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Chromium spinel in Late Quaternary volcanic rocks from Kamchatka: Implications for spatial compositional variability of subarc mantle and its oxidation state

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Type of Publication:
Cr-spinel, Olivine, Kamchatka, Redox conditions, Mantle wedge
Nekrylov, Nikolai; Portnyagin, Maxim V.; Kamenetsky, Vadim S.; Mironov, Nikita L.; Churikova, Tatiana G.; Plechov, Pavel Yu.; Abersteiner, Adam; Gorbach, Natalia V.; Gordeychik, Boris N.; Krasheninnikov, Stepan P.; Tobelko, Daria P.; Shur, Maria Yu.; Tetroeva, Sofia A.; Volynets, Anna O.; Hoernle, Kaj; Wörner, Gerhard
212 - 224
The Kamchatka volcanic arc (Russia) is one of best-studied, but most complex tectonic margins on Earth, with an extensive geologic history extending back to the Late Cretaceous. Unlike many other subduction zones, primitive basalts with Mg# > 65 are abundant in Kamchatka, thereby allowing characterization of the mantle source through compositional analyses of near-liquidus minerals in the rocks. In this paper, we present a comprehensive dataset on the composition of Cr-spinel inclusions in olivine for all main Late Quaternary volcanic zones in Kamchatka, comprising 1604 analyses of spinel inclusions and their host-olivine in 104 samples from 30 volcanic complexes (single volcanoes and volcanic fields). The studied rocks are basalts, basaltic andesites and high-Mg andesites, which cover the whole compositional range of the primitive Late Quaternary volcanic rocks in Kamchatka. The spinel composition shows large variability. Spinel inclusions with the lowest Cr# and Fe3+/Fe2+ ratios were found in basalts from Sredinny Range and Northern Kamchatka, whereas the most Cr-rich and oxidized spinel inclusions occur in basalts and high-Mg andesites from the Central Kamchatka Depression. Intermediate Cr-spinel compositions characterize the Eastern Volcanic Belt of Kamchatka. The compositions of olivine-spinel pairs were used to quantify the oxidation state of parental Kamchatka magmas and the degree of partial mantle melting. The redox conditions recorded in spinel compositions range from ΔQFM = +0.7 to +3.7. ΔQFM for spinel from the Sredinny Range and Northern Kamchatka correlates with a number of whole-rock proxies for the involvement of slab-derived components (e.g., La/Nb and Ba/La), which suggests a coupling between mantle oxidation state and slab-derived fluid/melt metasomatism. These correlations were not observed in frontal Kamchatka volcanoes with the highest estimated ΔQFM, which possibly indicates buffering of the mantle oxidation state by sulfur. The estimated degrees of partial mantle melting range from 8 to >20% for Kamchatka volcanoes. Spinel from the Central Kamchatka Depression has the highest Cr# and could crystallize from magmas generated from the most depleted sources. In contrast to the Eastern Volcanic Belt, spinel Cr# and the inferred degrees of melting in the Central Kamchatka Depression do not correlate with spinel TiO2 content. The apparent decoupling between the proxies of mantle depletion in the CKD spinel is interpreted to reflect refertilization of the CKD mantle by oxidized Ti-rich slab- or mantle lithosphere-derived melts near the northern edge of the subducting Pacific Plate. This study demonstrates that the composition of Cr-spinel in volcanic rocks in combination with bulk-rock compositions can be a powerful tool to map regional variations of the mantle source depletion, oxidation state, and involvement of various slab-derived components in island-arc magmatism.
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