Tracing the effects of high-pressure metasomatic fluids and seawater alteration in blueschist-facies overprinted eclogites: Implications for subduction channel processes

Eclogites from the Tian Shan high-pressure/low-temperature (HP/LT) metamorphic belt show evidence for successively increasing metasomatic alteration with increasing retrograde, blueschist-facies overprint. To constrain the source(s) of the metasomatizing fluid and to evaluate elemental and isotopic changes during this overprint, two sequences of eclogiteblueschist transitions were investigated: A layered transition from eclogite to blueschist (FTS 9-1 sequence) and blueschist-facies overprinted pillow metabasalts (FTS 4 samples). Geochemical trends based on the relationships of K, Ba, Rb and Th are consistent with HP metasomatism, but distinct from typical seafloor alteration trends. In contrast, oxygen isotope ratios in garnet ( 18 OV-SMOW = 7.3–8.7‰) and omphacite ( 18 OV-SMOW = 8.2–9.7‰) are similar to  18 OV-SMOW in bulk low-temperature altered oceanic crust (AOC), suggesting O isotopic preservation of a seafloor alteration signature. Carbonate crystallization related to the metasomatic overprint demonstrate CO2 mobility during subduction and potential C storage in HP metamorphic rocks. Carbon isotope ratios in the two sequences differ markedly: Disseminated calcite in the layered FTS 9-1 sequence has  13 CV-PDB = -9.14±0.19‰, whereas vein-forming ankerite in the pillow metabasalts has  13 CV-PDB = -2.08±0.12‰. The ankerite reflects an inorganic marine/hydrothermal signature, as observed in ophiolites, whereas the low  13 CV-PDB values from the calcite point to a contribution of organic carbon. The time when the metasomatic overprint occurred is estimated to be ~320±11 Ma based on a Rb-Sr isochron age of six blueschist samples from the pillow metabasalts, which is in agreement with active subduction in this region. Initial (T=320 Ma) 87 Sr/ 86 Sr ratios for all HP/LT rocks range from 0.7059 – 0.7085, and Nd320Ma varies from -0.4 to +10.9. Both eclogite-blueschist sequences have initial Sr isotope compositions ( 87 Sr/ 86 Sr ~0.707) that are significantly higher than those of typical oceanic mantle-derived basalts. They are thought to derive from a fluid that preserved the Sr isotopic signature of seawater by fluid-rock interaction with seawater-altered oceanic lithosphere in a subduction channel. Mixing models between eclogite and various fluids suggest that the contribution of a sediment-derived fluid was likely less than 20%. A fluid predominantly derived from seawater-altered oceanic lithosphere is also supported by the calculated O isotope composition of the fluids (10.2 – 11.2‰). It is thus evident that subduction channel fluids carry complex, mixed elemental and isotopic signatures, which reflect the composition of their source rocks modified by interaction with various other lithologies.