Si-metasomatism in serpentinized peridotite:The effects of talc-alteration on strontium and boron isotopes in abyssal serpentinites from Hole 1268a,

Ultramafic rocks recovered from Hole 1268a, Ocean Drilling Program Leg 209, to the south of the 15 degrees 20'N Fracture Zone on the Mid-Atlantic ridge have experienced a complex history of melt depletion and subsequent interaction with a series of fluids under varying temperature and pH conditions. After intense melt depletion, varying degrees of serpentinization at 100-200 degrees C took place, initially under seawater-like pH conditions. Subsequently, interaction with a higher temperature (300-350 degrees C) fluid with low (4-5) pH and low MgO/SiO2 resulted in the heterogeneous alteration of these serpentinites to talc-bearing ultramafic lithologies. The proximity of the currently active, high temperature Logatchev hydrothermal field, located on the opposite flank of the Mid-Atlantic ridge, suggests that unlike more distal localities sampled during ODP Leg 209, Hole 1268a has experienced Si-metasomatism (i.e. talc-alteration) by a Logatchev-like hydrothermal fluid. Serpentinite strontium isotope ratios were not materially shifted by interaction with the subsequent high-T fluid, despite the likelihood that this fluid had locally interacted with mid-ocean ridge gabbro. Sr-87/Sr-86 in the ultramafic lithologies of Hole 1268a are close to that of seawater (c.0.709) and even acid leached serpentinites retain Sr-87/Sr-86 in excess of 0.707, indistinguishable from Logatchev hydrothermal fluid. On the other hand, boron isotope ratios appear to have been shifted from sea-water-like values in the serpentinites (delta B-11 = c.+40 parts per thousand) to much lighter values in talc-altered serpentinites (delta B-11 = +9 to +20 parts per thousand). This is likely a consequence of the effects of changing ambient pH and temperature during the mineralogical transition from serpentine to talc. Heterogeneous boron isotope systematics have consequences for the composition of ultramafic portions of the lithosphere returned to the convecting mantle by subduction. Inhomogeneities in delta B-11, [B] and mineralogy introduce significant uncertainties in the prediction of the composition of slab fluids released during the early-to mid-stages of subduction. (C) 2013 Elsevier Ltd. All rights reserved.