Pervasive Hydrothermal Events Associated with Large Igneous Provinces Documented by the Columbia River Basaltic Province

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.

The life cycle of large igneous provinces

“…The crust surrounding LIP magmas -both continental and oceanic -hosts hydrothermal activity that might, in some instances, rival mid-ocean ridge hydrothermal systems in scale 133 , but LIP hydrothermal systems have received comparatively little attention. Weathering of mafic LIP rocks has been proposed as an important but controversial factor in Earth's silicate weathering cycle 134,135 .…”

“…Signatures of hydrothermal activity provide a powerful tool for inferring temporal progression of LIPs. For example, heat loss from CRB dikes drove fluid flow 133 , and the pulsed cooling, heating and remelting of the ~300-km 3 LIP-related Skaergaard intrusive complex and its host rocks are encoded in crystal to km-scale variability of oxygen isotopes 139,140 .…”

The Magmatic Architecture of Continental Flood Basalts: 2. A New Conceptual Model

“…Following Roland et al (2010); Cao et al (2019), we implement the effect of hydrothermal cooling in the upper crust (top 8 km) by modifying the thermal conductivity based on a Nusselt number (see the respective papers for details). Given the significant MITTAL AND RICHARDS 10.1029/2021JB021807 11 of 54 thermal input associated with a CFB and observations of shallow hydrothermal systems, we choose Nu = 8 for all our calculations (Bindeman et al, 2020). In the model, with continued melt emplacement, the crustal thermal profile evolves from a steady state geotherm to an elevated temperature due to the additional heat input from dikes.…”

Timescales of magma transport in the Columbia River flood basalts, determined by paleomagnetic data

“…In particular, dikes near Maxwell Lake (Fig. 1c) have been the subject of previous studies (Petcovic and Grunder, 2003;Karlstrom et al, 2019;Bindeman et al, 2020). Much of the overlying CRB flows in the Wallowa Mountains have been eroded away so the true paleodepths of the dikes are not known, but Petcovic and Dufek (2005) argue that depths could be as great as 2.5 km.…”