3 of Van derWal and Vissers 1993), presumably associated with theNeogene detachment of a subducted slab underneathS Spain, leading to extensional collapse of the pre-viously thickened lithospheric mantle wedge (Van derWal and Vissers 1993). The structural and geochemicalfeatures associated with the transition from low to highmelt-fraction porous flow may be viewed as an exampleof progressive thermal erosion of cold, deformed het-erogeneous lithosphere into more homogeneous unde-formed asthenospheric-type mantle.

Acknowledgements D.vdW thanks the colleagues in the formerCGG and other departments for their hospitality during his post-doctoral stay in Montpellier. The help of Guy Vasseur, JacquesVernieres and Marguerite Godard in modelling matters is greatlyacknowledged. Discussions with Carlos Garrido and Malika Re-maıdi have contributed significantly to this work. Special thanksgo to Eliane Nadal, Simone Pourtales, Liliane Savoyant andErna Huisman in logical, technical and moral support. PeterKelemen and an anonymous referee are thanked for their con-structive reviews. This project was sponsored by the Dutch Organi-sation for Scientific Research (NWO), dossier S 75—343. Additionalfinancial support for the analytical work came from the MinistereFraniais des Affaires Etrangeres (dossier CIES no. 094325H and‘‘Action Integree Franco-Espagnole Programme Picasso 1993’’).Financial support to J.L.B. from the Institut National des Sciencesde l’Univers (CNRS) reference DBTF5/03 is greatly acknowledged.

Appendix

The standard chromatographic model

When porous melt flow is not associated with significant modaland/or melt-volume changes (due to melt-rock reaction), the stan-dard chromatographic approach (Navon and Stolper 1987) may beused to describe time and space distribution of trace elements in themelt fraction and the solid phases. In this situation, melt percolationis associated with the formation of chromatographic fronts whichcorrespond to the concentration changes of trace elements due to thechemical exchange between peridotite minerals and the infiltratedmelt in disequilibrium with them. If instantaneous local equilibriumis assumed, the velocity of a chromatographic front (»

CF) is related

to that of the melt (»M) by the equation:

»CF

"»M

/(1##D), with #"(1!')oS/'o

M(1)

where D is the bulk peridotite/melt partition coefficient of the traceelement considered and ' the rock porosity ("melt fraction). Theterms o

Sand o

Mstand for peridotite and melt densities, respectively.

Upstream (or behind) the chromatographic front, i.e. in the t (time)and z (distance) domain characterized by z/t(»

CF, the composition

of the peridotite (CS,1

) is controlled by that of the infiltrated melt(C

S,1"DC

M,1). Conversely, downstream (or ahead of ) the front

(z/t'»CF

), the composition of the melt fraction (CM,0

) is controlledby that of the original peridotite (C

M,0"C

S,0/D). Equation (1)

shows that chromatographic fronts of the more incompatibleelements move faster than those of less incompatible and compatibleelements. This is thought to account for trace element fractionationobserved in percolated mantle rocks (e.g. REE—Navon and Stolper1987; Bodinier et al. 1990; Takazawa et al. 1992).

Published chromatographic models generally involve more com-plex processes of trace element transfer between melt and peridotite,such as instantaneous equilibrium at grain boundaries coupled withdiffusion in mineral grains (Vasseur et al. 1991). As schematicallyillustrated in Fig. 9, taking into account that diffusion is oftenessential for fitting mantle rock data (as for the spinel tectonites ofthe studied area). However, it should be noted that the strongestchromatographic effects are observed with the instantaneous equi-librium approach. This is because diffusion delays peridotite-meltre-equilibration which results in significant smoothing of thechromatographic fronts (Navon and Stolper 1987; Vasseur et al.1991). Hence, the simplified instantaneous equilibrium approach isjustified when the aim of modelling is mainly to understand theoutstanding features of the percolation process, such as chromato-graphic effects and sink/source effects of melt-rock reactions (seebelow). In addition this approach may be further justified whenmineral-melt equilibration is triggered by recrystallisation (as in thegranular peridotites of the studied area) or by dissolution-precipita-tion processes associated with melt-rock reactions.

The percolation-reaction model: sink-source effects coupled withchromatographic effects

A numerical model of percolation taking into account the effects ofmodal and/or melt-fraction ("porosity) changes associated withmelt-rock reactions was recently proposed by Godard et al. (1995).For the purpose of this study, we shall limit ourselves to the situ-ation where the reaction zone has a negligible thickness and corres-ponds to a sharp front moving at a velocity»

R. This simplified version

of the general percolation-reaction model assumes both instantaneousperidotite-melt equilibration at local scale and instantaneous reaction(both critical diffusion and critical reaction times are negligible).

Modal and/or porosity changes occuring at the reaction frontare responsible for significant changes in chromatographic frontvelocities across the reaction front, as a result of changing D and/or# values in Eq. (1). In the case of olivine-forming reactions(D

1(D

0), pyroxene-hosted elements (such as Cr and the incompat-

ible elements) are characterised by higher chromatographic velo-cities in the reacted peridotite than in the protolith (»

CF,1'»

CF,0).

The opposite is observed for olivine-hosted elements such as Ni(D

1'D

0), and also for clinopyroxene-producing reactions.

Porosity variations without mineral-forming reactions can beconsidered as follows: because constant volume is assumed for thepercolation system (i.e. no compaction or expansion occurs duringthe time considered, the magmatic flux being constant across thereaction front), porosities and magmatic velocities are related by»

M,1'

1"»

M,0'

1. The terms '

0and '

1represent the porosities

downstream and upstream of the reaction front, respectively, while»

M,0and »

M,1are the melt velocities in the same domains. It follows

that incompatible elements are characterised by lower chromato-graphic velocities in the reacted peridotite than in the protolith forpositive melt-fraction variations (melting — '

1''

0, #

1(#

0,) and

lower chromatographic velocities for negative melt-fraction vari-ations (crystallisation — '

1('

0, #

1'#

0). The opposite is ob-

served for compatible elements.These changes in chromatographic front velocities at the reac-

tion front are responsible for transient sink/source effects superim-posed on the chromatographic effects described above. The resultingdistribution of trace element concentrations along a percolation-reaction column may be predicted according to six different situ-ations determined by the relative magnitudes of »

R, »

CF,0and »

CF,1.

These situations are described by Godard et al. (1995) for modalchanges, and by Godard (1993) for porosity changes. For the pur-pose of this study, we will focus only on the three situations cor-responding to increasing porosity across the reaction front, withmodal compositions considered to be constant (D

1+D

0+D).

These situations are described by Godard (1993) and may be sum-marised as follows:

403

(i) For »CF,0

'»CF,1

'»R

(incompatible elements) and»

CF,1'»

CF,0'»

R(compatible elements), the reaction front is pre-

ceded by a chromatographic front moving at a velocity »CF,0

(notethat the chromatographic front in the reacted zone cannot overtakethe reaction zone). These two fronts define three time and spacedomains of trace element concentrations. One domain, locateddownstream from the chromatographic front (z/t'»

CF,0), has

a composition controlled by the protolith (CM,0

"CS,0

/D). Anotherdomain, located upstream from the reaction front (z/t(»

R), is

compositionally controlled by the infiltrated melt (CS,1

"DCM,1

).The third domain, located between the chromatographic front andthe reaction front (»

R(z/t(»

CF,0), is affected by sink/source ef-

fects resulting from porosity changes at the reaction front. Accordingto Godard (1993), the compositions of peridotite and melt in thisdomain (C

M{and C

S{, respectively) are related to those of the infil-

trated melt (CM,1

) by the following equation:

CM{

/CM,1

"CS{/C

S,1"

'1(»

M,1/»

R)!'

1!D(1!'

1)

'1(»

M,1/»

R)!'

0!D (1!'

0)

(2)

For simplicity, this equation assumes similar densities for solidand liquid phases (o

S+o

M+o). Note that Eq (2) is valid only for

D lower than the critical value D*"('1(»

M,1/»

R)!'

1)/(1!'

1),

for which CM{

/CM,1

"CS{/C

S{,1"0. It may be shown that D*

corresponds to the condition for which the chromatographic front inthe reacted domain moves at the same velocity as the reaction front(»

CF,1"»

R). All elements characterised by D(D* have their

chromatographic front downstream of the reaction front. Incompat-ible elements are depleted between these two fronts (C

M{/C

M{,1(1),

while compatible elements are enriched (CM{

/CM,1

'1). As illus-trated in Fig. 8, the degree of depletion varies as a function of D, witha minimum depletion for DP0 [C

M{/C

M,1P('

1(»

M,1/»

R)!

'1)/('

1(»

M,1/»

R)!'

0)], and a maximum depletion for DPD*

[CM{

/CM,1

P0]. This variation results from a ‘‘dilution’’ of thesink effect within a large volume of percolated medium for themost incompatible elements, because their chromatographic frontis located at great distance from the reaction front. Note thatthe ‘‘dilution’’ effect is more important for elevated values of»

M,1/»

R.

(ii) For »R'»

CF,0'»

CF,1(incompatible elements) and

VR'»

CF,1'»

CF,0(compatible elements), the chromatographic

front is located upstream from the reaction front and moves ata velocity »

CF,1. The two domains located downstream and up-

stream from the percolation-reaction column (z/t'»R

andz/t(»

CF,1respectively) have similar trace element compositions as

in the previous situation. The composition of the intermediate do-main (»

CF,1(z/t(»

R) is related to that of the protolith by the

equation (Godard 1993):

CM{

/CM,0

"CS{/C

S,0"

'1(»

M,1/»

R)!'

0!D(1!'

0)

'1(»

M,1/»

R)!'

1!D(1!'

1)

(3)

As in the previous situation, sink/source effects coupled withchromatographic effects are responsible for depletions of incompat-ible elements and enrichments of compatible elements in this do-main.

(iii) Finally, two particular situations are predicted when thechromatographic fronts progress roughly at the same rate as thereaction front. When »

CF,1'»

R'»

CF,0, the reaction front and

the chromatographic fronts are merged into one single front wherecompatible trace elements are accumulated. The instantaneousconcentration profile of a given trace element is characterised bya Dirac distribution, the amplitude of which can be calculatedfrom the budget of the trace element fluxes throughout the per-colation-reaction column (Godard et al. 1995). On the other hand,when »

CF,0'»

R'»

CF,1, two chromatographic fronts may be

distinguished downstream and upstream from the reaction front,respectively. Between them, the concentration of incompatible traceelements is zero. However, it should be noted that both the Dirac

distribution and the zero concentration level predicted by the instan-taneous equilibrium model are very unlikely to occur in naturalpercolation-reaction systems, where these extremes will be dimin-ished by diffusion processes.

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