Cr-spinel, an excellent micro-container for retaining primitive melts ^ implications for a hydrous plume origin for komatiites Kenji Shimizu a; *, Tsuyoshi Komiya a , Kei Hirose a , Nobumichi Shimizu b , Shigenori Maruyama a a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan b Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 24 October 2000; received in revised form 1 March 2001; accepted 18 April 2001 Abstract Ultramafic melt inclusions were discovered in Cr-spinels of 2.7 Ga Al-undepleted komatiites from the Belingwe Greenstone Belt, Zimbabwe. The inclusions consist of glass and sub-micrometer-size quench crystals of olivine and clinopyroxene. Homogenized melt inclusions are highly magnesian, ranging from 12.5 to 19.5 wt% in MgO content, and are also close to host komatiitic magma in other compositions. This fact indicates entrapment of melt into host spinel during the early stages of crystallization. The water content of two melt inclusions was determined using an ion probe; the high magnesian melt inclusion, 17.5 wt% in MgO, contains 1.1 wt% H 2 O, whereas the moderately magnesian melt inclusion, 11.8 wt% in MgO, has 1.7 wt% H 2 O. This evidence suggests that the primary komatiite melt contained 0.8^ 0.9 wt% H 2 O and 23.4^25.0 wt% MgO. The water content is about five times greater than previous estimates from melt inclusions in olivine [McDonough and Danyushevsky, EOS Trans. AGU 76 (1995) S266]. In addition, even the high H 2 O content preserved in melt inclusions within Cr-spinel may represent the minimum estimates of the parental composition, because part of the water should be dehydrated from parental magma during crystallization in the magma chamber. If the komatiite melt was formed by a high degree of partial melting of a peridotite, the source mantle should contain considerable amounts of water (V0.5 wt%). However, recent melting experiments of hydrous peridotite indicate that the addition of 0.5 wt% H 2 O to mantle peridotite would not significantly decrease komatiite liquidus temperature [Asahara et al., Geophys. Res. Lett. 25 (1998) 2201^2204]. Petrological study of melt inclusions together with experimental data suggest that Belingwe komatiites were formed from a hydrous plume at high temperatures. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: komatiite; chrome spinel; inclusions; Belingwe greenstone belt; plumes; high pressure; transition zones 1. Introduction Komatiites are the most magnesian extrusive rocks on the Earth, and are restricted to Archean greenstone belts except for the Cretaceous ko- matiite on Gorgona Island. It has been widely 0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII:S0012-821X(01)00359-4 * Corresponding author. Tel.: +81-3-5734-2618; Fax: +81-3-5734-3558; E-mail: shimmy@geo.titech.ac.jp Earth and Planetary Science Letters 189 (2001) 177^188 www.elsevier.com/locate/epsl