THE GLOBAL CHLORINE CYCLE OVER THE LAST 3.7 GA; CHLORINE ISOTOPE CONSTRAINTS

It is unknown whether the excess Cl in the exosphere (crust + ocean) accumulated from continual degassing of the Earth's mantle or by an early, catastrophic event. A thorough understanding of the global chlorine cycle provides important bounds on the behavior of volatile elements in the overall tectonic cycle. Here, the chlorine isotope compositions of meteorites, mantle, and early Earth are presented, and the secular global chlorine cycle is evaluated using a model that incorporates both chlorine isotope and concentration data. δ³⁷Cl values of sodalite grains from Allende carbonaceous chondrite and bulk sample (measured using ion microprobe and gas source IRMS, respectively) are -2.1 to -0.4‰ (SMOC), in agreement with some published values for meteorites and mantle. Evaporites (13-540 Ma) all average 0.1±0.2‰ (n=77), suggesting minimal variation in the Phanerozoic. The δ³⁷Cl value of 3.7 Ga sedimentary apatite from Isua, Greenland is +0.9±0.6‰, only slightly less than a 110 Ma sedimentary carbonate hydroxyapatite (+1.8‰). Overall, these data suggest that the δ³⁷Cl value of the terrestrial environment has changed by less than 1‰ since 3.7 Ga.

Subducted serpentinite has a bimodal δ³⁷Cl distribution. Peridotite serpentinized by sedimentary porewater and seawater are -1.3 to -0.5‰ and 0.1 to 0.4‰, respectively. With the boundary conditions outlined here, the following conclusions can be made using appropriate box models: 1) The Cl flux to and from the mantle is equal, requiring that Cl was catastrophically degassed from the mantle in Earth's early history. 2) If the δ³⁷Cl value of the mantle is -1‰, the average value of subduction must also be -1‰, suggesting extensive serpentinization in the forearc mantle sourced by sedimentary pore waters.

A δ³⁷Cl = 0‰ at 3.7Ga is inconsistent with an assumed mantle of -1‰, unless isotopically light Cl was lost to space during early meteorite bombardment. Assuming an early nearly complete degassing, the Cl concentration in the early oceans must have been at least double the modern value due to a lack of an evaporitic terrestrial 'sink', with implications for conditions of early life.