Humphreys. 2011. Silicate Liquid Inmmiscibility Within the Crystal Mush ... Etc

7/27/2019 Humphreys. 2011. Silicate Liquid Inmmiscibility Within the Crystal Mush ... Etc

1/28

Silicate Liquid Immiscibility within theCrystal Mush: Evidence fromTi in Plagioclasefrom the Skaergaard Intrusion

MADELEINE C. S. HUMPHREYS*

DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK

RECEIVED JANUARY 8, 2010; ACCEPTED NOVEMBER 2, 2010

A key target in the study of layered intrusions is to constrain theliquid line of descent of the magma. However, the evolution of the

interstitial liquid is rarely considered, and its liquid line of descent

is often assumed to be equivalent to that of the bulk magma.

Because of extensive sub-solidus and diffusional changes that

occur in slowly cooled rocks, clues to the composition of the intersti-

tial liquid can only be obtained using very slowly diffusing trace

elements and components. This study uses the Ti concentrations

and anorthite contents of interstitial plagioclase to consider the

compositional evolution of the interstitial liquid in the Skaergaard

Intrusion. Ti^XAn zoning of interstitial plagioclase does not

follow the same cryptic variations that develop in plagioclase pri-

mocrysts as a function of stratigraphic height, demonstrating that

the bulk and interstitial liquid lines of descent are not equivalent.After Fe^Ti oxides start to crystallize, Ti concentrations decrease

in both primocryst and interstitial plagioclase as a result of decreas-

ing melt Ti. However, in the interstitial plagioclase within a

single thin section, divergent trends develop adjacent to fine-grained

interstitial pockets containing diverse mineral assemblages, which

are interpreted to represent the crystallized products of late-stage

immiscible liquids. These trends vary systematically as a function

of stratigraphic height and spatial location within the intrusion.

The distribution and compositions of these plagioclase zoning

trends are used to comment on the spatial distribution and differen-

tial movement of interstitial immiscible liquids within the

intrusion.

KEY WORDS: layered igneous rock; immiscibility; plagioclase; zoning;

cumulate; Skaergaard

I N T R O D U C T I O NLayered intrusions are of interest for many reasons: for

example, they reveal information about differentiation

and fractionation processes that may be operating beneath

currently active volcanoes, they provide insights into the

effects of reactive fluids moving within a porous medium

and they are commonly host to economic precious metal

deposits. The cumulate rocks that define layered intrusions

can accumulate by various mechanisms, including the set-

tling of primocrysts (early formed crystals) onto the crys-

tal pile, in situ growth of crystals in the cooling boundary

layers, or deposition from crystal-laden plumes (e.g. Wager

& Brown, 1968; McBirney & Noyes, 1979; Irvine et al.,

1998; Marsh, 2006). In most instances, the result is a layerof cumulus crystals surrounded by interstitial liquid.

Once this crystal mush has formed, the interstitial liquid

is slowly eliminated through various processes that may in-

clude overgrowth on primocrysts, nucleation and growth

of oikocrysts and other interstitial material (including

new minerals), consolidation and compaction of the mush

owing to the weight of overlying crystals and composition-

al convection, which can result in adcumulus textures.

Although post-depositional processes acting within

the mush have been discussed in many previous studies

(e.g. Tait et al ., 1984; Barnes, 1986; Boudreau &

McCallum, 1992; Haskin & Salpas, 1992; Meurer &

Boudreau, 1998; Tegner et al., 2009), relatively little work

has addressed the differentiation of the interstitial liquid

itself, compared with that of the bulk magma. Studies spe-

cifically considering the development of intercumulus

*Corresponding author. Present Address: Department of Earth

Sciences, University of Oxford, South Parks Road, Oxford, OX13AN.

Telephone:44 (0)1865 272020. Fax:44 (0)1865 272072.

E-mail: [email protected]

The Author 2010. Published by Oxford University Press. All

rights reserved. For Permissions, please e-mail: journals.permissions@

oup.com

JOURNALOF PETROLOGY VOLUME 52 NUMBER1 PAGES147^174 2011 doi:10.1093/petrology/egq076


2/28


3/28

equivalent to the LZ, MZ and UZ of the LS, and UBS-t,

which refers to the outermost part of the UBS and is essen-

tially equivalent to the HZ (Naslund, 1984; Douglas, 1961).

In addition to the changes in primocryst assemblage, the

compositions of the primocrysts change with stratigraphic

height. For example, plagioclase compositions vary from

$An70 in HZ to An25 at the Sandwich Horizon (e.g.

Wager & Brown, 1968; Maale, 1976). This cryptic vari-

ation is generally accepted to reflect closed-system, frac-

tional crystallization of essentially a single batch of

magma. Whereas the LS has been heavily studied since

early descriptions (Wager & Brown, 1968), the MBS andUBS have been relatively neglected apart from the studies

of Naslund (1984) and Hoover (1989). The comprehensive

early petrological study of the MBS (Hoover, 1989)

showed t hat primocryst olivine compositions are up to

9 mol % more Fa-rich, clinopyroxene is 5 mol % richer in

the diopside component, and plagioclase is 5^8 mol %

more calcic, than primocrysts from equivalent stratigraph-

ic levels in the LS (Hoover, 1989). Minor element contents

of plagioclase were not analysed. These observations were

ascribed to re-equilibration of primocrysts with Fe-rich

interstitial liquid flowing down the margins of the intru-

sion (Hoover, 1989). In addition, mineral compositions

from the upper parts of the MBS (i.e. adjacent to the

Upper Border Series) were also observed to be more

Fe-rich, and plagioclase more Na-rich, than elsewhere in

the intrusion. This was interpreted as the result of either

more effective fractionation near the roof or modified

magma composition in the upper parts of the chamber

(Hoover,1989).

For Skaergaard, there is still disagreement over changesin oxygen fugacity (e.g. Toplis & Carroll, 1995, 1996; Thy

et al., 2009), and whether the residual liquid follows the

Fenner or Bowen compositional trends; that is, whether it

becomes Fe-rich at low SiO2 or tends towards SiO2-rich

compositions at relatively lower FeO contents (e.g.

McBirney, 1975; Hunter & Sparks, 1987; Brooks & Nielsen,

1990; McBirney & Naslund, 1990; Toplis & Carroll, 1995,

1996; Thy et al., 2006, 2009). It seems plausible that both

Fig. 1. Geological map of the Skaergaard Intrusion, after McBirney (1989). The locations of the 1966 Cambridge dri ll core and the Platinovadrill cores 90-22 and 90-10 are shown. Continuous lines (KR02 and SP) indicate the locations of traverses through the Marginal BorderSeries. The star indicates the location of LZc sample 458279 (f romTegner et al., 2009).

HUMPHREYS INTERSTITIAL LIQUID IMMISCIBILITY

149


4/28

the Bowen and Fenner trends are represented in the com-

positions of two immiscible liquids that have been shown

to coexist by at least UZb (McBirney & Nakamura, 1974;

Hanghj et al., 1995; Jakobsen et al., 2005). The postulated

earlier occurrence of immiscibility in the residual liquid

(LZc, Veksler et al., 2007) is disputed (e.g. Morse, 2008a;

McBirney, 2008; Philpotts, 2008; Veklser et al ., 2008).Furthermore, the effect, if any, of liquid immiscibility on

the resulting fractionation trend is unknown, as is the

stage at which immiscibility might occur within the inter-

stitial liquid.

Textural indicators of evolved interstitialliquidRecent work has demonstrated that evolved, late-stage

interstitial liquid can be present in glass-bearing nodules

entrained in lava flows as thick films of glass on grain

boundaries. In cumulate rocks, pyroxene commonly forms

thick grain boundary films and it has been suggested

that these represent an original liquid film (e.g. Holness,

2005; Holness et al., 2007a). At Skaergaard, crystallized

traces of evolved liquids can also be seen in trails of tiny

oxide and apatite crystals, clustered particularly along the

margins of plagioclase chadacrysts within clinopyroxene

oikocrysts (Fig. 2a). Holness et al . (2011) demonstrate

that, following immiscibility in the bulk magma, the so-

lidification of large pockets of interstitial liquid results in

the formation of paired (spatially associated) intergrowths

of granophyre and an ilmenite-rich assemblage including

apatite, skeletal Fe^Ti oxides and clinopyroxene (see

Holness et al., 2011, figs 10^12). Granophyre-rich pockets

tend to be associated with the plagioclase-rich regions of a

sample whereas ilmenite-rich pockets tend to be associated

with regions rich in olivine and clinopyroxene (Holnesset al., 2011). The paired intergrowths commonly occupy

planar-sided pockets, which shows that the liquid was not

reactive, and the variations in their spatial distribution

are clearest in the MBS. In the Skaergaard Peninsula,

which represents the most complete chemical sequence,

they become abundant near the MZ*ÛZa* boundary.

At the same point, evidence of localized reaction between

oxides and pyroxene disappears (Holness et al., 2011). In

combination the paired intergrowths represent up to

44% of the sample by area in the Layered Series, and

more in the evolved parts of the MBS (Stripp, 2009).

