Abstract:
We report two generations of low-18O zircons from the Himalayan eclogites and their host gneisses. In situ SIMS 18O analyses on single zircon crystals (with known age and Hf isotope ratios) from two populations of chemically distinct zircons demonstrate a complex history: (1) an early low-18O mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-18O minerals during the ultrahigh-pressure metamorphism. Magmatic zircon (269 Ma) in Group I eclogites yielded 18O values from 1.9 to 4.6‰ VSMOW with an average value of 4.0 ± 0.2 (n = 35, the error is 2SD analytical precision and “n” represents number of analyzed spots), which is lower than the typical mantle values (5.3 ± 0.6, 2SD). In contrast, metamorphic zircons (45 Ma) in Group II eclogites preserve unusually low, negative 18O values from −3.9 to −2.7‰ (average: −3.4 ± 0.4, n = 35, 2SD). Zircons in felsic gneiss that surround Group II eclogites have inherited magmatic cores (ca. 260 Ma) with 18O values of ca. 2.9‰, which decrease to −0.1‰ in metamorphic (ca. 45 Ma) rims. These zircons preserve lower 18O values than would be equilibrated with typical mantle. The low-18O values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative 18O values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons. This study reports evidence for melting of subducted low-18O ocean crust to form low-18O mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578-581) for Proterozoic Scourie Dikes.
