Chlorine chemistry of altered oceanic crust

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Author(s): Barnes, J. D.
Author Affiliation(s): Primary:
University of Texas at Austin, Department of Geological Sciences, Austin, TX, United States
Volume Title: Goldschmidt 2011 abstract volume
Source: Mineralogical Magazine, 75(3), p.488; Goldschmidt2011, Prague, Czech Republic, Aug. 8-14, 2011. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Note: In English. 3 refs.
Summary: Chlorine strongly partitions into aqueous fluids, thereby making Cl and its stable isotopes (37Cl and 35Cl) powerful tracers of fluid-rock interaction, including interaction between oceanic lithosphere and hydrothermal fluids. Although altered oceanic crust (AOC) is considered to be a major Cl reservoir estimated to subduct ≈2.5-3 x 1012 g of Cl worldwide each year [1, 2], work on the Cl chemistry of AOC is surprisingly limited. Here I present new Cl concentration and isotopic data for AOC from seven DSDP/ODP/IODP drill sites (801C, 735B, 894F/G, 504B, 1256D, 417A/D/418A, 332A/B), greatly expanding the previous data set. Chlorine concentration and δ37Cl values of AOC are heterogeneous among and within individual drill sites. Cl concentrations range from <0.01 to 0.09 wt% (n = 26) and δ37Cl values range from -0.8 to +1.5 ppm (error < ± 0.2 ppm) vs. SMOC (Standard Mean Ocean Chloride) (n = 20). These data greatly expand the range of previously reported AOC δ37Cl values (-1.6 to -0.9 ppm; n = 3; Hole 504B; [3]). Neither Cl concentration nor isotopic composition is correlated with tectonic setting or crustal age. The Cl concentration decreases with depth in ODP Hole 735B from the SW Indian Ridge from 0.09 wt% at the top of the hole to <0.01 wt% at depth. A similar pattern is seen for δ37Cl values ranging from -0.6 to +1.5 ppm with the most positive values located near the top of the hole. The high Cl concentrations and δ37Cl values are correlated with increased hydrothermal alteration (increased amphibole abundance) near the top of the hole. However, this trend is not consistent among all the holes implying that Cl chemistry is influenced by a variety of factors. Detailed future petrography and additional geochemical work will further test the correlation between Cl concentration and isotopic composition and mineralogy, as well as, examine the role of water-rock ratios, deformation, and temperature of hydrothermal alteration. These data can ultimately be used to reevaluate mass balance calculations improving our understanding of subduction recycling.
Year of Publication: 2011
Research Program: DSDP Deep Sea Drilling Project
IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 18 Geophysics, Solid-Earth; Atlantis II fracture zone; Chlorine; Cl-37/Cl-35; Concentration; Cores; Crust; DSDP Site 504; Deep Sea Drilling Project; East Pacific; Equatorial Pacific; Geochemistry; Halogens; Hydrothermal alteration; Indian Ocean; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Metasomatism; North Pacific; Northeast Pacific; ODP Site 735; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Southwest Indian Ridge; Stable isotopes
Coordinates: N011335 N011338 W0834348 W0834357
S324327 S324318 E0571618 E0571557
Record ID: 2012077778
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland