کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6428210 | 1634727 | 2015 | 13 صفحه PDF | دانلود رایگان |
- Data set of Br and Cl emissions from 29 large CAVA eruptions (VEI > 5).
- Melt inclusions are strongly enriched in chlorine and bromine compared to their respective matrix glasses.
- Fluid partitioning is 4 to 68 times more efficient for Br than for Cl.
- Subducted calcareous sediments are the major control on the arc-magmatic bromine contents.
- Average CAVA eruption would add 368% EESC of recent annual loading to stratosphere.
Large explosive volcanic eruptions inject gases, aerosols, and fine ashes into the stratosphere, potentially influencing climate and atmosphere composition on a global scale. Although the potential climate effect of chlorine (Cl) and bromine (Br) injections into the stratosphere is known, the global mass fluxes are poorly constrained. In this study we focus on the magmatic degassing systematics and budgets of Br and Cl, and on constraining the major sources of Br in a subduction setting. We therefore present a regional time series of Br and Cl emissions from 29 highly explosive eruptions throughout the Central American Volcanic Arc (CAVA), covering the last 200 ka, and a range of magmatic compositions and eruption magnitudes. We have measured Br and Cl in matrix glasses and melt inclusions using synchrotron radiation micro X-ray fluorescence spectrometry (SR micro-XRF) and electron microprobe, respectively. Melt inclusions of the CAVA tephras generally have higher Br (0.9 to 17.9 ppm) and Cl (770 to 3800 ppm) contents than the matrix glasses (0.39 to 1.5 ppmâBr, 600 to 2800 ppmâCl). Moreover, the difference between maximum and minimum concentrations observed in melt inclusions of a given sample ranges between 9 and 90% of the maximum observed concentration for Br, and between 2 and 40% for Cl. Such intra-sample variations arise from variable pre-eruptive degassing of these halogens into a magmatic fluid phase. The relative loss of Br from the melt is 4 to 68 times higher than that of Cl.The masses of Br (2-1100 kt) and Cl (0.1 to 800 Mt) emitted by the eruptions generate instantaneous additions to the stratosphere potentially amounting to â¼6-5600% of the present-day stratospheric annual global loading of Equivalent Effective Stratospheric Chlorine. As the size of the stratospheric impact is primarily a function of eruption magnitude, we use magnitude-frequency relationships to estimate that eruptions adding â¼10% to resident EESC loading would occur every <40 years while every â¼200 years an eruption would double the EESC loading.Comparing the variations in Br and Cl concentrations and particularly minimum Cl/Br ratios in melt inclusions with geochemical trace-element proxies (e.g. U/La, Ba/Th) and lead-isotope compositions, which change along the arc in response to changing subduction conditions, we suggest that subducted calcareous sediment is a major source of magmatic Br but also infer an important role of fluids expelled from serpentinized subducted mantle. Extrapolation of CAVA volcanic Br emissions to the global subduction system thus needs to consider variations in the nature of subducted lithologies.
Journal: Earth and Planetary Science Letters - Volume 429, 1 November 2015, Pages 234-246