1School of Earth Sciences, The University of Queensland, St. Lucia, Queensland 4072, Australia
2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
3Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, USA
4Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada
5Department of Earth Sciences, School of Education, Waseda University, Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan
6Institute of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Abstract(#br)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.