Mass recycling and thermodynamic variation during subduction initiation from the perspective of ophiolites

Abstract

Supra-subduction zone (SSZ) ophiolites have been regarded as analogues of oceanic forearc lithosphere which formed at the initial stage of subduction. In this study, typical Neo-Tethys SSZ ophiolites, i.e., the Troodos ophiolite in the island of Cyprus and the ophiolites in the Yarlung-Tsangbo suture zone (Tibet, China), are investigated to constrain mass recycling and thermodynamic variation during subduction initiation. The Troodos ophiolite is the most classic SSZ ophiolite in the world. Melt inclusions in olivine of the upper pillow lavas of the Troodos ophiolite have major compositions consistent with high-Ca boninites, including high MgO (11.1%-18.2%) and SiO2 (51.2%-54.8%) but low TiO2 (0.1%-0.5%) contents. The melt inclusions exhibit significant variation in Pb isotopic (208Pb/206Pb=2.0593-2.1216; 207Pb/206Pb=0.8275-0.8618) and fluid-mobile trace elements, indicating that they were originated from a refractory mantle source that had been metasomatised by slab fluids. Mantle equilibration temperature and pressure are about 1400 ℃ and 1.5 GPa, respectively. In comparison, recalculation of literature data suggest that the lower pillow lava derived from a less refractory and shallower source (1270 ℃ and 0.7 GPa). A subduction initiation model involving melting of two sources is further proposed to account for the chemostratigraphy of the Troodos lavas. Two independent methods, oxybarometer based on equilibrium between olivine and spinel and oxybarometer based on V distribution coefficient (DV) between olivine and melt inclusion, are applied to investigate the redox state of boninites. Calculation by using olivine-spinel oxybarometer indicates that the primary boninitic magma was generated under a redox state (△QFM= 0.2 ± 0.2) similar to that of MORBs. In contrast, calculation by using DV oxybarometer provides high fO2 values in accordance with different types of groundmass, that fO2 of lavas with vitrophyric groundmass and lavas with holocrystalline groundmass are △QFM= 0.6 ± 0.2 and △QFM= 1.1 ± 0.2, respectively. The discrepant fO2 value reflects diffusive re-equilibrium of melt inclusions due to the oxidation of the host magma during its ascent. Therefore, the initial redox state of boninite was close to MORBs but could have been raised during the ascent of magma. Peridotites from ophiolites in the Yarlung-Tsangbo suture zone are studied to evaluate the efficiency of Re-Os recycling in early stage of subduction. Whole-rock and mineral compositions reflect that these peridotites represent asthenospheric mantle which flowed into the subduction zone and then suffered fluid metasomatism without further modification. Therefore, these peridotites can serve as an important snapshot of interaction between slab fluid and arc mantle at a shallow depth. 187Os/188Os ratios and Os concentrations of the peridotites are comparable with those of abyssal and forearc peridotites, indicating that there is no significant input of crustal Os into the forearc mantle through fluid infiltration. Meanwhile, low Re contents and low Re/Os ratios of the peridotites were mainly controlled by melting processes. Thus, it is suggested that limited Re was added into the forearc mantle. A new model involving the storage of Re and Os by serpentine was proposed to explain recycle of Re and Os in the subduction zone.