Abstract The mechanism controlling the volcanic eruption frequency is yet poorly understood. The key to a better understanding this mechanism comes from integrating accurate geochronology with numerical models. In many silicic volcanic systems, the eruption frequency is studied for short timescales of <500 kyr. Here, we combine two published numerical models (Caricchi et al., 2014, https://doi.org/10.1038/ngeo2041 ; Degruyter & Huber, 2014, https://doi.org/10.1016/j.epsl.2014.06.047 ) to improve our understanding of the eruption frequency in a long‐lived (>3 Ma) felsic magmatic system, the Milos volcanic field. We use the published Milos geochronological data as input into the Caricchi et al. (2014, https://doi.org/10.1038/ngeo2041 ) model to derive average magma supply rates ( Q av ) and eruptible volumes. These two parameters are then used in the Degruyter and Huber (2014, https://doi.org/10.1016/j.epsl.2014.06.047 ) model to compute the chamber growth rate ( G mc ) for the Milos magmatic system. The results of the Caricchi et al. (2014, https://doi.org/10.1038/ngeo2041 ) model indicate that the time intervals between magma pulses into the subvolcanic reservoir ( t i ) and Q av are the key parameters controlling the eruption frequency. During the time intervals of 1.48–1.04 and 0.97–0.63 Ma the t i is longer than 1000 years and the volcanic quiescence periods are longer than 350 kyr. Furthermore, these periods are characterized by low values for Q av (≤0.001 km 3 · yr −1 ) and G mc (<0.0001 km 3 · yr −1 ). In contrast, during the time intervals of 2.0–1.5 and 0.60–0.06 Ma, the t i is shorter (<0.5 kyr) and the values for Q av (>0.001 km 3 · yr −1 ) are higher corresponding to frequent eruptions.
A large portion of Earth's crust is formed at convergent plate boundaries that are accompanied by the subduction of sediments that can contain evolved crust-derived detritus. Partial melting of such sediments can strongly affect the trace element and isotope geochemistry of new arc rocks. Here, we present high-precision Lu–Hf–Zr concentration data and Hf isotope compositions for a series of volcanic rocks from the Banda arc, East Indonesia, to quantify the transfer of subducted Hf to the Banda arc crust and address the influence of recycled Hf in subduction zones on the Hf isotope systematics of arc rocks. Along-arc from NE to SW, the 176Hf/177Hf decreases from 0.28314 to 0.28268 ranging from predominantly mantle-like ratios towards more crustal signatures. Hf–Nd isotope co-variations require low Nd/Hf in the arc magma source, inconsistent with fluid addition to the arc melts, but in agreement with the involvement of partial sediment melts. The systematic decrease in Hf–Nd isotopes infers a NE–SW along-arc increase in the involvement of subducted continental material (SCM) in the arc magma source, consistent with δ18O and Nd–Sr–Pb isotope constraints. The along-arc decrease in Hf isotopes coupled with increasing Zr/Hf (from 31.9 to 36.1) and decreasing Lu/Hf suggests that the newly produced arc lavas contain crustal-derived Hf as a result of partial melting of SCM associated with the breakdown of zircon. Based on recent experimental estimates of temperatures required to achieve zircon breakdown, we infer that slab surface temperatures in the Banda arc region need to be as high as 925 °C. As a consequence of the inheritance of non-radiogenic Hf from SCM, the juvenile Banda arc crust exhibits Hf isotope model ages biased by hundreds of millions of years. We conclude that crust-formation ages derived from Hf isotope ratios of convergent margin rocks and their constituent minerals (such as zircon) can be geologically meaningless mixing ages, even when they readily preserve low δ18O values (i.e., < 6.5). These findings are discussed with respect to the inferred origin of Hadaean zircons at convergent plate boundaries, which appear consistent with an origin in a convergent margin setting.
Abstract. The island of Patmos, in the eastern Aegean Sea, consists almost entirely of late Miocene to Pliocene volcanic rocks. The magmatism in the Aegean is governed by subduction of the African plate below the Eurasian plate, back-arc extension, slab roll-back, slab edge processes and westward extrusion of central Anatolia to the west along the Northern Anatolian Fault into the Aegean domain, The evolution of the Aegean basin is that of a back arc setting, with a southerly trend in the locus of both convergent tectonics, and back arc stretching, allowing intermittent upwelling of arc, lithospheric and asthenospheric magmas. Here, we present new 40Ar/39Ar age data for Patmos and the nearby small island of Chilomodi to place this volcanism in a new high resolution geochronological framework. High resolution geochronology provides a key to understanding the mechanisms of both the tectonic and magmatic processes that cause the extrusion of magma locally, and sheds light on the tectonic evolution of the larger region of the back-arc basin as a whole. The volcanic series on Patmos is alkalic, consistent with a back arc extensional setting and ranges from trachybasalt, to phonolites, trachytes and rhyolites, with SiO2 ranging from 51.6–80.5 wt.% and K2O from 2–11.8 wt.% with extrusion ages ranging from 6.59 ± 0.14 Ma–5.17 ± 0.11 Ma. Volcanism on Patmos and adjacent Chilomodi can be understood by a combination of mantle and crustal tectonic processes including influence of transform faults and rotational crustal forces that also caused the opening of the south Aegean basin due to roll back of the subducting slab south of Crete.
Sample preparation methods for MC-ICP-MS silicon-isotope measurements often involve a cation-exchange purification step. A previous study has argued that this would suffice for geological materials, as the occasional enrichment of anionic species would not compromise silicon-isotope analysis. Here we report significant offsets in MC-ICP-MS silicon-isotope measurements induced by the presence of sulfur. We show that offsets in δ30Si become significant above SO4/Si ratios (wt.) of 0.02, reaching up to ca. +1.4‰ at SO4/Si ratios above ∼0.2. This finding is particularly relevant for studies of sulfur-rich waters and silicified rocks where alteration was accompanied by sulfur-enrichments. We propose an additional purification step to remove sulfur from solid sample material.
Abstract Motuhora (Whale Island) lies c. 11 km offshore from Whakatane in the Bay of Plenty, New Zealand, and comprises tuffaceous marine sediments of the Camp Bay and Motuhora Formations separated by lavas, volcanic breccias, and slope‐wash deposits of the Whale Volcanics. Whale Volcanics can be divided into East Dome, Central Dome Complex, and Pa Hill Dome. East Dome is a flow banded, chaotically jointed dacite that is probably extrusive. Central Dome comprises lava flows, and extensive volcanic breccias and tuffs which thicken into a local depression to the north of the central high, suggesting rapid growth and erosion of the dome. Pa Hill Dome is largely intrusive into Camp Bay Formation, although blocks of Pa Hill dacite in an upper slope‐wash cobble bed suggest it was partially extrusive. The lavas are porphyritic with phenocrysts of plagioclase, orthopyroxene, and titanomagnetite with subordinate clinopyroxene and amphibole (particularly in Pa Hill Dome), and rare biotite. Rounded or broken and embayed quartz crystals are found in the Central Dome Complex and Pa Hill domes. Magmatic xenoliths are common in all lavas. Chemically the lavas are medium‐K, calc‐alkaline andesites and dacites, and show relative LILE enrichment and HFSE depletion typical of arc volcanics. Isotopically, samples tend to have more radiogenic Sr and less radiogenic Nd than volcanics from neighbouring White Island. It is likely that Motuhora lavas were formed by a multi‐stage process involving partial melting of N‐MORB‐type mantle that had been fluxed by fluids rich in incompatible elements derived from the dehydrating downgoing slab and followed by crystal fractionation of the magma. As the magma rose through the lower continental crust it was contaminated, probably by Torlesse metasediment. Petrographic textures and mineral chemistry indicate that magma mixing, while in an upper crustal magma chamber, is the norm for Motuhora lavas.
Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.
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