Ilvaite from a serpentinized peridotite in the Asama igneous complex, Mikabu greenstone belt, Sambagawa metamorphic terrain, central Japan

Abstract

Ilvaite is sparsely disseminated in a serpentinized plagioclase wehrlite of the Asama igneous complex that underwent the Sambagawa regional metamorphism of pumpellyite-actinolite to greenschist facies. Ilvaite in the Asama complex is monoclinic (a = 13.019(5), b = 8.808(2), c = 5.850(4) A, 13 = 90.19(4)~ and its composition is similar to the ideal end-member composition (CaFe~+Fe3+SizO8OH). Ilvaite occurs in mats of serpentine (chrysotile); it probably formed during serpentinization, which might have accompanied the Sambagawa metamorphism. The associated secondary minerals include salitic clinopyroxene, magnetite and andradite. The ilvaite-free mineral assemblage that formed during the serpentinization is usually serpentineclinopyroxene-magnetite, which is widespread in the complex. The phase relations between coexisting minerals suggest that the conditions during the formation of the ilvaite-bearing assemblage were reducing when compared to those of the assemblage serpentine-clinopyroxene-magnetite. The reducing conditions during the ilvaite formation were presumably brought about by hydrogen gas thatl was generated during the serpentinization of olivine.

Unravelling the root zone of ultramafic‐hosted black smokers‐like hydrothermalism from an Alpine analog

“…On the deepest parts, the mineralogical zonation from barren serpentinite to Fe‐Ca‐silicates to semi‐massive sulphides and then to Cu‐rich MS (Figures e and b) has never been reported before this study. The ilvaite‐andradite‐diopside assemblage has been described in low‐grade metamorphic serpentinites (Agata & Agachi, ) and in oceanic ultramafic settings (Gaggero, Cortesogno, & Gazzoti, ). Gaggero et al () suggested that this assemblage precipitated from early serpentinization‐derived fluids.…”

Geochemistry of ilmenite from the Asama ultramafic-mafic layered igneous complex, Mikabu greenstone belt, Sambagawa metamorphic terrane, central Japan.

“…Poikilitic ilmenite grain (black) encloses olivine grains that have been entirely altered to serpentine. Serpentine in core of pseudomorph after olivine is associated with fibrous clinopyroxene, chlorite and magnetite that were formed during serpentinization (Agata and Adachi, 1995). Width of field is 1.7 mm.…”

“…Mn-rich ilmenite occurs also as veins along cracks in many ilmenite grains, and is apparently not magmatic. Mn-rich ilmenite was found in rims of chromite-ilmenite intergrowth and is associated with "Ferritchromit" (Fe3+-rich Cr spinel formed as the result of alter ation of magmatic chromite; Ramdohr, 1969), which was formed during the Sambagawa meta morphism (Agata and Adachi, 1995). Mn-rich il menite was probably an alteration product during the Sambagawa metamolphism.…”

“…Mn-rich il menite was probably an alteration product during the Sambagawa metamolphism. The MnO contents of olivine and serpentine in the Asama complex are 0.15-0.27 and <0.02 wt.%, respectively (Agata, 1994;Agata and Adachi, 1995); most Mn in Mn rich ilmenite might have come from olivine that was altered to serpentine.…”

“…A brief account of geology of the Toba area Nakamura (1971), Agata (1994), and Agata and Adachi (1995). has been given by Nakamura (1971), who has first described layered structures in the Asama igneous complex.…”