Origin of josephinite

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Josephinite is a nickel-iron alloy-bearing rock from Josephine County, southwestern Oregon. Josephinite had been thought to be the same as the metallic nickel-iron mineral awaruite (KRISHNARAO, 1962, 1964; RAMDOHR, 1950), but recently it has been shown to be a complex assemblage of minerals (BIRD and WEATHERS, 1975a). Although most samples of josephinite are somewhat altered, least-altered josephinite consists of several nickel-iron alloys, one intergrown with andradite garnet, an iron-cobalt alloy, and minor amounts of iron-nickel arsenides, iron-nickel-copper sulphides, and various unidentified phases. The intergrown metal-garent assemblage is apparently unique to josephinite. Alteration phases include serpentine minerals and magnetite. Josephinite occurs as detritus in several streams that originate and flow on the Josephine Peridotite; bedrock includes sheared serpentinite zones and basaltic to dioritic dikes within the peridotite. It is a reasonable assumption that the josephinite pebbles must be associated with one or more of these rocks; however, no unequivocal, large josephinite samples have been found within bedrock. BIRD and WEATHERS (1975a) proposed that because of its remarkable and apparently unique composition and textures, and its association with obducted mantle, josephinite has been derived from the mantle, conceivably the deep-mantle. Subsequently, DICK (1974), THORNBER and HAGGERTY (1976) and BOTTO and MORRISON (1976), following RAMDOHR (1950), argued that josephinite was formed by reduction of silicates, or sulphides, during serpentinization of the Josephine Peridotite. Currently, there are two views concerning the origin of josephinite: 1) josephinite is a product of low-temperature, synserpentinization processes and is comparable with awaruite 2) because of its mineral composition, textures, and rock associations, josephinite pre-dated serpentinization of the Josephine Peridotite (obducted ophiolite), was related to the origin of the Josephine Peridotite, and was derived from the primitive mantle, conceivably from as deep as the core-mantle boundary. Excesses of ³He, ²¹Ne and ¹²⁹Xe have recently been found in josephinite and have been interpreted to indicate a mantle residence and, perhaps, a primordial age for this rock (DOWNING et al., 1977; BOCHSLER et al., 1978). We continue to propose a mantle derivation of josephinite; additionally, we suggest that josephinite was primitive material incorporated into the earth during earth accretion.