Predominantly recycled carbon in Earth's upper mantle revealed by He-CO2-Ba systematics in ultradepleted ocean ridge basalts

Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions of mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, erupted within the Garrett Transform Fault (GTF) and to its north and south along the East Pacific Rise (EPR). Analyses of He-3/He-4, and He and CO2 concentrations in vesicles and glass were performed, through a series of crushing and melting experiments plus FTIR spectroscopy. Trace elements were analyzed by laser ablation ICP-MS. The GTF basalts provide further tests for the origin of volatile-undersaturated basalts using CO2/Ba and CO2/Nb systematics. CO2 is highly correlated with Ba and Nb in basalts erupted in the transform domain (n = 13, including 5 undersaturated basalts) and does not show the variability expected from mixing between undegassed and variably degassed melts. Rather, the melts appear to originate from a heterogeneous mantle source that was variably depleted through partial melting, and limited mixing of melts is involved in their generation. The CO2/Ba and CO2/Nb weight ratios of 106 +/- 8 and 308 +/- 27, respectively, are similar to values determined previously for a global suite of undersaturated mid-ocean ridge basalts (MORBs). The ridge and transform domains have distinct He-3/He-4 ratios. Along the nearby EPR, He-3/He-4 = 8.5 - 9.1 R-A, while within the Garrett Transform Fault He-3/He-4 = 9.2 - 10.1 R-A. These two basalt populations are also distinct in their Pb-Sr-Nd isotope compositions based on earlier regional studies. The distinct populations result from partial melting of two different mantle source compositions. Melting of depleted mantle containing a small amount (similar to 1 to 5%) of enriched, ancient heterogeneities occurs beneath the EPR. Melting of ultradepleted mantle (in which the heterogeneities have been removed by earlier melting beneath the EPR) occurs beneath the GTF. This explains the distinction between intra-transform and spreading ridge domains for He-3/He-4, if the heterogeneities were enriched in U, Th and He and had low He-3/He-4 as would be found in tectonically recycled material. The enriched mantle component sampled by the EPR basalts has molar CO2/He-3 = 2 x 10(9), and it dominates the CO2/He-3 ratio generally ascribed to the upper mantle source for mid-ocean ridge basalts. In contrast, the ultradepleted MORE mantle component sampled by the GTF basalts has CO2/He-3 = 3 x 10(8) or less. This indicates that most of the carbon in Earth's upper mantle originates from tectonic recycling. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, providing constraints on the proportions of mantle carbon that are juvenile vs. tectonically recycled. Different isotopic compositions of basalts from GTF and EPR suggest they are derived from two different mantle source compositions.