A GEOCHEMICAL AND ISOTOPIC STUDY OF THE MAGMATIC-HYDROTHERMAL TRANSITION IN THE LOWER OCEANIC CRUST (ODP HOLE 735B)

We conducted a mineral and bulk rock chemical and isotopic study of veins, alteration halos, and proximal fresh rocks from ODP Hole 735B to distinguish between the effects of late-stage magmatic crystallization and alteration in response to reaction with magmatic fluids and seawater-derived fluids. 42 whole rock analyses of fresh-"altered" pairs (where "altered" ranges from amphibole vein halos to felsic vein halos to mylonites to pegmatitic gabbros) reveal that, in every case, the trace element pattern of the "altered" rock is markedly different from the precursor. The most extreme enrichments in HFSE include two felsic vein halos, two mylonites and a pegmatitic gabbro, all of which exhibit major element chemistries very similar to the unmodified precursor, suggesting that addition of siliceous or oxide-rich melts can be ruled out We suggest that magmatic fluids have affected the trace element budget (and textures) of these rocks. The idea that these fluids were aqueous solutions rather than siliceous melts is supported by the presence of secondary calcic plagioclase (An up to 95 Mol%) in some alteration halos. Vein amphiboles, with the exception of rare late-stage actinolite, have REE and stable isotope compositions similar to intergranular magmatic amphibole. These data are inconsistent with a pure hydrothermal origin of the vein amphiboles. Instead, they suggest that magmatic contributions have a large control on the chemistry of deep crustal fluids. We propose that the rocks in Hole 735B record a wide range of late-stage magmatic and subsolidus processes starting with the mobilization of highly evolved melts that produce oxide gabbros along their flow paths and finally crystallize to form granitic pods and felsic veins. Magmatic fluids exsolve during the latest stages of crystallization, flow along brittle fractures and react with host rock resulting in the formation of a metasomatically altered rock with the trace element signature of a late-stage melt. Near-vertical amphibole veins are of a mixed hydrothermal-magmatic origin and may reflect increasing inflow of seawater-derived fluids as the system cools and cracks propagate downward.