Geochemistry and petrogenesis of forearc peridotites, ODP Leg 125

Abstract

ODP Leg 125 recovered peridotites from Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. The peridotites recovered comprise about 95% harzburgites and about 5% dunites, which are variably serpentinised (mostly 60-100%). The Leg 125 peridotites represent some of the first extant peridotites recovered from a forearc selling. A detailed petrographic, mineral and bulk-rock chemical study of the peridotites has been undertaken in order to elucidate information about melting and fluid processes in the forearc mantle wedge. The harzburgites are highly refractory in terms of their mineralogy and geochemistry. They have low modal clinopyroxene, highly magnesian olivine (Mg# = 91.1-93.6) and orthopyroxene (Mg# = 91.6-93.2) and chrome-rich spinels (Cr# = 60-80) and very low incompatible element contents (Ti <80 ppm). Furthermore they are more refractory than the most depleted abyssal peridotite, which suggests that the harzburgites can be interpreted as residues to extensive partial melting (>20%). The Ti concentrations in the clinopyroxene indicate that the harzburgites are residues to -25% fractional melting. However, petrographic and other geochemical information show that the Leg 125 harzburgites have had a complicated melting and enrichment history. Many samples have olivine fabrics which are interpreted as having formed beneath a spreading ridge. Orthopyroxenes have lobate grain boundaries often associated with fresh olivine neoblasts. This texture is interpreted as showing the incongruent melting of orthopyroxene, a process which happens at low pressures (-3 kb) and high water pressures. These two types of textures indicate that the peridotites have had a two stage melting history. Moreover, the V concentrations in the clinopyroxenes can be explained by -15% partial melting at low oxygen fugacities (FMQ-1), followed by 5- 10% melting at high oxygen fugacities (FMQ+1). Oxygen thermobarometry calculations are in accordance with the peridotites last equilibrating under oxygen fugacities of greater than FMQ+1.The bulk-rocks have chondrite-normalised REE patterns showing extreme U-shapes with [La/Sm](_N) ratios in the range 5.03-250.0 and [Sm/Yb](_N) ratios in the range 0.05 to 0.25; several samples have possible small positive Eu anomalies. On extended chondrite-normalised plots the bulk-rocks also show enrichments in Sr and Zr relative to their neighbouring REEs and are enriched in LREE, Rb, Cs, Ba, Sm, and Eu relative to abyssal peridotites. Covariation diagrams based on clinopyroxene data show that Sr, Ce, Nd, Sm, Eu and Zr are enriched in the clinopyroxenes and that the enrichment took place during or after melting. The enrichment component is most likely a melt derived from the underlying subduction zone. A multistage melting and enrichment model is proposed for the peridotites where they first melt 10-15% beneath a spreading ridge. The resulting depleted spinel Iherzolite is enriched and then melted again 10- 15% above the subduction zone to produce the spatially associated boninites. A final enrichment event takes place during and after this melting event to produce the characteristic trace element enrichments in the Leg 125 peridotites.