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SERPENTINE-CARBONATEMINERALIZATION IN KIMBERLITESV. T. Podvysotskiy aa East Siberian Research Institute for Geology, Geophysics, andMineral Raw Materials, IrkutskPublished online: 29 Jun 2010.
To cite this article: V. T. Podvysotskiy (1985): SERPENTINE-CARBONATE MINERALIZATION INKIMBERLITES, International Geology Review, 27:7, 810-823
To link to this article: http://dx.doi.org/10.1080/00206818509466468
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SERPENTINE-CARBONATE MINERALIZATION IN KIMBERLITES
V. T. Podvysotskiy
Translated from "Serpentino-karbonatnaya mineralizatsiya v kimberlitakh," Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, 1985, No. 2, pp. 234-247. The author is with the East Siberian Research Institute for Geology, Geophysics, and Mineral Raw Materials, Irkutsk. The article was selected for translation and publication because it provides new insight into the processes controlling the formation of serpentine and carbonate minerals in kimberlite bodies.
Carbonates and minerals of the serpentine group are widely distributed in kimberlites and frequently constitute 80-90% of their volume. There are no diamond-bearing kimberlites known that lack serpentine and carbonates, and thus these are not only rock-forming minerals (in most cases) but also typical of this rock type. In spite of the extensive occurrence of serpentines and carbonates in kimberlites, little attention has been paid to these minerals, much more having been given to accessory minerals (pyrope, picroilmenite, etc.), as well as to abyssal ultra-basic xenoliths. A detailed study is required of serpentine-carbonate mineralization to deduce the primary kimberlite composition, and also to improve mineralogical methods of locating diamond deposits by the use of minerals from the light fraction of kimberlites.
Occurrence of Serpentine and Calcite in Kimberlites
Serpentine-carbonate mineralization has been examined in kimberlite bodies to the deepest accessible levels, 1200 m from the surface. The distributions of the serpentine and carbonates in plan and with depth in multiphase pipes are uneven and are determined mainly by the orientation of the particular kimberlite body, as kimberlites are usually distinguished on the degree and character of metasomatism. The character of the latter determines the kimberlite's color (blue, green, or yellow), which are used in some cases (particularly for African kimberlites) in distinguishing petrographic types
in multiphase pipes. In a single-phase body (composed of one type of kimberlite), there are no sharp fluctuations in the serpentine and calcite distributions, but the degree of alteration of olivine decreases somewhat with depth; this also occurs within the individual kimberlite varieties in the multiphase pipe. The relation between serpentine mineralization and carbonate varies considerably from one kimberlite body to another. For example, serpentine predominates in some pipes, whereas carbonate is extensive in others. High levels of carbonates (calcite ± dolomite) are characteristic of kimberlite veins linked to the pipes. Vein kimberlites often are not visually distinguishable from marmorized limestones, and can be identified only by the presence of pyrope and picroilmenite phenocrysts.
Serpentine and carbonates occur as pseudo-morphs after primary minerals (mainly after olivine phenocrysts, and also after olivine and pyroxene in ultrabasic xenoliths), and along with other minerals they form ground-mass and late hydrothermal monomineralic or polymineralic segregations, and also develop after xenoliths of sediments and metamorphites. We consider the features of serpentine-carbonate mineral formation by reference to various components.
Olivine phenocrysts and ultrabasic xenoliths. The serpentine occurring after olivine phenocrysts appears in thin section as colorless, pale green, golden-yellow, or greenish-yellow, often being stained by iron hydroxides
810 Copyright © 1985 by V. H. Winston & Sons, Inc. All rights reserved.
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FIGURE 1. Relationships between secondary minerals in pseudoraoiphs after olivine. Thin sections, polarizers crossed, X 30 (a), X 60 (b), and X 20 (c and d): a) various serpentine generations, b) earliest mesh-textural lizardite, c) serpentine (meshed and isotropic) and coarse-grained calcite, d) incomplete pseudomorph; at the center olivine, white calcite, dark serpentine.
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FIGURE 2. Scanning electron micrograph showing (a) altered olivine xenolith and micro-structure of isotropic serpentine from a mesh core (b): a) thin section, polarizers crossed, X 20, b) carbon replica, X 16000.
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to brown or rust-brown. In many cases, pseudo-morphs, particularly large ones, are composed of several structural varieties of serpentine, which indicates multiple stages of serpentiniza-tion (Fig. la). The earliest for mesh-textured colorless or greenish lizardite of transverse platy habit (Fig. lb). The contacts with olivine are sharp and relatively even. There are also strips of chrysotile after olivine, whose margins differ from those in lizardite in being uneven and indented (sawtooth shape).
Relict olivine in the central parts of pseudo-morphs or in cores of the mesh serpentine has been replaced by calcite in many varieties of kimberlite, or sometimes by isotropic serpentine (Fig. lc). The calcite replaces the olivine from the boundary with the serpentine, and if the replacement of the olivine is incomplete, one gets zoned pseudomorphs particularly characteristic of kimberlites (Fig. ld). Some varieties of kimberlite, and usually kimberlite veins, do not show early serpentini-zation, and olivine is completely converted to calcite pseudomorphs. The calcite in the kimberlites also replaces phlogopite, perovskite, apatite, and other minerals.
Ultrabasic xenoliths (dunites and lherzolites) have a meshed replacement structure, and serpentine pseudomorphs do not inherit the shapes of the primary minerals but instead form continuous masses (Fig. 2a). The serpentine in these is more strongly colored and has a higher birefringence than does the colorless isotropic serpentine in the mesh cores, which has a shortfiber (columnar) micro-structure (Fig. 2b). Relict olivine in the cores is replaced by calcite along with isotropic serpentine, the calcite in many cases filling each separate relict as a single crystal (Fig. 2a). Pyroxene, if present, may be replaced by calcite, or less often by colorless or slightly greenish bastite.
Kimberlite groundmass. The serpentine in the groundmass is usually present in two varieties: interstitial (filling the gaps between grains of calcite and other minerals) and pseudo-morphous (replacing carbonates and other groundmass minerals). Interstitial serpentine is usually isotropic and structureless or
spherulitic, and may represent the final component in the crystallization of the residual kimberlite liquid. Non-pseudomorphous (primary) serpentine in the groundmass has been assumed by some researchers on South African kimberlites to arise by reaction between compounds of silica, MgO, and water at temperatures below 500°C [34J, and the same applies to researchers on North American [31, 33] and Yakutian kimberlites [19, 21, 13]. According to Kornilova et al. [13], the isotropic serpentine in the Yakutian kimberlite ground-mass is virtually always lizardite, whose particles sometimes have rounded edges as seen in the electron microscope, which indicate the conversion of lizardite to chrysotile. The second type of serpentine occurring in the ground mass has a close spatial association with carbonates, but follows them paragene-tically [23].
Carbonates are present in the groundmass as fine-grained aggregates of xenomorphic grains and also as tabular crystals (micro-liths). Calcite microliths are single crystals with straight extinction and bounded at the ends by rhombohedral faces; some of them form crossed twins (Fig. 3). Zhabin has shown that tabular crystals with these end faces form early and represent the highest-temperature habit of calcite. Microliths frequently encompass phenocrysts and xenoliths, resulting in a fluid groundmass texture. Microstructure analysis [10] has shown that microliths to be oriented subparallel to the vertical pipe axis. The proportion of calcite microliths generally increase with depth.
In addition to the idiomorphic calcite, which is assumed to have crystallized from a liquid, the kimberlite groundmass contains globular carbonate segregations, which may indicate a liquid-immiscibility origin [23]. These fairly large carbonate globules (up to 2-3 mm in size or sometimes 5 mm) have been observed in kimberlites in one of the pipes in the Malaya Botuoba region. The globules are rounded and variously deformed. It is notable that globules are smaller and less frequent at depth. The size of the globules increases near the surface, and the number of them increases substantially, attaining a
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FIGURE 3. Forms of carbonate segregation in kimberlite groundmass: a) microliths, Udachnaya pipe, b and c) globules, pipe No. 3 (b) and pipe Tayezhnaya (c), d) silicate particles (dark) in carbonate matrix (light), Udachnaya pipe. Thin sections (a, b, and d), polarizers crossed, X 30 (a), X 40 (b), and X 15 (d); c) hand specimen.
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maximum in the upper part of the pipe, where in places they may constitute up to 40-50% of the rock volume. Within each part, the globules are unevenly distributed, forming streaks and clumps of irregular shape. Sometimes the kimberlite has an emulsion texture due to silicate particles enclosed in the carbonate matrix.
