Oxygen-osmium isotope systematics of West Maui lavas: A record of shallow-level magmatic processes

New δ18O and 187Os / 188Os data from late shield-building stage lavas from West Maui volcano, Hawai'i, indicate that a range of distinct shallow-level processes can affect compositions of plume-generated ocean island magmas. The relative ages of the samples studied were determined either on the basis of field relationships, or from collection depth in a water well. The stratigraphically lower lavas have [Os] = 61-399 ppt, initial 187Os / 188Os = 0.1311-0.1324 and δ18Oolivine = 4.53-4.88‰. The stratigraphically higher lavas have [Os] = 20-273 ppt, initial 187Os / 188Os = 0.1323-0.1390 (with the exception of one low [Os] sample with 187Os / 188Os = 0.1517), and δ18Oolivine = 4.73-5.21‰. These compositions are generally similar to those of Kea-type Hawaiian lavas from Mauna Kea and Kilauea volcanoes, but in detail, there are some significant differences between the West Maui lavas and other Kea-type lavas. The stratigraphically lower lavas have 187Os / 188Os similar to those previously observed for Mauna Kea and Kilauea lavas, and define a nearly flat 87Sr / 86Sr vs. 187Os / 188Os trend that is consistent with assimilation by these magmas of small degree melts of Pacific oceanic crustal gabbros. The stratigraphically higher West Maui lavas extend to 187Os / 188Os higher than observed at Mauna Kea or Kilauea. This probably results from interaction of these magmas with the hydrothermally altered volcanic edifice during the waning phases of the shield building stage. This process is likely to affect the isotope systematics of Os and O, but will have little effect on those of Hf, Nd, Pb or Sr.