Dunite-pyroxenite xenoliths from South African kimberlites: former cumulates of Archean oceanic crust?

Dunite, wehrlite and pyroxenite xenoliths are widespread
in South African kimberlites, but are less well studied than
lherzolites and eclogites.
During the early Archean the Earth was probably hotter
and more melt was generated at mid-ocean ridges, leading to a
thicker crust with a more magnesian bulk composition [1].
Eclogites are widely accepted to represent basaltic and
gabbroic parts of Archean oceanic crust, whereas rocks
representing former ultramafic cumulates within the oceanic
crust have not yet been identified. Following high pressure
metamorphism, these ultramafic cumulates and picritic rocks
would yield pyroxenites [2].
Four xenolith types in South African kimberlites are
potential candidates for cumulates, namely pyroxenites,
spinel-bearing wehrlites, spinel-free wehrlites and low-Mg
dunites. These types differ in modal composition and
structure, as well as major and trace element composition.
Pyroxenites have equigranular structures with coarse grained
garnet (Grt) in some samples and fine grained Grt in others,
the latter crystallising along grain boundaries. Spinel-free
wehrlites and dunites have equigranular or porphyroclastic
structures, whereas spinel-bearing wehrlites have
equigranular, porphyroclastic or foliated porphyroclastic
structures. Silicate minerals in pyroxenites and spinel-free
wehrlites have the lowest Mg# (100*Mg/(Mg+SFe)) of the
four xenolith types; Ol has Fo89 and cpx Mg# 87 to 92.
Pyroxenites also contain Opx with Mg# 88 to 91 and Grt with
Mg# 72 to 77. Oxide minerals in pyroxenites and spinel-free
wehrlites are magnetite, whereas dunites have Ol with Fo89-
93 and contain ilmenite and magnetite. Spinel-bearing
wehrlites have silicate minerals with the highest Mg# of the
four xenolith types: Ol = Fo90-93, Cpx = Mg# 90 to 93, Opx
= Mg# 92 to 93 and Grt = Mg# 82 to 85. Cr-spinel is always
present, and magnetite, ilmenite and/or rutile may occur in
different abundances. Pyroxenites and wehrlites have Cpx, Grt
and Opx with consistently different REE patterns. In
comparison to wehrlitic Cpx, pyroxenitic Cpx is more
enriched in LREE and has a steeper REE slope. Spinel-free
and spinel-bearing wehrlites have similar REE patterns.

References
[1] Zegers T.E. and van Keken P.E. (2001) Geology 29, 1083-
1086.
[2] Foley S.F., Buhre S., and Jacob D.E. (2003) Nature 421,
249-252.