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Arc basalts have a higher proportion of Fe in an oxidized state (Fe^(3+)) relative to Fe^(2+) compared to mid-oceanic ridge basalts (MORBs), likely because slab-derived fluids oxidize the mantle wedge where subduction zone magmas originate. Yet, the time scales over which oxygen fugacity of the mantle wedge changes during subduction initiation and margin evolution are unknown. Fe speciation ratios show that magmas produced during the early stages of subduction in the Mariana arc record oxygen fugacities ∼2× more oxidized than MORB. Mantle wedge oxygen fugacity rises by ∼1.3 orders of magnitude as slab fluids become more involved in melt generation processes, reaching conditions essentially equivalent to the modern arc in just 2–4 m.y. These results constrain existing models for the geochemical evolution of the mantle wedge and suggest that oxidation commences upon subduction initiation and matures rapidly in the portions of the mantle wedge that produce melts. This further implies that sulfide or other reduced phases are not present in the mantle wedge in high enough abundance to prevent oxidation of the magmas that form upon subduction initiation. The arc mantle source is oxidized for the majority of a subduction zone’s lifetime, influencing the mobility of multivalent elements during recycling, the degassing of oxidized volcanic volatiles, and the mechanisms for generating continental crust from the immediate onset of subduction.

Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confirmed by measurements of Li contents as high as 207 µg/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess (210Pb)/(226Ra), implying that measurable Li enrichments may occur despite significant differences in the timing and/or extent of magmatic degassing. Diffusion modeling shows that Li enrichment occurred within ∼1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short flux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides.

In the Andean Central Southern volcanic zone, basalts from small eruptive centers near the large composite center Volcan Villarrica are poor in fluid mobile elements, such as B, Cs, Rb, K, Pb, Ba, and U, compared with concurrently erupted Villarrica basalts. New 1 0 Be and U-series isotopic data for these centers show that fluid mobile element-poor small eruptive center basalts have small 1 0 Be/ 9 Be ratios (1.6-1.9 × 10 - 1 1 ) and ( 2 3 8 U/ 2 3 0 Th) activity ratios near 1.0, whereas basalts from Villarrica show 2 3 8 U enrichment and larger 1 0 Be/ 9 Be ratios (4.0-6.4 × 10 - 1 1 ). These results suggest that small eruptive center basalts include materials derived from the subducted lithosphere that were stored in the mantle wedge for 350 k.y. to 3 m.y. That these materials are poor in fluid mobile elements may reflect fluid expulsion during solidification or their formation in an initially hotter subduction setting. In contrast, the composite center basalts sample materials rich in fluid mobile elements that were recently transferred into the mantle wedge from the subducted lithosphere. The results confirm that mantle wedges in subduction zones include subducted materials added to the wedge over both long and short time scales.

Isotopic analyses of three stalagmites from Cold Water Cave, northeast Iowa, United States, reveal nearly identical δ 13 C trends from ca. 7 to 2 ka. However, δ 18 O patterns differ by as much as 3‰ from 5.7 to 3.2 ka. These disparate δ 18 O values reflect evaporative 18 O enrichment in meteoric water prior to infiltration, suggesting that previously calculated temperatures based on a single Cold Water Cave stalagmite overestimated middle Holocene warming. The coincidence of elevated middle Holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle Holocene was marked by a predominance of coolweather precipitation during a period of increased overall aridity.

Existing trace element and isotopic data, as well as new U-series data for Nicaragua and Costa Rica, are examined in light of the recent suggestion that asthenospheric mantle is flowing northward behind the subducting Nazca plate around the northwest corner of South America. Lavas from central Nicaragua have trace element and isotopic compositions suggesting that they are derived from relatively depleted mantle with a long history of interaction with the subducting Cocos plate. Lavas from northern and central Costa Rica have compositions indicating a more enriched mantle source that has been influenced little by the subducting slab. These compositional traits vary irregularly between the two regions, suggesting that mantle compositions and subduction influences intermingle in a complex manner beneath northern Costa Rica. The dip of the subducting slab decreases abruptly southward, and accretion of sediment begins in this same vicinity. These changes in composition and physical parameters may all be induced or influenced by an influx of asthenospheric material from behind the retreating Nazca plate.

Lavas from volcanoes in the Mariana arc show a range of K 2 O-SiO 2 trends even though the volcanoes are all nearly the same distance above the Benioff Zone. The preferred petrologic explanation for the K 2 O-SiO 2 variations invokes differences in the degree of partial melting required to produce lavas for each volcano from a relatively homogeneous source material. The Mariana arc volcanoes appear to evolve chemically in a manner similar to oceanic volcanoes (e.g., Hawaii) with early and late alkalic stages dominated by an intermediate tholeiitic stage. If volcanoes in other arcs also undergo this evolution, many of the anomalies in K 2 O-depth relations in modern arcs could be explained.