SR ISOTOPIC COMPOSITION OF PORE-WATERS, SERPENTINE MUDS, AND MANTLE ROCK CHIPS IN THE CHOMORRO SEAMOUNT, MARIANAS FOREARC REGION: IMPLICATIONS FOR SUBDUCTION ZONE PROCESSES, MUD FLOW RATES, AND EQUILIBRATION TIMES

Subduction of the Pacific Plate beneath the Mariana forearc releases water to the overlying mantle wedge causing serpentinization of harzburgitic mantle rocks. The reaction product, serpentinite "mud," moves upward along fractures to discharge with cold water as springs on the ocean floor. There, the discharged mud forms seamounts tens of km wide and up to two km high. To understand processes occurring in the subduction zone and also during ascent of fluids, we measured the isotopic composition of Sr in water squeezed from cores drilled on the Chomorro Seamount and also in the solid serpentine and included harzburgitic clasts. The Sr87/Sr86 ratio in the upper meter below the seawater-mud interface is about 0.7091, indicating equilibration with seawater, but it falls to about 0.7056 over another one to two meters and remains constant at that value except for an upward excursion to about 0. 7087 between 13 and 16 mbsf in core 1220E. This excursion probably marks a fossil carbonate surface formed during the growth of the mud volcano at which time Sr in the solid equilibrated with seawater. Sr in aragonite in core 1200D at a depth of 1.35 m has a roughly comparable 87Sr/86Sr of 0.7091. Strontium concentrations in the upper few meters are typically 60-90 umol/kg, but decrease below this to ~6 umol/kg, remaining essentially constant except for an excursion to about 80 umol/kg in core 1200 D between 11 and 14 mbsf. The abrupt excursions in 87Sr/86Sr and Sr concentration are not correlated. Leaching experiments and analysis of residual solid material indicate that the porewater fluids have equilibrated isotopically everywhere with adjacent solids. The shapes of profiles of Sr isotopic composition and concentration with depth are inconsistent with diffusive or advective transport from the subduction zone towards the seafloor because Sr is chemically reactive. The mud flow rate is on the order of cm/yr, which constrains the time necessary for Sr in pore water to equilibrate with mud to a maximum of a few hundred years, geologically instantaneous given the thousands of years that the mud volcano has been in existence. We conclude that the Sr isotopic composition of the fluids mostly reflects devolatilization of subducted oceanic crust and sediment, modified by equilibration with overlying mantle rocks during serpentinization reactions.