59 research outputs found

    New constraints from Central Chile on the origins of enriched continental compositions in thick-crusted arc magmas

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    Magmas from continental arcs built on thick crust have elevated incompatible element abundances and ā€œenrichedā€ radiogenic isotope ratios compared to magmas erupted in island and continental arcs overlying thinner crust. The relative influence of the slab, mantle, and upper plate on this variability is heavily debated. The Andean Southern Volcanic Zone (SVZ; 33-46Ā° S) is an ideal setting to investigate the production of enriched continental arc compositions, because both crustal thickness and magma chemistry vary coherently along strike. However, the scarcity of primitive magmas in the thick-crusted northern SVZ has hindered previous regional studies. To better address the origin of enriched continental compositions, we investigate the geochemistry (major and trace element abundances, 87Sr/86Sr and 143Nd/144Nd ratios) of new mafic samples from Don Casimiro and Maipo volcanoes in Diamante-Maipo Caldera Complex of the northern SVZ. While evolved Diamante-Maipo samples show evidence for crustal assimilation, the trace element and isotopic enrichment of the most mafic samples cannot result from crustal processing, as no known regional or global basement lithologies are enriched in all of the necessary incompatible trace elements. Subduction erosion models similarly fail to account for the enriched isotopic and trace element signature of these samples. Instead, we suggest that the enrichment of northern SVZ magmas is derived from an enriched ambient mantle component (similar to EM1-type ocean island basalts), superimposed on a northward decline in melt extent. A substantial, but nearly uniform contribution of melts from subducting sediment and altered oceanic crust are required at all latitudes. The EM1-like enrichment may arise from recycling of metasomatized subcontinental lithospheric mantle (M-SCLM), as the isotopic trajectory of primitive rear-arc monogenetic cones trend towards the compositions of SCLM melts sampled across South America. Isotopic data from spatially distributed rear-arc centres demonstrate that the arc-parallel variations in the degree of EM1-type enrichment observed in arc-front samples are also present up to 600 km behind the trench in the rear-arc. Rear-arc trace element systematics require significant but variable quantities of slab melts to be transported to the mantle wedge at these large trench distances. Overall, we show that a unified model incorporating variable mantle enrichment, slab additions, and melt extents can account for along and acrossarc trends within the SVZ. The recognition that mantle enrichment plays a key role in the production of enriched continental compositions in the SVZ has important implications for our understanding of the chemical evolution of the Earth. If ambient mantle enrichment is not taken into account, petrogenetic models of evolved lavas may overestimate the role of crustal assimilation, which, in turn, may lead models of continental crust growth to overestimate the amount of continental material that has been recycled back into the mantle

    Explosive Activity on KÄ«lauea's Lower East Rift Zone Fueled by a Volatile-Rich, Dacitic Melt

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    Abstract: Magmas with matrix glass compositions ranging from basalt to dacite erupted from a series of 24 fissures in the first 2 weeks of the 2018 Lower East Rift Zone (LERZ) eruption of KÄ«lauea Volcano. Eruption styles ranged from low spattering and fountaining to strombolian activity. Major element trajectories in matrix glasses and melt inclusions hosted by olivine, pyroxene and plagioclase are consistent with variable amounts of fractional crystallization, with incompatible elements (e.g., Cl, F, and H2O) becoming enriched by 4ā€“5 times as melt MgO contents evolve from 6 to 0.5 wt%. The high viscosity and high H2O contents (āˆ¼2 wt%) of the dacitic melts erupting at Fissure 17 account for the explosive Strombolian behavior exhibited by this fissure, in contrast to the low fountaining and spattering observed at fissures erupting basaltic to basalticā€andesite melts. Saturation pressures calculated from melt inclusion CO2ā€H2O contents indicate that the magma reservoir(s) supplying these fissures was located at āˆ¼2ā€“3 km depth, which is in agreement with the depth of a dacitic magma body intercepted during drilling in 2005 (āˆ¼2.5 km) and a seismically imaged low Vp/Vs anomaly (āˆ¼2 km depth). Nb/Y ratios in erupted products are similar to lavas erupted between 1955 and 1960, indicating that melts were stored and underwent variable amounts of crystallization in the LERZ for >60 years before being remobilized by a dike intrusion in 2018. We demonstrate that extensive fractional crystallization generates viscous and volatileā€rich magma with potential for hazardous explosive eruptions, which may be lurking undetected at many ocean island volcanoes

