Chlorine Isotope Constraints on the Origin and Distribution of Earth's Chlorine

Details Chlorine Isotope Constraints on the Origin and Distribution of Earth's Chlorine Chlorine Isotope Constraints on the Origin and Distribution of Earth's Chlorine

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v14c..02b&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #V14C-02

Mathematics

Logic

1025 Composition Of The Mantle, 1028 Composition Of Meteorites (3662, 6240), 1041 Stable Isotope Geochemistry (0454, 4870), 8413 Subduction Zone Processes (1031, 3060, 3613, 8170), 8416 Mid-Oceanic Ridge Processes (1032, 3614)

Scientific paper

Volatile elements exert a strong influence over the chemical and physical properties of the Earth's mantle. Due to its incompatible, soluble and volatile element chemical characteristics, chlorine is especially valuable in understanding the current and past evolution of the Earth (e.g., melting, recycling, degassing, differentiation). Comprehensive understanding of the exchanges of chlorine among Earth's reservoirs may help constrain the origin and the budget of chlorine and other volatile elements on Earth. Due to the large uncertainties in the estimated range of mantle Cl flux inputs and outputs, we present here the chlorine isotopic compositions (δ37Cl) of mantle and subducted materials as well as those in chondrites with the aim of better understanding the global Cl cycle. Mantle, subduction and chondrites δ37Cl. Based on fresh N- and E- MORB samples affected by various degrees of assimilation of seawater-derived materials (e.g., intergranular brines; Bonifacie et al., Chem Geol, 2005), we estimate that the δ37Cl value of the mean upper mantle is inferior or equal to - 1.9‰. Analyses on HP metaperidotites from the Alps suggest that no Cl-isotopes fractionation occurs during the Cl loss associated with the dehydration of serpentines throughout prograde subduction. Considering HP metaperidotites as suitable candidates for Cl transfer to the mantle, and excluding the possible contribution of sediments, we estimate that the subducted material has δ37Cl values superior or equal to -1.4‰. Various types of chondrites show relatively homogeneous δ37Cl values (~ - 1.7\mp‰). The global chlorine cycle and implications on the origin of Earth's Cl. The slight but significant difference between the Cl-isotopic signature of recycled Cl and upper mantle implies that the δ37Cl value of the mantle increased while that of exogenous reservoirs decreased over geological time. Box modeling predicts for early Earth: i/ a large amount of Cl in exogenous reservoirs, and ii/ a significant δ37Cl difference between exogenous reservoirs and mantle. This argues in favor of an early and catastrophic degassing of Cl from the mantle (however not sufficient to explain the predicted early δ37Cl differences) and/or a "late veneer" supply of Cl (i.e. heterogeneous accretion). Our data also show evidence for a δ37Cl difference between the "Upper Earth" (i.e. exogenous reservoirs and upper mantle; -0.3‰) and chondrites. This might be explained by a preferential loss of ^{35}Cl during Earth accretion and/or a "late veneer" supply of Cl with positive δ37Cl value.

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