Composition of matrix glasses and melt inclusions of fallout tephra from ODP Hole 125-782A

We studied the systematics of Cl, F and H2O in Izu arc front volcanic rocks using basaltic through rhyolitic glass shards and melt inclusions (Izu glasses) from Oligocene to Quaternary distal fallout tephra. These glasses are low-K basalts to rhyolites that are equivalent to the Quaternary lavas of the Izu arc front (Izu VF). Most of the Izu glasses have Cl ~400-4000 ppm and F ~70-400 ppm (normal-group glasses). Rare andesitic melt inclusions (halogen-rich andesites; HRA) have very high abundances of Cl (~6600-8600 ppm) and F (~780-910 ppm), but their contents of incompatible large ion lithophile elements (LILE) are similar to the normal-group glasses. The preeruptive H2O of basalt to andesite melt inclusions in plagioclase is estimated to range from ~2 to ~10 wt% H2O. The Izu magmas should be undersaturated in H2O and the halogens at their preferred levels of crystallization in the middle to lower crust (~3 to ~11 kbar, ~820° to ~1200°C). A substantial portion of the original H2O is lost due to degassing during the final ascent to surface. By contrast, halogen loss is minor, except for loss of Cl from siliceous dacitic and rhyolitic compositions. The behavior of Cl, F and H2O in undegassed melts resembles the fluid mobile LILE (e.g.; K, Rb, Cs, Ba, U, Pb, Li). Most of the Cl (>99%), H2O (>95%) and F (>53%) in the Izu VF melts appear to originate from the subducting slab. At arc front depths, the slab fluid contains Cl = 0.94+/-0.25 wt%, F = 990+/-270 ppm and H2O = 25+/-7 wt%. If the subducting sediment and the altered basaltic crust were the only slab sources, then the subducted Cl appears to be almost entirely recycled at the Izu arc (~77-129%). Conversely, H2O (~13-22% recycled at arc) and F (~4-6% recycled) must be either lost during shallow subduction or retained in the slab to greater depths. If a seawater-impregnated serpentinite layer below the basaltic crust were an additional source of Cl and H2O, the calculated percentage of Cl and H2O recycled at arc would be lower. Extrapolating the Izu data to the total length of global arcs (~37000 km), the global arc outflux of fluid-recycled Cl and H2O at subduction zones amounts to Cl ~2.9-3.8 mln ton/yr and H2O ~70-100 mln ton/yr, respectively - comparable to previous estimates. Further, we obtain a first estimate of global arc outflux of fluid-recycled F of ~0.3-0.4 mln ton/yr. Despite the inherent uncertainties, our results support models suggesting that the slab becomes strongly depleted in Cl and H2O in subduction zones. In contrast, much of the subducted F appears to be returned to the deep mantle, implying efficient fractionation of Cl and H2O from F during the subduction process. However, if slab devolatilization produces slab fluids with high Cl/F (~9.5), slab melting will still produce components with low Cl/F ratios (~0.9), similar to those characteristic of the upper continental crust (Cl/F ~0.3-0.9).

本研究以渐新世至第四纪远端降落火山碎屑中的玄武质至流纹质玻璃碎屑与熔体包裹体（melt inclusions，伊豆玻璃，Izu glasses）为研究对象，探究了伊豆弧前带火山岩中氯（Cl）、氟（F）及水（H₂O）的地球化学系统特征。该类玻璃对应低钾玄武岩至流纹岩，与伊豆弧前火山岩带（Izu VF）的第四纪熔岩一致。绝大多数伊豆玻璃的氯含量为400~4000 ppm，氟含量为70~400 ppm，属于正常组玻璃（normal-group glasses）。罕见的安山质熔体包裹体（富卤素安山岩，halogen-rich andesites, HRA）的氯（~6600~8600 ppm）与氟（~780~910 ppm）含量极高，但其不相容大离子亲石元素（large ion lithophile elements, LILE）的含量与正常组玻璃相近。斜长石中玄武质至安山质熔体包裹体的喷发前水含量估算为~2%~10%（重量百分比）。伊豆岩浆在中-下地壳（~3~11千巴，~820℃~1200℃）的最优结晶条件下，水与卤素均处于不饱和状态。在最终上升至地表的过程中，原始水的大部分因脱气作用而流失。相较之下，卤素的流失量极少，仅硅质英安质与流纹质组分中的氯会发生流失。未脱气熔体中氯、氟与水的地球化学行为与流体活动性大离子亲石元素（LILE，如钾K、铷Rb、铯Cs、钡Ba、铀U、铅Pb、锂Li）相似。伊豆弧前火山岩带熔体中，绝大多数氯（>99%）、水（>95%）与氟（>53%）均源自俯冲板片。在弧前深度下，俯冲板片流体的氯含量为0.94±0.25 wt%，氟含量为990±270 ppm，水含量为25±7 wt%。若仅以俯冲沉积物与蚀变玄武质地壳作为板片的物质来源，则俯冲的氯在伊豆弧几乎完全得到循环（循环率约77%~129%）。与之相反，水（弧内循环率约13%~22%）与氟（循环率约4%~6%）要么在浅俯冲过程中流失，要么被保留在板片中并随其进入更深的地幔深度。若将玄武质地壳下方的海水浸染型蛇纹岩作为氯与水的额外来源，则弧内循环的氯与水的计算占比会更低。将伊豆数据外推至全球弧系总长度（约37000 km），则俯冲带内全球弧系流体循环的氯与水输出通量分别为~2.9~3.8百万吨/年与~70~100百万吨/年，这一结果与此前的估算值相当。此外，本研究首次估算了全球弧系流体循环氟的输出通量约为0.3~0.4百万吨/年。尽管存在固有不确定性，本研究结果仍支持俯冲带内板片的氯与水发生强烈亏损的相关模型。与之相反，大部分俯冲的氟似乎会被返还至深部地幔，这表明俯冲过程中氯与水会从氟中发生高效分馏。但若板片脱挥发分作用产生的板片流体具有高氯氟比（~9.5），则板片熔融仍会形成低氯氟比（~0.9）的组分，这与上地壳的特征氯氟比（0.3~0.9）相近。