The Fe3+/ΣFe Ratios of East Scotia Back-Arc Basin Lavas: New Insights Into Subduction Inputs

Abstract:

We present new wet chemical determination of the Fe³⁺/ΣFe ratio for 41 glassy samples from the East Scotia back-arc basin (South Atlantic) showing a chemical continuum from MORB-like to arc-like compositions. The Fe³⁺/ΣFe ratios of East Scotia samples vary from 0.05 to 0.21. Proxies of subduction inputs such as H_8.0 (H₂O content corrected for fractional crystallization) and Ba/Y ratios display along the ridge axis three arc-like spikes (H_8.0> 1.5 wt.%; Ba/Y >2) that are correlated to high Fe³⁺/ΣFe ratios (0.21, 0.18, and 0.20). MORB-like samples (H_8.0<0.3; Ba/Y<2) display lower Fe³⁺/ΣFe ratios of 0.08±0.02. We show that fractional crystallization processes cannot account solely for the observed Fe³⁺/ΣFe variation and propose that the high Fe³⁺/ΣFe ratios of arc-like samples reflect higher oxidation states of primary magmas compared to MORB-like samples.

Previous studies have shown that the northernmost subduction spike is characterized by a complex interplay between various subduction inputs and the westward inflow of enriched mantle derived from the Bouvet hotspot. We did not find evidence for any influence of the OIB-like enriched mantle on the Fe³⁺/ΣFe ratios of lavas samples. Correlations between Fe³⁺/ΣFe and trace element ratios diagnostic of subduction inputs (e.g. Ba/Y, r=0.83) indicate that input of water-rich subduction components is related to high oxidation states in arc-like samples. Further south, the inflow of Bouvet mantle fades to almost no influence and reveals the central spike that show the most arc-like influence with the contribution of water-rich and sediment-melts components. In this section, the Fe³⁺/ΣFe is also strongly and positively correlated with Ba/Y ratios (r=0.85) and shows, for a given Ba/Y ratio and compared to northern and central samples, lower Fe³⁺/ΣFe ratios. Finally, when the southern samples are characterized by high H₂O/[LILE] ratios. The Fe³⁺/ΣFe ratio of southern samples is also strongly correlated with the Ba/Y ratio (r=0.94) showing, for a given Ba/Y higher Fe³⁺/ΣFe ratios compared to samples from the northern spike. East Scotia back-arc lavas display 3 different relationships between Fe³⁺/ΣFe data and proxies of subduction inputs. We discuss the origin of those 3 different trends and the potential mantle wedge oxidation by subduction fluids.