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Highly Siderophile Elements and 187Os/188Os from Hawaiian Cumulate Xenoliths Show Potential Mixing Between the Loa and Kea Trends

Brian Oller, James M.D. Day

Abstract

Studies of shield-stage lavas have established that there exist two distinct geochemical trends in Hawaiian volcanoes referred to as the 'Loa' and 'Kea' trends [1, 2]. These trends are isotopically distinct endmembers that have been best defined by long-lived Rb-Sr, Sm-Nd and U-Th-Pb isotopes [1, 2]. We report new 187Os/188Os, highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances, and bulk rock major and trace element abundance data for ultramafic cumulate xenoliths from Mauna Kea (Kea trend), Mauna Loa, and Hualalai (Loa trend). These samples provide the strongest constraints yet on 187Os/188Os for the Loa and Kea trends. Loa trend xenoliths have more radiogenic Os (Avg = 0.1342 ±0.0043; 2σ n = 8) than Kea trend xenoliths (Avg = 0.1270 ±0.0062; 2σ n = 10), consistent with higher time integrated Re/Os for the Loa trend mantle and a recycled oceanic crustal component. These new results show there are no clear distinctions between absolute or relative HSE abundances between the two trends, but there is clear distinction in long-term Re/Os between the mantle sources. However, seismic and gravity studies suggest the presence of shared magma chamber and/or plumbing systems between volcanoes of Loa and Kea trend composition [3]. These observations seem to contradict the available geochemical and isotopic data. Here, 187Os/188Os mixing models are used to address the viability of different mixing scenarios.   [1] Abouchami et al. (2005) Nature, 434(7035), 851; [2] Weiss et al. (2011) Nature Geoscience, 4(12), 831; [3] Wilding et al. (2023) Science, 379(6631) 462-468.