Samples from the less evolved, outermost parts of the

Skaergaard Peninsula traverse contain small, isolated,fine-grained, planar-sided pockets of late-stage interstitial

phases (Fig. 2b and c). These are distinct from the paired

intergrowths of Holness et al. (2011) in that they are smaller,

commonly very fine-grained and highly altered; however,

these fine-grained pockets probably also represent the crys-

tallized products of late-stage liquids. Where the contents

of the pockets can be identified, they typically include

some of quartz, K-feldspar, apatite, non-skeletal Fe^Ti

oxides, pyroxenes, biotite and zircon, as well as plagioclase

growing on the margins of the pockets. Their mineralogy

is highly variable (assuming that the 2D distribution is

representative of the 3D volume), and the pockets repre-

sent between 027 and 106% of the samples by area in the

Skaergaard Peninsula (Table 1). At lower structural levels

in the MBS, near the base of the intrusion (e.g. northernKraemer Island), the interstitial pockets are more sparsely

developed, representing at most 020 area % of the sam-

ples (Table 1). Abundant interstitial low-Ca pyroxene is

present, commonly surrounding resorbed olivine or adja-

cent to Fe^Ti oxides (Fig. 2d), and olivine may be partially

replaced by oxy-symplectites of magnetite and orthopyrox-

ene (see Holness et al., 2011).

In the LS, the interstitial pockets are typically present at

020^035 area % (Table 1), but in the main replacive

grain-boundary microstructures are seen instead in the

central parts of the LS (LZc^MZ, Holness et al., 2011; see

below and Table 1). At the base of the LS low-Ca pyroxene

is also present, and the interstitial pockets are lessabundant. The coarse-grained, paired granophyric and

ilmenite-rich intergrowths become abundant around the

MZÛZa boundary near the margins of the LS and near

the UZaÛZb boundary in the centre of the intrusion

(Holness et al., 2011). Between LZc and the point at which

the coarse-grained, paired intergrowths dominate, there is

abundant evidence of reactive melt occupying grain

boundaries, manifest as irregular (stepped) clinopyrox-

ene^plagioclase grain boundaries, fish-hook intergrowths

of pyroxene and plagioclase, olivine rims on oxide grains,

and mafic symplectites; grains of apatite, amphibole

and oxides are commonly found within the microstruc-

tures. This spatial distribution, as well as the microstruc-

tures involved, is described in detail by Holness et al.

(2011), who interpret the microstructures as the result

of chemical reaction following physical separation of

Fe-rich and Si-rich immiscible liquids within the floor

cumulates.

S A M P L E S A N D M E T H O D S

Samples studiedThis study investigates a suite of samples from the LS and

MBS of the Skaergaard Intrusion (Table 1). The LS was

sampled mainly using drill core material, with samples se-

lected to cover the range of stratigraphy. The lower partsof the LS (HZ^LZb) are covered by the 1966 Cambridge

Drill Core (Maale, 1976; Holness et al., 2007b), and the

upper parts (UZaÛZb) by drill core 90-22 (collected by

Platinova Resources Ltd, 1990; Tegner, 1997), with one

LZc sample (458279) from a surface reference profile

(Nielsen et al., 2000; Tegner et al., 2009). Drill core 90-22

samples the centre of the intrusion (Fig. 1). Four samples

from MZÛZa were also examined from drill core 90-10,

JOURNAL OF PETROLOGY VOLUME 52 NUMBER1 JANUARY 2011

150


5/28

which is located at the western margin of the intrusion

(Fig. 1).Plagioclase compositions in the MBS were analysed

in selected samples from two surface traverses, one on

Kraemer Island (sample numbers beginning KR02),

which includes zones HZ*^LZb*, and one on the

Skaergaard Peninsula (sample numbers beginning SP),

which samples the base of LZa* to UZb* (the contact is

unexposed, so it is unclear how much of HZ*^LZa* is

missing, but the fine-grained sample SP60 is assumed to

represent the base of LZa*). The figures presented here

also include plagioclase data for MZ (drill core 90-22)collected by Stripp (2009) as well as data from the

Cambridge drill core from Humphreys (2009).

Analytical methodsPlagioclase analyses were obtained using a Cameca SX-100

electron microprobe at the University of Cambridge. A

2 mm, 15 kV beam was used, with 10 nA current for major

elements and 100 nA for minor elements. Typical peak

pl

pl

pl

pl

qz

K-fsp

200 m

(b)

(c)

pl

100 m

pl

(a)

0.25 mm

ox

ox

ap

px

ap

ap

apap

(d)

pl

pl

pl

ol

ol

ol

opx opx

0.25 mm

Fig. 2. Textural characteristics of interstitial material from the Skaergaard cumulates. (a) Cross-polarized light photomicrograph showingstrings of small, grain-boundary oxides (arrowed) and apatite crystals (labelled) along the margins of plagioclase within clinopyroxeneoikocrysts, marking the location of late interstitial liquids (from HZ*, north Kraemer Island). (b) Back-scattered SEM image of fine-grainedinterstitial pocket containing mafic minerals including apatite, Fe^Ti oxides and pyroxenes, from sample SK46 (LZa*^LZb* boundary) inthe Skaergaard Peninsula. (c) Back-scattered SEM image of fine-grained interstitial pocket containing felsic minerals including quartz andalkali feldspar, as well as unidentifiable, brightly coloured weathered material, from sample SK46 (LZa*^LZb* boundary) in the SkaergaardPeninsula. (d) Cross-polarized photomicrograph showing orthopyroxene rims around olivine crystals, characteristic of the lower parts of theMBS (Kraemer Island) and the Layered Series.


151


6/28

Tab

le1:Locat

ionsand

detailsofsamplesstud

ied

Sample

Strat.

height(m)

Series

Zone

Primocryst

mineralogy

Evolved

pockets?

Area%

Pockets

Replacive

sympl?

Non-

replacive

sympl?

Location

Latitude

(N)

Longitude

(W)

90-22-421

1671

LayeredSeries

UZb

plolcpx

ox

ap

Y

Drillcore90-22

90-22-461.8

1630

LayeredSeries

UZb

plolcpx

ox

ap

Y

026

T1

Drillcore90-22

90-22-481.8

1610

LayeredSeries

UZa

plolcpx

ox

T1

Y

Drillcore90-22

90-22-824.2

1268

LayeredSeries

UZa

plolcpx

ox

(Y)

0006

T1

Drillcore90-22

458279

752

LayeredSeries

LZc

plolcpx

ox

Y

0032

T1

KraemerIsland

68811265"

31842563"

90-10-212

LayeredSeries

UZa

plolcpx

ox

Y

028

(Y?)

Drillcore90-10

90-10-445

LayeredSeries

MZ

plcpx

ox

Y

035

(T1)

Drillcore90-10

90-10-456

LayeredSeries

MZ

plcpx

ox

T1

Drillcore90-10

90-10-524.1

LayeredSeries

MZ

plcpx

ox

Y

013

T1

Drillcore90-10

118590

197

LayeredSeries

LZb

plolcpx

Y

033

Cambridgedrillcore

118601

1681

LayeredSeries

LZb

plolcpx

(Y)

0061

Cambridgedrillcore

118605

1636

LayeredSeries

LZb

plolcpx

Y

030

(T1)oxy

Cambridgedrillcore

118653

43

LayeredSeries

LZa

plol

Y

020

oxy

Cambridgedrillcore

118678

22

LayeredSeries

HZ

plol

Y

011

Cambridgedrillcore

Sample

Dist.from

margin

(m)

Series

Zone

Primocryst

mineralogy

Evolved

pockets?

Area%

pockets

Replacive

sympl?

Non-

replacive

sympl?

Location

Latitude

(N)

Longitude

(W)

SP29

594

MBS

UZb*

plolcpx

ox

ap

Y

SkaergaardPeninsula

688849

5"

318453

2"

SP20

4718

MBS

MZ*

plcpx

ox

Y

027

T1

SkaergaardPeninsula

688849

5"

31845138"

SP16

4273

MBS

LZc*

plolcpx

ox

Y

036

T1

SkaergaardPeninsula

688849

7"

31845177"

SP8

3042

MBS

LZb*

plolcpx

Y

067

(T1)

SkaergaardPeninsula

688850

2"

31845286"

SP46

133

MBS

LZa*-LZb*

plolcpx

Y

106

SkaergaardPeninsula

688850

6"

31845444"

SP60

10

4

MBS

BaseLZa*

plol

Y

047

SkaergaardPeninsula

688848

9"

31845542"

KR02-21

2227

MBS

LZb*

plolcpx

Y

014

KraemerIsland

68811362"

31844224"

KR02-37

80

5

MBS

LZa*

plol

Y

020

oxy

KraemerIsland

68811378"

31844340"

KR02-4

10

2

MBS

HZ*

plol

(Y)

0015

oxy

KraemerIsland

68811382"

31844401"

Stratigraphicheightsof

90-22andlocationof458279takenfro

m

thereferenceprofileofTegneretal.(2009).Y,yesthismicrostructureispresent(parentheses

indicaterarely);T1,Typ

e1replacivesymplectites;oxy,oxy-symplectites(Holnesset

al.,2011).


