Magnesium isotopic systematics in postshield lavas from Mauna Kea Volcano, Hawaii
Abstract
The magnesium isotopic effect during crystal fractionation of tholeiitic lavas has been studied using samples recovered from Kilauea Iki lava lake, which have homogeneous δ26Mg (-0.36 to -0.20). We explore the Mg isotopic effect during crystal fractionation of alkalic lavas using the well-studied, compositionally distinct Mauna Kea postshield lavas. The subaerial surface of Mauna Kea is essentially covered by postshield lavas, which can be divided into two substages based on their eruption timing and compositions. The older Hamakua Volcanics erupted basaltic lavas while the younger Laupahoehoe Volcanics erupted hawaiites and mugearites. The compositional variations observed in Mauna Kea's postshield substages are thought to be caused by the segregation of mineral assemblages involving clinopyroxene, olivine, plagioclase, and Fe-Ti oxides in moderate pressure conditions. Compared to Laupahoehoe lavas, Hamakua lavas experienced lower degrees of crystallization resulting in higher MgO contents. The major and trace element abundances of the evolved magmas, hawaiites, are sensitive to pressure due to the plagioclase/clinopyroxene ratio in the fractionating assemblage decreasing as pressure increases from that of the mid-crust, 0.6-1.5 kbar, to upper mantle pressure of 4.4 kbar. Our results show that at high MgO content (>8 wt. %), there is no resolvable variation in δ26Mg (-0.27 ± 0.07); values that are identical to those of Mauna Kea shield stage tholeiites and the average mantle value (δ26Mg= -0.25). However, at lower MgO (<8 wt. %) there is an observable variation in δ26Mg ranging from -0.09 to -0.39. We will utilize major and trace element data coupled with thermodynamic modeling to explore the crystal fractionation history of Mauna Kea's postshield lavas and to understand the Mg isotopic variations found within these lavas. Our results will assist in the understanding of Mg isotopic variations in oceanic and continental basalts, many of which are alkalic in composition.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V12A..06H
- Keywords:
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- 1037 Magma genesis and partial melting;
- GEOCHEMISTRYDE: 1038 Mantle processes;
- GEOCHEMISTRYDE: 1040 Radiogenic isotope geochemistry;
- GEOCHEMISTRYDE: 1065 Major and trace element geochemistry;
- GEOCHEMISTRY