Xenoliths. Serpentinization is extensive (carbonatization less so) in xenoliths from various rocks in kimberlite breccias. The serpentinization involves the edges of the xenoliths, where rims are formed from a fraction of a millimeter to several centimeters. The serpentine develops (frequently together with chlorite) after grains of dolomite and calcite, after clay particles in xenoliths of sedimentary carbonate rocks, and after feldspars, pyroxenes, and other minerals in fragments of dolerites (basalts) and various xenoliths of basement rocks. The feldspars, pyroxenes, and amphiboles at the edges of the xenoliths are also replaced by sericite, carbonates, magnetite, and other secondary minerals.
The latest generations of serpentine and carbonates are monomineralic and polymin-eralic hydrothermal veinlets, druses, and nodules. Serpentine and calcite also impregnate the kimberlite, being deposited from hydro-thermal solutions in small pores and cavities.
Characteristics of Serpentine and Calcite from Kimberlites
Serpentine. X-ray examination shows that the various generations of serpentine differ one from another structurally and on the whole tend to be intermediate between lizar-dite and chrysotile [25]. In that respect, serpentine from kimberlites may correspond to one of the varieties of serpentine from ultrabasites described by Varlakov and Babitsyn [7], which they call chrysotiloid. This serpentine is analogous in chemical composition to chrysotile but differs in structure, being intermediate between lizardite and clinochrysotile. However, this type of diffraction pattern for serpentine from kimberlites
may indicate that it belongs to the six-layer orthoserpentines [2].
The electron microscope shows that a fibrous structure predominates for serpentine from kimberlites, with the fibers varying in length and thickness.
The composition of the serpentines is closely related to the modes of formation of the various generations, as are their structural and morphological features. Serpentines after ultrabasic xenoliths and olivine phenocrysts are enriched in iron and aluminum but are depleted in magnesium and silica by comparison with the late hydrothermal serpentines filling rock cavities. The apoolivine serpentines also contain appreciable amounts of nickel, cobalt, and chromium (Table 1). The highest levels of nickel, cobalt, and iron occur in looped serpentine (Table 2), which is the earliest and consequently represents the highest temperature. Optical spectroscopy [3] indicates that the iron in the serpentine is in present divalent and trivalent forms, with Fe2+ occupying only octahedral positions, i.e., replacing Mg, while Fe3+ is distributed between octahedral and tetrahedral positions. The isomorphous substitution of iron as well as aluminum and nickel is confirmed by micro-probe analysis: by laser methods (Table 2) and x-ray spectral analysis (Table 3). Characteristically, the groundmass serpentine (primary) contains not only a substantial amount of iron but also sometimes very large amounts of aluminum (Table 3, analysis 6), and therefore cannot always strictly speaking be called serpentine [30].
Hydrothermal-metasomatic serpentine usually inherits the chemical features of the replaced minerals. The isomorphous components entering apoolivine serpentine are responsible for pleochroism in the colored varieties, somewhat elevated refractive indices, and strengthened color. Apocarbonate and late hydrothermal serpentines differ from apoolivine serpentines in having low levels of trace elements.
There is no clear-cut correlation between chemical composition and the structure of serpentine (the fibrous serpentines of the
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TABLE 1. Chemical Compositions (weight percent) of Serpentines from Kimberlites and Contents (g/t) of Certain Trace Elements
Component
SiO2 TiO2
A12O3 F e 2 O 3
FeO MnO MgO CaO Na2O K2O P 2 O 5
CO2 S H 2 O +
Ign. loss F H2O-
Sum Ni Co Cr
Analyses
1
35.90 0.14 0.53 5.04 2.05 0.05
34.03 4.73 0.91 0.28 0.06
Not det. 0.30
12.68 3.24
Not det. Not det.
99.54 1300
34 310
2
33.95 0.13 0.53 4.28 3.45 0.09
32.44 5.61 0.99 0.39 0.05
Not det. 0.39
13.18 3.40
Not det. Not det.
98.49 1600
34 550
3
35.86 0.20 2.27 2.63 2.74 0.08
39.19 0.49 0.08 0.09 0.06 0.56 0.47
13.85 0.48 0.28 1.84
99.34 3400
160 1340
4
34.61 0.12 1.20 5.35 1.84 0.10
38.07 0.99 0.08 0.04 0.03 0.56 0.24
13.24 2.20 0.15 2.62
99.60 1980
200 2250
5
36.80 0.12 1.06 4.75 2.17 0.10
38.76 1.02 0.04 0.06 0.03 0.56 0.02
12.95 1.07 0.15 2.16
99.65 2200
65 1460
6
39.35 0.02 0.00 2.16 0.83 0.05
41.72 0.00 0.03 0.03 0.03 0.56 0.12
13.77 0.43 0.42 1.66
99.44 8.2 --
7
40.50 0.05 0.69 3.38 4.88 0.03
35.06 0.50 1.10 0.25 0.06
Not det. Not det.
12.74 0.20
Not det. Not det.
99.39 120 44 25
8
30.95 2.72
13.10 4.91 7.72 0.14
23.89 2.35 0.47 0.36 0.86 0.55 0.10
11.28 0.06 0.19 0.80
99.65 29 15 49
9
30.27 1.25
14.39 3.72 7.25 0.14
26.81 0.81 0.06 0.10 0.40 0.20 0.33
13.42 0.47 0.11 1.56
99.61 54
-34
Note: Analyses 1 and 2 are for pseudomorphs after olivine phenocrysts, analyses 3-5 are for serpentinites (xenoliths), analysis 6 is for serpophite (nodule), analysis 7 veinlet, analyses 8 and 9 serpentinized basement xenoliths, H2O- not entering into sum. Analysts: analyses 1, 2, and 7 S. Ya. Bolozneva (East Siberian Geology, Geophysics, and Mineral Raw Materials Research Institute, Irkutsk), analyses 3-5, 6, 8, and 9, T. N. Anisimova (Institute of Geology, Yakutia Branch, Siberian Division, USSR Academy of Sciences, Yakutsk), and Ni, Co, and Cr determined by a quantitative spectral method, analyst I. G. Yegranova (East Siberian Geology, Geophysics, and Mineral Raw Materials Research Institute, Irkutsk).
various generations differ substantially in composition), although there is evidently some effect from the chemical composition on the bending or twisting of the serpentine mineral layers. High levels of iron (and aluminum) replacing magnesium in the brucite layers of lizardite may tend to cause the platelets to twist. Bending in lizardite particles has been observed in the electron microscope [13] , evidently due to iron being released from the serpentine and segregated as magnetite. Because of differences in the sizes of the silicon-oxygen and brucite layers (the first being smaller than the second), there are forces tending to convert each
structural layer to a cylinder, with the silicon-oxygen layer on the inside.
Thus, the kimberlite serpentine relates compositionally to the serpentine-ferroser-pentine series and has a content of up to 8-10% iron. The differences in composition, structure, and morphology between serpentine generations indicate differences in physico-chemical conditions of formation.
Serpentine in kimberlites may be divided into two main groups by mode of formation: a) hydrothermal metasomatic after primary silicates (mainly olivine), and b) late
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TABLE 2. Ratios of Element Concentrations to Magnesium Contents in Various Generations of Serpentine from the Udachnaya Pipe
(LMA-10 Laser Microanalyzer, GDR)
Analysis
1 2
3
4 5
6
7 8 9
Serpentine generation
Nodular serpophyte segregation Looped serpentine in incomplete pseudomorph
after olivine Mesh-textured serpentine in a serpentine-calcite
pseudomorph after olivine Same Serpentine pseudomorph after olivine
Mesh Cores
Homogeneous pseudomorph after olivine composed of enamel-type serpentine
Same Same Serpentinized olivinite xenolith
Mesh Cores
Ni
0.00034 0.096
0.0059
0.0078
0.093 0.00055 0.0021
0.0032 0.00066
0.022 0.0043
Co
-0.00018
0.00029
0.00011
0.00072 -—
--
0.0021 -
Fe
0.035 0.19
0.45
0.17
0.19 0.15 0.07
0.34 0.10
0.67 0.042
Ca
0.0044 0.031
0.060
0.028
0.038 0.020 0.0086
0.11 0.11
0.18 —
Note: Analyst I. G. Yegranova (East Siberian Geology, Geophysics, and Mineral Raw Materials Research Institute).
hydrothermal filling pores, cavities, and joints (veinlets, nodules, and so on). Hydrothermal metasomatic apoolivine serpentine is the most common in kimberlites.
Carbonates (calcite). Calcite is the most common carbonate in kimberlites, with dolomite less prominent. Aragonite, strontianite, pyroaurite, and certain other carbonates are also present. The proportion of hydrothermal carbonates forming veinlets and geodes in kimberlites is small; fairly detailed descriptions have been given by Bobriyevich et al. [4, 5] and by Marshintsev et al. [19] . The main role in the kimberlites is played by calcite pseudomorphous after primary minerals and by calcite forming the groundmass together with magnetite, serpentine, and phlogopite.
Isotope data indicate that the carbonate component is heterogeneous. The Sr and Ba distributions in calcite from kimberlites [8] indicate that the carbonates are derived from two sources: plutonic (kimberlite proper) and surficial (the country rocks). The
kimberlite calcite proper has high Sr levels (up to 1 wt.%) and low 8 7Sr/ 8 6Sr ratios (0.703-0.704, in [12 ,6] ) .
Carbon and oxygen isotope data also indicate several CO2 sources [35, 32, 18, 16] . Mamchur et al. [18] distinguished four genetic groups of carbonates in kimberlites: 1) those formed from solutions saturated in magmatic CO2 (δ1 3C from -10 to - 5 % o , 2) those formed by CO2 from oxidized magmatic material (δ1 3C less than - 1 0 ‰ ) , 3) calcite deposited from solutions in which methane and carbonaceous material were formed (δ1 3C up to 33.6%o), and 4) recrystallized limestones (δ1 3C from -5 to 0%o) .
Kuleshov and Ilupin [15] examined sediment xenoliths in Siberian pipes and found wide ranges in δ13C and δ1 8O, with the peripheral zones in the xenoliths enriched in 12 C. It was concluded that the carbonate-rock xenoliths in the kimberlite substrate had been reworked and enriched in 1 2C as a result of exchange with carbon compounds of mantle and crustal origin.
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TABLE 3. Microprobe Analysis Data for Serpentines from Kimberlites
Component
SiO2 TiO2 A12O3 Cr2O3 FeO MgO CaO Na2O NiO Sum
Analyses
1
39.61 0.03 0.23 0.00 5.77
39.18 0.09 0.00 0.00
85.03
2
39.24 0.00 0.70 0.05 3.50
39.84 0.09 0.21 1.02
84.76
3
37.41 0.00 0.20 0.00 5.36
40.20 0.15 0.00 0.28
83.61
4
32.15 0.00 0.27 0.00 9.83
37.93 1.48 0.07 0.26
81.96
5
40.50 0.36 0.30 0.07
11.16 33.50
0.04 0.00 0.09
86.49
6
42.5 0.2
11.6 0.0 2.8
27.9 1.7 0.11 0.0
86.1
7
40.0 0.0 2.7 0.0
13.0 25.9
1.1 0.03 0.03
83.3
8
40.6 0.0 0.4 0.0 8.1
33.6 0.3 0.4 0.0
83.4
9
39.5 0.0 0.4 0.0
10.0 33.0 0.1 0.4 0.0
83.4
Note: Analysis 1 from groundmass, analyses 2-4 after olivine; analysis 5 isotropic structureless serpentine from groundmass, analysis 6 serpentine (saponite) from groundmass, analysis 7 apoolivine serpentine, analysis 8 the same (mean of three analyses), analysis 9 from groundmass (mean of three analyses), analysis 10 pseudomorph (after olivine?); analyses 1-4 kimberlites from Elvin Bay, Canada [33], analysis 5 kimberlite dike from Greenland [31], analyses 6-9 South African kimberlites [30].
There have been many reports of considerable fluctuations in δ13C (PDB) for carbonates from kimberlites (from -11.7 to +0.24%o) and also for diamonds (from -34 to + 2 . 5 ‰ ) [35, 32, 11] . The values of δ18O (SMOW) for carbonates from kimberlites range from 7 to 21 ‰ [ 35 ] , or from 6 to 24%o according to [32] . These researchers observed that the higher levels of 18O in carbonates from kimberlites compared to primary eruptive carbonatites may be due to the kimberlite substrate interacting with meteoric waters during injection. The
variations in C and O isotope composition are probably associated not only with uptake of sedimentary material but also, as Plyusnin et al. have shown [22] , with changes in physicochemical conditions of formation (particularly temperature).
A likely mechanism for kimberlite carbonates being enriched in 18O is the formation of some of the CO2 by the oxidation of a hydrocarbon component in the kimberlite fluid under subsurface conditions via the reaction
TABLE 4. Types of Carbonates Present in Kimberlites
According to CaO and CO2 source
Abyssal, kimberlite proper
Introduced by solutions from country carbonate rocks
According to mode of formation
Primary, late magmatic
Secondary, hydrothermal-metasomatic
Hydrothermal (redeposited)
According to segregation form
Microliths, idiomorphic (xeno-morphic ?)
Aggregates of xenomorphic and polygonal grains in pseudo-morphs after primary minerals in phenocrysts and in ground-mass
Veinlets, nodules, druses
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CH4 + 2O2 = CO2 + 2H2O
The water formed in this process may cause early serpentinization in the olivine pheno-crysts.
Therefore, the carbonates present in kim-berlites can be divided into several types (Table 4).
Kimberlite carbonates proper are usually not observed in pure form and are virtually always substantially diluted with sedimentary carbonates. The primary magmatic cal-cite has elevated contents of strontium, barium, and certain other elements. Metasomatic calcite inherits the trace-element compositions of the replaced minerals, as does serpentine. An important distinctive feature of kimberlite carbonates is the elevated content of nickel (hundredths or tenths of a percent) in the apoolivine calcite [24] . The secondary (hydrothermal metasomatic) calcite may have been produced from CaO and CO2 of abyssal and surface origin. The late hydrothermal calcite filling joints, pores, and cavities in most cases is redeposited and contains no trace components at all.
The extensively autocarbonatized (calcite) kimberlites do not lose their geochemical character either in relation to lithophile elements (carbonatite ones) such as Nb, Ta, Sr, and Ba or as regards the siderophile (ultra-basic) components Ni, Co, Cr, and Sc. In that respect, autocalcitization for kimberlites is similar to the formation of metasomatic carbonatites after silicate (ultrabasic) rocks. Armbrustmacher [28] remarked that one of the features distinguishing metasomatic carbonatites (after lamprophyres) from primary magmatic ones is that the former are enriched in cobalt, chromium, scandium, nickel, and manganese in addition to the elements typical of these rocks (Ba, La, Nb, Sr, and Ce), i.e., in elements from the replaced rocks (Table 5).
Discussion
For a long time, the concept of a serpentine-carbonate mineralization in kimberlites was equated with that of secondary mineralization.
TABLE 5. Mean Contents of Minor Elements (percent) in Kimberlites, Carbonatites, and Lamprophyres
Element
Ni Co Cr Ti Sr Ba Nb Zr P
Analyses
1(10)
0.073 0.0056 0.103 1.78 0.046 0.084 0.028 0.031 0.9
2(38)
0.020 0.0033 0.047 0.61 0.088 0.53 0.014 0.015 0.2
3(52)
0.0008 0.0017 0.0048 0.5 0.34 0.23 0.2 0.11 0.85
4 (2 )
0.07 0.007 0.07 1.2 0.02 0.3 0.03 0.01 0.1
Note: Analysis 1 calcite (carbonatite) kimberlites (veins conjugate with pipes), analyses 2-4 from [28]: analysis 2 metasomatic carbonatites, analysis 3 primary magmatic carbonatites, analysis 4 lamprophyres. The number of analyses is given in parentheses.
Many researchers considered all carbonates in kimberlites as secondary, the source being taken as calcium-bearing silicates (diopside, monticellite, melilite) and to a considerable extent the country carbonate rocks. However, recent data indicate that some of the calcite in the groundmass is primary (mag-matogenic). Syngenetic carbonate segregations (calcite and dolomite) have been observed in many kimberlite minerals. MaPkov [17] considers primary magmatic calcite as a typical mineral in the kimberlite ground-mass, occurring characteristically with phlogo-pite, magnetite, apatite, and perovskite. This is confirmed by the presence of calcite in the groundmass of the freshest kimberlites. However, geological observations, together with geochemical and isotopic data, indicate that much of the carbonate (possibly most of it) is derived from the country carbonate rocks. The African kimberlites emplaced in granites, granite-gneisses, and other rocks practically free from CaO and CO2 have carbonate contents less by more than factors of two than those in kimberlites emplaced in carbonate rocks. Nevertheless, American kimberlites (in the Colorado-Wyoming region)
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in granites include varieties with carbonate contents up to 45% by volume [36].
