TY - JOUR
T1 - Silica and volatile-element metasomatism of Archean mantle
T2 - A xenolith-scale example from the Kaapvaal Craton
AU - Bell, David R.
AU - Grégoire, M.
AU - Grove, T. L.
AU - Chatterjee, N.
AU - Carlson, R. W.
AU - Buseck, P R
N1 - Funding Information:
Acknowledgments This work was supported by US National Science Foundation grants EAR-9526702, EAR-9526840, EAR-0003533 and EAR-0310330. DRB gratefully acknowledges support at MIT from a Crosby Fellowship. We thank Sam Bowring (MIT) for discussions, Anton le Roex (University of Cape Town) for hosting the ICP-MS analyses in his laboratory, the Carnegie Institution of Washington for access to electron microprobe facil- ities, and De Beers Consolidated Mines for field support and permission to sample the Kimberley dumps.
PY - 2005/10
Y1 - 2005/10
N2 - 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.
AB - 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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U2 - 10.1007/s00410-005-0673-8
DO - 10.1007/s00410-005-0673-8
M3 - Review article
AN - SCOPUS:27744515131
SN - 0010-7999
VL - 150
SP - 251
EP - 267
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 3
ER -