Carbonatites are defined as an igneous rock containing > 50% carbonate minerals, although not all rocks labelled as carbonatite fit strict mineralogical criteria. Carbonatites are rare, very unusual in terms of mineralogy and geochemistry, compositionally variable and extraordinarily diverse. These enigmatic rocks are also highly prospective, hosting large resources of Nb, REE, phosphates and other commodities now classed as 'critical minerals'. Ideas on the genesis and evolution of carbonatite are divergent and many observations or hypotheses remain controversial. This review focuses mostly on geochemical aspects, notably as applied to models for the generation of carbonatite parental magmas, and the wider implications of such data. These rare and fascinating rocks provide insights into the secular evolution of the sub-continental mantle, the role of metasomatism at both crustal and mantle levels, and the nature of deeper parts of the mantle.
The Grande Ronde Basalt (GRB) is the main volcanic phase of the Columbia River Basalt Group. It erupted in less than half a million years yet makes up more than 72% (150,000 km(3)) of this famous Cenozoic flood-basalt sequence. The Teepee Butte Member is one of the most representative components of the Grande Ronde Basalt. It includes extensive lava flows, a well-preserved similar to 100 km-long system of dykes and a vent complex. The vent complex consists of tephra, "Pele's tears" (frozen lava droplets), bombs, shelly pahoehoe flows, a lava lake and a feeder dyke. The Teepee Butte Member includes two high-MgO basalt flows with two intercalated low-MgO flows, collectively representing two cycles of high-MgO to low-MgO eruptions. The first high-MgO flow (Joseph Creek flow) was followed by a hiatus during which the magma chamber fractionated olivine, plagioclase and clinopyroxene (i.e. gabbroic fractionation) and later erupted a low-MgO flow and remnant lava lake. The first low-MgO flow, the Joseph Creek Lava Lake flow, advanced over 350 km nearly reaching the Columbia River Gorge. The magma chamber was subsequently recharged with a high-MgO magma which then erupted as the Pruitt Draw flow, which advanced nearly 500 km to the Portland Basin in Oregon. After the Pruitt Draw eruption waned, the remaining magma again fractionated and then erupted as a separate low-MgO flow termed the Dog Mountain flow. This also reached the Columbia River Gorge. The flows of the Teepee Butte Member exemplify multiple cycles of high-MgO-low-MgO lava flows that persisted throughout the eruption of the Grande Ronde Basalt. Field relationships and geochemical data suggest that the composition of the Grande Ronde Basalt magma chamber was relatively consistent over time, remaining close to the composition of the highest-MgO GRB flow, but periodically fractionated to produce low-MgO flows. We document at least ten such cycles of high-MgO to low-MgO flows within the GRB. Each cycle is characterized by recharge of high-MgO magma into the magma chamber and eruption followed by fractionation in the chamber, producing a low-MgO magma that subsequently erupted, prior to the next recharge event. Poor correlations between pairs of compatible and incompatible elements support a model of periodic recharge, tapping and fractionation. The low-MgO magma compositions that mark the end of each fractionation and recharge cycle are tightly clustered, suggesting that the low-MgO magmas represent the steady-state composition of the GRB magma system. Variations in Zr/Nb and Zr/Y through the 10 cycles are broadly consistent with gabbroic fractionation; however, they also suggest the presence of subtle heterogeneities within the mantle source region supplying the recharging magma. The end of the Grande Ronde Basalt eruptions is marked by a hiatus and compositional change of the magma source, the latter resulted in the eruption of the younger Fe-and Ti-rich flows of the Wanapum Basalt.
Geological Carbon Sequestration (GCS) is viable within the Cambrian sandstone reservoir of southwestern Ontario but GCS will likely be limited in sequestration capacity and CO2 injectivity. Capacity and injectivity limitations will arise during CO2 injection because of the fluid-pressure responses induced within the Cambrian reservoir and the subsequent geomechanical-hydrogeological responses in the Precambrian basement. Any coupling between a pressurized reservoir overlying a critically stressed Precambrian basement can be expected to cause local microseismicity. In this context, the challenge is to optimize CO2 injection while minimizing microseismic responses. The principal risks and responses to the development of the Cambrian sandstone that must be resolved beforehand are (i) the robust estimation of accessible pore space for the injected super-critical CO2; (ii) the limitation of the potential for fracturing or shear damage to the caprock that might allow leakage; (iii) the induced failure of seals in active, suspended or legacy oil and gas wells penetrating the reservoir allowing upward brine or CO2 migration; (iv) the buoyant transmission of CO2 along permeable or unclamped faults penetrating the reservoir; and, (v) any microseismicity induced by pressure increases penetrating critically-stressed basement rocks. Notwithstanding these risks, we believe the Cambrian sandstone is a viable CO2 reservoir for at least 10 megatonnes of CO2 sequestration annually.The Ontario Government’s 2018 goal of 30% CO2 emissions reduction by 2030 must partly occur though industrial sequestration of CO2 within the Cambrian reservoir. However, to provide information critical for regulatory management of GCS, research is needed immediately to pave the way for CO2 injection operations if they are to have any role in industrial decarbonization. This article summarizes our weak understanding of these critical issues and proposes the development of a research borehole to provide much of the necessary information for regulatory management of CO2 injection. It will also aid the design and operation of CO2 injection hubs along the north shore of Lake Erie. In short, a site similar to the Aquistore site is needed for development of the Cambrian GCS reservoir in southwestern Ontario.
The Grande Ronde Basalt (GRB) is the main volcanic phase of the Columbia River Basalt Group. It erupted in less than half a million years yet makes up more than 72% (150,000 km3) of this famous Cenozoic flood-basalt sequence. The Teepee Butte Member is one of the most representative components of the Grande Ronde Basalt. It includes extensive lava flows, a well-preserved ~100 km-long system of dykes and a vent complex. The vent complex consists of tephra, “Pele’s tears” (frozen lava droplets), bombs, shelly pāhoehoe flows, a lava lake and a feeder dyke. The Teepee Butte Member includes two high-MgO basalt flows with two intercalated low-MgO flows, collectively representing two cycles of high-MgO to low-MgO eruptions. The first high-MgO flow (Joseph Creek flow) was followed by a hiatus during which the magma chamber fractionated olivine, plagioclase and clinopyroxene (i.e. gabbroic fractionation) and later erupted a low-MgO flow and remnant lava lake. The first low-MgO flow, the Joseph Creek Lava Lake flow, advanced over 350 km nearly reaching the Columbia River Gorge. The magma chamber was subsequently recharged with a high-MgO magma which then erupted as the Pruitt Draw flow, which advanced nearly 500 km to the Portland Basin in Oregon. After the Pruitt Draw eruption waned, the remaining magma again fractionated and then erupted as a separate low-MgO flow termed the Dog Mountain flow. This also reached the Columbia River Gorge. The flows of the Teepee Butte Member exemplify multiple cycles of high-MgO - low-MgO lava flows that persisted throughout the eruption of the Grande Ronde Basalt. Field relationships and geochemical data suggest that the composition of the Grande Ronde Basalt magma chamber was relatively consistent over time, remaining close to the composition of the highest-MgO GRB flow, but periodically fractionated to produce low-MgO flows. We document at least ten such cycles of high-MgO to low-MgO flows within the GRB. Each cycle is characterized by recharge of high-MgO magma into the magma chamber and eruption followed by fractionation in the chamber, producing a low-MgO magma that subsequently erupted, prior to the next recharge event. Poor correlations between pairs of compatible and incompatible elements support a model of periodic recharge, tapping and fractionation. The low-MgO magma compositions that mark the end of each fractionation and recharge cycle are tightly clustered, suggesting that the low-MgO magmas represent the steady-state composition of the GRB magma system. Variations in Zr/Nb and Zr/Y through the 10 cycles are broadly consistent with gabbroic fractionation; however, they also suggest the presence of subtle heterogeneities within the mantle source region supplying the recharging magma. The end of the Grande Ronde Basalt eruptions is marked by a hiatus and compositional change of the magma source, the latter resulted in the eruption of the younger Fe- and Ti-rich flows of the Wanapum Basalt.
