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 basaltic lavas from the Honomanu Gulch section of Haleakala Volcano, Maui spanning the shield-building, post-shield and post-erosional stages of the typical Hawaiian volcanic cycle. Os is shown to behave compatibly during fractional crystallization of the basalts, with a bulk crystal-melt partition coefficient of 10 +/- 5. Os isotopic compositions are inversely related to Os concentrations and exhibit no simple temporal trend, in contrast to Sr, Nd, Pb and He isotopic data from the suite [1]. The Os, Sr and He isotopic systematics in the Haleakala basalts are explained by the presence of three components. The dominant component 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 reflect 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 representative of depleted upper mantle (MORB source). Temporal and petrologic trends in the isotopic data support a model for progressive mixing between plume-derived melts and up to 30% of small-degree partial melts of the MORE source during the later stages of volcanic activity. Direct exchange of plume-derived melts with MORE-source peridotite is precluded. 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 oceanic crust beneath the volcano. This study demonstrates that Os isotopes in ocean island basaltic magmas are sensitive to interactions with oceanic crust and mantle. As a consequence, the Os isotopic compositions of such basalts can record a variety of magmatic processes both within and outside their mantle plume source regions.