TY - JOUR
T1 - Majoritic garnet
T2 - A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond
AU - Collerson, Kenneth D.
AU - Williams, Quentin
AU - Kamber, Balz S.
AU - Omori, Soichi
AU - Arai, Hiroyoshi
AU - Ohtani, Eiji
PY - 2010/10
Y1 - 2010/10
N2 - A means for estimating pressures in natural samples based on both the coupled substitution (Na+)[1+] (Ti+[VI]Si)[4+]=(M)[2+] (Al+Cr)[3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites (n=4) yield uniform pressures of 23.8±0.2GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4±0.6GPa; n=5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2±1.1GPa; n=3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7±2.7GPa (n=30) and 8.3±1.6GPa (n=15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3±0.8GPa and 16.9±1GPa), Monastery (15.7±7GPa) and Kankan (15.5±0.2GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.
AB - A means for estimating pressures in natural samples based on both the coupled substitution (Na+)[1+] (Ti+[VI]Si)[4+]=(M)[2+] (Al+Cr)[3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites (n=4) yield uniform pressures of 23.8±0.2GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4±0.6GPa; n=5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2±1.1GPa; n=3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7±2.7GPa (n=30) and 8.3±1.6GPa (n=15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3±0.8GPa and 16.9±1GPa), Monastery (15.7±7GPa) and Kankan (15.5±0.2GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.
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U2 - 10.1016/j.gca.2010.07.005
DO - 10.1016/j.gca.2010.07.005
M3 - Article
AN - SCOPUS:79952146580
SN - 0016-7037
VL - 74
SP - 5939
EP - 5957
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
IS - 20
ER -