Chlorine isotope variability in subglacial glasses from Iceland

Chlorine concentrations tend to be significantly enriched in surface reservoirs relative to that of the mantle. This large contrast in chlorine contents makes primary asthenospheric melts, highly susceptible to contamination by surficial chlorine in shallow-level crustal environments. Indeed, on the basis of both chlorine abundance systematics [e.g., 1], and chlorine isotopes [2], previous researchers have argued for a surficial chlorine component in controlling the chlorine systematics of MORB. In contrast, other studies suggest recycling of ancient chlorine via a subducted, altered oceanic lithospheric component as a means of controlling the chlorine isotope composition of the mantle [3]. So far, very few high precision chlorine isotope data are available from key oceanic islands, which potentially provide access to deeper parts of the mantle and thus allow for testing of the relative role of these ideas.

We report new chlorine isotope (reported as δ³⁷Cl; n=22) and abundance data (SIMS) derived from fresh subglacial glasses from Iceland. The glasses, which cover all the currently active volcanic zones of Iceland, span a wide range in their major element composition with MgO contents between 2.1 and 10.0 wt% and chlorine contents, that vary by almost two orders of magnitude, of 17 to 1270 ppm. Chlorine contents show significant correlations (R² > 0.9) with incompatible elements such as potassium, consistent with earlier observations from Iceland and the adjacent Reykjanes Ridge [4, 5]. δ³⁷Cl values range from -0.6‰ to +1.4‰. Significantly, δ³⁷Cl values strongly correlate with Cl and MgO contents, with low δ³⁷Cl values in samples with low Cl and high MgO concentrations. The data are consistent with mixing between two different reservoirs: a upper mantle reservoir with low Cl concentration and a slightly negative δ³⁷Cl value and a crustal reservoir with a high Cl concentration and enriched in ³⁷Cl. To further investigate the origin of these chlorine isotope variations, we are currently analyzing a range of other isotopic tracers, including oxygen and radiogenic isotopes.

[1] Michael & Cornell, JGR, 103, (1998), [2] Bonifacie et al., Science 319, (2008), [3] John et al., EPSL, 298, (2010), [4] Sigvaldason & Óskarsson, GCA, 40, (1976), [5] Unni & Schilling, Nature, 272, (1978).