Strontium isotope compositions of anorthite and olivine phenocrysts in basaltic lavas and scorias of Miyakejima volcano, Japan.

Abstract

Sr isotope compositions were determined for anorthite megacrysts, olivine phenocrysts and host basalts of Miyakejima volcano, Izu-Ogasawara arc, Japan. The anorthite megacrysts are of An97-94 in composition. The olivine phenocrysts have almost the same composition (Fo84-82) as olivine inclusions in the anorthite megacrysts.The 87Sr/86Sr ratios of anorthites range within a narrow variation of 0.70340-0.70352, while those of olivine phenocrysts and olivine inclusions show distinctively higher values (0.70357-0.70374) than those of anorthites. Host rock samples excluding megacrysts yield intermediate 87Sr/86Sr values between anorthite and olivine . This disequilibrium in 87Sr/86Sr ratios between phenocrysts and host rocks demonstrates that anorthites and olivines did not crystallize directly from the host magmas but were probably xenocrysts carried up by the magmas. The present data further suggest that the olivine phenocrysts and olivine inclusions in anorthite megacrysts have the same origin and that the crystallization of anorthite occurred posterior to olivine crystallization. The difference in 87Sr/86Sr ratios between olivine inclusions and host anorthite megacrysts provide an evidence for magma mixing between two types of magmas (both of basaltic in composition) with different 87Sr/86Sr ratios. The basaltic magmas which formed the Miyakejima volcano may also be explained by the mixing of above two end member magmas, at least in some stage (or stages) of magma evolution.

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

“…The ratios of the 1940 and 1962 rocks are limited in the ranges of 0.70346-0.70352 and 0.70341-0.70347, respectively. 87 Sr/ 86 Sr of the 1983 rocks (0.70338-0.70361) was reported in several papers (Notsu and Aramaki 1984;Kuritani et al 2003;Arakawa et al 1992;Yokoyama et al 2003), which is wider than that of 1940 or 1962. 87 Sr/ 86 Sr of the 18 August 2000 summit bombs (0.70343-0.70348) is similar to that of the 1940-1983 rocks of this study (0.70341-0.70347), whereas that of the 2000 submarine spatter (0.70345-0.70356) is more scattered.…”

“…The M-type clots consist of plagioclase, olivine with occasional clinopyroxene of the least differentiated compositions. The clots show petrographic features that they cannot be comagmatic phenocrysts of the host magmas: (1) kink-banding of olivine megacrysts, (2) spherical olivine in anorthite, (3) Notsu and Aramaki (1984) and Kuritani et al (2003) for the 1983 rocks, Amma-Miyasaka and Nakagawa (2002) for the 1940 rock, Arakawa et al (1992) for the 1983, 1940 and pre-historic rocks, Notsu et al (1983) for the pre-historic rocks, and Yokoyama et al (2003) for the historic and pre-historic rocks except the 2000 samples The B-type clots consist of plagioclase, clinopyroxene and olivine, characterized by the absence of orthopyroxene and magnetite. The A-type clots are characterized by magnetite and orthopyroxene as well as plagioclase, clinopyroxene and olivine.…”

H2O-rich island arc low-K tholeiite magma inferred from Ca-rich plagioclase-melt inclusion equilibria

“…It is possible to assume that Ca-rich plagioclase is not in equilibrium with matrix (melt) composition. For example, Arakawa et al (1992) and Kimata et al (1995) proposed that a xenocrystic origin for the Ca-rich plagioclase in terms of its Sr isotopic composition. Falloon and Green (1986) attributed its origin at the Tongan arc to Ca-rich magma (CaO > 13 wt.%, Na 2 O < 0.8 wt.%) found in less differentiated melt inclusions hosted by olivine, although Danyushevsky et al (1997) excluded their suggestion later by analyses of plagioclase-hosted melt inclusions.…”

Single crystal X-ray and electron microprobe study of Al/Si-disordered anorthite with low content of albite

“…Hence, for the exposure to order-promoting agents, such as long-time kinetics, high-temperature, or highpressure annealing and hydrothermal alternation, the unstable Al/Si disordered anorthite part was likely confined within the anorthite megacryst. Although the formation mechanism of anorthite megacryst remains unclear [16,77,78], crystal growth under temperature cycling [79] would be one possible mechanism that produces anorthite megacryst in which both Al/Si ordered part and Al/Si disordered parts can coexist as discussed in the following paragraphs.…”