HR: 15:10h
AN: V12F-06    [PDF]
TI: Garnet from diamondiferous metamorphic rocks of Kokchetav massif, Kazakhstan as a peak pressure recorder
AU: * Sobolev, N V
EM: sobolev@uiggm.nsc.ru
AF: Institute of Mineralogy and Petrography, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russian Federation
AU: Palyanov, Y N
AF: Institute of Mineralogy and Petrography, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russian Federation
AU: Shatsky, V S
AF: Institute of Mineralogy and Petrography, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russian Federation
AU: Sokol, A G
AF: Institute of Mineralogy and Petrography, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russian Federation
AU: Tomilenko, A A
AF: Institute of Mineralogy and Petrography, Russian Academy of Sciences, Siberian Branch, Novosibirsk, 630090 Russian Federation
AB: Garnet is a key mineral coexisting with diamond both in kimberlite (as xenocrysts, in diamondiferous garnet peridotites and eclogites, as inclusions in diamond) and in UHP metamorphic rocks of Kokchetav massif (diamondiferous gneisses, garnet-pyroxene rocks, dolomitic marbles and diamond facies eclogites). In UHPM rocks garnets are of particular importance as inclusions in zircons protected from retrograde metamorphism. Diamond formation conditions in eclogitic (E-type) upper mantle environment are estimated based upon Grt-Cpx thermometry and coesite barometry (e.g. Sobolev et al., PNAS, 2000, 97:11875) at P=5.5-6.0 GPa and T=1000-1300$\deg$C. These data are supported by diamond synthesis in carbonate-silicate fluid (e.g. Palyanov et al., Nature, 1999, 400: 417). E-type garnet may dissolve up to 0.3 wt.$%$ Na$_{2}$O (Sobolev, Lavrentyev, Contr. Min. Petr., 1971, 31:1) depending on pressure and Na$_{2}$O contents in coexisting pyroxene and melts (fluids). Majorite component (pyroxene solid solution) was reported in rare garnets from diamonds (e.g. Moore, Gurney, Nature, 1985, 318:553) and UHP conditions were experimentally confirmed for such garnets (Irifune, Phys. Eart. Pl. Int., 1987, 45:324; Gasparik, Phys. Chem. Min., 2002, 29:170; Luth, Am. Miner., 1997, 82:1198). Garnets from Kokchetav diamondiferous metamorphic rocks demonstrate considerably lower Na$_{2}$O solubility (up to 0.2 wt.$%$ in rare samples) and absence of majorite component. However, coexisting pyroxenes may contain up to 50 mol.$%$ jadeite. Several UHP experiments performed with Kokchetav eclogites and dolomitic marbles using a split-sphere apparatus resulted in detection of up to 0.3-0.4 wt.$%$ Na$_{2}$O in newly formed eclogitic garnets at P=5.7 and 7.0 GPa, T=1400 and 1700$\deg$C respectively. Majorite component was also determined in newly formed garnets reaching about 5$%$ with Si (pfu)=3.05-3.06. Similar garnets without Na$_{2}$O were also obtained in UHP experiments with diamondiferous dolomitic marbles (e.g. Palyanov et al., Dokl. Earth. Sci., 2001, 380:671). Based on the difference in Na$_{2}$O and majorite contents in natural Kokchetav garnets, and those coexisting with diamonds in kimberlite and obtained in UHP experiments, we conclude that the peak of metamorphism at Kokchetav massif occurred at P about 4.5-5.0 GPa and T=900-1000$\deg$C (Sobolev, Shatsky, Nature, 1990, 343:742; Shatsky et al., Contr. Min. Petr., 1999, 137:185; Sobolev et al., Dokl. Earth. Sci., 2001, 380:237) but not exceeding 6.0 GPa (e.g. Ogasawara et al., Island Arc, 2000, 9:400).
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting