Origin of garnet and clinopyroxene in Kaapvaal low-T peridotite xenoliths:
implications from secondary ionisation mass spectrometry (SIMS) data
N. S. C. Simon1* , D. G. Pearson2 , R. W. Carlson3 , G. R. Davies 1
1
Faculty of Earth Science, University of Amsterdam, De Boelelaan 1085, 1181 HV
Amsterdam, The Netherlands. * e-mail: simn@geo.vu.nl
2
Department of Geological Sciences, Durham University, South Rd, Durham DH1 3LE, UK
3
Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad
Branch Road N.W., Washington, D.C. 20015, USA
Introduction
Most peridotite xenoliths from the Kaapvaal craton are strongly depleted in major elements.
The mean Mg/(Mg+Fe) ratio in Kaapvaal peridotite olivines is ~0.93. Recent Re-Os isotope
studies (e.g. [1]) show that the major melting event causing this depletion took place in the
Archaean, and that the lithosphere was coupled to the overlying crust since that time [2].
Seismic tomography has revealed that the Kaapvaal craton is underlain by a thick, cold and
stable lithospheric keel, which is consistent with the chemical composition of a depleted
harzburgitic residue formed by large degrees of partial melting [3]. Thus, from melting
experiments and phase relation studies it would be expected that the lithospheric mantle
underneath the Kaapvaal is very poor in garnet and clinopyroxene [4]. Instead, lherzolites
with abundant garnet and clinopyroxenes are very common as xenoliths in kimberlites. These
rocks still have depleted major element chemistry, notably high Mg/(Mg+Fe), but are
enriched in incompatible trace elements. This enrichment has frequently been attributed to
the infiltration of metasomatic fluids or melts, modifying the pre-existing mineral assemblage
(cryptic metasomatism, e.g. [5]), or even introducing new minerals like phlogopite or
clinopyroxene (patent metasomatism). However, this cannot explain the modal composition
of Kaapvaal garnet lherzolites [4].
In this study we have performed detailed petrographic and electron microprobe (EPMA)
analyses on 20 lherzolites and harzburgites from Lesotho (Matsoku, Letseng-la-Terai,
Liqhobong, and Thaba Putsoa). Seven garnet lherzolites were selected for trace element
analyses by SIMS, where single spots and traverses were measured on garnets,
clinopyroxenes, orthopyroxenes and phlogopite (if present). Our results show that both,
garnet and clinopyroxene, were re-introduced to the depleted residue at different times after
the original melting of the lithosphere in the Archaean.
Results
Sample description
All selected samples are typical coarse Kaapvaal peridotites and contain between 3 and 10%
modal garnet, and 1-8% modal clinopyroxene. Large euhedral phlogopite is present in M2
and LET64. Garnet in all samples is kelyphitized to different extents. Some garnets
(especially in the samples from Letseng-la-Terai) are surrounded by up to 3 distinct kelyphite
rims. The inner one is very fine grained. The outer rim consists of coarser (5-100 mm) spinel,
clinopyroxene, orthopyroxene and phlogopite. Most garnets have fractures, which are often
connected to the coarse kelyphite rim and filled with the same material. In samples GP404
and M2 some very fresh and almost unaltered garnets are preserved.
Clinopyroxene is often spatially related to garnet but also occurs as distinct single grains. In
all samples a second generation of clinopyroxene (in addition to the high-Al kelyphitic cpx)
could be identified, having distinctively higher Ti contents and highly variable major and
trace element composition. These secondary clinopyroxenes occur on grain boundaries and in
cracks and have a vein-like appearance. They are often accompanied by remnants of melt that
approximates to kimberlitic major and trace element compositions [6]. Large clinopyroxene
porphyroblasts often show a spongy overgrowth rim of up to 100 mm that has the same
composition as the vein-like clinopyroxenes.
Re-Os whole rock analyses are available for all samples but M2 and GP404. Melt depletion
ages (TRD = minimum ages) are Archaean and vary from 2.7 – 2.9 Ga [1]. Most samples were
affected by later Re addition, which can be correlated with the amount of fracturing and the
presence of kimberlitic material in the rocks.
Major element composition
Where compositional variations in phases were detected, quantitative traverses through grains
were measured. X-ray mapping of single grains gives a detailed picture of zoning and
compositional variations close to inclusions or fractures. Except for the late-stage high-Ti
overgrowth rims on clinopyroxene and
phlogopite, and slight compositional changes
in the very outer rims of the other minerals,
all phases are rather homogeneous. Most
garnets display some variations in Cr, Al and
Ca, following the trend described as “type I”
zonation by Burgess and Harte [7] for the
deformed high-temperature peridotite suite
from Jagersfontein. (Fig. 1). All garnets
show this compositional zonation parallel to
the “lherzolite trend”, except sample M2
(distinct low-Ca outer rim). This low-Ca rim
is related to fracturing of the garnet and is
Fig. 1: Major element variation in garnet. Diagram
absent in unfractured grains that occur as
modified after [7].
inclusions in orthopyroxene or olivine.
Average Mg/(Mg+Fe) ratios are 0.93 for olivine, orthopyroxene and clinopyroxene and 0.85
for garnet and so these samples are representative of the Kaapvaal low-T peridotite suite [8].
P,T
Equilibration temperatures and pressures (for mineral cores calculated with [9]) for the
selected samples ranges from 928 – 1123°C and 2.9 – 4.5 GPa.
Trace elements
Although significant variations in absolute abundances of trace elements exist for garnets, cpx
and opx, in different samples (Fig. 2a, b), the shape of their chondrite normalised REE
patterns are very similar. The cores of all garnets show sigmoidal REE patterns with a
maximum between Nd and Eu (Fig. 2a). The rims of some garnets evolve towards more
“normal” patterns with low LREE and high HREE. Zonation in Ba, Sr, Hf, Zr, Ti and Y is
also observed. The cores of all minerals are generally homogeneous in trace element
concentrations. Variations occur only in the outer part of the grain, or near inclusions and
fractures. In addition to showing features common to other samples, sample M2 also displays
very complex zoning in some garnet rims, indicating that it was affected by additional latestage processes. All clinopyroxenes are LREE enriched with a maximum at Nd and show
generally less variation
in REE concentrations
within
individual
samples compared to
garnets (Fig. 2b). REE
concentrations in opx
vary widely between
the samples, and two
types of pattern can be
Fig 2: REE variation in, a) garnet; b) cpx and opx for Lesotho Low-T
recognised:
i)
a
peridotite suite. Data includes both core and rim compositions.
straight
slope
decreasing from LREE to HREE; ii) a slightly curved pattern with a maximum at Nd – Eu,
and only slightly lower chondrite-normalised HREE than LREE concentrations (Fig. 2b).
Discussion
Trace element equilibrium
Sigmoidal REE patterns in garnet, comparable to those observed here, have been described
from G10 garnets found as inclusions in diamond and in harzburgite xenoliths (e.g., [10] and
references therein). LREE enriched clinopyroxenes are also common in Kaapvaal low-T
lherzolites. In this study, garnet and clinopyroxene cores seem to be fairly equilibrated in
terms of major elements, but they are definitely not in
trace element equilibrium (Fig. 3). We calculated trace
element distribution coefficients for garnet/cpx (Dgt/cpx )
pairs (e.g. LQ6) and compared these to equilibrium D’s
from the literature ([11, 12] and references therein). This
comparison shows that for mineral cores, Dgt/cpx LREE are
higher than and Dgt/cpxHREE are lower than the expected
equilibrium values. In contrast, garnet-cpx partitioning
relationships for the rims are consistent with equilibrium.
Dgt/cpx Sr is identical for cores and rims and lies within the
Fig. 3: Comparison of D(gt/cpx) with
equilibrium range. This is in agreement with a much
literature data of [11 & 12].
higher diffusivity of divalent cations in contrast to 3+ ions
[13, 14]. Across the sample suite, different stages of re-equilibration between garnet and
clinopyroxene are preserved. Some rims seem to be completely equilibrated, but others do
not differ much from the cores. Taking textural observations into account, it becomes clear
that equilibration is far more advanced where garnets are significantly fractured and altered.
“Clean” garnets usually preserved their LREE enriched sigmoidal pattern.
Origin of garnet and clinopyroxene
To put further constraints on the origin of clinopyroxene
and garnet in these rocks, we calculated hypothetical
melts in equilibrium with these minerals (Fig 4). The
hypothetical melt, parental to the garnet cores, is very
similar to the “megacryst melt” proposed by Burgess and
Harte [15]. These authors studied garnets with sigmoidal
REE pattern from a suite of garnet harzburgites from
Jagersfontein and proposed that they might have
crystallised from a melt that was fractionated by
Fig. 4: Calculated equilibrium melts
precipitation
of Cr-poor garnet megacrysts. The Burgess
for garnet, clinopyroxene and
and Harte model fits well with our observations, and we
orthopyroxene. D data from [12 &
also believe that garnets in our samples are introduced to a
references therein..
garnet-free melt residue for the following reasons:
1. melting experiments for a large pressure range show that it is not possible to
generate a residue with the modal composition observed for Kaapvaal peridotite
xenoliths (opx-rich and garnet bearing) [16, 17 & 4];
2. experiments [14] show that the diffusivities of REE in garnet do not depend on
ionic radius. Thus, sigmoidal REE patterns in garnet can not be produced by
differences in the kinetic behaviour of REE, as proposed by [18] and [10];
3. to our knowledge, no zonation from REE depleted cores towards LREE
enriched rims, which would indicate the modification of pre-existing residual
garnet by a LREE enriched metasomatic fluid, was ever observed in Kaapvaal
garnets;
4. the REE zonation observed in the garnets is consistent with re-equilibration of
the garnets with clinopyroxene introduced during a later metasomatic event.
The calculated “cpx melt” has a much less fractionated REE pattern and, in contrast to the
calculated ”garnet melt”, closely resembles a kimberlitic melt. Thus, both textural and trace
element evidence indicates that clinopyroxene crystallised significantly later than the garnet.
The calculated “opx melt” has significantly lower REE contents, which is probably mainly
due to the fact that the partitioning of elements into orthopyroxene is strongly dependent on
pressure, temperature, and probably also modal composition [11]. The results calculated here
are based on partitioning data from experiments performed at maximum 2 GPa [12].
However, the slope of the “opx melt” REE pattern is slightly steeper than that of the “cpx
melt”, which might be a further indication of incomplete REE equilibrium, this time between
orthopyroxene and clinopyroxene (and orthopyroxene and garnet).
Summary
Based on a detailed petrographic trace element study, we propose that garnet grew from a
highly fractionated melt in the absence of clinopyroxene. In a second event, clinopyroxene
was introduced (eventually by a hydrous metasomatic melt together with phlogopite). Major
elements re-equilibrated (almost) entirely, but trace element disequilibrium between garnet
and clinopyroxene could be at least partly preserved in all samples studied here. Fracturing of
the rocks and introduction of hydrous fluids, possibly related to kimberlite volcanism,
facilitated trace element equilibration along these zones of weakness [19].
Shortly before or during eruption of the kimberlite, some samples were affected by infiltrating
kimberlitic melts forming either a glassy matrix or crystallising high-Ti secondary
clinopyroxenes and overgrowth rims on other minerals. Based on recent diffusion
experiments for REE and other trace elements in pyrope and diopside [13, 14], we conclude
that REE disequilibrium between garnet and clinopyroxene in the (relatively cold)
lithospheric mantle might be preserved over long periods of time in the absence of fluids.
Resetting occurs rapidly as soon as fluids and fractures facilitating the exchange and transport
of REE are present.
Future single crystal and core-rim Nd isotope studies on thoroughly characterised garnets and
clinopyroxenes from a variety of Kaapvaal peridotites are aimed at constraining the relative
and absolute time scales of these processes.
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