Ingrid Aarnes

The impact of host-rock composition on devolatilization of sedimentary rocks during contact metamorphism around mafic sheet intrusions

Sedimentary rocks represent vast reservoirs for hydrous and carbonaceous fluids (liquid or gas) that can be generated and released during contact metamorphism following the emplacement of igneous sill intrusions. A massive release of these fluids may impose perturbations in the global climate. In this study we assess the influence of varying host-rock compositions on the magnitude and type of fluids generated from thermal devolatilization, with particular emphasis on carbon and halogens released from heated limestone, coal and rock salt, and the different timescales of metamorphism. In limestones the generated fluids are dominated by H2O with limited CH4 and CO2 production on a time-scale of 600–3000 years. Cracking of organic matter and CO2 production (8000–28,000 years) dominates the fluid products from a coal sequence. In the case of evaporites, the presence of reactive organic matter or petroleum results in the generation of CH4 and CH3Cl (260–1000 years). In order to compare the basin scale impacts of the differing host-rocks, two plausible scenarios are explored in which a 100 m thick and 50 000 km2 large sill is emplaced into 1) organic-rich shale and coal, and 2) limestones and rock salt. The results show the formation of 1) >1600 Gt CH4, and 2) >700 Gt of CH3Cl, demonstrating that the sill emplacement environment (i.e., the composition of the host rocks) is of major importance for understanding both gas generation in sedimentary basins and the environmental impact of a Large Igneous Province. By evaluation of the isotopic signature of carbonaceous fluids from shales and coals, we show that intrusions into coal-rich sediments are potentially of much less importance for perturbing the atmospheric carbon isotope values than shales.

Contact metamorphism and thermogenic gas generation in the Vøring and Møre basins, offshore Norway, during the Paleocene–Eocene thermal maximum

Voluminous volcanic intrusive activity took place in the Vøring and Møre basins at the Paleocene–Eocene boundary at about 56 Ma. This event caused thermal maturation of Cretaceous sedimentary rocks in the basins. We have estimated the resulting thermogenic gas generation potential from contact metamorphism using numerical simulations calibrated using borehole data. The borehole 6607/5-2 from the Utgard sill complex in the Vøring Basin contains two c. 100 m thick sills and is used as a case study. We present both new and compiled data showing that (1) the bulk organic content is reduced towards the sill intrusions, (2) a c. 1 km thick stratigraphic interval is thermally affected, based on vitrinite reflectance data, (3) relative emplacement timing can affect the gas yield by up to 25%, and (4) some of the thermogenic methane is still present in the aureoles. The numerical model is calibrated using data from 11 wells. We estimate that the total gas generation potential for the two Utgard sills equals that of the Troll field (c. 10 Gt CH4), the largest producing gas field offshore Norway. We show that in the Vøring and Møre basins, the total gas generation potential is up to 1500 Gt CH4 (c. 1100 Gt C), even from the relatively organic-poor Cretaceous source rocks with c. 1 wt% organic carbon, with implications for the carbon cycle at the Paleocene–Eocene boundary.

Magmatic differentiation processes in saucer-shaped sills: Evidence from the Golden Valley Sill in the Karoo Basin, South Africa

Analysis of compositional variations along profiles from tholeiitic sills provides insights into syn- and post-emplacement magmatic differentiation processes. We present here 18 whole-rock compositional profiles sampled from a saucer-shaped sill emplaced in the Karoo Basin (South Africa), the Golden Valley Sill. We show that different compositional profile patterns previously described in basic-ultrabasic sills may be found in different parts of a single saucer-shaped sill. The detailed examination of the mineral grain assemblage and compositions suggests that processes taking place in hundred-meter-thick sills relate to early and late fractional crystallization. Our observations in the Golden Valley Sill suggest that a significant part of fractionation takes place at a late stage of cooling when a crystalline skeleton or mush zone is formed. We show that porous flow of interstitial melt driven by forces related to the particular geometry (saucer-shaped) of the sill may result in a post-emplacement compositional evolution. We propose that the process of post-emplacement melt flow regionally overprinted compositional patterns produced by earlier crystal segregation from the cooling magma at fluid-like stages during the emplacement.

Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidification of magmatic sheet intrusions in sedimentary basins

Sediment dikes are common within dolerite sill intrusions in the Karoo Basin in South Africa. The dikes are subvertical and as much as 2 m wide, sometimes with abundant fragments of sedimentary rocks and dolerite. The matrix consists of contact-metamorphic sandstone. There is no petrographic evidence for melting within the sediment dikes. The maximum temperature during heating is restricted to the plagioclase and biotite stability field, or above ∼350 °C. Thermal modeling of a sandstone dike in a dolerite sill shows that a temperature of 350–450 °C is reached in the dike after a few hundred years of sill cooling. The calculated pressure history of a cooling sill and its contact aureole shows that substantial fluid pressure anomalies develop on a short time scale (1–15 yr) and are maintained for more than 100 yr. Calculated pressure anomalies in the sill (-7 to -22 MPa) and the aureole (4–22 MPa) are significant and may explain sill fracturing and sediment mobilization from the aureole into the sill. We conclude that sediment dikes represent common features of sedimentary basins with sill intrusions in which fluid pressure gradients have been high. Sediment dikes thus signify that pore fluids may escape from the aureoles on a short time scale, representing an intermediate situation between fluid loss during formation of microfractures and fluid loss during violent vent formation.

Sub-Volcanic Intrusions and the Link to Global Climatic and Environmental Changes

Most of the Large Igneous Provinces (LIPs) formed during the last 260 million years are associated with climatic changes, oceanic anoxia, or extinctions in marine and terrestrial environments. Current hypotheses involve (1) degassing of carbon from either oceans or shallow sea-bed reservoirs, (2) degassing from flood basalts, or from (3) sedimentary basins heavily intruded by LIP-related sills. These hypotheses are based on detailed geological and geochemical studies from LIPSs or relevant proxy data sequences. Here we present new data on gas generation and degassing from a LIP, based on the LA1/68 borehole north of the Ladybrand area in the Karoo Basin, South Africa. The borehole was drilled in the middle of a phreatic breccia pipe and penetrated 11 sills before reaching the basement at 1710 m depth. We present new data on the lowermost 15 m thick sill emplaced in shale, and on the breccia comprising the uppermost 154 m of the core. We show that (1) a reduction in organic matter within a contact aureole can be explained by heating and the formation of CH4, (2) a phreatic eruption and breccia formation was initiated from pore fluid boiling around sills emplaced in Beaufort Group sandstones at 420–570 m depth, (3) the phreatic eruption cut through a cover of solidified and partly molten lava flows that subsequently filled the crater, and (4) the pipe has been used as a fluid flow pathway for millions of years, demonstrated by fossil and active oil seeps. We conclude that the sub-volcanic LIP environment hold the key to understand the relationships between large scale volcanism and rapid environmental perturbations.