HR: 1330h
AN: V42C-0369    [PDF]
TI: Os and Sr Isotope Signatures in Kamchatka Adakites, Nb-Rich Arc Basalts and Mantle Pyroxenites: Inferences on Mantle and Crustal Processes
AU: * Widom, E
EM: widome@muohio.edu
AF: Miami University, Department of Geology, Oxford, OH 45056 United States
AU: Kepezhinskas, P
EM: pkepezhi@tampabay.rr.com
AF: Geoprospects International, Inc., Tampa, FL 33620 United States
AU: Defant, M J
EM: defant@tampabay.rr.com
AF: University of South Florida, Geochemistry CPR 107, Tampa, FL 33620 United States
AB: A suite of adakites, Nb-enriched arc basalts (NEABs), and mantle pyroxenites from Kamchatka have been analyzed for Os and Sr isotopes in order to investigate the relationship between the lavas and their respective veined-mantle source regions, and the extent to which crustal contamination has contributed to the enriched isotopic signatures in the lavas. The mantle pyroxenites range in $^{187}$Os/$^{188}$Os from 0.1280 to 0.4138, and in $^{87}$Sr/$^{86}$Sr from 0.70305 to 0.70339. The adakites and NEABs for which both Sr and Os data are available fall within the compositional range of the pyroxenites for $^{187}$Os/$^{188}$Os, varying from 0.1384 to 0.2983, and have $^{87}$Sr/$^{86}$Sr ranging from 0.70294 to 0.70320. Os abundances in these lavas range from 12 to 112 ppt. Two distinct positive Os-Sr isotope correlations are observed amongst these samples, including a shallower trend that comprises pyroxenites and lavas from the northern arc front, and a steeper trend that comprises pyroxenites and lavas from behind the arc front in the south. In each of these groups, it is notable that the pyroxenites, rather than the lavas, define the radiogenic endmember. This relationship is consistent with melting of veined mantle (peridotite-pyroxenite) as the origin of the radiogenic Os signatures in the lavas. The two distinct Os-Sr isotope trends displayed by the respective pyroxenite-lava groups are further consistent with previous studies of Kamchatka peridotite xenoliths, which indicate the involvement of compositionally distinct slab-derived metasomatic fluids in the northern versus southern Kamchatka sub-arc mantle, related to differences in the age of the respective subducting slabs (Kepezhinskas et al., 1996; Widom et al., 2002). Other Kamchatka lavas, with Os abundances between 1.3 and 2.5 ppt and $^{187}$Os/$^{188}$Os between 0.7888 and 1.2908, are significantly more radiogenic than any of the pyroxenites, and therefore most likely derive their radiogenic Os by crustal assimilation. References: Kepezhinskas, P.K., Defant, M.J. and Drummond, M.S. 1996, GCA 60, 1217-1229. Widom, E., Kepezhinskas, P. and Defant, M. 2003, Chemical Geology 196, 283-306.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting