Terry Plank

Peridotite xenoliths commonly exhibit melt veining, variably attributed to mantle metasomatism, host magma infiltration, or decompression melting. In order to test these origins and constrain the P–T–t path of xenolith ascent, we study spinel-lherzolite xenoliths and host scoria from a cinder cone north of the Grand Canyon (Sullivan Ranch). Micro-X-ray computed tomography (μCT) scans of the xenoliths show that melt veins are ubiquitous at this locality. They make up ~10% of bulk xenoliths, are highly interconnected at the mm scale and include bubbles. Bubbles are surrounded by silica-rich residual glass, although the veins are largely crystalline, composed of secondary plagioclase, clinopyroxene (cpx), olivine, and Cr-spinel (sp). The reconstructed bulk melt vein is basaltic (50–52% SiO2), and mineral compositions within the melt vein (high-Fo olivine, high-Ti spinel, high-Cr, and lower-La/Sm cpx) are distinct from primary minerals away from melt veins. The reconstructed incongruent melting reactions for two different xenoliths (63cpx + 72opx + 13sp ➔ 100liq + 48ol, on average) are similar to those from equilibrium peridotite melting experiments at 1 GPa, even though minerals are not in equilibrium across the 3–10 cm scale xenoliths. The timescale for xenolith melting was derived from Fe–Mg zonation profiles in primary olivine (Fo89–90) adjacent to melt veins (with Fo91–92 rims) that indicate diffusion over 1–3 weeks. This timescale is similar to that derived for olivine grains at the xenolith exterior that were diffusively equilibrating with the host magma for ~1 week. The similarity of timescales from the interior melt veins to the exterior olivine indicates that in situ melting occurred during entrainment and transport. Ascent rates from the entrainment depth (~40 km) to the surface are 0.05–0.15 m/s, similar to other primitive cinder cone eruptions and consistent with Stokes settling velocities. Xenocrysts in the host scoria yield diffusion timescales of hours to days, consistent with xenolith disaggregation late in the ascent history. Evidence for limited interaction with the host magma comes from trace element zonation in melt veins that approaches the host composition in the outer 1 cm of the xenolith. The interaction does not appear to be driven by diffusion, as elements with very different diffusivities (e.g. Cl vs Zr) show similar length scales of zonation. Instead, host melt infiltration could have occurred due to a combination of capillary forces and inflation of the xenolith driven by the volume change of melting and bubble growth in the melt veins. Given the P–T–t path of these lherzolite xenoliths, which should be similar to those erupted from other alkali basalt cinder cones from around the world, decompression melting is inevitable. The extent of decompression melting will be a function of the host magma P–T path, the xenolith water content, and the proportion of touching cpx-opx-sp. Such melting consumes cpx and may lead to systematic misclassification of xenoliths as refractory harzburgites that were originally lherzolites. Geochemical studies of xenoliths should take care to avoid the outer cm (or more) that may have been contaminated by the carrier magma, and secondary minerals formed from infiltration, reaction, melting and/or crystallization.

Sulfur plays a critical role in modulating redox cycling on Earth. Yet, sulfur’s behavior during subduction and in mantle redox reactions is debated. We analyzed ³⁴ S/ ³² S in mafic arc melt inclusions from contrasting subduction zones and modeled slab-mantle interaction to investigate the subduction zone sulfur cycle. We find that degassing may enrich or deplete the melt strongly in ³⁴ S as a function of melt redox state. After correction for this effect, arc magmas have a substantially narrow range of δ ³⁴ S values (+3 ± 2‰), higher than the ambient upper mantle (−1‰). Slab-derived sulfur is oxidized, evidenced by a concurrent increase in mantle Fe ³⁺ and sulfur contents, and contributes up to 86% of the mantle wedge’s sulfur budget. Arc magma δ ³⁴ S values reflect a common slab source for sulfur: the oceanic crust. Subduction zones act as a “filter” for oxidative power and ³⁴ S, effectively returning these to the surface over geological timescales.

Basaltic Plinian eruptions challenge our understanding of explosive volcanism. The 122 B.C. Plinian eruption of Etna ranks among the most powerful mafic explosive events known. Here, we combine volatile barometry of 122 B.C. from olivine‐hosted melt and fluid inclusions with comparative data from the sub‐Plinian Fall Stratified eruption at Etna (3930 BP) to assess the storage depths and degassing paths that govern explosive eruptive styles. Our results indicate that the 122 B.C. magma storage started deep (∼22 km) within Etna's plumbing system, had a complex ascent history, and was finally stored pre‐eruptively at shallow levels (∼2–5 km) for a minimum of ∼3 weeks, resulting in low equilibrated H ₂ O (∼2 wt%) and CO ₂ (≤500 ppm in bubble‐free melt inclusions) contents. The Fall Stratified event, in contrast, records deeper storage (∼24–30 km), high magmatic volatile contents (CO ₂ concentrations up to 9600 ppm, H ₂ O concentrations up to 6.3 wt%), and exceptionally rapid magma ascent rates (17.5 m/s). We propose that the 122 B.C. eruption likely resulted from multiple episodes of replenishment by basaltic magma with a much lower CO ₂ concentration (2 , is thought to drive the eruption.

Here we provide comprehensive estimates for the subducted sedimentary flux of inorganic and organic carbon and δ ¹³ C at 16 trench segments worldwide. Our calculations provide minimum (incoming sediment), maximum (trench fill) and best estimates (underthrust) of carbon fluxes. Notably, our estimates are granular, at a scale of ∼10 km along the trench, relevant to individual volcanoes. Results highlight extreme regional heterogeneity, where the sedimentary carbon input to some trench segments is zero, while for others it exceeds that in the altered oceanic crust by more than a factor of four. The fraction of carbon that is organic is 70% in others, with a resulting large range in subducting sedimentary δ ¹³ C from +1 to −17‰. We find that the δ ¹³ C of high‐temperature volcanic gases at different arc volcanoes correlates with that in the local subducted sediment. We also provide a new estimate of the carbon flux in global subducting sediment (C‐GLOSS) of 26.5 ± 1 MtC/yr, more than a factor of two lower than previous estimates. Given the uncertainties in the global volcanic output rate of carbon, mantle inputs and outputs of carbon are still roughly in balance. The fraction of organic carbon relative to total carbon in C‐GLOSS is 21%, yielding an isotopic composition of −4.2 ± 0.4‰ δ ¹³ C, which is by coincidence within the mantle range.