HR: 1330h
AN: V12C-0607    [PDF]
TI: Hydrous and anhydrous garnet-bearing mantle xenoliths from Hawaii: Isotopic Heterogeneity?
AU: * Mukherjee, S
EM: smukh002@fiu.edu
AF: Earth Sc, Florida Intl' Univ, Miami, FL 33199 United States
AU: Bizimis, M
EM: bizimis@magnet.fsu.edu
AF: Earth Sc, Florida Intl' Univ, Miami, FL 33199 United States
AU: Keshav, S
EM: skesha01@fiu.edu
AF: Earth Sc, Florida Intl' Univ, Miami, FL 33199 United States
AU: Sen, G
EM: seng@fiu.edu
AF: Earth Sc, Florida Intl' Univ, Miami, FL 33199 United States
AB: This study focuses on the petrography, major element mineral chemistry, and bulk-rock trace element and Sr-Nd isotopes compositions of a suite of garnet-bearing xenoliths from Oahu, Hawaii, in an attempt to answer the following questions: (1) Are these xenoliths restites, cumulates, or frozen melts? (2) Are they related to Koolau-like (shield stage) magmas or post-erosional Honolulu Volcanics (HV)-type magmas? Large clinopyroxene (cpx) is the dominant phase; however, large garnet (gt), orthopyroxene (opx), olivine (ol), and spinel (sp) are also present. Some xenoliths also have trace (less than 1 percent) amounts of phlogopite of different shapes and sizes. Cpx has exsolved opx, sp, and gt. Mineral chemically, cpx is a low-Cr, high-Al, Na, Ti, and Fe/Mg variety. Garnet is a low-Cr and high Fe/Mg type. The Mg\#s of cpx and gt range from 74-85 and 61-74, respectively. The Mg\# of opx ranges from ~80-84. Spinels are of the pleonaste type. Chemical traits of cpx, gt, and sp indicate that these xenoliths cannot be treated as either restites or frozen melts. Post-exsolution and "igneous" stage temperatures based on cpx-gt thermometry range from ~ 950-1350oC and ~ 1290-1470oC. Cpx and gt are in Mg\# equilibrium with each other. Opx has not equilibrated with cpx or gt in terms of Mg/Fe. The anhydrous xenoliths are distinct from the Pacific-MORB and Koolau, but are virtually identical to the HV lavas, confirming previous suggestions that the garnet pyroxenites are genetically related to parental magmas. However, some hydrous xenoliths have more radiogenic Sr isotope (for a given Nd) than both HV and the anhydrous xenoliths, and fall outside the compositions of the Hawaiian lavas in the Sr-Nd isotope space. From Sr-Nd data on these xenoliths, the following becomes clear: (1) All the anhydrous and some hydrous xenoliths pyroxenite are virtually identical to the HV; (2) Higher Sr isotope compositions of the hydrous variety indicates that water-bearing phases may be partially responsible for the isotopic heterogeneity among this suite of xenoliths. It is proposed here that later melts that are perhaps hydrous infiltrate these lower lithospheric xenoliths, resulting in the precipitation of phlogopite (of different Sr isotopic composition) in relatively anhydrous, high-pressure crystal extracts.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting