A Depleted Component in Shield Lavas from Mauna Kea, Hawai'i
Abstract
To constrain the evolution of a Hawaiian volcano, the Hawaii Scientific Drilling Project (HSDP) drilled and cored a 3.5km hole on the flank of Mauna Kea volcano. Geochemical studies show that several distinct primary magmas contributed to the shield stage of Mauna Kea. These include high- and low-SiO2 tholeiitic groups that also differ in He and Pb isotope ratios. Another distinctive geochemical shield stage group occurs in a depth range from 1765-1810 mbsl. Relative to other tholeiitic groups, this group has higher CaO at a given MgO content, higher abundances of highly incompatible elements, and similar abundances of moderately incompatible elements. It is inferred that high-CaO lavas are mixtures of a low degree ( 2%) partial melt of the peridotite source of low-SiO2 basalt and a mafic component rich in cumulate clinopyroxene. The clinopyroxene-rich cumulates may have formed beneath arcs during continent formation, at the lower continental crust, as lower oceanic crust or as high pressure cumulates. To further constrain their petrogenesis, we analyzed 24 high-CaO basalts for Sr-Nd-Hf-Pb isotope ratios. These samples are isotopically the most depleted lavas ever reported from Mauna Kea having low 206Pb/204Pb (18.02-18.26), higher ɛNd (7.26-9.33) and ɛHf (14.44-16.80), and lower 87Sr/86Sr (0.70341-0.703240) at a given 206Pb/204Pb. Surprisingly, Mauna Kea high-CaO basalts are isotopically identical to and covers a significant range of reported Hawaiian rejuvenated stage lavas isotope ratios in spite of their contrasting compositional difference, i.e., tholeiitic vs. alkalic. Compared to shield stage lavas, rejuvenated stage lavas have higher abundances of incompatible elements but lower 87Sr/86Sr and higher ɛNd. This compositional difference is commonly interpreted as a result of low extent of partial melting of a depleted source. Interestingly, this depleted component has also been proposed to be present in post-shield lavas at Hualalai. Our study, for the first time, finds the occurrence of such distinctive isotopic signatures in both shield stage lavas (Mauna Kea high-CaO basalts) and rejuvenated stage lavas, which may imply that this depleted component is intrinsic to the plume.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMDI11A2334D
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY