Explore the vast range of titles available.

Alkaline rock and carbonatite complexes, including the Prairie Lake complex (NW Ontario), are widely distributed in the Canadian region of the Midcontinent Rift in North America. It has been suggested that these complexes were emplaced during the main stage of rifting magmatism and are related to a mantle plume. The Prairie Lake complex is composed of carbonatite, ijolite and potassic nepheline syenite. Two samples of baddeleyite from the carbonatite yield U-Pb ages of 1157.2±2.3 and 1158.2±3.8 Ma, identical to the age of 1163.6±3.6 Ma obtained for baddeleyite from the ijolite. Apatite from the carbonatite yields the same U-Pb age of ∼1160 Ma using TIMS, SIMS and laser ablation techniques. These ages indicate that the various rocks within the complex were synchronously emplaced at about 1160 Ma. The carbonatite, ijolite and syenite have identical Sr, Nd and Hf isotopic compositions with a 87Sr/86Sr ratio of ∼0.70254, and positive εNd(t)1160 and εHf(t)1160 values of ∼+3.5 and ∼+4.6, respectively, indicating that the silicate and carbonatitic rocks are co-genetic and related by simple fractional crystallization from a magma derived from a weakly depleted mantle. These age determinations extend the period of magmatism in the Midcontinent Rift in the Lake Superior area to 1160 Ma, but do not indicate whether the magmatism is associated with passive continental rifting or the initial stages of plume-induced rifting.

This paper examines arbitrage opportunities available in one of the largest wholesale electricity markets in the world, the Midcontinent Independent System Operator (MISO) electricity exchange. While prior research suggests that market efficiency on the exchange has increased over time, this study reveals that historical pricing information can still be used to generate positive returns. I find that a trading rule based on prior spot and forward prices generates statistically and economically significant risk-adjusted returns across the entire MISO footprint. These returns may in part be explained by the relatively small number of financial traders in the market and the ability of generation owners to exercise market power.

Invasive species are often assumed to have measurable impacts throughout their potential range, but this is not always true. This study evaluated whether zebra mussels ( Dreissena polymorpha ) were causing an increase in water clarity near the southern edge of their North American expansion using a 40+ year dataset. We looked for abrupt shifts and long-term trends in water clarity and compared these to the estimated zebra mussel invasion date for 7 reservoirs in Missouri, USA. We also looked at water clarity in 26 non-invaded reservoirs to evaluate whether zebra mussel impacts were being masked by variation in environmental factors. Collectively, our analyses provide a weight of evidence based on a lack of consistent shifts in water clarity corresponding with zebra mussel invasions. Mussel establishment did not increase water clarity, likely because densities were too low to result in a measurable impact. The highest zebra mussel density we observed was 65 mussels m −2 , an order of magnitude less than in systems where they have had a sustained impact. Low densities could be due to a combination of sublethal environmental conditions such as warm water temperatures, suspended inorganic particulates, fluctuating water levels, and reduced oxygen concentrations, as well as limited hard substrata or predation by fish. While additional study is required to understand which, if any, of these factors may be limiting zebra mussel establishment in Missouri, our study shows that dreissenids are not having an impact as elsewhere throughout their range, suggesting that how an invader alters ecosystem functions is critical to determining its impact.

The effect of microbial binding for the stabilization of steep carbonate slopes is well documented in Cenozoic examples but its significance and relationship with abiotic marine cements in Paleozoic reef systems and steep slopes is not clearly established. Here, samples from a Late Silurian (Ludlovian) reef complex are evaluated by using an integrated approach that involves petrographic and isotope analyses, cathodoluminescence microscopy, and environmental scanning electron microscopy. This study reveals the in situ production of mineral fabrics of microbial origins, including micrite, peloidal micrite with dendritic fabrics, meniscus, and bridge-like cements. This study stresses the leading role of microbes in the early lithification stages that led to the stabilization of Silurian steep carbonate slopes. These findings are further supported by the occurrence of fossilized microbes and extracellular polymeric substance. The micritic cements are the first step in the diagenetic paragenesis followed by abiotic fibrous and equant calcite cements whose stable isotope values are in concert with estimated values of calcite precipitated in equilibrium with Silurian seawater. Results from this project provide insights into the relationship between microbial binding and early abiotic marine cements in ancient reef systems and, further, provide evidence for a consortia of microbes that existed 440 Ma ago.

