Changes in magmatic oxidation state induced by degassing

Abstract:

Temporal variations in the oxygen fugacity (fO₂) of the mantle may have been transmitted to Earth’s atmosphere and oceans by volcanic degassing. However, it is unclear how redox states of volatiles relate to their source magmas because degassing and assimilation can impact fO₂ before or during eruption. To explore this, we present µ-XANES measurements of the oxidation states of Fe and S and laser fluorination measurements of ¹⁸O/¹⁶O ratios in submarine glasses from two settings where degassing is recorded: 1) submarine glasses from the Reykjanes Ridge as it shoals to Iceland, including subglacial glasses from the Reykjanes Peninsula; and 2) submarine glasses from Mauna Kea recovered by the Hawaii Shield Drilling Program (HSDP).

Glasses from both settings are basalts with 5.5-9.9 wt% MgO and 350-1790 ppm S. Submarine Reykjanes glasses are sulfide saturated. Subglacial Reykjanes and HSDP glasses are not sulfide saturated, and S and H₂O contents are consistent with S+H₂O degassing.

Submarine Reykjanes glasses have ¹⁸O/¹⁶O indistinguishable from MORB and become progressively ¹⁸O-depleted as MgO decreases. Subglacial glasses have lower ¹⁸O/¹⁶O than submarine glasses at a given MgO, but both sample types project to a common ¹⁸O/¹⁶O near 10 wt% MgO, suggesting that ¹⁸O-depletion in these lavas is generated by fractional crystallization and assimilation of an ¹⁸O-depleted crustal component. The oxidation state of Fe increases only slightly as ¹⁸O/¹⁶O decrease, suggesting that the assimilant is not oxidized enough to change magmatic fO₂. Fe and S do not oxidize or reduce with decreasing S or H₂O, suggesting that relatively reduced magmas at depth degassed S+H2O without changing magmatic fO₂, and that the fO₂ of these lavas reflect the fO₂of their mantle source.

The oxidation states of Fe and S in HSDP glasses are broadly correlated and samples with the highest S concentrations are the most oxidized. Both Fe and S reduce with decreasing S and H₂O contents. This suggests that relatively oxidized magmas at depth degassed S+H2O with open-system removal of oxidized species, reducing magmatic fO₂ and producing a degassed volatile inventory more oxidized than its parent magma. Degassing may reduce fO₂ more in H and S rich magmas that degas at high pressure, creating large differences between the redox states of magmas and volatiles.