Lying in wait: deep and shallow evolution of dacite beneath Volcán de Santa María, Guatemala
Lying in wait: deep and shallow evolution of dacite beneath Volcán de Santa María, Guatemala
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等待：危地马拉圣玛丽亚火山下方英安岩的深浅演化
DOI：
10.1144/sp385.2
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
Nick Rogers
中科院分区：
文献类型：
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作者：
Brad S. Singer;B. Jicha;J. Fournelle;B. Beard;C. Johnson;Kate E. Smith;S. E. Greene;Noriko T. Kita;J. Valley;M. Spicuzza;Nick Rogers
1902年10月，在危地马拉的Volcán de Santa María火山喷发了8.5 km3的英安质浮石和少量玄武质安山岩的碎屑和火山灰，剧烈地中断了25 kyr的休止期，该休止期是通过挤压8 km3的玄武质安山岩熔岩而形成的~ 75 kyr的火山锥生长。双氧化物和辉石质测温显示一个氧化（Ni-NiO+2测井单元）热分带岩浆体，其中1020℃SiO2含量为54 wt%的玄武岩安山岩和870℃SiO2含量为65 wt%的英安岩共存。英安岩浮石中的斜长石和玄武岩安山岩中的斜长石表现出非常相似的分带，其特征是向高钙长石反复漂移，镁、铁和锶的增加与吸收面有关，吸收面沿英安岩至流纹岩熔体包裹体被捕获。当SiO2从69 wt%增加到77 wt%时，熔体包裹体在K2O中略有增加，而H2O含量在5.2 wt%至1.4 wt%之间，随着K2O的增加而下降。这些观测结果表明，斜长石的结晶和高硅流纹岩残余熔体的演化主要发生在成分分带岩浆体从b> ~ 180mpa （b> ~ 5km）向地表的减压脱气过程中。浮石和矿渣中斜长石组成、分带和熔体包裹体组成的相似性表明，最初生长在较冷的英安岩中的晶体在英安岩和玄武岩安山岩岩浆混合或部分混合的过程中发生了交换。自1922年以来，与1902年的浮石相似的1亿立方千米的英安岩岩浆喷涌而出，在1902年的火山口形成了桑提亚圭托圆顶。微量元素和Sr-Nd-Pb-O和U-Th同位素数据表明，在地壳深部发生的锥状玄武岩安山岩熔岩记录过程中，岩壁的加热足以引起部分熔融和同化，其中包括至少50 kyr的地幔源玄武岩的几次再补给脉冲。这一过程的根本转变与25 ka时锥形构造的终止相吻合：1902年英安岩反映了斜长石+角闪孔+斜辉石+磁铁矿的分晶化，这些斜长石+角闪孔+斜辉石+磁铁矿来自于锥形构造后遗留下来的玄武岩安山岩岩浆，这些岩浆缓慢冷却，没有吸收额外的地壳。Sr-Nd-Pb比值差异较小，δ18O（WR）差异不大，（238U/230Th）活度比值差异较大，表明1902年的岩屑与英安岩并非同源岩浆，而该玄武岩安山岩可能是新近进入该体系的。1972年圣亚吉托英安岩熔岩的238U-230Th等时线为9.5±2.5 ka，显示出较深的、被封闭的硅岩浆体部分在1902年喷发之前至少在地壳中孵化了10 kyr。桑提亚圭托英安岩熔岩穹丘中的玄武岩安山岩包裹体被解释为1902年英安岩母源的锥形岩浆的改良残余物。补充材料：电子探针分析的玻璃标准，和SIMS数据的标准和熔融夹杂物的氢测量可在http://www.geolsoc.org.uk/SUP18606
Abstract The Plinian eruption in October 1902 of 8.5 km3of dacitic pumice and minor basaltic andesite scoria and ash at Volcán de Santa María, Guatemala violently interrupted a 25 kyr period of repose that had followed ∼75 kyr of cone-growth via extrusion of 8 km3 of basaltic andesite lava. Two-oxide and pyroxene thermometry reveal an oxidized (Ni-NiO+2 log units) and thermally-zoned magma body in which basaltic andesite with 54 wt% SiO2 at 1020 °C and dacite with 65 wt% SiO2 at 870 °C coexisted. Plagioclase in dacite pumice and basaltic andesite scoria shows remarkably similar zoning characterized by repeated excursions toward high anorthite and increases in Mg, Fe, and Sr associated with resorption surfaces along which dacitic to rhyolitic melt inclusions are trapped. The melt inclusions increase slightly in K2O as SiO2 increases from 69 to 77 wt%, whereas H2O contents between 5.2 and 1.4 wt% drop with increasing K2O. These observations suggest that crystallization of the plagioclase, and evolution of a high-silica rhyolitic residual melt, occurred mainly in the conduit as the compositionally-zoned magma body decompressed and degassed from >180 MPa, or >5 km depth, toward the surface. The similarity of plagioclase composition, zoning, and melt inclusion compositions in pumice and scoria suggests that crystals which grew initially in the cooler dacite, were exchanged between dacitic and basaltic andesite magma as the two magmas mingled and partially mixed en route to the surface. Since 1922>1 km3 of dacitic magma similar to the 1902 pumice has erupted effusively to form the Santiaguito dome complex in the 1902 eruption crater. Trace element and Sr–Nd–Pb–O and U–Th isotope data indicate that cone-forming basaltic andesite lavas record processes operating in the deep crust in which wallrock heating sufficient to induce partial melting and assimilation involved several pulses of recharging mantle-derived basalt over at least 50 kyr. A fundamental shift in process coincides with the termination of cone-building at 25 ka: the 1902 dacite reflects >40% fractional crystallization of plagioclase+amphibole+clinopyroxene+magnetite from ∼20 km3 of basaltic andesite magma left-over following cone-building that cooled slowly without assimilating additional crust. Small contrasts in Sr–Nd–Pb ratios, a modest contrast in δ18O(WR), and a large difference in the (238U/230Th) activity ratio between the 1902 scoria and dacite indicate that these two magmas are not consanguineous, rather this basaltic andesite is likely a recent arrival in the system. A glass–whole rock–magnetite–amphibole 238U–230Th isochron of 9.5±2.5 ka for a 1972 Santiaguito dacite lava suggests that deeper, occluded portions of the silicic magma body, not erupted in 1902, incubated in the crust for at least 10 kyr prior to the 1902 eruption. Basaltic andesite inclusions in the Santiaguito dacite lava domes are interpreted to be modified remnants of the cone-forming magma parental to the 1902 dacite. Supplementary material: Electron probe analyses of glass standards, and SIMS data from standards and melt inclusions for the hydrogen measurements are available at http://www.geolsoc.org.uk/SUP18606