CO2-rich komatiitic melt inclusions in Cr-spinels within beach sand from Gorgona Island, Colombia

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Shimizu, Kenji
Shimizu, Nobumichi
Komiya, Tsuyoshi
Suzuki, Katsuhiko
Maruyama, Shigenori
Tatsumi, Yoshiyuki
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Melt inclusion
Gorgona komatiite
The volatile content of komatiite is a key to constrain the thermal and chemical evolution of the deep Earth. We report the volatile contents with major and trace element compositions of ~ 80 melt inclusions in chromian spinels (Cr-spinels) from beach sands on Gorgona Island, Colombia. Gorgona Island is a ~ 90 Ma volcanic island, where picrites and the youngest komatiites known on the Earth are present. Melt inclusions are classified into three types on the basis of their host Cr-spinel compositions: low Ti (P type), high Ti with high Cr# (K1 type) and high Ti with low Cr# (K2 type). Chemical variations of melt inclusions in the Cr-spinels cover all of the island's lava types. P-type inclusions mainly occur in the picrites, K1-type in high-TiO2 komatiites (some enriched basalts: E-basalts) and K2-type in low-TiO2 komatiites. The H2O and CO2 contents of melt inclusions within Cr-spinels from the beach sand are highly variable (H2O: 0.03–0.9 wt.%; CO2: 40–4000 ppm). Evaluation of volatile content is not entirely successful because of compositional alterations of the original melt by degassing, seawater/brine assimilation and post-entrapment modification of certain elements and volatiles. However, the occurrence of many melt inclusions with low H2O/K2O ratios indicates that H2O/K2O of Gorgona komatiite is not much different from that of modern mid-oceanic ridge basalt (MORB) or oceanic island basalt. Trend of CO2/Nb and Zr/Y ratios, accounted for by two-component mixing between the least degassed primary komatiite and low-CO2/Nb evolved basalt, allow us to estimate a primary CO2/Nb ratio of 4000 ± 2200 or a CO2 content of 0.16 ± 0.09 wt.%. The determined CO2/Nb ratio is unusually high, compared to that of MORB (530). Although the presence of CO2 in the Gorgona komatiite does not affect the magma generation temperature, CO2 degassing may have contributed to the eruption of high-density magmas. High CO2/Nb and the relatively anhydrous nature of Gorgona komatiite provide possible resolution to one aspect of the hydrous komatiite debate.
Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 288 (2009): 33-43, doi:10.1016/j.epsl.2009.09.005.
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