Restoration of premetasomatic protolith compositions in mantle xenoliths

Tatjana Rehfeldt, S. F. Foley, D. E. Jacob

Research output: Contribution to journalMeeting abstract

Abstract

Metasomatic overprints in peridotites of the Kaapvaal Craton
document mantle refertilization. Compositions of most mantle
peridotites document depletion in Al, Fe, Ca due to earlier partial
melting events, whereas their incompatible trace element contents
indicate reenrichment. Additionally, metasomatism may introduce
clinopyoxene (cpx) and garnet (grt) as well as phlogopite
(phl), transforming harzburgites into lherzolites. The original
mantle composition can be restored from metasomatised peridotites
by subtracting the effects of metasomatising agents.
We report results on wehrlite and websterite xenoliths from the
Kimberley kimberlites, South Africa. These contain olivine (ol),
cpx, opx, grt and phl in different abundances and exhibit mineral
chemistry similar to peridotite xenoliths. While wehrlites comprise
Mg-rich minerals (pyroxene Mg# 0.90–0.93), websterites can be
subdivided into high-Mg (Mg# 0.90–0.92) and low-Mg (Mg#
0.87–0.90) groups. Pyroxene and grt in both wehrlite and websterite
xenoliths have worm-like embayed forms which may indicate
healed veinlets. Cpx exhibits convex upward rare earth element
(REE) patterns, similar to cpx in lherzolite xenoliths, and garnets
are light REE depleted and heavy REE enriched, similar to grt
rims in lherzolite and harzburgite xenoliths. This indicates that
these chemical signatures are characteristic for metasomatically
introduced cpx and grt even where textural evidence for their
metasomatic origin has been lost. Additionally, oxygen isotope
composition of ol, opx, cpx and grt in the wehrlite and websterite
xenoliths lie within the mantle range, but show disequilibrium
partitioning between the minerals. Incomplete equilibration is
also documented by thermometry: temperatures within grt–cpx–
opx clusters are similar (Tgrt–cpx = 1043 C; Tcpx–
opx = 1075 C), whereas the Tgrt–ol thermometer gives 100
higher (1150 C) for mineral pairs at the edges of clusters.
The texture of cpx in wehrlite xenoliths can be used to classify
them as pervasively enriched mantle peridotites, whereas websterites
are recognisable as vein assemblages crystallized from percolating
metasomatic melts. The simpler mineralogy of wehrlites
compared to lherzolites makes it easier to recalculate their premetasomatic
protolith composition. Subtracting the metasomatic
minerals cpx and grt and adding opx to the wehrlites results in
harzburgitic to dunitic compositions with major element compositions
that lie upon the mantle depletion trend.
Original languageEnglish
Pages (from-to)A523-A523
Number of pages1
JournalGeochimica et Cosmochimica Acta
Volume70
Issue number18 supplement
DOIs
Publication statusPublished - 2006
Externally publishedYes
EventGoldschmidt Conference (16th : 2006) - Melbourne, Australia
Duration: 27 Aug 20061 Sep 2006

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