OS-ISOTOPIC VARIATION IN BASALTS FROM HALEAKALA VOLCANO, MAUI, HAWAII- A RECORD OF MAGMATIC PROCESSES IN OCEANIC MANTLE AND CRUST

Os isotopic data are presented for a suite of tholeiitic and alkalic b asaltic lavas from the Honomanu Gulch section of Haleakala Volcano, Ma ui spanning the shield-building, post-shield and post-erosional stages of the typical Hawaiian volcanic cycle. Os is shown to behave compati bly during fractional crystallization of the basalts, with a bulk crys tal-melt partition coefficient of 10 +/- 5. Os isotopic compositions a re inversely related to Os concentrations and exhibit no simple tempor al trend, in contrast to Sr, Nd, Pb and He isotopic data from the suit e [1]. The Os, Sr and He isotopic systematics in the Haleakala basalts are explained by the presence of three components. The dominant compo nent has Os-187/Os-188 = 0.132+/-0.002 (Os-187/Os-186 = 1.10 +/- 0.02) , Sr-87/Sr-86 = 0.704 and elevated He-3/He-4 and is interpreted to ref lect melts derived from the Hawaiian mantle plume. A second component with low Os-187/Os-186, Sr-87/Sr-86 and He-3/He-4 is considered repres entative of depleted upper mantle (MORB source). Temporal and petrolog ic trends in the isotopic data support a model for progressive mixing between plume-derived melts and up to 30% of small-degree partial melt s of the MORE source during the later stages of volcanic activity. Dir ect exchange of plume-derived melts with MORE-source peridotite is pre cluded. A third component with Os-187/Os-188 > 0.135 (Os-187/Os-186 > 1.12) is diagnostic of old crustal materials with high time-integrated Re/Os. It is present only in some of the most differentiated basalts and is probably derived through contamination of melts with aged ocean ic crust beneath the volcano. This study demonstrates that Os isotopes in ocean island basaltic magmas are sensitive to interactions with oc eanic crust and mantle. As a consequence, the Os isotopic compositions of such basalts can record a variety of magmatic processes both withi n and outside their mantle plume source regions.