Lying in wait: deep and shallow evolution of dacite beneath Volcán de Santa María, Guatemala

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.

1902年10月，危地马拉圣玛丽亚火山（Volcán de Santa María）发生体积达8.5 km³的英安质浮石（dacitic pumice）喷发，伴随少量玄武安山岩火山渣（basaltic andesite scoria）与火山灰，此次普林尼式喷发（Plinian eruption）猛烈打断了此前持续约25千年的喷发静止期——该静止期始于约75千年以来通过喷出8 km³玄武安山岩熔岩完成的火山锥生长阶段。二氧化物与辉石测温法（two-oxide and pyroxene thermometry）结果显示，存在一个氧化程度相对于Ni-NiO氧逸度缓冲剂高2个对数单位、且具热分带特征的岩浆体：其中SiO₂含量为54 wt%的玄武安山岩温度达1020 ℃，SiO₂含量为65 wt%的英安岩温度为870 ℃，二者稳定共存。英安质浮石与玄武安山岩火山渣中的斜长石（plagioclase）呈现出极为相似的分带特征：反复向高钙长石组分偏移，伴随Mg、Fe、Sr含量升高，且发育熔蚀面，英安质至流纹质的熔体包裹体（melt inclusions）即捕获于这些熔蚀面之上。随着SiO₂含量从69 wt%升至77 wt%，熔体包裹体中的K₂O含量略有升高；而H₂O含量介于5.2 wt%至1.4 wt%之间，并随K₂O含量升高而降低。上述观测结果表明，随着成分分带的岩浆体从大于180 MPa（即大于5 km深度）向地表减压脱气，斜长石的结晶以及高硅流纹质残余熔体的演化主要发生在火山通道中。浮石与火山渣中斜长石的成分、分带特征以及熔体包裹体成分均极为相似，这暗示最初在较冷的英安岩中结晶的晶体，在两种岩浆混合并部分混熔向地表运移的过程中，于英安质与玄武安山质岩浆之间发生了交换。自1922年以来，与1902年喷发的浮石成分相似的英安质岩浆已喷溢超过1 km³，形成了坐落于1902年喷发火山口内的圣地亚古多穹丘群（Santiaguito dome complex）。微量元素与Sr-Nd-Pb-O、U-Th同位素数据显示，形成火山锥的玄武安山岩熔岩记录了地壳深部的作用过程：在至少50千年的时间里，多次幔源玄武岩补给脉冲引发了足以诱发部分熔融与围岩同化作用的围岩加热。作用过程的根本性转变与25 ka时火山锥建造的终止相吻合：1902年的英安岩反映了，在火山锥建造阶段残留的约20 km³玄武安山岩岩浆中，发生了超过40%的斜长石+角闪石+单斜辉石+磁铁矿分离结晶作用，且该岩浆缓慢冷却，未同化额外的地壳物质。1902年的火山渣与英安岩在Sr-Nd-Pb比值上仅有微小差异，δ¹⁸O(WR)存在适度差异，而(²³⁸U/²³⁰Th)活度比则存在巨大差异，这表明这两种岩浆并非同源（consanguineous）；相反，该玄武安山岩可能是该岩浆系统中近期到达的岩浆。对1972年圣地亚古多英安岩熔岩进行的玻璃-全岩-磁铁矿-角闪石U-Th等时线（isochron）年龄为9.5±2.5 ka，这表明1902年未喷发的、封闭留存于长英质岩浆体深部的部分，在1902年喷发前已于地壳中孕育演化了至少10千年。圣地亚古多英安岩穹丘中的玄武安山岩包体，被解释为1902年英安岩母源火山锥形成岩浆的改造残余体。