Chemical and lithium isotope characteristics of murakamiite and Li–rich pectolite from Iwagi Islet, Southwest Japan

• Published in: Journal of Mineralogical and Petrological Sciences Vol. 116; no. 1; pp. 9 - 25
• Main Authors: IMAOKA, Teruyoshi, KIMURA, Jun–Ichi, CHANG, Qing, ISHIKAWA, Tsuyoshi, NAGASHIMA, Mariko, TAKESHITA, Natsuki
• Format: Journal Article
• Language: English
• Published: Sendai Japan Association of Mineralogical Sciences 01.01.2021; Japan Science and Technology Agency
• Subjects: Augite; Cations; Cretaceous; Crystal structure; Fluids; Granite; Isotopes; Iwagi Islet; Japan; Li isotopes; Lithium; Lithium isotopes; Li–rich pectolite; Manganese; Metasomatism; Minerals; Murakamiite; Rocks; Sodium; Solid solutions; Subduction; Trace elements; Japan
• ISSN: 1345-6296; 1349-3825
• DOI: 10.2465/jmps.200721

We report in situ major and trace element and Li isotope analyses of murakamiite and Li–rich pectolite in an albitite and whole–rock analyses of the albitite and host granite from Iwagi Islet, SW Japan. The albitite forms small bodies that are several tens of centimeters to tens of meters in size, disseminated in a host granite of Late Cretaceous age. The studied murakamiite–bearing albitite contains albite, sugilite, aegirine–augite, quartz, murakamiite–Li–rich pectolite, microcline, katayamalite, and accessory minerals. It shows conspicuous strain–induced textures. The murakamiite and Li–rich pectolite form a solid solution with Li × 100/(Li + Na) atomic ratios ranging from 44.2 to 60.1, and the Na line profiles show a zoning structure in which Na decreases from core to rim. Major and trace element compositions of murakamiite–pectolite normalized to that of albitite indicate the enrichments of some elements, particularly in Mn, Ca, Li, Sr, and REEs, roughly on the same order of magnitude (~ 10 times). The albitite–normalized element concentrations vary systematically with ionic radius of the element; the normalized concentrations of cations with the same valence roughly form a simple convex parabolic curve when plot against the ionic radius. This indicates that the element partitioning of murakamiite and pectolite during metasomatism to form albitite took place under a strong control of crystal structure, quasi–equilibrated with metasomatic fluids and coexisting minerals. The δ7Li values of murakamiite and Li–rich pectolite show a wide range from −9.1 to +0.4‰ (average −2.9‰), and no obvious correlation with Li contents was observed. These δ7Li values should have resulted from hydrothermal fluid–rock interactions at the temperatures of 300–600 °C. The very low δ7Li values down to −9.1‰ may have originated from intra–crystalline Li isotope diffusion, or involvement of deep–seated, Li–Na–enriched subduction–zone fluids with low δ7Li values.