We report palaeomagnetic and 40Ar/39Ar dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by ~50 Myr: one set lies within the Cretaceous normal superchron at 112–115 Ma and a second at 59–70 Ma spanning the Cretaceous‐Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex, multipolar field. However, the time‐dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

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Geophysical imaging of the Yellowstone supervolcano shows a broad zone of partial melt interrupted by an amagmatic gap at depths of 15-20 km. We reproduce this structure through a series of regional-scale magmatic-thermomechanical forward models which assume that magmatic dikes stall at rheologic discontinuities in the crust. We find that basaltic magmas accumulate at the Moho and at the brittle-ductile transition, which naturally forms at depths of 5-10 km. This leads to the development of a 10-to 15-km thick midcrustal sill complex with a top at a depth of approximately 10 km, consistent with geophysical observations of the pre-Yellowstone hot spot track. We show a linear relationship between melting rates in the mantle and rhyolite eruption rates along the hot spot track. Finally, melt production rates from our models suggest that the Yellowstone plume is~175°C hotter than the surrounding mantle and that the thickness of the overlying lithosphere is~80 km. Plain Language SummaryWe present a series of supercomputer models which we use to investigate the origins of the two-level magmatic system revealed by recent geophysical observations of the Yellowstone supervolcano. We show that the distribution of melt which matches these observations arises when we assume that rising magmas preferentially accumulate at depths where there are strong contrasts in the ratio of melt overpressure to the effective viscosity of the surrounding crust. Melt accumulates at the major rock strength discontinuities which occur at the base of the crust and at the brittle-ductile transition which forms above the developing magmatic system at approximately 10-km depth. This second boundary captures the considerable majority of the melt and produces a basaltic sill complex which resides between depths of 10 and 25 km and which provides heat which melts the surrounding crust. This sill complex cools and solidifies and separates the partially molten crust above and below it into the two magmatic systems seen in the geophysical images. Finally, we are able to constrain the temperature of the Yellowstone mantle plume to be 175°C hotter than the surrounding mantle and the thickness of the overlying lithosphere to be approximately 80 km. Model SetupWe performed several dozen 2-D numerical experiments exploring a broad parameter space using a 1,000 × 300-km finite difference grid with a resolution of 2 km, with 16 or more Lagrangian markers per cell, a time step of 5 kyr, and a total duration of 7-8 Myr. We are interested in the behavior of the crustal magmatic system which develops over a relatively stable mantle plume tail, rather than over the plume head which is COLÓN ET AL. 3873

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the degree and extent of crustal hydrothermal alteration related to the eruption of large igneous provinces is poorly known and not easily recognizable in the field. We here report a new δ 18 o dataset for dikes and lavas from the Columbia River Basalt Group (16-15 Ma) in the western USA, and document that dikes on average are 1-2‰ more depleted in δ 18 O than basalt flows. We show that this observation is best explained with the involvement of heated meteoric waters during their cooling in the crust. the largest 6-8‰ depletion is found around and inside a 10 m-thick feeder dike that intruded the 125 Ma Wallowa tonalitic batholith. This dike likely operated as a magma conduit for 4-7 years, based on the extent of heating and melting its host rocks. We show that this dike also created a hydrothermal system around its contacts extending up to 100 m into the surrounding bedrock. A model that considers (a) hydrothermal circulation around the dike, (b) magma flow and (c) oxygen isotope exchange rates, suggests that the hydrothermal system operated for ~150 years after the cessation of magma flow. In agreement with a previously published (U-Th)/He thermochronology profile, our model shows that rocks 100 m away from such a dike can be hydrothermally altered. Collectively, our sample set is the first documentation of the widespread hydrothermal alteration of the shallow crust caused by the intrusion of dikes and sills of the columbia River Basalt province. it is estimated that heating and hydrothermal alteration of sediments rich in organic matter and carbonates around the dikes and sills releases 18 Gt of greenhouse gases (cH 4 and co 2). Furthermore, hydrothermal δ 18 o depletion of rocks around dikes covers 500-600 km 3 , which, when scaled to the total CRB province constitutes 31,000 km 3 of low-δ 18 o rocks. these volumes of crust depleted in δ 18 O are sufficient to explain the abundant low-δ 18 o magmas in eastern oregon and western idaho. this work also demonstrates that the width and magnitude of δ 18 O depletion around dikes can identify them as feeders. Given this, we here interpret Paleoproterozoic dikes in Karelia with the world's lowest δ 18 o depletions (−27.8‰) as feeders to the coeval large igneous province aged 2.2-2.4 Ga that operated under the Snowball Earth glaciation conditions.

The Kamchatka Peninsula of eastern Russia is currently one of the most volcanically active areas on Earth where a combination of >8 cm/yr subduction convergence rate and thick continental crust generates large silicic magma chambers, reflected by abundant large calderas and caldera complexes. This study examines the largest center of silicic 4-0.5 Ma Karymshina Volcanic Complex, which includes the 25 × 15 km Karymshina caldera, the largest in Kamchatka. A series of rhyolitic tuff eruptions at 4 Ma were followed by the main eruption at 1.78 Ma and produced an estimated 800 km 3 of rhyolitic ignimbrites followed by high-silica rhyolitic post-caldera extrusions. The postcaldera domes trace the 1.78 Ma right fracture and form a continuous compositional series with ignimbrites. We here present results of a geologic, petrologic, and isotopic study of the Karymshina eruptive complex, and present new Ar-Ar ages, and isotopic values of rocks for the oldest pre-1.78 Ma caldera ignimbrites and intrusions, which include a diversity of compositions from basalts to rhyolites. Temporal trends in δ 18 O, 87 Sr/ 86 Sr, and 144 Nd/ 143 Nd indicate values comparable to neighboring volcanoes, increase in homogeneity, and temporal increase in mantle-derived Sr and Nd with increasing differentiation over the last 4 million years. Data are consistent with a batholithic scale magma chamber formed by primarily fractional crystallization of mantle derived composition and assimilation of Cretaceous and younger crust, driven by basaltic volcanism and mantle delaminations. All rocks have 35-45% quartz, plagioclase, biotite, and amphibole phenocrysts. Rhyolite-MELTS crystallization models favor shallow (2 kbar) differentiation conditions and varying quantities of assimilated amphibolite partial melt and hydrothermally-altered silicic rock. Thermomechanical modeling with a typical 0.001 km 3 /yr eruption rate of hydrous basalt into a 38 km Kamchatkan arc crust produces two magma bodies, one near the Moho and the other engulfing the entire section of upper crust. Rising basalts are trapped in the lower portion of an upper crustal magma body, which exists in a partially molten to solid state. Differentiation products of basalt periodically mix with the resident magma diluting its crustal isotopic signatures. Bindeman et al. Isotopic and Thermomechanical Model of Karymshina Caldera At the end of the magmatism crust is thickened by 8 km. Thermomechanical modeling show that the most likely way to generate large spikes of rhyolitic magmatism is through delamination of cumulates and mantle lithosphere after many millions of years of crustal thickening. The paper also presents a chemical dataset for Pacific ashes from ODDP 882 and 883 and compares them to Karymshina ignimbrites and two other Pleistocene calderas studied by us in earlier works.

The Columbia River Flood Basalts (CRB) of the northwestern USA are coeval with eruptions of several thousand km 3 of rhyolite. A broad survey of major phenocryst oxygen isotopes and of O and Hf isotopes in zircons from these rhyolites reveals significant diversity in inferred d 18 O melt values, ranging from +1.9 to +10.5& (SMOW), and in zircon Hf isotope compositions, which range from e Hf = À39 to +9. This newly identified isotopic diversity shows that the syn-CRB rhyolites were derived from high-percentage melting of the crust. Low-d 18 O rhyolites, which fingerprint the melting of hydrothermally altered crust, are concentrated at the edge of the North American craton. This suggests that the conditions of crustal heating, faulting, and hydrothermal alteration required for the production of these rhyolites were concentrated there by the contrasts in crustal thickness and rheology associated with the boundary between the North American craton and younger accreted terranes.