Trace element characteristics of the Kerguelen ultrabasic and basic xenoliths: a window on the petrogenetic processes occurring in the deep part of the oceanic lithosphere
M. Gregoire and S.Y. O'Reilly, GEMOC, Macquarie
Alkali basalts from the Kerguelen Islands have entrained many
mantle peridotites (harzburgites and dunites) and various meta-igneous
xenoliths (deep tholeiitic- transitional and alkali segregates)
which commonly show reequilibration to the granulite facies conditions.
Clinopyroxenes separated from type I xenoliths show large trace-element
variations, for example, Sr ranges from less than 1 to 1440 ppm,
Ti from 45 to 11225 ppm and Zr from less than 1 to 2365 ppm. The
most of the samples show LREE-enriched patterns but few samples
display LREE-depleted patterns. The amphiboles and phlogopites
coexisting with clinopyroxene (cpx) in a few samples show high
amounts of Nb and Ta. These trace-element characteristics emphasize
the origin of the Kerguelen harzburgites as a result of high degrees
of partial melting (20-30 %). Then multi-metasomatic events associated
with the activity of the Kerguelen plume explain the modification
of this depleted mantle to varying degrees. The metasomatic agents
probably range from basaltic melts to carbonatitic melts and may
sometimes correspond to Ti- and alkali-rich, H2O-poor fluid.
Clinopyroxenes separated from type II xenoliths display lesser
trace-element variations, for example, Sr ranges from 8 to 293
ppm and Zr from 2 to 85 ppm. Titanium alone shows a wide range
of composition, from 630 to 13180 ppm. The subsolidus garnet occurring
in some samples shows two-types of patterns: one, a high degree
of LREE depletion, the other a low degree thereof with positive
Eu anomalies. The trace-element characteristics of the cpx of
the type II Kerguelen xenoliths confirm their origin as cumulates
of tholeiitic-transitional or alkaline magmas and evidence (with
garnet data) the importance of the subsolidus reequilibration
in the granulite facies stability field.
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