ALBERT

Description: Disequilibrium in the short-lived U-series isotopic system occurs during partial melting, differentiation, and volatile transport; therefore, the U-series decay chain is a unique tool to examine the magnitude and timing of magmatic processes. However, our understanding of U-series fractionation in subduction zones is incomplete. We use published data from volcanoes around the world and new data from volcanic systems in the Kamchatka Arc (Bezymianny, Klyuchevskoy, and Karymsky) to examine two theories regarding the behavior of U-series nuclides: 1) that Th-excess in arc magmas, (230Th)/(238U) 〉1, is a function of arc-thickness/garnet in the melting region, or magnetite fractionation and 2) that 210Pb deficits, (210Pb)/(226Ra) 〈1, are the result of continuous magma degassing. Our results show that neither of these theories explains the complete dataset produced by the U-series community. Th-excess is generally observed in MORB and attributed to decompression melting; however, global data also record Th-excess in fluid-fluxed subduction zones. Limited experimental data suggest preferential U transport over Th in subduction zone fluids and, therefore, U-excess rather than Th-excess should exist in arcs. The common explanation for arc Th-excess is melt interaction with thick continental crust where phases such as garnet retain high U/Th in crystalline residues. We record Th-excess at Bezymianny, (230Th)/(238U) from 1.04-1.06 and Klyuchevskoy, (230Th)/(238U) from 1.01 and 1.08, volcanoes, which are located on relatively thin (~35 km), primitive crust. These magmas have low Sr/Y (15.5-19.9) that preclude a significant influence of garnet. In addition, LA-ICP-MS measurements of in-situ U and Th mineral-melt partitioning on erupted mineral phases (plagioclase, pyroxene, Fe-Ti oxides, apatite) suggest that U-series disequilibria are transparent to shallow crustal processing. Th-excess at Klyuchevskoy inversely correlates with Ba/Th, Sr/Th, Dy/Yb, and Ce/Pb. We suggest that Th-excess is a function of decompression mantle melting beneath arcs and/or phase fractionation by lower crustal mineral assemblages. 210Pb deficits are interpreted as the result of 222Rn gas loss in magmatic systems. With 222Rn loss, the daughter product, 210Pb, cannot grow into magma. Two published models (Gauthier and Condomines 1999 and Condomines et al. 2010) simulate this process where the magnitude of 210Pb deficit increases with the duration of degassing. We record 210Pb deficits at Bezymianny and Karymsky volcanoes (0.846-0.966). However, for known eruption intervals, these 210Pb deficits are larger than current models predict. If physical constraints for gas behavior in magma are added to these models, the problem is magnified. Our results highlight a need for studies quantifying the fractionation of U-series nuclides among all phases (crystals, melts, and volatiles) relevant to subduction zones. Primitive, thin volcanic arcs that have a well-characterized volcanic history through time, similar to the Kamchatka Arc, are ideal places to begin.

Description: The formation of the Pannonian basin by major lithospheric extension during the early to mid-Miocene was accompanied by intensive volcanic activity. Among it, explosive volcanism fed by silicic (dacite to rhyolite) magmas is considered to have been the largest (accumulated tephra volume exceeded 4000 km3) in Europe in the last 20 Myr. Zircon U-Pb geochronology indicates that this occurred between 18.2 and 14.4 Ma (Bükkalja volcanic field; BVF) and involved at least four major eruption episodes. Distal ash materials related to these volcanic eruptions are recognized around the Pannonian basin and even farther, over thousand kilometre away, based on zircon and glass shard data. Another major silicic volcanic activity developed eastward (Tokaj Mts., TM) from 13.2 to 11.5 Ma. In this case, explosive and effusive volcanic products were equally preserved. Explosive events in the TM are also clearly distinguished by zircon geochronology as well as zircon and glass shard trace element compositions. The diverse zircon and glass geochemical data from these two areas suggest that reservoirs with distinct silicic magmas developed in the shallow continental crust for about 7 Myr. The epsilon-Hf values of zircon crystals are in a similar range from -10 to +2 and show a temporal increase, implying increasing role of mantle-derived magmas. In the BVF, an abrupt change in the epsilon-Hf values occurred after 16.2 Ma, suggesting that by this time the crust, and the lithospheric mantle was considerably thinned, while in the TM, this occurred later and in two smaller steps. MELTS modelling results, zircon and glass trace element data suggest different magma evolution in the BVF and TM as shown particularly by the Eu and Ce anomalies. In the TM, silicic magmas evolved under more reduced and drier magmatic environments. In contrast, wetter, and more oxidised conditions, with the presence of amphibole in the mineral assemblage, dominated in some of the BVF silicic magmas. The silicic magmatism had a major influence on the state of the continental crust due to the contemporaneous presence of large volume silicic magma reservoirs in the crust, and could have contributed to the crustal stretching beneath the Pannonian basin.

Description: The Mid-Miocene volcanism in the northern Pannonian basin has a peculiar nature. The first volcanic products (metaluminous andesite to peraluminous dacite-rhyodacite) are unique since the dacites and rhyodacites contain almandine garnet, which rarely occurs in volcanic rocks. The primary almandine crystals have a moderate Ca content (CaO=4.5–8.1 wt%), although low-Ca (CaO〈3 wt%) almandine derived from lower crustal metapelites is also found. Trace element variations in garnet are consistent with progressive crystal fractionation at high-pressure (〉700 MPa). Zircon U-Pb geochronology results suggest eruption ages between 15.6 and 15.0 Ma. Remarkably, this volcanism has no relationship with coeval subduction, but it occurred in response to continental rifting. The nature of these magmas was further explored with the help of trace element and Hf-isotope composition of zircon. Zircon in the peraluminous rhyodacites has a unique compositional feature, an elevated Al content (Al=10–15 ppm) not found in other silicic volcanic rocks in this region. Notably, zircon in the coeval metaluminous volcanic rocks has also relatively high Al-content (5–10 ppm). Zircon crystals in the peraluminous rocks have relatively high Dy/Yb and low Th/U ratios consistent with early garnet crystallization. Strong depletion of heavy rare earth elements is observed also in the glass shards. The epsilon Hf values of zircon crystals show a variation from -4 to +2, transitional values compared to the Miocene volcanic rocks of the region and suggest mixing of mantle-derived and crustal-derived magmas. This is consistent with petrogenetic modelling calculations based on bulk rock isotope and trace element data, implying interaction between mafic magmas derived from enriched lithospheric mantle and metasedimentary lower crust. Thus, the peraluminous garnet-bearing magmas in the northern Pannonian basin are hybrids, and are not of pure anatectic origin, as being proposed for compositionally similar magmas, such as in SE Spain. Preservation of high-pressure garnet can be explained by fast magma ascent enhanced by crustal extension. In contrast, some magmas stalled at shallow crust. They also contain Al-bearing zircon crystals suggesting peraluminous character of their parental magmas, whereas garnet phenocrysts were likely dissolved at low pressure (as also reflected in the glass composition).