152


7/28

counting times were 20 s for major elements and 30^40 s

for minor elements. Zoned grain margins, triple junctions,

the margins of planar-sided interstitial pockets (see

above) and traverses of optically strongly zoned grain

edges were preferentially studied, to identify the compos-

itional variations of interstitial plagioclase from the late

stages of fractionation. These features were identifiedoptically or by SEM observation using a JEOL JSM-820

instrument, with a 20 kV beam and c. 15 mm working dis-

tance. Data from Stripp (2009) from MZ are included for

completeness; these represent traverses across zoned

grains but probably do not sample the most extreme

compositions.

R E S U L T S

Primocryst plagioclaseThe cryptic compositional variation of plagioclase primo-

crysts that occurs through the Layered Series stratigraphyat Skaergaard is summarized here, for later comparison

with t he interstitial compositions. Anorthite contents of

primocrysts decrease steadily upwards through the stratig-

raphy, with a relatively restricted range of compositions

observed in each zone, from $An63^69 in HZ, through

$An48^53 at LZc, to $An31^39 at UZb (Fig. 3a). This is

in agreement with the results of several previous studies

(e.g. Maale, 1976; McBirney, 1989) and is consistent with

closed-system fractionation of basaltic magma. TiO2 con-

tents of plagioclase primocrysts increase to a maximum

of $013 wt % in the LZb sample, then decrease almost

linearly with increasing stratigraphic height (decreasing

XAn: the primocryst trend, Fig. 3a), as a result of satur-ation in Fe^Ti oxides (Humphreys, 2009). Primocryst FeO

contents decrease with decreasing XAn until approximately

UZa levels, then start to increase again (Fig. 3b). This

increase in FeO through the Upper Zone is consistent

with previous observations (Tegner, 1997) and has been

interpreted as reflecting the increasing FeO content of

the fractionating bulk magma (Tegner, 1997; Tegner &

Cawthorn, 2010). MgO contents are scattered but approxi-

mately constant until $An45 and then decrease with XAn(Fig. 3c). In general, primocrysts from Layered Series

rocks near the margins of the intrusion (i.e. drill core

90-10) contain slightly more FeO and MgO than those

from the centre of the intrusion (drill core 90-22).

In the Marginal Border Series, primocryst compositions

are very similar to those in the LS in terms of XAn,

but tend to be enriched in MgO and FeO, similar

to plagioclase from drill core 90-10. This is consistent

with the observations of Hoover (1989), who interpreted

the Fe-rich compositions as the result of flow of

Fe-rich residual liquids down the marginal walls of the

intrusion.

Interstitial plagioclase: Marginal BorderSeries, Skaergaard PeninsulaThe MBS as exposed on the Skaergaard Peninsula repre-

sents a nearly complete transect through the stratigraphy,

comprising the base of LZa* to UZb*. As expected, inter-

stitial plagioclase from any given stratigraphic zone typic-

ally evolves to substantially more sodic compositionsthan the primocrysts from that zone: to $An40 in HZ*,

through $An35 in LZc*, to $An15 in UZb* (see

Electronic Appendix 1, available for downloading at

http://www.petrology.oxfordjournals.org). The compos-

ition of the most evolved plagioclase analysed from any

sample is variable. Minor element concentrations show

complex but systematic variations. In general, many inter-

stitial plagioclase compositions plot at lowerTiO2 contents

for a given XAn than the primocrysts, and this will be

described in detail below. However, one of the most inter-

esting features of the interstitial compositions is that in

the lower stratigraphic zones, the interstitial plagioclase

trend itself splits into two divergent compositional trends,typically adjacent to the planar-sided interstitial pockets

described above. Strong normal zoning can be observed

adjacent to pockets containing abundant granophyre or

quartz, commonly with a clear concave-upward trend

of TiO2^XAn. In contrast, strong reverse zoning with

decreasing TiO2 is observed in the vicinity of clearly

calcic, Fe-rich pockets (typically containing Fe^Ti oxides,

apatite, clinopyroxenebiotite; Fig. 4). Single spot ana-

lyses also fall on these trends. There is a wide variation of

plagioclase compositions even around a single pocket, and

few plagioclase compositions represent the extreme ends

of these normal and reverse trends. Many of the interstitial

pockets are not clearly dominated by either Si-rich orFe-rich minerals, or are strongly altered. As far as can be

observed, however, reverse or calcic compositions are not

associated with pockets that contain quartz or granophyre,

whereas conversely not all pockets that contain Fe-rich

minerals (apatite, oxides, pyroxenes, etc.) show reverse

zoning.

In sample SP60 (base LZa*), the TiO2 and anorthite

contents of the interstitial plagioclase are initially indistin-

guishable from primocryst compositions from higher

stratigraphic zones. With decreasing XAn, TiO2 concentra-

tions rise to a maximum of c. 012 wt % at $An56, and

then diverge steeply from the primocryst trend, with inter-

stitial plagioclase dipping to lower TiO2 contents whereasthe primocrysts show a much shallower decrease (Fig. 5a).

The interstitial plagioclase trend then itself follows two

divergent compositional trends after An54 (Fig. 5a) as

described above. Interstitial pockets represent $047% of

the thin section by area in this sample (Table 1).

In sample SP46 (LZa*^LZb* boundary), interstitial

pockets account for $106% by area (Table 1). Plagioclase

compositions follow the XAn^TiO2 primocryst trend


153
http://www.petrology.oxfordjournals.org/http://www.petrology.oxfordjournals.org/


8/28

0.0

0.2

0.4

0.6

0.8

1.0

0.00

0.02

0.04

0.06

0.08

0.10

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

20 30 40 50 60 70 80

mol % Anorthite

wt%

TiO

2

wt%MgO

wt%FeO

(a)

(b)

(c)

20 30 40 50 60 70 80

HZ

LZa

LZb

LZc

MZ

UZa

UZb

20 30 40 50 60 70 80

Fig. 3. Mole per cent Anorthite vs (a) TiO2, (b) FeO and (c) MgO compositions of primocryst plagioclase from the Layered Series. Data fromHolness et al. (2011) and this study. Approximate zone boundaries are indicated.


154


9/28

until $An50, when the interstitial compositions diverge

to lower TiO2, as for SP60. The two interstitial trends

observed in SP60 are particularly well defined for sample

SP46. Adjacent to pockets containing abundant

granophyre or quartz, there is a very clear, curving trend

towards very low TiO2 at An28, whereas plagioclase adja-

cent to pockets containing mafic phases defines a sharply

reversed trend towards $An75 (Fig. 5b). There is also one

100 m

400 m

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

towards Si-richpockets

towards Fe-richpockets

X An

wt%T

iO2

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 20 40 60 80 100 120 140 1600

10

20

30

40

50

60

70X An

wt%

TiO2

Distance from pocket margin (m)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 50 100 150 200 250 300 3500

10

20

30

40

50

60

70

X An

wt%T

iO2

Distance from pocket margin (m)

(a)(b)

(c)

(d)

(e)

ox

pl

px + ap

pl

qz

K-fs

Fig. 4. Continuous zoning profiles towards Fe-rich (a) and Si-rich (d) interstitial pockets from sample SP46 (LZa*^LZb* boundary,

Skaergaard Peninsula). Zoning at the margins of Fe-rich pockets shows decreasing TiO2 (filled symbols, b) but increasing (reverse) XAn (opensymbols, b) adjacent to the margin (550mm). In c ontrast, zoning at the margins of Si-rich pockets shows a decrease in TiO2 (filled symbols, e)and decreasing (normal) XAn (open symbols, e). This results in two diverging Ti^XAn trends (c; shows four separate profiles; normal zoning pro-files, circles; reverse zoning profiles, diamonds).


155


10/28

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.040.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

HZ* LZa*

LZb* LZc*

MZ* UZa*

UZb*

X An

X An

wt%T

iO2

w

t%T

iO2

wt%T

iO2

wt%T

iO2

(a) (b)

(c) (d)

(e)

(g)

(f)

(no samples analysed)

SP60KR02_4

SP46KR02_37

SP16SP8

KR02_21

SP20

SP29

Fig. 5. TiO2^XAn core and interstitial compositional data for traverses through the Marginal Border Series on the Skaergaard Peninsula(grey circles) and northern Kraemer Island (black triangles), with Layered Series primocryst compositions for comparison (grey crosses, asshown in Fig. 3). Black continuous lines indicate representative continuous zoning profiles from each dataset. No data are available for UZa.Grey arrow in UZb* (g) points to analyses from ilmenite-rich intergrowths (up to 023wt % TiO2). Sample SP60 is from base LZa*,Skaergaard Peninsula, shown in (a) for clarity.


156


11/28

point at low TiO2 and intermediate anorthite content

(An45).

In LZb* (SP8), interstitial pockets may contain any

of apatite, quartz, clinopyroxene, biotite/hornblende,

plagioclase, zircon and opaque minerals, and account for

$067% by area of the section (Table 1). Interstitial plagio-

clase compositions lie on the primocryst trend until$An47. Fewer analyses show very evolved compositions

than in the LZa*^LZb* sample (SP46), but several ana-

lyses plot on a similar normal zoning trend to that identi-

fied in SP46, reaching very low TiO2 at $An28. Highly

calcic, low-TiO2 compositions were not observed, although

points plotting below the primocryst trend at An49,

005 wt % TiO2 are arguably part of a steeper trend to-

wards calcic compositions (Fig. 5c).

