科研之友
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摘要
We measured the magnesium isotopic compositions of endoskarns (i.e., the metasomatically altered interior margin of the pluton) generated by contact metamorphism between granodioritic magma and dolomitic wallrock. The endoskarns were sampled as crustal xenoliths in Pleistocene basaltic cinder cones erupted in eastern California and provide a record of pluton-wallrock interactions at depth. The endoskarns consist of an outer zone made of pyroxenite (Mg-rich) and an inner zone represented by a plagioclase-quartz lithology with relict plutonic textures and minor pyroxene. The inner and outer zones are separated by a thin selvage of phlogopite. The Mg isotopic compositions of these endoskarns are all significantly lighter (delta Mg-26 %26lt; -0.8 parts per thousand relative to the Dead Sea magnesium (DSM-3) standard) than canonical peridotitic mantle (delta Mg-26 = -0.25 parts per thousand). In particular, the outer pyroxenite zones of the endoskarn are consistently N1% lighter (-1.69 to -2.09 parts per thousand; mean = -1.74 parts per thousand;SD = 0.07 parts per thousand; n = 8) than the mantle, andMgisotopic values increase in the inner, Mg-poor and plagioclase-quartz dominated zone (delta Mg-26 of -0.82 parts per thousand). Because of the high temperatures associated with endoskarn formation, it is unlikely that the light Mg isotopic compositions result from equilibrium isotopic fractionations between mineral phases. The Mg isotopic signatures of the endoskarns are most easily interpreted by the mixing of Mg between Mg-rich and Mg-26-depleted dolomitic wallrock with Mg-poor and Mg-26-enriched magma. Mg isotopes may thus be useful in tracking magma-carbonate interactions in magmas.
- 出版日期2013-10-9
- 单位北京大学