As regards serpentinization, the view most widely taken is that this process begins after the rock has consolidated in the channels and is caused by hydrothermal solutions. Various suggestions have been made about the source of the serpentinization water. Many geologists consider serpentinization as an autometamorphic process occurring in response to magmatogenic solutions. According to other workers, the content of 5-10% water in the kimberlites of the pipe facies does not correspond to the water content of kimberlite magmas and is much higher than the original H2O content at depth. The conclusion that the water level in kimberlite magmas is low agrees well with calculations and experimental data indicating that kimberlite magmas cannot contain more than 1-2% H2O under mantle conditions. Magmas saturated with water are not capable of substantial vertical displacement [26]. Also, diamonds are readily corroded by water vapor, and therefore could not persist in a water-saturated medium [35]. Using hydrogen-isotope data, Barrett et al. [29] and Sheppard et al. [35] have shown for South African kimberlites that mass serpentinization is related to water of meteoric origin; Ukhanov and Devirts [27] have done the same for Yakutian kimberlites.
Most geologists consider kimberlite volume to be constant during serpentinization but that considerable amounts of material (mainly magnesium) are lost to the country rocks. Kryatov [14] and Zolnikov [9] have shown that the country carbonate rocks at the contacts with kimberlites have elevated contents of magnesium, iron, water, and silica, which participate in the formation of the serpentine replacing calcite, dolomite, and limestone clayey particles, and which is deposited along joints with the formation of serpentine veinlets and nodules. Anodin [ 1 ] found an increase in the degree of dolomitization in the country limestones as certain pipes were approached. There are also no bulk-expansion effects in minerals showing extensive serpentinization. Bobriyevich et al. [4] in this connection
reported goniometric studies on pseudo-morphs of serpentine after olivine. It was found that symmetry elements in the pseudo-morphs and the angles between the normals to the faces correspond exactly to the rhombo-dipyramidal class of the orthorhombic system for olivine crystals. There is no cracking, swelling, or displacement of partially serpentin-ized olivine grains, and the idiomorphic appearance of the completely replaced crystals persists, as has been reported by other researchers on kimberlites [20].
Therefore, the production of serpentine and carbonates in kimberlites is much more complicated than had previously been considered. It is generally accepted that kimberlite magmas are saturated in volatile, but there has been no complete elucidation of the relation between autometamorphism and superimposed metamorphism in kimberlites, particularly since the relationships differ for the various types of kimberlite and are dependent on many factors. Certain other aspects of this complicated problem have also not been resolved.
Conclusions
Our own data and the data from many other sources concerning kimberlites in Yakutia, Africa, and America yield the following conclusions on serpentine-carbonate mineral formation in kimberlites.
1. The formation of serpentine and carbonates is characteristic and regular in the production of kimberlites, and it involves magmatic as well as hydrothermal-metasomatic process.
The latter play the main role. The primary (essentially olivine) mineral assemblage in the kimberlites becomes a nonequilibrium assemblage as P and T fall and (particularly) in the presence of considerable amounts of CO2, H2O and other components, and it gives way to a hydrosilicate-carbonate assemblage more stable under these conditions.
2. The early mesh-textured serpentinization in olivine is comparatively limited in
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extent and is uniform in occurrence (within individual varieties of kimberlite), and may be produced by the water bound in the kimberlite magma. Extensive serpentinization is due to the entry of mineralized meteoric or hydrothermal waters from the crust into consolidated kimberlites, as is indicated by geological, petrographic, isotopic, and geo-chemical data. Kimberlite serpentinization involves material influx and efflux at constant volume, and it is therefore a typical meta-somatic process. The constancy of the kimberlite volume during serpentinization is indicated by the almost universal preservation of primary textures and mineral shapes without regard to the extent of the process, as well as by the products, from which brucite and magnesite are absent.
3. Kimberlites show extensive calcitization of the primary minerals in phenocrysts (mainly olivine) and in the groundmass, due to the high carbon dioxide activity in calcium-bearing hydrothermal solutions.
4. Extensive secondary processes in most kimberlite bodies have led to substantial redistribution not only on local scales (between phenocrysts and groundmass or between xenoliths and the cementing kimberlite) but also on a larger scale within the kimberlite bodies because of displacement of components by solution flow.
References
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