Sulfur Isotopic Composition of Ocean Island Basalts: New insights into the composition of the primitive mantle and mantle recycling.

Abstract:

Here, we report the S isotopic composition of bulk sulfide inclusions in olivine and pyroxene isolated from lavas originating from 11 oceanic hotspot volcanoes, including Hawaii, Samoa, Pitcairn, and Mangaia. We also report the S isotopic composition of a peridotite xenolith from a Hawaiian lava.

We digested between 200 and 1500 mg of sample using an HF chemical extraction technique (Labidi et al., 2012) and analyzed for S isotopes on a Thermo-Finnigan MAT 235 IRMS.

A total of 35 samples from 11 different islands were analyzed, but only 14 samples yielded sufficient S for analysis. S concentrations range between 22 and 555 ppm S (±10%) in magmatic olivines and pyroxenes from Mangaia. We extracted 47 ppm S from the Hawaiian xenolith. For Mangaia, δ³⁴S values range between -4.90±0.20‰ and +0.42±0.20‰, and Δ³³S values range between +0.001±0.008‰ and +0.044±0.008‰. Δ³⁶S values range between -0.03±0.20‰ and +0.32±0.20‰ (all 1σ).

We measured 20 ppm S in the Baffin Island olivine separate. We observe a δ³⁴S value of -1.97±0.20‰ and a Δ³³S value of +0.010±0.008‰ in this sample. This lava may tap into the most primitive mantle source, based on the primordial He isotopic composition of this locality. Therefore, our S isotopic data suggest a sub-chondritic δ³⁴S value for the primitive mantle, in agreement with results from Labidi et al. (2014).

Mangaia and Baffin phenocrysts are observed to host significant S amounts. However, samples from Pitcairn, Samoa, Hawaii, and others, all show S contents < 5 ppm S. This suggests negligible sulfide saturation for their host lavas.

Lastly, we observe mass-dependent S isotope ratios for all Mangaia samples investigated here (n=8), inconsistent with the Cabral et al. (2013) report. However, we did not analyze the same samples from Mangaia. A lack of MIF in our sample suite may be attributed to: 1. Heterogeneities within the same hotspot and 2. Dilutions of the MIF signature when a single sulfide may contain MIF while hundreds of other sulfides are mass-dependent. This will be further discussed at the conference.Here, we report the S isotopic composition of bulk sulfide inclusions in olivine and pyroxene isolated from lavas originating from 11 oceanic hotspot volcanoes, including Hawaii, Samoa, Pitcairn, and Mangaia. We also report the S isotopic composition of a peridotite xenolith from a Hawaiian lava.

We digested between 200 and 1500 mg of sample using an HF chemical extraction technique (Labidi et al., 2012) and analyzed for S isotopes on a Thermo-Finnigan MAT 235 IRMS.

A total of 35 samples from 11 different islands were analyzed, but only 14 samples yielded sufficient S for analysis. S concentrations range between 22 and 555 ppm S (±10%) in magmatic olivines and pyroxenes from Mangaia. We extracted 47 ppm S from the Hawaiian xenolith. For Mangaia, δ³⁴S values range between -4.90±0.20‰ and +0.42±0.20‰, and Δ³³S values range between +0.001±0.008‰ and +0.044±0.008‰. Δ³⁶S values range between -0.03±0.20‰ and +0.32±0.20‰ (all 1σ).

We measured 20 ppm S in the Baffin Island olivine separate. We observe a δ³⁴S value of -1.97±0.20‰ and a Δ³³S value of +0.010±0.008‰ in this sample. This lava may tap into the most primitive mantle source, based on the primordial He isotopic composition of this locality. Therefore, our S isotopic data suggest a sub-chondritic δ³⁴S value for the primitive mantle, in agreement with results from Labidi et al. (2014).

Mangaia and Baffin phenocrysts are observed to host significant S amounts. However, samples from Pitcairn, Samoa, Hawaii, and others, all show S contents < 5 ppm S. This suggests negligible sulfide saturation for their host lavas.

Lastly, we observe mass-dependent S isotope ratios for all Mangaia samples investigated here (n=8), inconsistent with the Cabral et al. (2013) report. However, we did not analyze the same samples from Mangaia. A lack of MIF in our sample suite may be attributed to: 1. Heterogeneities within the same hotspot and 2. Dilutions of the MIF signature when a single sulfide may contain MIF while hundreds of other sulfides are mass-dependent. This will be further discussed at the conference.