Continental rifts and reactive lids: Pervasive hydrothermal alteration and carbonation of volcanic rocks in the Oslo Rift

The Oslo Rift formed about 300 million years ago and is characterized by emplacement of subaerial lavas and an extensive sub-volcanic system of sills, dykes, and plutons. The earliest stages of volcanism were dominated by fissure-fed plateau basalts, trachytes and latites (including rhomb porphyries), followed by later stage caldera-related basalts, rhyolites and ignimbrites. More than 1.5 kilometer of rhomb porphyry stratigraphy is preserved in the ca 250 km long subaerially exposed rift system, but primary igneous minerals are usually completely altered except from apatite and rare zircons. Interestingly, the sedimentary rocks that form the substrate for the lavas remain unaffected by hydrothermal alteration, questioning conventional models for fluid flow and temperature evolution in rifts.

Here we use new boreholes and cores from the Oslo Rift to characterize and further understand rift-scale pervasive alteration of lava flows. We focus on two cores, where the first includes 350 meters of interbedded sandstones and lava flows, and the second is 50 meters long and drilled through a fault zone in the two lowermost flow units. We present wireline logs, geochemical data and petrography in order to further understand the hydrothermal alteration and porosity-generation in these rocks. Overall, the primary igneous minerals (e.g. pyroxene, feldspar, ilmenite) are hydrated and replaced by assemblages including chlorite, albite, K-feldspar, quartz, calcite, rutile, monazite, and magnetite. Vesicle-rich horizons in the lavas (flow bases and tops) are filled by chlorite and calcite, with minor dolomite, epidote, fluorite, barite, and bitumen. Vein minerals includes calcite, quartz, and epidote. In the presentation we also show the carbon and oxygen isotope systematics of the carbonates.

Our results shed light on the complex history of fluid-rock interactions in a continental rift. Lavas have acted as reactive lids, trapping water and light and mobile elements such as carbon and alkalies, whereas apatite-hosted REE still preserve igneous geochemical signatures. Hydrothermal circulation took place across a large temperature range, where the maximum temperature is recorded by quartz plus epidote assemblages (ca. 300 C). Conceptual models need to include the variability of fluid sources during rift progression, including igneous/mantle, contact metamorphic, basinal brines, meteoric and seawater sources. Finally, the pervasive hydrothermal alteration resulted in high porosity in the lavas that still has a significant influence on the content and flow of ground water.