The Anomalous C/N Ratio of the Bulk Silicate Earth – A Record of Which Early Planetary Catastrophe?

Abstract:

The C/N ratio of the bulk silicate Earth (BSE) is anomalously high compared to cosmochemical sources and this anomaly is likely a remnant of early cataclysmic events of either core formation or catastrophic atmospheric loss. However, this important ratio is subject to large uncertainties and the processes affecting it remain poorly understood. Recent estimates of the C/N mass ratio of the BSE range from 39 [1] to 313 [2]. Reconsideration of mantle C/N, with C derived from C/Nb and C/Ba ratios of oceanic basalts [3], combined with N derived from N/⁴⁰Ar basalt ratios with the terrestrial K and Ar budgets, propagating all uncertainties, yields a revised C/N estimate of 42±12, which is ~2X greater than CI, CO, or CM chondrites, but plausibly similar to more degassed CV chondrites. N is a siderophile element, leading some to suggest that the N depletion of the BSE is owing to core formation, but C is more siderophile than N, suggesting that core formation cannot account for the high C/N of the BSE. Catastrophic atmospheric loss can account for high BSE C/N, assuming that primitive atmospheres were comparatively N-rich, as should be the case because, at least under oxidizing conditions, N is much less soluble in magma than C. However, core sequestration of volatile elements cannot be estimated from metal/melt partition coefficients alone; solubilities and atmospheric storage must also be considered. If the atmosphere overlying a magma ocean is oxidizing, N₂ is much less soluble in magma than CO₂, and so metal is separated from silicate that is already N-depleted, and core formation lowers BSE C/N. For a reduced atmosphere, the situation reverses: N is more soluble than C in the magma, and core/mantle/atmosphere partitioning can raise BSE C/N, even if C is much more siderophile than N. Similarly, catastrophic atmospheric loss will have opposite effects if the vapor is oxidized (low C/N vapor) or reduced (high C/N vapor). Thus, the significance of Earth’s anomalously high C/N cannot be determined without constraints on the chemical conditions prevailing during large scale vapor/silicate/metal. Finally, the CK chondrites remind us that some of the C/N fractionation could have occurred on precursors, prior to accretion. [1] Halliday (2013) GCA 105, 146-171. [2] Marty (2012) EPSL 313: 56-66. [3] Rosenthal et al. EPSL submitted.