Sean Scott

U-series and 40Ar/39Ar ages of Holocene volcanic rocks at Changbaishan volcano, China

Accurately dating Holocene volcanic rocks poses many challenges but is critical to assessing magmatic evolution and hazard risks at highly active volcanoes. Here we use Ra/Th and 40 Ar/ 39 Ar geochronology to date very young eruptions at Changbaishan volcano, northeastern China, a recently active stratovolcano responsible for one of the most voluminous eruptions in the past ∼2000 yr. For Holocene eruptions, 40 Ar/ 39 Ar ages are consistently older than those of both independently determined ages and maximum Ra/Th ages. Overall, Ra/Th ages are most consistent with historical accounts and indicate inaccurate 40 Ar/ 39 Ar ages that are due to extraneous argon in various forms. Ra/Th geochronology also confirms the highly active nature of Changbaishan and supports the continued presence of trachytic magma residing under the volcano that appeared more than ∼1100 yr ago.

Geodynamic implications of crustal lithologies from the southeast Mariana forearc

The deep submergence research vehicle Shinkai 6500, diving on the Challenger segment of the Mariana forearc, encountered a superstructure of nascent arc crust atop a younger mantle with entrained fragments of metamorphosed crust. A plutonic block from this crust collected at 4900 m depth has a crystallization age of 46.1 Ma and mixed boninitic-arc tholeiitic geochemical signatures. A hornblende garnetite and two epidote amphibolites were retrieved from depths between 5938 m and 6277 m in an area dominated by peridotite. The garnetite appears to represent a crystal cumulate after melting of deep arc crust, whereas the amphibolites are compositionally similar to enriched mid-ocean ridge basalt (MORB). The initial isotopic compositions of these crustal fragments are akin to those of Eocene to Cretaceous terranes along the periphery of the Philippine plate. The garnetite achieved pressures of 1.2 GPa or higher and temperatures above 850 °C and thus could represent a fragment of the delaminated root of one of these terranes. This sample has coeval Sm-Nd, Lu-Hf, and 40Ar-39Ar ages indicating rapid ascent and cooling at 25 Ma, perhaps in association with rifting of the Kyushu-Palau arc. Peak P-T conditions were lower for the amphibolites, and their presence on the ocean floor near the garnetite might have resulted from mass wasting or normal faulting. The presence of relatively fusible crustal blocks in the circulating mantle could have contributed to the isotopic similarity of Mariana arc and backarc lavas with Indian Ocean MORB.

Spreading Dynamics of an Intermediate Ridge: Insights from U-series Disequilibria, Endeavour Segment, Juan de Fuca Ridge

We present new U-series disequilibria results for 37 basalts collected from the flanks and axial valley of the Endeavour segment of the Juan de Fuca Ridge, making Endeavour one of the most wellcharacterized spreading ridges along the global ridge system. Detailed studies of Endeavour basalts provide a geological framework for the tectonic, magmatic, and hydrothermal evolution of the ridge, and the geochemical diversity of basalts that result from short spatialand temporal-scale variations in mantle sources and melting processes. Because the geologic and geochemical context of these basalts is well known, new uranium series disequilibria and model ages place additional constraints on the recent magmatic evolution of the ridge. While Endeavour basalts vary widely and include depleted, normal, transitional, and enriched compositions, relatively little variation is observed in Th/ U ratios or U–Th disequilibria in near zero-age basalts. The largest variation in both exists in lavas erupted most recently in the axial valley (the ‘Graben Trend’) with overall higher Th excesses. Some basalts with lower Th excesses collected from pillow mounds on the flanks of the ridge (the ‘Inflated Trend’) have aged significantly since eruption and document the episodic nature of volcanic and tectonic regimes at Endeavour. Uranium-series melting models using two lithologies (peridotite and eclogite) can account for U–Th systematics in near zero-age basalts. These models indicate that mixing of diverse, deeply-generated, enriched melts and homogeneous, shallowlygenerated, depleted melts can account for the geochemistry of Endeavour basalts. In addition, decreasing porosity and melting rate of the eclogite component is primarily responsible for chemical and physical changes occurring at the Endeavour segment through time.

Evidence of Young Crystal Ages in Andesitic Magmas from a Hyperactive Arc Volcano—Volcan de Colima, Mexico

The residence time of crystals in magmatic systems is an essential parameter to describe the dynamics of these systems and to evaluate the temporal representativeness of the mineral data used to document the physical conditions in the magmas. Uranium-series disequilibria in mineral separates from young volcanic rocks with a known eruption date provide unique insights into these residence times. We present U–Th–Ra measurements for plagioclase separates and groundmass from magmas erupted at Volc an de Colima, Mexico in 2004 and 2007. The (Th/Th) activity ratios in plagioclase show limited variations within the range measured in groundmasses and previously documented for whole-rocks. The (Th/Th) and (U/Th) activity ratios in plagioclase are predominantly controlled by glass present as inclusions in the crystals or adhering to their rims, even in high-purity crystal separates with less than 2% glass. Variations in these activity ratios are best explained by crustal assimilation during crystallization and do not require ageing of the crystals. One plagioclase separate with very limited contamination by glass impurities has a demonstrable zero Th–U crystal age. Precise Ra–Th model ages are difficult to obtain owing to the strong influences of uncertainties in the partition coefficients, the temperature of crystallization and the correction for glass impurities in crystal separates. Nevertheless, if these uncertainties are taken into account, Ra–Th model ages in the range 0–2000 years are obtained for the plagioclase separates. More complex models elaborated on the basis of the compositional zoning observed in plagioclase phenocrysts suggest that the cores of the crystals are at most 8500 years old. Overall, these results indicate that the plagioclase phenocrysts are relatively young and are not sourced from a long-lived near-solidus crystal mush as is often suggested for arc volcanoes. A more dynamic system in which crystallization predominantly occurs in response to degassing during ascent of the melts is conceivable at Volc an de Colima.