[S-GC33] Solid Earth Geochemistry, Cosmochemistry

[SGC33-06] The characteristics of highly siderophile elements in the high-Mg andesite and basalts and its implications for the origin of the Quaternary arc magmatism in the forearc region, Kamchatka

*Tatsuji Nishizawa¹, Yuri Mitsude², Hitomi Nakamura³, Tatiana Churikova⁴, Boris Gordeychik⁵, Akira Ishikawa⁶, Hikaru Iwamori² (1.Volcanic Fluid Research Center, School of Science, Tokyo Institute of Technology, 2.Earthquake Research Institute, The University of Tokyo, 3.Institute of Earthquake and Volcano Geology, Geological Survey of Japan, AIST, 4.Institute of Volcanology and Seismology, Far East Branch, RAS, 5.Institute of Experimental Mineralogy, RAS, 6.Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology)

Keywords:Kamchatka, Highly Siderophile Elements (HSE), Arc magmatism, High-Mg andesite, Seamount

Kamchatka is a massive volcanic arc that corresponds to the subduction of the northern part of the Pacific Plate (PAP), which forms three sub-parallel volcanic belts; the Eastern Volcanic Front (EVF), the Central Kamchatka Depression (CKD), and the Sredinny Range. The Emperor Seamount Chain connected to the Hawaiian hot spot is subducting along with the PAP beneath the central part of Kamchatka. The subducting dip decreases sharply from 50° to 35° at the northern edge of the PAP slab, which causes the volcanic front from to bend from the EVF to the CKD at 55.5°N. On the other hand, in the northestern extension of the EVF corresponding to the forearc region, there are Quaternary monogenetic cones dated at <1 Ma, the East Cone volcanic group (EC). The 16 fresh EC lavas sampled from 8 cones could be categorized into 5 rock types, including high-Mg andesite (HMA) and basalt (B). The EC lavas are primitive with low FeO*/MgO (<~1) and show typical arc-signatures such as positive Pb spikes. Based on the petrological and geochemical characteristics of EC lavas, the previous study suggests that the thermal and geochemical contributions from the subducted seamount chain have caused the temporal evolution of local arc magmatism in EC with a wide compositional range in the forearc region. For example, HMA including ultra-high-Ni olivine was derived from pyroxenite veins formed by the metasomatism of the mantle peridotite caused by silica-enriched fluids derived from the subducted seamount. On the other hand, the primitive B was generated by flux melting of the mantle peridotite source caused by silica-depleted slab fluids due to the previous metasomatism (Nishizawa et al., 2017).
In order to further elucidate the genesis of EC lavas, we focused on the highly siderophile elements (HSE) that are strongly partitioned into metal or sulfide phases. Therefore, HSE concentration in lavas could provide important information about melting and fractional processes. In this study, we analyzed the bulk-rock concentrations of Re and the platinum-group elements (PGE) including Ru, Pd, Os, Ir, and Pt. As results, the depleted mantle (DM; Salters & Stracke, 2004)-normalized HSE spidergram for EC lavas are different in each rock-type or cone, but all EC lavas are relatively depleted in iridium-PGE (I-PGE; Os, Ir, and Ru) compared to the platinum-PGE (P-PGE; Pt, Pd) and Re, which are common in volcanic rocks such as MORB and arc lavas (e.g. Gannoun, 2016). Especially, HMA and the primitive B show almost the same pattern and concentration of P-PGE and Re, but the concentration of I-PGE is in contrast. In the I-PGE concentration of EC lavas, HMA shows the lowest concentration, while the primitive B shows the highest concentration. HMA seems to have been generated by I-PGE “nugget” fractionation from the primitive B melt. In other words, the factors that determine P-PGE and Re concentration of the melts are the almost same while the factors that determine I-PGE concentration in the melts are different between HMA and the primitive B, which is an important feature of the genesis of EC lavas.
To elucidate the factors that determined the HSE concentration in the EC lavas, we performed forward calculations of primitive melt assuming the four different melting conditions: 1. bulk rock batch melting model, 2. sulfide melt entrainment model, 3. sulfide melt partitioning model, 4. sulfide melting model. These results suggest that the differences in HSE concentration between HMA and the primitive B cannot be explained by a simple difference in the degree of partial melting. In addition, in any case, it is difficult to reproduce the HSE concentration of the EC lavas without assuming the sulfide phase that is the main host of HSE in the mantle rocks.