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Carbonatite metasomatism in the mantle : sources and roles of carbonate in metasomatic enrichment processes in the lithosphere

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Yaxley, Gregory M (1993) Carbonatite metasomatism in the mantle : sources and roles of carbonate in metasomatic enrichment processes in the lithosphere. PhD thesis, University of Tasmania.

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Abstract

Recent high pressure experimental work in the Hawaiian pyrolite+H20+C02 system
(Wallace and Green, 1988; Falloon and Green, 1990) has revealed the presence of a
stability field for high Mg/(Mg+LFe), sodic dolomitic carbonatite melt in equilibrium
with pargasitic lherzolite at pressures greater than the decarbonation reaction opx + dol
= ol + cpx + C02. Carbonatite melts could therefore be formed as low degree primary
melts of carbonated peridotite at depths greater than -60km, or by crystallization and
reaction of amphibole+clinopyroxene from carbonated, undersaturated silicate melts
within the upper mantle at depths of 60-90 km. Such carbonatite melts could segregate
and ascend from their source regions at very low melt fractions ( <0.02%; Hunter and
MacKenzie, 1988). Analogy with natural eruptive carbonatites suggests that primary
carbonatites would possess extreme LILE enrichment, and may therefore have an
important role in metasomatism of the lithosphere at pressures of 15-20 kbars (Green
and Wallace, 1988).
A suite of mantle xenoliths (apatite+amphibole bearing magnesian wehrlites,
lherzolites and harzburgites) from western Victoria, Australia, display the predicted
(Green and Wallace, 1988) petrographic and geochemical signatures of carbonatite
metasomatism, namely, an unusual assemblage of magnesian olivine and diopsidic
clinopyroxene in the absence of orthopyroxene (wehrlites), textural evidence of
replacement of primary orthopyroxene by jadeitic clinopyroxene and forsteritic olivine
(lherzolites and harzburgites), the almost ubiquitous presence of significant accessory
apatite,.unusually high whole-rock CaO/Al203 and Na20/Al203 values~ and extreme
LILE enrichment without concomitant TiO2 enrichment.
Comparison of the trace element and isotopic characteristics of this carbonatite
metasomatised suite of apatite+amphibole-bearing magnesian spinel wehrlites with
other, previously reported (O'Reilly and Griffin, 1988; Stolz and Davies, 1988),
southeastern Australian spinel peridotite xenoliths, suggests that those containing
apatite+low-Ti pargasite have also been metasomatised by ephemeral, dolomitic
carbonatite melts. Low Ti/Eu, high Zr/Hf, and restricted bulk earth-like Sr-Nd
isotopic characteristics are characteristic of t~is metasomatic style within the
southeastern Australian lithosphere. This con~rasts with metasomatism related to
hydration or basaltic intrusion of the lithosphere, in which Ti/Eu and Zr/Hf remain
unfractionated at primitive mantle values of around 7740 and 36 respectively.
A model for carbonatite metasomatism is proposed in which undersaturated mafic
silicate melts, derived from asthenospheric upwelling or diapirism, crystallize
clinopyroxene + amphibole ± phlogopite as they ascend into the stability field of
hydrous garnet peridotite+carbonatite melt at P~30 kbars. The residual liquid evolves
to sodic dolomitic carbonatite with low Ti/Eu, high Zr/Hf, and extreme enrichment in
Sr and REE. This carbonatite melt segregates at low melt fractions and ascends
rapidly, finally being absorbed by the lithosphere as it intersects a series of end-member
decarbonation reactions at around 15-20 kbars. The nett effect of these is to
react the enstatite and Mg-Tschermak's components of primary orthopyroxene, and
MgAl204 component in spinel, with sodium carbonate and dolomitic components of
the carbonatite, producing jadeite-ureiite solid solution in clinopyroxene, forsteritic
olivine and C02-rich fluid. Evolution of the reacting carbonatite leads to deposition of
pargasitic amphibole and apatite. In some cases, replacement of primary
orthopyroxene by sodic Cr-diopsidic clinopyroxene and forsteritic olivine was
complete. Subsequent annealing produced the texturally equilibrated wehrlites.
Interruption of the metasomatic process by entrainment in the host magma produced
apatite+amphibole-bearing spinel lherzolites and harzburgites which retain direct
textural evidence of the decarbonation reactions, namely rims and veins of
olivine+clinopyroxene on primary orthopyroxene grains. The extreme LILE
enrichment of the carbonatite, and the ephemeral style of metasomatism resulted in
imposition of its distinctive trace element and isotopic signature onto refractory spinel
peridotite at this level.
This model was tested experimentally by reversing the decarbonation .. reactions.
Synthetic amphibole±apatite bearing wehrlite compositions with 1, 5 or 7 wt% C02
were run at PT conditions within the carbonatite melt stability field of Wallace and
Green (1988). This produced lherzolite or harzburgite residue in equilibrium with
sodic dolomitic melt. The model implies the release of large C02 fluxes of the order of
several wt% in the lithosphere in regions of carbonatite metasomatism (<15-20
kbars). This may provide a mechanism for incorporating a carbonatite component into
some intra-plate volcanic products as has recently been proposed by Nelson et al.
(1988) and Dupuy et al. (1992).
A second major theme of this study relates to the fate of subducted carbonate. Many
DSDP reports allude to the presence of calcitic carbonate in veins and vugs in oceanfloor
basalts, related to low temperature alteration processes. A series of high pressure
experiments was conducted at 15<P<35 kbars, in which a composition representative
of altered ocean-floor basalt (synthesized with excess H20 at 20 kbars and 700°C,
producing a quartz-bearing garnet amphibolite assemblage) was run with 0, 3 or 10
wt% calcite. These experiments revealed that carbonate is stable as a residual phase to
temperatures of at least 1000°C at 20-35 kbars. At these conditions, a hydrous silicate
melt coexists with a carbonate+rutile eclogite assemblage. An end-member exchange
reaction between garnet and carbonate (calcite+ pyrope = grossular +dolomite)
resulted in an increased grossular component in garnet, and increasing MgO content in
carbonate, with increasing pressure at near-solidus temperatures. Dolomite was found
to be the stable carbonate phase at P>25 kbar and Tg50°C.
Reconnaissance experiments using magnesite instead of calcite revealed the presence
of a bulk compositional control on the equilibrium carbonate composition. A shift to
more magnesian bulk compositions resulted in stabilization of Ca-bearing magnesite,
whereas runs conducted at identical PT conditions using calcite contained Mg-bearing
calcite as the equilibrium carbonate composition.
The results of the hydrous basalt+carbonate experiments are supported by a study of a
suite of natural carbonate-bearing eclogites from the Krusne hory Mountains in the
Bohemian part of the Czechoslovakian segment of the Saxothuringicum (Klapova,
1990). The suite contains textural and mineralogical evidence of the above
garnet+carbonate reactions in both the prograde and retrograde senses. For example,
some samples show patches of calcite, surrounded by magnesite, surrounded in tum
,by thin rims of dolomite, in contact with garnet. The garnet is often markedly pyroperich
on the rim, compared with the core. Textures and compositions imply prograde
conversion of primary calcite to magnesite, with retrograde conversion to dolomite.
Analysis of garnet and carbonate compositions and textures in these samples, and
comparison with the basalt+carbonate experimental data suggest that the rocks
achieved peak metamorphic conditions of 25 kbar and 650-700°C, followed by a near-isobaric
heating event, and subsequent uplift and retrogression.
The study suggests that at least some carbon involved in the Earth's carbon budget
can be recycled through subduction related processes. The ultimate fate of subducted
carbon may depend on oxygen fugacity conditions in the deeper mantle. Recycled
carbonate may result in diamond formation at low f02 conditions, in particular those
diamonds included in eclogitic xenoliths hosted by kimberlites, or containing
garnet±omphacite inclusions. Alternatively, if f02 is sufficiently high, carbon could
be fixed in the mantle as dolomite or magnesite, and may be involved in carbonatite or
fluid metasomatism, or in petrogenesis of undersaturated silicate melts (melilitites,
nephelinites) or carbonatites (Brey and Green, 1976).

Item Type: Thesis (PhD)
Keywords: Metasomatism (Mineralogy), Carbonatites
Copyright Holders: The Author
Copyright Information:

Copyright 1993 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1994. Includes bibliographical references (p. 154-166)

Date Deposited: 04 Feb 2015 23:21
Last Modified: 24 Aug 2016 04:04
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