A Calcium-in-Olivine Geohygrometer and its Application to Subduction Zone Magmatism (original) (raw)
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Magmatic origin of low-Ca olivine in subduction-related magmas: Co-existence of contrasting magmas
Chemical Geology, 2006
Unravelling the origin of different components contributing to subduction-related magmas is a prerequisite to understanding the sources and processes involved in their origins. Mafic, high-Ca subduction-related magmas from geographically-diverse areas (Indonesia, Solomon Islands, Kamchatka, Valu Fa Ridge) contain two populations of olivine crystals, of which only the high-Ca population (CaO = 0.3–0.5 wt.%) crystallized from the melt that dominantly contributed to the whole rock composition. Forsterite-rich (Fo90–94), low-Ca (CaO < 0.15 wt.%), high-Ni (NiO > 0.3 wt.%) olivine crystals, which constitute 16–37 vol.% of total olivine population, are generally interpreted as mantle or lithospheric xenocrysts. However, in these samples, the olivine shape and chemical zoning, the composition of included minerals (orthopyroxene, clinoenstatite and Cr-spinel) and presence of melt inclusions, are indications that these crystals are phenocrysts from a mafic magma with high silica and low calcium contents. The coexistence of contrasting magmas (mafic high-Ca silica-poor versus low-Ca silica-rich) within a number of arc systems and their mixing may not be a rare event, and should be taken into account when developing models of arc petrogenesis.
The James Ross Island Volcanic Group (JRIVG) at the northern tip of the Antarctic Peninsula is composed of various types of volcanic rocks erupted during the Late Miocene to Late Pleistocene. The sub-aerial lavas are the most abundant erupted products within the suite and are represented by alkali olivine basalts that contain significant amounts of ultramafic xenoliths. Precise determination of chemical compositions of olivine by electron micro-probe and LA-ICP-MS reveals more than one compositionally distinct olivine population in the xenoliths and the host lavas. The majority of the olivine grains from the xenolith suite are remarkable for their high Mg/Fe ratios (NFo 88) and low-Ca contents (b500 ppm) and are similar in composition to the mantle olivine, while some others, despite their similarly low-Ca abundances, are characterized by significantly lower Mg/Fe (bFo 85) reflecting some degree of differentiation. We interpret the coexistence of both olivine types with low-Ca contents as reflecting a magmatic cumulate origin of the xenoliths through olivine-dominated fractional crystallization from a H 2 O-rich parent magma, during which the presence of water affects the partitioning behavior of elements and reduces the D CaO olivine/melt significantly. The cores of the most primitive olivine macrocrysts (NFo 88) from the basaltic lavas also have low Ca contents compared to MORB olivine at similar Fo, indicating an arc-melt like volatile content of the primary magma. Evaluation of minor and trace element relative abundances in olivine further indicate that the alkaline basalts in the JRIVG are the products of peridotite-dominated partial melting of a volatile-rich mantle source with signatures of mantle hydration most probably promoted by preceding subduction events. The results from olivine chemistry, when combined with the evaluation of primary melt compositions, appear to be consistent with the view that the primary magmas from which the JRIVG basalts were derived are the results of partial melting of a mantle domain that has experienced hydrous silicate melt metasomatism through interaction of pe-ridotitic upper mantle rocks with melts of a slab-derived component, most probably generated by dehydration melting of subducted oceanic crust.
Genesis of ultra-high-Ni olivine in high-Mg andesite lava triggered by seamount subduction
The Kamchatka Peninsula is a prominent and wide volcanic arc located near the northern edge of the Pacific Plate. It has highly active volcanic chains and groups, and characteristic lavas that include adakitic rocks. In the north of the peninsula adjacent to the triple junction, some additional processes such as hot asthenospheric injection around the slab edge and seamount subduction operate, which might enhance local magmatism. In the forearc area of the northeastern part of the peninsula, monogenetic volcanic cones dated at <1 Ma were found. Despite their limited spatiotemporal occurrence, remarkable variations were observed, including primitive basalt and high-Mg andesite containing high-Ni (up to 6300 ppm) olivine. The melting and crystallization conditions of these lavas indicate a locally warm slab, facilitating dehydration beneath the forearc region, and a relatively cold overlying mantle wedge fluxed heterogeneously by slab-derived fluids. It is suggested that the collapse of a subducted seamount triggered the ascent of Si-rich fluids to vein the wedge peridotite and formed a peridotite–pyroxenite source, causing the temporal evolution of local magmatism with wide compositional range. The Kamchatka Peninsula is one of the largest volcanic arcs in the world. It corresponds to the subduction of the northernmost part of the Pacific Plate and it comprises three volcanic chains 1. The volcanic front is continuous from the Kurile Arc to the Eastern Volcanic Front (EVF, Fig. 1a). The volcanic front is bent toward the Kliuchevskoy Volcanic Group (KVG) at around 55°N along the 100–180-km slab-depth contour (Fig. 1a) 2. The northern end of the volcanic front is the Shiveluch Volcano, which is located on the slab edge of the Pacific Plate where the mantle wedge opens to the north 3, 4 and where asthenospheric flow around the slab edge might heat the slab and cause melting 3. The extension of the Emperor Seamount Chain is subducted from the southeast 5, 6 , which contributes to the northward shallowing of the subduction dip angle 2 and influences the magmatism in northern Kamchatka 6–8. The Kamchatka Peninsula is a unique place that has undergone these extremely dynamic processes, where the geological structure and igneous materials continue to reflect the interaction between the subducting Pacific Plate and the overlying arc system via material cycling and structural evolution. This study investigated the East Cone volcanic group 9, 10 (hereafter, EC) in the northeastern forearc area in the northern termination of the EVF 1 and to the east of the KVG. The EC is located ~60 km above the subducting slab, which is presumed old and cold (~100 Ma 11). In this case, a supply of slab-derived fluid and corresponding Published: xx xx xxxx OPEN
Growth of, and diffusion in, olivine in ultra-fast ascending basalt magmas from Shiveluch volcano
Scientific reports, 2018
Complex core-rim zoning of Mg-Fe-Ni-Ca-Cr-Al-P in high-Mg olivine crystals from a tuff ring of Shiveluch volcano, Kamchatka, enables reconstruction of the entire olivine crystallization history from mantle conditions to eruption. Bell-shaped Fo and Ni profiles in crystal cores were formed by diffusion after mixing with evolved magma. Diffusion proceeded to the centres of crystals and completely equilibrated Fo and Ni in some crystals. Diffusion times extracted from Fo and Ni core profiles range from 100 to 2000 days. During subsequent mixing with mafic mantle-equilibrated melt, the cores were partially dissolved and overgrown by Fo olivine. Times extracted from Fo and Ni diffusion profiles across the resorption interface between the core and its overgrowth range within 1-10 days, which corresponds to the time of magma ascent to the surface. The overgrowth shows identical smooth Fo-Ni decreasing zoning patterns for all crystals towards the margin, indicating that all crystals shared ...