Geochemical Modelling of Shallow Fractionation and Deep Mantle Melting Below Mauna Loa Volcano, Hawaii
Date
2020
Authors
Prichard, Jennifer
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Abstract
Hawaii is the archetypal example of intra-plate 'hotspot' volcanism, yet the mechanisms of
plume formation, hotspot volcano formation, and the nature of chemical heterogeneity in
hotspot lavas such as those at Hawaii remain in question. Of particular interest is understanding
what physical and chemical processes lead to the distinctive 'double-track' geochemical trend,
whereby the volcanoes on the south-west side of the Hawaiian volcanic chain ('Loa' track) vary
systematically compared with those on the north-east side ('Kea' track) with respect to major
elements, trace elements, and isotopic ratios. In order to make meaningful inferences about how
the parallel distinction may arise, it is necessary to understand how the chemistry of magma
evolves at Hawaii starting with the composition and melting of the mantle source, followed by
emplacement of the magma into the crust with associated residence times and mixing, the
progression of the crystallisation process as the magma cools, and finally the eventual eruption
through the volcanic edifice. Here, major element, trace element, and Sr and Nd isotopic
analyses of tholeiites from submarine Mauna Loa volcano are used alongside compiled literature
data of Mauna Loa and Kilauea Volcanoes to investigate both shallow magmatic processes in
the crust and deep mantle melting in the upwelling plume. Evidence from modal olivine, olivine
forsterite content, glass compositions, and whole rock Ni suggest that none of the whole rock
samples are direct parental magmas, but instead reflect an evolved magma in which olivine has
remained entrained or become subsequently entrained to varying degrees. Computational
geochemical modelling suggests the occurrence of two distinct fractionation sequences: ol + sp
+ opx + cpx + plag and another with ol + sp + cpx, which could be achieved with minor
variations in the water content (0 - 0.5 wt. %). Furthermore, a compilation of experimental melt
studies and trace element models are presented, showing that the source of Hawaii's lava is most
probably a garnet-bearing peridotite at ~4 GPa with a major element composition similar to that
of primitive mantle or Hawaiian pyrolite, contradicting some current hypotheses which argue
for a pyroxenitic source. Trace element modelling reveals a the differences between Mauna Loa
and Kilauea can broadly be explained by variable degrees of melting of a fertile, garnet
peridotite, with Mauna Loa resulting from ~12% melting and Kilauea resulting from ~6%. It
was shown that low-olivine pyroxenites cannot produce the required high Ni content of Mauna
Loa lavas, and that the Ni may be better explained by the high temperatures and high MgO
contents associated with the Hawaiian plume which cause an associated decrease in the bulk
DNi. This negates the need for a low-olivine pyroxenite as the source. Despite this, a component
with high radiogenic lead and 143Nd/144Nd with low 87Sr/86Sr is required in the Kilauea source,
while the Mauna Loa source lies isotopically between Bulk Silicate Earth and Depleted MORB
mantle.
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