Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton |
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Authors: | Email author" target="_blank">D?R?BellEmail author M?Grégoire T?L?Grove N?Chatterjee R?W?Carlson P?R?Buseck |
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Institution: | (1) Department of Geological Sciences and Department of Chemistry/Biochemistry, Arizona State University, 871404, Tempe, AZ 85287-1404, USA;(2) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;(3) Observatoire Midi-Pyrénéés, UMR - CNRS 5562, 14 Avenue E. Belin, 31400 Toulouse, France;(4) Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road N.W., Washington, DC 20015, USA |
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Abstract: | Textural evidence in a composite garnet harzburgite mantle xenolith from Kimberley, South Africa, suggests metasomatism of
a severely melt-depleted substrate by a siliceous, volatile-rich fluid. The fluid reacted with olivine-rich garnet harzburgite,
converting olivine to orthopyroxene, forming additional garnet and introducing phlogopite, and small quantities of sulfide
and probable carbonate. Extensive reaction (>50%) forming orthopyroxenite resulted from channelized flow in a vein, with orthopyroxene
growth in the surrounding matrix from a pervasive grain-boundary fluid. The mineralogy of the reaction assemblage and the
bulk composition of the added component dominated by Si and Al, with lesser quantities of K, Na, H, C and S, are consistent
with experimental studies of hybridization of siliceous melts or fluids with peridotite. However, low Na, Fe and Ca compared
with melts of eclogite suggest a fluid phase that previously evolved by reaction with peridotitic mantle. Garnet and phlogopite
trace element compositions indicate a fluid rich in large-ion lithophile (LIL) elements, but poor in high field-strength elements
(HFSE), qualitatively consistent with subduction zone melts and fluids. An Os isotope (TRD) model age of 2.97 ± 0.04 Ga and lack of compositional zonation in the xenolith indicate an ancient origin, consistent with
proposed 2.9 Ga subduction and continental collision in the Kimberley region. The veined sample reflects the silicic end of
a spectrum of compositions generated in the Kimberley mantle lithosphere by the metasomatizing effects of fluids derived from
oceanic lithosphere. These results provide petrographic and chemical evidence for fluid-mediated Si-, volatile- and trace-element
metasomatism of Archean mantle, and support models advocating large-scale modification of regions of Archean subcontinental
mantle by subduction processes that occurred in the Archean. |
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