HR: 1330h
AN: V42A-0317    [PDF]
TI: OLIVINE LAMELLAE AND INTERSTITIAL BLEBS OF DIOPSIDE AND ENSTATITE EXSOLVED FROM MAJORITIC GARNET DURING DECOMPRESSION IN MULTIANVIL APPARATUS
AU: * Dobrzhinetskaya, L F
EM: larissa@ucrac1.ucr.edu
AF: Inst. Geophys. & Planet. Phys. and Dept. of Earth Sciences, University of California, Riverside, CA 92521 United States
AU: Green, H W
EM: hgreen@mail.ucr.edu
AF: Inst. Geophys. & Planet. Phys. and Dept. of Earth Sciences, University of California, Riverside, CA 92521 United States
AB: We present preliminary experimental data on the decompression of majoritic garnet primarily synthesized from a mineral mix of garnet peridotite bulk chemistry showing exsolution from majoritic garnet of olivine (Ol) in the form of oriented plates and pyroxenes as interstitial blebs. Experiments conducted at 14GPa/1673K demonstrate that all enstatite (En) and about 85% of diopside (Di) were dissolved into garnet yielding run products of approximately 40% Ol + 55% Grt + 5% Di. Garnet was found to be supersilicic with Si=3.17-3.31 p.f.u. Repeat of such experiments followed immediately by re-annealing at 13 and 12 GPa yielded exsolution of both Di and Ol. Olivine exsolved as micron-size plates nucleated within garnet on low-angle boundaries. In contrast, diopside exsolved abundantly as tiny blebs at garnet grain boundaries, exhibiting no typical exsolution microstructures. Similarly, in specimens annealed at 5 GPa after previous equilibration at 8GPa/1673K, En exsolved as small blebs at garnet boundaries. Under conditions similar to the latter experiments, interstitial blebs of natural enstatite also occur at garnet grain boundaries (Van Roermund et al., 2001) in Norwegian deep-seated ($>$200 km) subduction zone grt-peridotite. Our experiments show that Ol as well as En and Di may exsolve during decompression of majoritic garnets in the course of Grt peridotite exhumation. Examples of preservation of pyroxene exsolution lamellae in former majoritic garnets come from both xenoliths in kimberlites (Haggerty and Sautter, 1990; Sautter et al., 1991) as well as from very large garnets in subduction-zone peridotites (van Roermund and Drury, 1998). However, many other garnet peridotites from subduction zones contain Di, En, and/or Ol along grain boundaries within larger polycrystalline garnets and within embayments at the margins of smaller amoeboid garnets (e.g. Dobrzhinetskaya et al, 1996, Green and Dobrzhinetskaya, 2003). Such garnets also may contain rounded non-oriented inclusions of each of these minerals, or all three together, consistent with the results of majoritic garnet decompression presented above.
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 3694 Instruments and techniques
DE: 3924 High-pressure behavior
DE: 8102 Continental contractional orogenic belts
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting