HOW DOES SLAB-DERIVED SULFUR AFFECT MAGMA REDOX IN THE SOUTHERN CASCADES? INSIGHTS FROM THE MELT INCLUSION RECORD

Magma oxidation state is an important parameter that plays a role in nearly every stage of melting and differentiation, but magma redox is difficult to track and is often poorly constrained. Globally, arc magmas are oxidized relative to MORB, but the causes for this are uncertain. One possibility is that arc magmas inherit this oxidized signature from slab-derived material present in the sub-arc mantle during melting. The addition of sulfur in particular has the potential to drive large changes in redox state. The other possibility is that arc magmas become oxidized during differentiation within the crust.

To investigate the origins of oxidized magmas in warm-slab subduction zones, we use Lassen Volcanic Field, located in the southern portion of the Cascades, as a natural laboratory. We analyzed primitive olivine-hosted melt inclusions collected from the tephra of basaltic cinder cones to characterize the composition of magmas present during the earliest stages of crystallization, minimizing the influence of crustal differentiation. To build a high resolution data set targeted towards unraveling the processes that oxidize arc magmas, we integrate evidence from major elements, trace elements, volatile contents, and XANES-based fO₂ estimates. Specifically, we focus on the sulfur content, metal content, and Fe valence state of primitive magmas, all of which directly relate to redox state and can be used to track how it varies during melting and early differentiation.

Sulfur concentrations in individual melt inclusions range from 468 to 3853 ppm. Cu, Zn, and Sn concentrations in individual melt inclusions range from 17 to 167 ppm, 65 to 124 ppm, and 0.5 to 1.5 ppm, respectively. Minimum fO₂ values inferred from S XANES range from QFM + 0.8 to QFM + 1.5, whereas those calculated from Fe XANES range from QFM + 0.8 to QFM + 2. The fO₂ values for different cinder cones correlate with S/Dy and Sr/Nd values and are consistent with the addition of oxidized, sulfur-carrying slab material to the Lassen sub-arc mantle. Cu/Sc values approach average MORB values and are similar between cinder cones despite significant differences in fO₂ and sulfur concentrations. This suggests that chalcophile element behavior is moderated by the presence of a residual sulfide phase during melting, even at relatively high oxygen fugacities.