    Spatial and Temporal Variations in SOā‚‚ and PMā‚‚.ā‚… Levels Around KÄ«lauea Volcano, Hawai'i During 2007ā€“2018

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    Among the hazards posed by volcanoes are the emissions of gases and particles that can affect air quality and damage agriculture and infrastructure. A recent intense episode of volcanic degassing associated with severe impacts on air quality accompanied the 2018 lower East Rift Zone (LERZ) eruption of KÄ«lauea volcano, Hawai'i. This resulted in a major increase in gas emission rates with respect to usual emission values for this volcano, along with a shift in the source of the dominant plume to a populated area on the lower flank of the volcano. This led to reduced air quality in downwind communities. We analyse open-access data from the permanent air quality monitoring networks operated by the Hawai'i Department of Health (HDOH) and National Park Service (NPS), and report on measurements of atmospheric sulfur dioxide (SO2) between 2007 and 2018 and PM2.5 (aerosol particulate matter with diameter <2.5 Ī¼m) between 2010 and 2018. Additional air quality data were collected through a community-operated network of low-cost PM2.5 sensors during the 2018 LERZ eruption. From 2007 to 2018 the two most significant escalations in KÄ«lauea's volcanic emissions were: the summit eruption that began in 2008 (KÄ«lauea emissions averaged 5ā€“6 kt/day SO2 from 2008 until summit activity decreased in May 2018) and the LERZ eruption in 2018 when SO2 emission rates reached a monthly average of 200 kt/day during June. In this paper we focus on characterizing the airborne pollutants arising from the 2018 LERZ eruption and the spatial distribution and severity of volcanic air pollution events across the Island of Hawai'i. The LERZ eruption caused the most frequent and severe exceedances of the Environmental Protection Agency (EPA) PM2.5 air quality threshold (35 Ī¼g/m3 as a daily average) in Hawai'i in the period 2010ā€“2018. In Kona, for example, the maximum 24-h-mean mass concentration of PM2.5 was recorded as 59 Ī¼g/m3 on the twenty-ninth of May 2018, which was one of eight recorded exceedances of the EPA air quality threshold during the 2018 LERZ eruption, where there had been no exceedances in the previous 8 years as measured by the HDOH and NPS networks. SO2 air pollution during the LERZ eruption was most severe in communities in the south and west of the island, as measured by selected HDOH and NPS stations in this study, with a maximum 24-h-mean mass concentration of 728 Ī¼g/m3 recorded in Ocean View (100 km west of the LERZ emission source) in May 2018. Data from the low-cost sensor network correlated well with data from the HDOH PM2.5 instruments, confirming that these low-cost sensors provide a robust means to augment reference-grade instrument networks

    Rapid metal pollutant deposition from the volcanic plume of KÄ«lauea, Hawaiā€™i

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    AbstractLong-lived basaltic volcanic eruptions are a globally important source of environmentally reactive, volatile metal pollutant elements such as selenium, cadmium and lead. The 2018 eruption of KÄ«lauea, Hawaiā€™i produced exceptionally high discharge of metal pollutants, and was an unprecedented opportunity to track them from vent to deposition. Here we show, through geochemical sampling of the plume that volatile metal pollutants were depleted in the plume up to 100 times faster than refractory species, such as magnesium and iron. We propose that this rapid wet deposition of complexes containing reactive and potentially toxic volatile metal pollutants may disproportionately impact localised areas close to the vent. We infer that the relationship between volatility and solubility is an important control on the atmospheric behaviour of elements. We suggest that assessment of hazards from volcanic emissions should account for heterogeneous plume depletion of metal pollutants.</jats:p
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