Abstract
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.