In LZc* (SP16), the few pockets observed were highly

altered or contained opaque minerals, biotite, plagioclase

and apatite, and accounted for $036% by area.

Interstitial plagioclase follows the primocryst trend until

$An45 ( 3 mol % anorthite), when the interstitial com-positions diverge towards lower TiO2. In this sample a

reverse interstitial trend is not observed, only steep

normal zoning (Fig. 5d). Several compositions have anom-

alously high TiO2 contents; these analyses were from

traverses along thin interstitial plagioclase cusps between

clinopyroxene and Fe^Ti oxides, and probably reflect

diffusive contamination or secondary fluorescence effects.

These compositions show very slight reverse zoning

(Fig. 5d).

For MZ* (SP20), the interstitial plagioclase compos-

itions lie on the primocryst trend with no divergence to

lower TiO2; there is no evidence for either of the two dis-

tinct interstitial zoning trends seen in the lower strati-graphic zones (Fig. 5e). Instead, zoning profiles towards

interstitial pockets ($027% by area) follow the compos-

itional trend defined by the primocrysts.

Interstitial plagioclase from UZb* (SP29) again lies

on the compositional trend defined by the primocrysts

(Fig. 5g). Plagioclase adjacent to the granophyric non-

replacive microstructures described by Holness et al. (2011)

has very low XAn ($An25), whereas plagioclase from

within the coexisting ilmenite-rich intergrowths has anom-

alously high TiO2 (4011wt % and up to 025 wt % TiO2,

An18^38). These data were excluded on the basis that these

anomalous compositions were probably caused by second-

ary fluorescence.

The other minor elements (Mg and Fe) typically show

less clear compositional differences thanTi. MgO contents

of interstitial plagioclase in the Skaergaard Peninsula are

low and decrease continuously with decreasing XAn, over-

lapping with the primocryst compositions. Zoning profiles

towards silicic pockets typically show constant or decreas-

ing MgO, and zoning profiles towards mafic pockets have

little change in MgO contents. For FeO, zoning towards

the felsic pockets or granophyre typically shows constant

or slightly decreasing FeO contents, and the up-turn

in FeO seen for cumulus minerals at $An40 is not seen

(Fig. 6). Zoning towards mafic pockets tends towards

higher FeO (Fig. 6). For MZ and above, Fe compositions

are scattered and show no clear trend.

Interstitial plagioclase: Marginal BorderSeries, Kraemer IslandKraemer Island has a low structural position within the in-

trusion, so the MBS comprises only HZ*, LZa* and part

of LZb* at this location. In terms of MgO and FeO con-

tent, the Kraemer Island plagioclase compositions overlap

with those from the Skaergaard Peninsula, but FeO con-

tents are scattered towards high values (Fig. 6; Electronic

Appendix 2). However, in TiO2 content, interstitial plagio-

clase from Kraemer Island samples shows some differences

compared with the Skaergaard Peninsula. The part of the

Skaergaard Peninsula interstitial trend that matches the

primocrysts is typically also displayed by interstitial

plagioclase from Kraemer Island. However, nearly all the

analyses from Kraemer Island plot on the reverse zoning

interstitial trend, similar to that defined adjacent to

Fe-rich pockets in the Skaergaard Peninsula (Fig. 4).

Many more reversed compositions are found compared

with the Skaergaard Peninsula, fleshing out the shape of

this trend and (for LZb*) extending it (Fig. 5). The

planar-sided pockets found in the Kraemer Island samples

are much less common than those in the Skaergaard

Peninsula (0015^020% by area; Table 1); typically the

identifiable minerals are apatite and pyroxenes as well as

plagioclase.

Interstitial plagioclase: Layered Series,Cambridge drill core and drill core 90-22The compositions of plagioclase from the central Layered

Series are shown in Figs 7 and 8, and extend earlier obser-

vations (Humphreys, 2009). The results show a complex

mixture of the different XAn^TiO2 trends observed in the

Marginal Border Series. In HZ (sample 118678, Fig. 7a),

the compositions are almost identical to those of sample

SP60 on Skaergaard Peninsula (see Fig. 5; Electronic

Appendices 3, HZ-MZ, and 4, UZ). Interstitial pockets

represent $011% by area (Table 1). LZa plagioclase com-

positions (sample 118653) are similar to those of HZ, the

primocryst trend being followed until $An52, with a max-

imumTiO2 content of 0

13 wt % observed. The interstitialcompositions separate to low and high XAn contents at

low TiO2 (Fig. 7b), as for HZ, but in contrast to LZa*

plagioclase. However, the lowest and highest XAn compos-

itions observed in these divergent trends are equivalent to

those seen in LZa* (An35 and An75, respectively).

Interstitial pockets represent$020% by area (Table 1).

Plagioclase compositions from LZb (samples 118605,

118601, 118590) follow the primocryst trend until An49,


157


12/28

HZ* LZa*

LZb* LZc*

MZ* UZa*

UZb*

X An

X An

wt%FeO

(a) (b)

(c) (d)

(e)

(g)

(f)

(no samples analysed)

w

t%FeO

w

t%FeO

wt%FeO

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90

SP60KR02_4

SP46KR02_37

SP16

SP8

KR02_21

SP20

SP29

Fig. 6. FeO XAn core and interstitial compositional data for the Skaergaard Peninsula and northern Kraemer Island traverses through theMarginal Border Series, with Layered Series primocryst compositions for c omparison, as in Fig. 3. Samples and symbols as for Fig. 5.


158


13/28

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 900.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 900.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.040.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 900.00

0.02

0.040.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80 90

X An

X An

wt%T

iO2

w

t%T

iO2

wt%T

iO2

wt%T

iO2

(a) (b)

(c) (d)

(e)

(g)

(f)

HZ LZa

LZb LZc

MZ UZa

UZb

90-22-461.890-22-421

90-22-824.2

90-22-481.890-22-983.97 and 995.09

90-22-966.16 and 1000.08

458279

1966-1186531966-118678

1966-1185901966-1186011966-118605

Fig. 7. TiO2^XAn core and interstitial compositional data for the central Layered Series from drill cores 1966 and 90-22 as well as surfacesample (458279), with Layered Series primocryst compositions for comparison (grey crosses; see Fig. 3). Samples are 118678, HZ (a); 118653,LZa (b); 118605, 118601 and 118590, LZb (c); 458279, LZc (d). MZ compositions ( e) include unpublished zoning traverses from samples90-22-966.16 and 90-22-1000.08 (diamonds, Stripp, 2009) and plagioclase zoning towards granophyre veins in samples 90-22-995.09 and90-22-983.97 (crosses, Stripp, 2009). Upper zone samples include 90-22-824.2 (f, lower UZa), 90-22-481.8 (f, UZa^UZb boundary), 90-22-461.8(g, lower UZb) and 90-22-421 (g, mid-UZb). Black continuous lines indicate representative continuous zoning profiles from each dataset.


159


14/28

X An

X An

wt%FeO

(a) (b)

(c) (d)

(e)

(g)

(f)

w

t%FeO

w

t%FeO

wt%FeO

HZ LZa

LZb LZc

MZ UZa

UZb

90-22-461.890-22-421

90-22-824.290-22-481.890-22-983.97 and 995.09

90-22-966.16 and 1000.08

458279

1966-1186531966-118678

1966-1185901966-1186011966-118605

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90

Fig. 8. FeO XAn core and interstitial compositional data for the central Layered Series. Samples and symbols as for Fig. 7.


160


15/28

where the interstitial trend diverges towards low XAn and

low TiO2. The interstitial trend is similar to that seen for

LZa, but no highly calcic compositions were observed

except for one point at An55 (Fig. 7c). The mode of intersti-

tial pockets is variable in the three LZb samples, represent-

ing $006 to $033% by area; this could reflect sampling

issues or inter-sample variations (see below).In LZc (sample 458279) the interstitial trend diverges

from the primocryst trend at $An48. The interstitial trend

is similar to that seen for LZb until $An55; most compos-

itions become more calcic with decreasing TiO2, although

there is also a hint of the normal zoning trend at around

An46 (Fig. 7d). One highly calcic interstitial plagioclase

composition (An78) was recorded at a pl^pl^cpx triple

junction. Interstitial pockets are very rare in this sample,

representing only $003% by area (Table 1).

In MZ (samples from 96616 m and 100008 m depth in

drill core 90-22; data from Stripp, 2009), the primocryst

trend is not followed substantially: late-stage interstitial

plagioclase plots below the primocryst trend at $An45.Few low-TiO2 compositions were observed, making it

difficult to assess how these data fit into the series. Zoning

profiles towards isolated granophyre veins in other MZ

samples (99509 m and 98397 m in drill core 90-22; data

from Stripp, 2009) lie on the primocryst trend until

$An38 and then diverge to lower TiO2 at $An32 (Fig. 7e).

A few analyses plot at anomalously high TiO2 contents

(above the primocryst trend); these may result from diffu-

sive contamination or secondary fluorescence effects and

appear similar to those from LZc* (see Fig. 5d).

Interstitial plagioclase compositions in UZa (8242 m

and 4818 m in drill core 90-22) show variable trends.

The lower sample, from 824

2 m in mid-UZa, shows twotrends of low-TiO2 interstitial compositions that diverge

at $An43, one towards $An30 and one towards $An55(Fig. 7f). In contrast, interstitial plagioclase from 4818 m

(at the UZa^UZb boundary) lies on the primocryst trend

up to very evolved compositions ($An20), with no evidence

of highly calcic, low-TiO2 compositions. Interstitial pock-

ets are very rare (5001% by area in sample from

8242 m; Table 1).

Plagioclase compositions from UZb (samples from

4618 m and 212 m in drill core 90-22), lie on a linear

trend of decreasing TiO2 and XAn, with compositions

very similar to those from the UZa^UZb boundary. The

trend is very slightly steeper than the equivalent trend for

the Skaergaard Peninsula (Fig. 7g). Again, there is no evi-

dence of highly calcic, low-TiO2 compositions. Two ana-

lyses plot at higher TiO2; these are probably similar to

high-TiO2 compositions from MZ. Interstitial pockets are

present in the sample from 4618 m ( 026% by area),

which has no non-replacive symplectites (Table 1).

As with the MBS (see above) and the lower parts of the

LS (Humphreys, 2009), MgO and FeO contents of

interstitial plagioclase also vary with XAn. MgO contents

are lower than in the MBS, and are highest in HZ^LZ

(005 wt % MgO), decreasing continuously to $001wt %

at UZb. FeO contents (Fig. 8) tend to be slightly lower

than in the MBS, as with the primocryst compositions,

and decrease from $045wt % in HZ to $015 wt % in

upper UZb. More evolved compositions tend either to-wards high FeO contents with little difference in FeO, or

towards lower FeO and lower XAn. As with the MBS sam-

ples, the most evolved plagioclase compositions (An30 and

below) do not show the increase in FeO observed in the

primocryst compositions. The two distinct trends are most

clear for MZ, UZa and UZb. For the MZ samples, the

low-Fe trend is defined by plagioclase adjacent to grano-

phyre veins (Stripp, 2009).

Interstitial plagioclase: Layered Series,drill core 90-10Four samples in MZ and UZa were examined from drill

core 90-10, to determine whether spatial position within

the Layered Series affects interstitial plagioclase compos-

itions and to look more closely at intra-zone variations

(Figs 9 and 10; Electronic Appendix 5). For equivalent

stratigraphic positions, the plagioclase compositions are

similar to those of drill core 90-22, with core compositions

in the range An40^48. In general, plagioclase from drill

core 90-10 is slightly more Fe-rich and more Mg-rich than

that from drill core 90-22, similar to plagioclase from the

MBS. In terms of Ti^XAn variations, there is limited evi-

dence of highly calcic, low-TiO2 compositions, except for

a few analyses from 90-10-524.1 (meso-gabbro, lower MZ)

and decreasing TiO2 at approximately constant XAn in

90-10-456 (oxide-rich layer, mid-MZ; Fig. 9). This behav-

iour is similar to that seen for drill core 90-22 in LZc orMZ. Samples 90-10-445 (meso-gabbro, top MZ) and

90-10-212 (mid-UZa) show clear normal zoning with a

steep decrease in TiO2 between An45 and An35 (Fig. 9),

similar to the zoning adjacent to veins of granophyre in

lower MZ (drill core 90-22, Stripp, 2009; see Fig. 7e).

There is no clear progression of composition with stratig-

raphy as expected in these MZ samples; the steepness of

the Ti^XAn trend varies non-systematically with strati-

graphic height and may depend on the bulk composition

of the sample, with the steepest trend observed in the

most oxide-rich sample (90-10-456; Fig. 9). The abundance

of interstitial pockets varies between these four samples,

from 0

13 to 0

28% by area where present, but the pocketsare absent in 90-10-456 (Table 1).

SummaryIn general, interstitial plagioclase from a given stratigraph-

ic zone shows a much greater compositional range than

primocrysts from that zone. Minor element concentrations,

in particularTiO2, vary systematically with XAn, but inter-

stitial plagioclase compositions do not always lie on the


161


16/28


17/28

the interstitial trend from that of the primocrysts therefore

indicates that differentiation of the mush liquid does

not always occur under conditions of near-perfect frac-

tional crystallizationthe interstitial liquid and the bulkmagma can follow different liquid lines of descent. This de-

parture from the primocryst trend typically occurs after

an initial period when interstitial and primocryst com-

positions coincide (e.g. Fig. 5b). The XAn composition

at which primocrysts and interstitial plagioclase trends

diverge decreases with increasing stratigraphic height, sug-

gesting a porosity or permeability control on crystalliza-

tion conditions. The gradient of the interstitial trends also

varies both within samples and with stratigraphic position.

However, the uppermost parts of the stratigraphy contain

interstitial plagioclase compositions that do not diverge

from the primocryst Ti^XAn trend: this suggests that in

the latest stages of crystallization, the mush undergoesfractional crystallization much as the bulk magma does.

A key question, therefore, concerns the processes con-

trolling the interstitial compositional variations from the

lower stratigraphic zones. Modelling is made difficult

by continuous solid solution in plagioclase, olivine and

pyroxenes; peritectic reactions (e.g. pyroxene^olivine); the

unusual, highly Fe-rich melt compositions reached;

and the lack of certainty about the original (primocryst)

compositions of mafic phases owing to sub-solidus diffu-

sion. However, in a simplified model that assumes negli-

gible compaction, compositional convection or diffusive

processes, Ti can be treated simply as a trace element ina liquid that is undergoing progressive fractional crystal-

lization, with XAn used as a passive marker of both tem-

perature (T) and F, the fraction of liquid remaining

(Humphreys, 2009). Using this approach, different bulk

partition coefficients for Ti (DBTi) can be used to test the ef-

fects of different crystallizing assemblages, from a starting

point at moderate XAn and high TiO2 (Fig. 11). In the

absence of a formula specific to crystallization of the inter-

stitial liquid, a polynomial function derived from experi-

mental petrology studies (Toplis & Carroll,1996)

TK 1295 265F 290F2 1753 1

is used to determine Tas a function of F, and thus deriveXAn and DTi

pl (following Humphreys, 2009):

TC 3:61XAn 899 2

ln DplTi 0:00535T

C 9:458: 3

For simplicity, only three phases are considered: plagio-

clase, Fe^Ti oxides and clinopyroxene. The DTi values

for olivine and plagioclase are very similar, so neglecting

10 20 30 40 50 60 70 80 90

90-10-456

90-10-212

0

0.2

0.4

0.6

0.8

10 20 30 40 50 60 70 80 90

0

0.2

0.4

0.6

0.8

10 20 30 40 50 60 70 80 90

0

0.2

0.4

0.6

0.8

90-10-445

90-10-524.1

MZ MZ

UZa

X An

w

t%FeO

wt%FeO

(a) (b)

(c)

X An

Fig. 10. FeO XAn core and interstitial compositional data for drill core 90-10, with primocryst compositions for comparison. Samples and sym-bols as for Fig. 9.


163


18/28

olivine may overestimate the modal abundance of plagio-

clase, but plagioclase clinopyroxeneFe^Ti oxides rep-

resent most of the interstitial material in most rocks.

Despite its over-simplicity (ignoring crystallization of all

accessory minerals, for example), the model is useful be-

cause it demonstrates that, to a first order, significant vari-

ations in Ti^XAn curvature can be produced simply bydifferent bulk partition coefficients (DBTi), which may cor-

respond to differing proportions of the interstitial phases

(Fig. 11). The curvature essentially depends on the propor-

tion of Fe^Ti oxides crystallizing relative to plagioclase

and olivine, because of their strong differences in DTi.

Importantly, the normal zoning trends observed in the

MBS can be reproduced well, although no unique results

can be obtained. For example, profiles from sample SP46

(LZa*^LZb* boundary, Skaergaard Peninsula) can be

matched by a low bulk DTi that could equate to crystalliza-

tion of 45% plagioclase, 40% clinopyroxene and 15% Fe^

Ti oxides (Fig. 11). Therefore, at least in places, interstitial

crystallization can be explained by relatively simple frac-tional crystallization processes. In principle, using the

same approach to model equilibrium crystallization will

result in a slower decrease of Ti concentrations compared

with fractional crystallization: equilibrium crystallization

in the mush cannot therefore explain the differences be-

tween interstitial and primocryst plagioclase compositions.

However, the interstitial reverse zoning cannot be

produced by simple fractional crystallization and requires

a different mechanism. This is of course because XAn is cal-

culated as a passive marker ofTand F, and therefore de-

creases monotonically with progressive crystallization.

The liquid Ca/(CaNa) is not modelled explicitly; the

simplified dependence of XAn on temperature is taken on

the basis of the available experimental data (Toplis &

Carroll, 1995, 1996; Thy et al., 2006). However, processesthat might cause changes in liquid composition must clear-

ly be considered (see below).

Given the apparently strong effects of modal mineralogy

on the Ti^XAn gradient, one might expect the primocryst

trend to be curved, as predicted by the modelling of Thy

et al. (2009), or to show significant changes in slope at each

sub-zone boundary to reflect the changes in mineralogy.

However, the primocryst trend is almost linear for zones

LZc and above, suggesting that DBTi changed little during

differentiation of the upper parts of the intrusion.

Origin of calcic plagioclase rims and

reverse zoningAs demonstrated above, the normal interstitial zoningtrends observed are consistent with a relatively simple pro-

cess of fractional crystallization, but the reverse zoning

observed in the lower parts of the intrusion is not and

therefore requires a different mechanism. In principle,

increased XAn in plagioclase could result from increased

temperature, increased pH2O, or an increase in the

anorthite component of the liquid. For the interstitial

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

10 20 30 40 50 60 70 80

wt%T

iO2

X An

1.

2.

4.

3.

Fig. 11. Numerical modelling results for different bulk Ti partition coefficients, superimposed on interstitial data from sample SP46 (LZa*^

LZb* boundary, Skaergaard Peninsula) for comparison (symbols as in figure 5b). The continuous curve (1) represents the model fromHumphreys (2009) using DBTi $ 32 after An54 (equivalent to 20% Fe^Ti oxides 40% plagioclase 40% clinopyroxene). The long-dashedline (2) represents DBTi$17 (equivalent to 10% Fe^Ti oxides 50% plagioclase 40% clinopyroxene). The dotted line (3) representsDBTi$ 12 (equivalent to 7% Fe^Ti oxides 70% plagioclase 23% clinopyroxene). The short-dashed line (4) indicates crystallization withDBTi$ 24 ( equivalent to 45% plagioclase40% clinopyroxene15% Fe^Ti oxides), with a starting composition of 42 wt % TiO2 and assum-ing initial Fof 0 28.


164


19/28

liquid of a slowly cooled basaltic crystal mush, an increase

in temperature during solidification seems unlikely, and

although reversely zoned plagioclase has been produced

experimentally by metastable crystallization during rapid

quenching (Lofgren, reported by Grove et al., 1973), this is

not applicable to intrusive gabbroic suites. If pH2O were

the cause of the increasing anorthite contents, significantquantities of H2O would need to be present in the liquid

(Sisson & Grove,1993; Koepke et al., 2005), and the absence

of hydrous phases associated with the calcic rims argues

against this. The presence of adjacent calcic and sodic

rims within the same thin section would require the forma-

tion of interstitial pockets with widely differing H2O con-

tents; this also seems unlikely. Furthermore, regions of

high pH2O should occur preferentially in the outermost

parts of the intrusion, where external water could most

easily infiltrate the rocks, or in the most fractionated parts

(i.e. UZc), where incompatible components are most

strongly enriched, and not in the centre of the LS. The

most likely cause of the reverse zoning and calcic rims

therefore seems to be a change in the composition of the

interstitial liquid, as argued by Humphreys (2009). There

are several possible causes of changing liquid composition,

including convective exchange within the crystal mush,

dissolution and reprecipitation during compaction, infil-

tration metasomatism, and silicate liquid immiscibility

within the mush.

Compositional convectionCompositional convection within the mush can occur if

the residual liquid becomes less dense than the adjacent

crystals or overlying melt (e.g. Sparks et al., 1984; Morse,

1986). Convection would result in the supply of fresh,

dense, unevolved melt to the crystallization site, and re-moval of more evolved, less dense melt, and could result

in adcumulus textures if the convective velocity is greater

than the growth rate of the crystallization front (e.g.

Tait et al., 1984; Kerr & Tait, 1986; Tait & Jaupart, 1992).

For floor cumulates, the melt composition at the magma^

mush interface will be essentially identical to that in the

main magma reservoir, with more evolved residual melt

near the base of the mush (lower Ca content, higher FeO,

higher alkalis, Thy et al., 2009; although see Boorman

et al., 2004). Compositional convection within the intersti-

tial liquid could therefore result in mineral zonation near

the base of the mush. This would correspond to decreasing

FeO, increasing Mg-number for mafic phases, and increas-ing XAn (reverse zoning) in plagioclase. However, once

the interstitial liquid is saturated with Fe^Ti oxides, TiO2concentrations in the interstitial liquid will fall below

those of the overlying bulk magma, so this reverse zoning

would also be associated with increasing TiO2 in plagio-

clase, whereas the observations show increasing FeO and

decreasing TiO2. Compositional convection would also be

strongly hindered by the low permeability near the base

of the crystal mush, as demonstrated by the very small pro-

portion of total interstitial plagioclase represented by

calcic compositions. This suggests that convection within

the crystal mush is not the reason for the observed grain

boundary zoning.

Infiltration metasomatismMetasomatism by expulsion of interstitial liquids fromlower parts of the crystal mush during compaction was

suggested by Irvine (1980) to explain discrepancies be-

tween observed modal and geochemical zoning patterns

in the cyclically layered Muskox intrusion, Canada.

Compaction also occurs at Skaergaard (e.g. McBirney,

1995; Tegner et al., 2009); however, unlike the Muskox

Intrusion, the Skaergaard Intrusion represents essentially

a single pulse of magma that solidified through continuous

fractional crystallization. Residual liquids deeper in the

crystal mush are therefore probably more evolved, not

more primitive, than those at shallow depths in the mush

or even in the bulk magma (although see Boorman et al.,

2004), so infiltration metasomatism at Skaergaard would

result in normal zoning, not reverse zoning. Injection of

new, primitive magma (e.g. Roelofse et al., 2009) is ruled

out because of a lack of supporting field evidence, because

the reversals are seen in samples covering a wide strati-

graphic range, and because the reversals are seen only in

the outermost grain margins.

Dissolution^reprecipitationLocal dissolution of plagioclase can result from com-

paction during post-cumulus growth, owing to dissolution

of unfavourably oriented plagioclase and reprecipitation

of the material in lower stress orientations, as suggested

for the Stillwater Intrusion (Meurer & Boudreau, 1998)and the Skaergaard Intrusion (Boudreau & McBirney,

1997). Maale (1976) showed that the dissolving plagioclase

grain is always more An-rich than that precipitating,

which should result in enrichment of the anorthite compo-

nent in the interstitial liquid, and could explain local

reverse zoning. However, this mechanism cannot account

for reversed plagioclase within the steeply dipping, and

relatively rapidly cooled MBS, where compaction should

be negligible, nor the transition to normal (i.e.

non-reversed) overgrowth rims in higher stratigraphic

zones of the MBS. Furthermore, Tegner et al . (2009)

showed that compaction is unimportant in HZ^LZa,

where strong reverse zoning is seen.

Silicate liquid immiscibilityImmiscibility of the interstitial liquid was not previously

considered (Humphreys, 2009) but could explain the dif-

fering compositional trends observed in interstitial plagio-

clase. Liquid immiscibility in silicate magmas has been

demonstrated unambiguously by petrographic observa-

tions (e.g. De, 1974; Philpotts, 1979, 1982), as well as


165


20/28

experimental studies (e.g. Roedder, 1951; Dixon &

Rutherford, 1979; Philpotts & Doyle, 1983). Late-stage im-

miscibility within the Skaergaard magma specifically has

also been demonstrated, both by the presence of two dis-

tinct populations of melt inclusions in apatite primocrysts

(UZb, Jakobsen et al., 2005) and by experimental observa-

tions (McBirney & Nakamura, 1974; Veksler et al., 2007). Ithas been suggested that in fact the magma intersects the

two-liquid miscibility gap much earlier in the fractionation

process, around upper LZ to upper MZ, when the

magma was $45^60% crystallized (Veksler et al., 2007;

Holness et al ., 2011), although this is controversial

(McBirney, 2008; Morse, 2008a; Philpotts, 2008; Veksler

et al., 2008). Irrespective of the stage of fractionation at

which immiscibility occurs, the magma at Skaergaard

exsolves into an Fe-rich liquid that is also rich in Ca and

P, and a Si-rich liquid that is also rich in alkalis (e.g.

McBirney & Nakamura, 1974; Jakobsen et al ., 2005;

Veksler et al., 2007).

At thermodynamic equilibrium, the chemical potential

of each component must be the same in every phase in the

system, including both conjugate liquids. However, this

does not equate to equal equilibrium concentrations,

hence the markedly different compositions of conjugate im-

miscible liquids and their different trace element concen-

trations (e.g. Veksler, 2009). Furthermore, each conjugate

liquid may crystallize different modal proportions of the

minerals present, and partition coefficients can also differ

(Roedder, 1978; Philpotts, 1979; Veksler et al ., 2006).

For minerals with complete solid solution, the effects of

liquid immiscibility on an intermediate composition can

be thought of in terms of the end-member components.

For example, in plagioclase feldspar the Si-rich,

alkali-rich liquid would crystallize dominantly the albite

(NaAlSi3O8) component whereas crystallization from the

Fe-rich, Ca-rich liquid would be dominated by the anorth-

ite component (CaAl2Si2O8). At equilibrium, when both

conjugate liquids are present in cotectic proportions, there

is no change in the feldspar composition compared with

crystallization from a single liquid, but disequilibrium con-

ditions could result in crystallization of more An-rich

plagioclase from the Fe-rich liquid and more An-poor

plagioclase from the silicic liquid. In the Skaergaard cu-

mulates, the observation of highly discrepant composition-

al zonation in different parts of the same samples

indicates that disequilibrium crystallization must be occur-

ring, resulting in the crystallization of anomalously calcicplagioclase.

C R YS TA L L I Z A T I O N F R O M

I M M I S C I B L E L I Q U I D SBecause of experimental difficulties, the composition of

plagioclase in equilibrium with each of the liquids

separately is not known independently, but can be esti-

mated if the anorthite component (XCaAl2Si2O8) and

albite component (XNaAlSi3O8) of the conjugate liquids

are known. These components can be calculated (Lange

et al., 2009) from known or estimated t wo-liquid pairs

(Table 2). The calculated anorthite components vary from

0064 to 1

175 for Fe-rich conjugate liquids and from 0

076

to 0247 for Si-rich conjugate liquids, whereas the calcu-

lated albite components vary from 0047 to 0162 for

Fe-rich and from 0286 to 0613 for Si-rich conjugate liquids

(Table 2), based on liquid compositions from

re-homogenized melt inclusions (Jakobsen et al., 2005), ex-

perimental compositions (Veksler et al., 2007) and melting

experiments (McBirney & Nakamura, 1974). The higher

factors for Si-rich liquids reflect their higher affinity for

both calcic and sodic feldspars, but in every case, the nom-

inal XAn for the liquid, calculated as XCaAl2Si2O8/

(XCaAl2Si2O8XNaAlSi3O8), is substantially lower for

the Si-rich liquids (0120^0464) than the Fe-rich liquids

(0

391^0

636, Table 2). This demonstrates that, if the liquidsare out of equilibrium with one another, the Fe-rich liquid

should crystallize more calcic plagioclase, whereas the

Si-rich liquid should crystallize more evolved (albite-rich)

plagioclase. Thus the observed reverse zoning and anomal-

ously high anorthite contents of some interstitial plagio-

clase are compatible with disequilibrium crystallization

from an emulsion with greater than cotectic proportions

of the Fe-rich component, or from the Fe-rich liquid itself.

It is concluded that strong disequilibrium can occur

within the mush, and that immiscible melts that exsolve

from the bulk residual liquid do not necessarily remain in

equilibrium with each other after exsolution, as suggested

by Crawford & Hollister (1977).This concept is supported by the results of several previ-

ous studies. Ripley et al. (1998) reported calcic plagioclase

(An71^79) in nelsonite bodies from the Duluth Complex,

Minnesota, which are thought to have formed from

an immiscible Fe-rich liquid. Similarly, Loferski &

Arculus (1993) reported highly calcic (An92) rims

around plagioclase-hosted melt inclusions that had crystal-

lized to cpx ilmap titanite, rutile and baddeleyite,

in rocks from the Stillwater Intrusion, Montana. In lunar

basalts, two distinct plagioclase compositional trends were

observed adjacent to patches of high-K mesostasis

(to An64) and low-K mesostasis (to An94^96, Crawford &

Hollister, 1977). The Loch Ba ring dyke, Scotland, formed

by mixing of rhyolitic and mafic melts; the melts contain

plagioclase of An21^32 (rhyolite) and An30^65 (mafic) re-

spectively (Sparks, 1988).

Disequilibrium crystallization could occur as a result of

physical separation, perhaps by gravitational separation of

two liquids of differing densities, or by diffusion rates fall-

ing below the threshold required to maintain equilibrium

(Roedder, 1978). This would require the liquid droplets to


166


21/28

be separated by more than the characteristic diffusion dis-

tance over the relevant crystallization timescale, and

could be achieved either by complete physical segregation

of the immiscible liquids, perhaps through gravitationalseparation (because the silicic liquid will have much

lower density than the Fe-rich liquid), or by partial but

incomplete separation at different temperatures and

crystallization rates. A droplet separation of 100mm, for

example, might permit equilibrium crystallization at slow

crystallization rates (when there is a longer time available

for diffusion) or at higher temperatures (when diffusivities

are higher), yet at more rapid crystallization rates or low

temperatures the same droplet separation could result in

disequilibrium and hence changes in mineral composition.

This means that, under certain conditions, an emulsion

that contains non-cotectic proportions of two immiscible li-

quids can undergo disequilibrium crystallization to formcrystals of anomalous composition. A quantitative decou-

pling of the effects of crystallization rate and diffusivity is

beyond the scope of this study. However, the compositions

of crystal rims will depend on the relative rates of droplet

exsolution, liquid^liquid segregation, diffusion and crystal-

lization with falling temperature.

The permeability of the crystal mush may also be im-

portant in driving disequilibrium, by producing pockets

of varying composition, in particular in the least evolved

cumulates. Immiscibility in the interstitial liquid will

occur at a very late stage of crystallization in the LZ cumu-

lates, when the residual porosity and permeability arevery low. Local differences in modal mineralogy could

cause the composition of the residual liquid in each pore

to vary widely, and residual liquids remaining in the

pores may not be in chemical equilibrium. The wide vari-

ation of plagioclase compositions that can occur around a

single pocket demonstrates that these final stages of solidifi-

cation can be highly localized. Differing proportions of

the two liquids in these residual pockets, or perhaps the

interstitial liquid locally entering the miscibility gap from

opposite sides of the solvus, could therefore result in the

disequilibrium trends observed. However, it is not entirely

clear why the initial interstitial trend is so steep (e.g.

Fig. 5a). It is possible that the steep gradient reflects highDTi and may also be related to some permeability thresh-

old, but this cannot be constrained further using the data

presented here.

Interstitial liquid immiscibility inthe Skaergaard PeninsulaThe clearest signature of plagioclase crystallization from

single conjugate liquids is in sample SP46 from the LZa*^

Table 2: Compositions of conjugate immiscible silicate liquids from Skaergaard

Jakobsen et al. (2005) McBirney & Nakamura (1974) Philpotts (1982) Dixon & Rutherford (1979)

Fe-rich Si-rich Fe-rich Si-rich Fe-rich Si-rich Fe-rich Si-rich

SiO2 4067 6558 514 655 415 733 439 685

TiO2 186 022 24 12 58 08 461 170

Al2O3 787 1295 67 96 37 121 70 111

FeOt 3085 863 266 119 310 32 2345 786

MnO 051 013 05 00 055 016

MgO 235 047 04 04 09 00 232 075

CaO 897 2 67 31 94 18 1018 382

Na2O 158 433 22 2 08 31 185 286

K2O 103 368 1 26 07 33 042 12

P2O5 025 003 17 03 35 007 487 096

Total 9594 9802 991 966 978 9767 9915 9891

n 54 17 15 16 7 7

XCaAl2Si2O8 011 008 010 011 006 008 012 015

XNaAlSi3O8 008 051 016 037 005 061 009 047

XAn liquid 058 014 038 023 055 012 057 024

The compositions are based on the compositions of homogenized melt inclusions in cumulus apatite (Jakobsen et al.,2005), melting experiments (McBirney & Nakamura, 1974; Dixon & Rutherford, 1979) and analysis of exsolvedglasses (Philpotts, 1982). Dataset from Jakobsen et al . (2005). Anorthite components (XCaAl2Si2O8) and albitecomponent (XNaAlSi3O8) of the liquids are calculated following Lange et al . (2009). XAn liquidXCaAl2Si2O8/[XCaAl2Si2O8XNaAlSi3O8].


167


22/28

LZb* boundary on the Skaergaard Peninsula. In this

sample, interstitial plagioclase compositions show clear

normal zoning in the vicinity of planar-sided interstitial

pockets containing quartzgranophyre, but reverse

zoning in the vicinity of a mafic pocket dominated by

apatitepyroxenesFe^Ti oxides (see above and Fig. 4);

single spot analyses also fall on these trends. Because ofthe disparate bulk major-element compositions of the

interstitial pockets, and their similarity to the crystallized

products of immiscible melt inclusions in apatite

(Jakobsen et al., 2005), the pockets are interpreted as the

crystallized products of trapped immiscible residual li-

quids (see above and Holness et al., 2011). The normal and

reverse plagioclase zoning trends form by disequilibrium

crystallization towards pores dominated by the Si-rich

and Fe-rich liquids, respectively. This is supported by the

compositional similarity to plagioclase in granophyre

and mafic replacive symplectites, which represent the crys-

tallized products of immiscible residual liquids (see

Holness et al., 2011, fig. 13). Thus any difference between pri-mocryst and interstitial compositions in a given zone is

related to whether disequilibrium crystallization occurs

within the mush of that zone, in contrast to the bulk

magma.

It is worth noting that calcic plagioclase or reverse

zoning may be present even in the absence of a nearby

interstitial pocketreverse-zoned plagioclase commonly

occurs on grain boundaries or at triple junctions, in con-

trast to the very An-poor compositions, which are almost

always associated with quartz-bearing interstitial pockets.

This could be related to the differing viscosities and/or

interfacial energies of the two liquids, with the inviscid

Fe-rich liquid able to form thin films on grain boundaries,and the highly viscous Si-rich liquid tending to sit in more

equant pore spaces. This is consistent with the morphology

of inferred residual liquid films as determined from

glass-bearing nodules and cumulate rocks (Morse &

Nolan, 1984; Holness, 2005; Holness et al., 2007a).

It is of interest to estimate the residual porosity when the

interstitial liquid intersects the miscibility gap, and this

can be done approximately using the modelling results of

Thy et al. (2009). According to their modelled liquids, the

Skaergaard magma reaches UZa (by which time experi-

ments and the presence of paired granophyric and

ilmenite-rich intergrowths indicate that immiscibility has

occurred in the bulk magma; Veksler et al., 2007; Holness

et al., 2011), after 66^68% fractional crystallization. An

LZa rock (comprising primocrysts plus LZa liquid and

following the same liquid line of descent as the bulk

magma) with an initial porosity (fi) of around 50%,

would reach immiscibility after 66^68% crystallization of

the interstitial liquid, leaving a residual porosity (fr) of

fi(1^066) to fi(1^068), or 17^16%. Using the same ap-

proach, even if immiscibility did not start until the

equivalent of UZb (Jakobsen et al., 2005), and allowing a

further 10% crystallization to account for the HZ (see

Thy et al., 2009) the interstitial liquid in an HZ rock

could still intersect the miscibility gap with a residual por-

osity in the region of 11^75% (this approach assumes

that porosity is not occluded by other processes such as

adcumulus growth). Therefore the observation that eventhe most primitive rocks undergo silicate liquid immiscibil-

ity in residual liquid retained in the last interstitial pores

is consistent with previous studies demonstrating immisci-

bility in the bulk liquid (McBirney & Nakamura, 1974;

Jakobsen et al., 2005). However, the permeability when

this occurs is probably very low, so the exsolved liquids

are likely to form in isolated pockets. In more evolved

rocks, interstitial liquid immiscibility will occur at increas-

ingly high fr until eventually, at the point where the bulk

magma itself exsolves into two liquids, even the initial por-

osity will be filled with an emulsion rather than a single

liquid.

S P A T I A L D I S T R I B U T I O N O F

I M M I S C I B L E L I Q U I D S I N M B SThe diverse compositions of interstitial plagioclase on

grain boundaries and adjacent to interstitial pockets are

consistent with disequilibrium crystallization from immis-

cible liquids during the late stages of solidification. The

spatial distribution of these diverse plagioclase compos-

itions can provide insights into the distribution of the two

liquids within the crystal mush and throughout the intru-

sion. The simplest sample traverse to consider is that

through the Marginal Border Series on the Skaergaard

Peninsula. The MBS crystallized more or less in situ onthe walls of the intrusion, and is therefore free from the ef-

fects of compaction and current-driven sedimentation,

unlike the floor cumulates (Layered Series). The

Skaergaard Peninsula traverse represents the most com-

plete cross-section through the evolving cumulates.

The presence of the two distinct interstitial plagioclase

compositional trends in HZ* indicates that, at the final

stages of crystallization, the residual interstitial liquid

reached the miscibility gap and the two liquids were sepa-

rated, probably into spatially distinct pockets, although

probably only the most evolved liquids reached immiscibil-

ity, at a very low residual porosity. In LZa*, the reverse

Ti^XAn trend appears nearly identical to that in HZ*, butthe two interstitial trends diverge earlier, suggesting that

the residual liquid formed coarser pockets and more

evolved compositions.

Samples from LZb* and LZc* show the same pattern,

although the most extreme compositions were not

observed, as reflected in the paucity of very low TiO2 con-

tents. This may reflect a sampling issue or could reflect a

genuine low abundance in the rocks (see discussion


168


23/28

below). However, despite a lack of definition, two distinct

trends are still apparent in LZb*, with one clearly heading

towards $An30 and the other discernible in several ana-

lyses below the primocryst trend at $An50 (see Fig. 5). In

LZc* there are two clear trends, one of which defines

normal zoning whereas the other (a zoning profile towards

a plagioclase^pyroxeneôxide triple junction) does showslight reverse zoning but is affected by diffusive modifica-

tion or secondary fluorescence leading to anomalously

high Ti contents (see Fig. 5). These patterns also indicate

that two liquids exsolved during disequilibrium crystalliza-

tion of the interstitial liquid.

For MZ* and above, interstitial plagioclase compos-

itions fall on the primocryst trend. The slope is shallower

than that of the normal zoning trend in stratigraphically

lower zones, which suggests a lower DBTi and a difference

in abundance of pyroxene or oxides in the crystallizing as-

semblage.The appearance of abundant, paired granophyre

and ilmenite-rich intergrowths within MZ* indicates that

the onset of immiscibility in the bulk magma also occurs

at approximately this point (Holness et al., 2011). This is

earlier than suggested by some researchers (e.g. McBirney

& Nakamura, 1974) but may be consistent with experi-

ments (Veksler et al., 2007; although see McBirney, 2008;

Morse, 2008a; Philpotts, 2008; Veklser et al., 2008) and com-

positional variations of melt inclusions in plagioclase near

the LZ^MZ boundary (Jakobsen et al., 2006). The coinci-

dence of the primocryst and interstitial compositional

trends at MZ* and above is interpreted as the result of

crystallization from equilibrium immiscible liquids, as in

the bulk magma, resulting in no change in feldspar com-

position and no distinction between interstitial and primo-

cryst compositions.

Disappearance of calcic plagioclaseor reverse zoningIt is suggested that the primocryst and interstitial compos-

itions coincide when the interstitial liquid no longer under-

goes disequilibrium crystallization, but crystallizes in

equilibrium as the bulk magma doesin other words,

when the feldspar composition is buffered by the presence

of both liquids in equilibrium. In the MBS this occurs

within upper MZ*, where the appearance of abundant

paired granophyre-rich and ilmenite-rich intergrowths

(Holness et al., 2011) also implies that the bulk magma has

reached the miscibility gap. These intergrowths represent

large pockets of Si-rich and Fe-rich liquid, respectively,that were present within the crystal mush in the higher

parts of the stratigraphy (Holness et al., 2011) so the absence

of two distinct interstitial zoning trends for plagioclase

there is surprising; one might have expected to see even

clearer evidence for two distinct trends than in LZ. This

suggests that chemical communication between the paired

intergrowths was sufficiently good to preclude disequilib-

rium crystallization, despite residing in different (albeit

adjacent) pockets. For the liquids to be in equilibrium,

they must be separated by less than the characteristic diffu-

sion distance relative to the crystal interface. The grano-

phyric and ilmenite-rich intergrowths do clearly occupy

separate pockets but these are closely associated spatially;

moreover, the porosity (and permeability) will be high

when the pockets form, allowing good communication,and cooling rates will be relatively low.

Alternatively, the paired intergrowths may in fact repre-

sent pockets of incompletely separated emulsion, with

varying proportions of the two liquids. Granophyre-rich

pockets may contain one or more of apatite, oxides, clino-

pyroxene, biotite or amphibole, and similarly the

ilmenite-rich intergrowths may be intergrown with grano-

phyre or K-feldspar as well as biotite, pyroxenes, apatite

and olivine (Holness et al., 2011). Ilmenite-rich intergrowths

form from the Fe-rich liquid with a subsidiary component

of Si-rich liquid, whereas granophyric intergrowths are

dominated by the Si-rich liquid with a lesser Fe-rich com-

ponent. Either way, both liquids are still in chemical equi-

librium, so no change in plagioclase composition resulted.

TheTi^XAn data presented in this study cannot differenti-

ate between these explanations; the reality is probably a

combination of both.

SummaryPrimitive cumulates from the Skaergaard Peninsula under-

go silicate liquid immiscibility during crystallization of

the interstitial liquid. This happens when the residual

porosity and permeability are low, resulting in isolated

pockets containing differing proportions of the two

immiscible liquids. Disequilibrium crystallization can

occur as a result of rapid crystallization and physical

separation of the two liquids, and leads to the formationof two distinct interstitial crystallization trends. At

approximately MZ* and above, slower crystallization and

inefficient separation of liquids results in equilibrium con-

ditions and precludes the formation of the discrepant

plagioclase compositions.

D I S C U S S I O N

Differences between Layered Series andMarginal Border SeriesIn comparison with the MBS, interstitial variations in LS

plagioclase from the 1966 and 90-22 drill cores appear to

evolve more slowly with increasing stratigraphic height(except for HZ compositions, which are indistinguishable

from those in HZ* on the Skaergaard Peninsula).

Specifically, the two distinct interstitial trends tend to split

at lower TiO2 in the LS, and the presence of two trends

persists right up to the UZaÛZb boundary, instead of

dying out in MZ as in the Skaergaard Peninsula.The pres-

ence of two distinct interstitial trends up to UZaÛZb sug-

gests that disequilibrium within the mush persisted until


169

7/27/2019 Humphreys. 2011. Silicate Li

Documents

Humphreys. 2011. Silicate Liquid Inmmiscibility Within the Crystal Mush ... Etc