Chromium spinel in Late Quaternary volcanic rocks from Kamchatka: Implications for spatial compositional variability of subarc mantle and its oxidation state
Nekrylov, N and Portnyagin, MV and Kamenetsky, VS and Mironov, NL and Churikova, TG and Plechov, PY and Abersteiner, A and Gorbach, NV and Gordeychik, BN and Krasheninnikov, SP and Tobelko, DP and Shur, MY and Tetroeva, SA and Volynets, AO and Hoernle, K and Worner, G, Chromium spinel in Late Quaternary volcanic rocks from Kamchatka: Implications for spatial compositional variability of subarc mantle and its oxidation state, Lithos, 322 pp. 212-224. ISSN 0024-4937 (2018) [Refereed Article]
The Kamchatka volcanic arc (Russia) is one of well-studied but complex tectonic margins on Earth, with an extensive geologic history stretching as far back as 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 of analyses of 1604 spinel inclusions and their host-olivine from 104 samples representing 30 volcanoes and volcanic fields. The studied rocks are basalts, basaltic andesites and high-Mg andesites, which cover the whole compositional range the Late Quaternary primitive volcanic rocks in Kamchatka. The composition of spinel 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-Mgandesites 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 samples from the Sredinny Range and Northern Kamchatka correlates with a number of proxies of the involvement of slab-derived components incorporated in the composition of their host-rocks (e.g., La/Nb and Ba/La), which suggests a coupling between the mantle oxidation and metasomatism by slab-derived fluids or melts. These correlations were not observed for frontal Kamchatka volcanoes with the highest estimated ΔQFM, which possibly indicates a 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-spinelin 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.