Journal
PHYSICS AND CHEMISTRY OF MINERALS
Volume 33, Issue 1, Pages 28-34Publisher
SPRINGER
DOI: 10.1007/s00269-005-0043-0
Keywords
in situ X-ray diffraction; MORB density; Le Bail method; hexagonal phase; calcium ferrite phase
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In situ X-ray diffraction study was conducted to identify the crystal structure of the Al-phase, which was previously reported to form in basaltic compositions at pressures and temperatures of the uppermost part of the lower mantle. Le Bail whole-pattern fitting method was adopted to investigate the structure of the Al-phase under high pressure and temperature as well as ambient conditions. Observed patterns were satisfactorily fitted using the hexagonal phase with space group P6(3)/m (plus minor amount of garnet) under both of these conditions. On the other hand, the calcium ferrite structure model proposed in some earlier studies based on quench experiments yielded profile-fitting results at significantly lower confidence levels, particularly at simultaneous high pressure and high temperature conditions, suggesting that this phase may not form in oceanic crust materials subducted in the uppermost lower mantle. The difference in densities of hexagonal and calcium ferrite phases, however, is only similar to 1% under pressures and temperatures of the uppermost part of the lower mantle conditions, which yields a negligible effect on the bulk density of the subducted oceanic crust.
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