Abstract The Plinian eruption in October 1902 of 8.5 km 3 of 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 km 3 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% SiO 2 at 1020 °C and dacite with 65 wt% SiO 2 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 K 2 O as SiO 2 increases from 69 to 77 wt%, whereas H 2 O contents between 5.2 and 1.4 wt% drop with increasing K 2 O. 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 km 3 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 km 3 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 δ 18 O(WR), and a large difference in the ( 238 U/ 230 Th) 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 238 U– 230 Th 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

Abstract The Middle Ordovician St. Peter Sandstone and Glenwood Formation of the Michigan basin are composed of alternating intervals of quartz sandstone, micritic carbonate, and an occasional thin shale. They contain abnormally pressured compartments in the deepest portion of the basin. Some of these pressure compartments are gas reservoirs and are bounded, above and below, by diagenetically banded sandstone and/or carbonate sedimentary units. Diagenetically banded sandstones are dominated by bands of quartz cement that formed as a result of chemical compaction, quartz dissolution, and quartz precipitation during burial. Petrographic and geochemical data suggest that quartz overgrowths precipitated from fluids with a slight meteoric component. Dolomite, the second most abundant authigenic mineral in diagenetic bands and elsewhere in the St. Peter and Glenwood, postdated quartz overgrowths and precipitated from hypersaline fluids at high temperatures. Values of 5 13 C from the dolomite indicate that the carbon was partially derived from the maturation of organic matter, and the carbon isotopic composition appears to be stratigraphically controlled. Bands of dolomite and quartz cement occur in horizontal and cross-bedded siliciclastic lithofacies that contain planar depositional discontinuities, such as grain size laminations. Original porosity in the St. Peter Sandstone and Glenwood Formation was relatively homogeneous within a given lithofacies. Diagenesis, especially the development of diagenetic bands, resulted in heterogeneous porosity distribution. The porosity was reduced to 0 to 3% in the tightly cemented bands (quartz and dolomite) and in intervals that experienced intense intergranular pressure solution. The most significant porosity modification occurred in the deep burial environment. Depositional controls on diagenesis include bedding style, degree of bioturbation, and original mineralogical composition of the sediments. The correlation between depositional facies and diagenetic banding may allow regions of low porosity and permeability to be predicted within a sequence stratigraphic framework in deeply buried sandstones.