Halogen (F, Cl, Br, and I) concentrations of the upper continental crust through time as recorded in ancient glacial diamictite composites

The continental crust is an important reservoir for incompatible elements, including the halogen elements (F, Cl, Br, and I), but their concentrations remain poorly known, thus hindering better understanding of the role of the crust in Earth’s halogen cycle. We present halogen data (F, Cl, Br, and I) for twenty-four well-characterized glacial diamictite composites that derive from the upper continental crust (UCC) and were deposited during discrete glacial events at ∼2.9 Ga, ∼2.4–2.2 Ga, 0.75–0.58 Ga, and ∼0.30 Ga. A good correlation between Cl and the highly soluble Na (R2 = 0.70), together with low and scattered Cl concentrations in the diamictites (2–279 ppm), indicates significant Cl loss during chemical weathering of the continents. The other halogens (F, Br, and I), however, are not strongly affected by chemical weathering as revealed by their correlations with less soluble elements like K, P, Nd, and Lu, which may be due to their retention in secondary minerals and/or organic matter. Increasing concentrations of F in the composites through time may reflect the evolving composition of the UCC. Using the median values of the Neoproterozoic and Paleozoic diamictite composites, halogen concentrations of the present-day weathered UCC are estimated to be: 575 ± 87 ppm F, 29 ± 20 ppm Cl, 0.65 ± 0.14 ppm Br, and 0.05 ± 0.01 ppm I (errors quoted at the median absolute deviation). Linear correlations between halogens and other elements provide estimates for minimum halogen concentrations of the present-day crystalline UCC: 394 ± 67 ppm F, 83 ± 24 ppm Cl, 0.41 ± 0.04 ppm Br, and 0.03 ± 0.01 ppm I (with 2σ uncertainties). These estimates are all lower than normalized concentrations of elements of similar incompatibility during igneous differentiation, which may reflect loss of halogens via magmatic degassing/exsolution and/or chemical weathering during the formation of the continental crust. The distinct behavior of Cl compared to Br and I during continental weathering leads to a wide range of Br/Cl (2–265 * 10−3) and I/Cl (122–197,952 * 10−6) ratios, and Br/I ratios that are distinctly higher than those of pelagic sediments and marine pore fluids. Similar halogen signals have been reported from the lithospheric mantle and may reflect recycling of terrigenous sediments. The calculated weathering flux of Cl from the continents is quite small compared to the total Cl content of seawater (<8–34 %), suggesting that Cl in seawater is mainly derived from outgassing of the mantle and/or late volatile accretion. On the other hand, the significantly higher Br and I contents in terrigenous organic-rich sediments compared to those of crystalline bedrocks suggest that significant amounts of Br and I were transported from the oceans to the continents.