HR: 1340h
AN: DI33A-1126    [Abstracts]
TI: Sulfides in the Garnet Pyroxenite xenoliths from Oahu, Hawaii
AU: Sen, I S
EM: isen001@fiu.edu
AF: Florida International University, Department of Earth Sciences 11200 SW 8th Street, Miami, Fl 33199, United States
AU: Sen, G
EM: seng@fiu.edu
AF: Florida International University, Department of Earth Sciences 11200 SW 8th Street, Miami, Fl 33199, United States
AU: * Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: Florida State University, National High Magnetic Field Laboratory, Isotope Geochemistry 1800 E. Paul Dirac Drive, Tallahassee, Fl 32306, United States
AB: Oahu is known for its garnet bearing xenoliths that occur in the Honolulu Volcanics. Clinopyroxene is the dominant minerals of these rocks, and modes of other silicate minerals â€" orthopyroxene, olivine, garnet, amphibole, and phlogopite vary considerably. Ilmenite and Spinels of diverse variety also occur (Keshav et al. 2007, J. Petrol.). In this report we present new electron microprobe and LA-ICPMS data on the sulfides that are always present in these xenoliths although they make up only trace amounts. In terms of morphology and mode of occurrence the sulfides can be divided fundamentally into two types â€" Type I occurs as poikilitic inclusions in the silicate phases mostly in clinopyroxene and Type II occurs in the interstitial spaces between the silicates, along grain boundaries and along cracks within individual silicate grains. Sizes of both types vary considerably. Type I sulfides are generally globular and appear to have formed from immiscible sulfide melts that got enclosed by the silicate minerals that grew from the main body of silicate melt. Keshav et al. (2007) estimate the average solidus temperatures of garnet pyroxenites from Oahu to range from 1215 to 1600°C (average 1325°C) at 3-5 GPa. Therefore, the Type I sulfides are high temperature sulfides that formed above the silicate solidus. Type II sulfides take various forms â€" from vein-like to dendritic. Compositionally, both types include Ni rich pyrrhotites (Ni content varies from 3-5 wt%) and monosulfide solid solutions(MSS). The MSS are divided into Ni rich MSS containing as much as 20 wt% of Ni, the average is 15 wt% while the Ni poor MSS has 5-9 wt% of Ni in it. We have limited data on PGE so far but the Type II sulfides have a very low PGE content. Two recent papers have noted that Hawaiian plume-derived shield tholeiites are too rich in Ni for a given SiO2% to be produced by partial melting of a peridotite and called for an unusual Ni-rich pyroxenite source in which the large Ni content is locked in clinopyroxene. The many experiments that have been conducted on pyroxenites have not been able to generate such high Ni clinopyroxenes. We propose that the Ni actually comes from the high Ni monosulfide solid solutions similar to those in the pyroxenites studied. However, we do not think that these pyroxenites are the source of Hawaiian shield lavas because their isotopic composition is distinct from shield lavas.
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting