Tracking carbon from subduction to outgassing along the Aleutian-Alaska Volcanic Arc.

Subduction transports volatiles between Earth's mantle, crust, and atmosphere, ultimately creating a habitable Earth. We use isotopes to track carbon from subduction to outgassing along the Aleutian-Alaska Arc. We find substantial along-strike variations in the isotopic composition of volcanic gases, explained by different recycling efficiencies of subducting carbon to the atmosphere via arc volcanism and modulated by subduction character. Fast and cool subduction facilitates recycling of ~43 to 61% sediment-derived organic carbon to the atmosphere through degassing of central Aleutian volcanoes, while slow and warm subduction favors forearc sediment removal, leading to recycling of ~6 to 9% altered oceanic crust carbon to the atmosphere through degassing of western Aleutian volcanoes. These results indicate that less carbon is returned to the deep mantle than previously thought and that subducting organic carbon is not a reliable atmospheric carbon sink over subduction time scales.

Hydrogen Incorporation in Plagioclase.

We conducted experiments at high pressure (P) and temperature (T) to measure hydrogen solubility in plagioclase (Pl) with a range of compositions (An15 to An94). Experiments were run at 700-850 °C, 0.5 GPa, and f O 2 close to either the Ni-NiO (NNO) or iron-wüstite (IW) oxygen buffers. Experiments at 700 °C on An15 (containing 0.03 wt% FeO) reveal no dependence of H solubility on f O 2 between IW and NNO, but experiments at 800-850 °C on other compositions (with 0.3-0.5 wt% FeO) demonstrate that H solubility is enhanced by a factor of ~2 to 3 at IW compared to NNO, consistent with previous experiments by Yang (2012a) on An58. By analogy with synthetic hydrogen feldspar (HAlSi3O8), we infer that the predominant mechanism for H incorporation in Pl is through bonding to O atoms adjacent to M-site vacancies, and we propose likely O sites for H incorporation based on M-O bond lengths in anhydrous Pl structures. Increased uptake of structurally bound H at low f O 2 is explained by the formation of defect associates resulting from the reduction of Fe3+ in tetrahedral sites to Fe2+, allowing additional H to be incorporated in adjacent M-site vacancies. This mechanism counteracts the expected effect of water fugacity on H solubility. We also speculate on possible substitutions of H on tetrahedral vacancies, as well as coupled H-F substitution. Enhanced incorporation of H in Pl at low f O 2 may have implications for estimating the water content of the lunar magma ocean. However, mechanisms unrelated to low f O 2 are needed to explain high H contents in terrestrial Pl xenocrysts, such as those found in basalts from the Basin and Range.