Cratonic lithospheres carry a long history of tectonic modifications that result in heterogeneous structures, as revealed by an increasing number of geophysical observations. The existence of cratonic basins indicates protracted periods of tectonic modification, causing subsidence within global continental interiors. An enigmatic aspect of this process is the cessation of subsidence in cratonic basins with unclear mechanisms. Here, using full‐wave ambient noise tomography, we reveal distinct seismic low‐velocity anomalies below 60 km beneath the Illinois and Michigan Basins, where subsidence terminated in the late Paleozoic to the early Mesozoic. These low‐velocity volumes, surrounded by distinctly higher velocities, are attributed to asthenospheric materials upwelling to shallow mantle depths during lithospheric foundering or delamination. This lithospheric modification may be associated with a major regional tectonic exhumation in the early Mesozoic that could have terminated basin subsidence and unroofed upper portions of basin stratigraphy. This timing coincides with the passage of this region over mantle plumes, which likely triggered lithospheric delamination and asthenospheric upwelling. Geodynamic modeling shows that the emplacement of these buoyant asthenospheric materials would lead to an uplift of about 3.5 km, sufficient to terminate the subsidence in the cratonic basins within this region. These findings document evidence of lithospheric delamination in the North American midcontinent and present important links between geodynamic drivers and geological records of the evolution of the cratonic lithosphere in North America and beyond. They also offer broader implications for understanding how deep Earth processes shape surface environments, influencing resource distribution and long‐term landscape evolution. Cratons are ancient and stable continental regions. Cratonic basins are depressed areas within these stable continental cores and would stop sinking over time, though the reason for the abrupt cessation of subsidence is not well understood. We used an advanced seismic imaging method to investigate the subsurface structure below the North American midcontinent, covering the Illinois and Michigan Basins, where sinking stopped between 350 and 300 million years ago. Below 60 km depth, we found volumes through which seismic waves propagate more slowly, surrounded by regions where seismic waves travel faster, particularly below the Illinois and Michigan Basins. These lower velocity regions suggest that material from the Earth's asthenosphere has replaced older continental lithosphere, potentially due to a process called lithospheric delamination, where parts of the Earth's lithosphere peel away. Our numerical model shows that the ascending mantle material could have caused the surface to rise by about 3.5 km, enough to stop the basins from sinking further and remove the uppermost layers of sedimentary rocks. This study highlights the role of lithospheric delamination in shaping cratonic basins and provides new insights into how Earth's deep processes influence surface geology over time. Full‐wave seismic tomography reveals low‐velocity anomalies below the Wisconsin Dome and Illinois and Michigan Basins below 50–60 km depth These scattered low velocities beneath the thinned lithosphere reflect emplaced hot asthenospheric materials after lithospheric delamination The topographic uplift due to lithospheric delamination, as shown in geodynamic models, is sufficient to end subsidence in cratonic basins

The W Horizon is a thin continuous zone of low sulfide-high-grade platinum group element (PGE) mineralization in the Two Duck Lake gabbro at the Marathon Cu-PGE deposit, Coldwell Complex, Canada, that resembles reef-type PGE deposits found in much larger intrusions. The Marathon deposit was previously shown to have grown by accretion of syn-magmatic, sulfide-bearing sill-like intrusions that formed each of the W Horizon, Main Zone, and Lower Zone mineralization. Each zone can be distinguished by features such as sulfide mineral or platinum group mineral assemblages, or by Cu/Pd, Pd/Se, and Pd/Au values. For example, the W Horizon is defined by the presence of bornite, very high PGE values, low Cu/Pd (< 1000), and high Pd/Au (34 ± 32), whereas the Main Zone and Lower Zone types are characterized by mantle-like Cu/Pd (1000 to 10,000), and Pd/Au of 10.3 ± 4.7. Model calculations based on Kerr-Leitch multistage dissolution upgrading show that PGE enrichment and decoupling of PGE from Au and chalcophile elements in the W Horizon can be related to progressive upgrading and recycling of sulfide liquid by successive pulses of silicate magma. For the W Horizon, extreme PGE enrichment can be explained by the interaction of a sulfide liquid with up to 25 pulses of magma and simultaneous sulfide liquid dissolution resulting in a loss of > 95% of the original sulfide liquid. In some samples, metal and S abundances indicate that PGE were transported as TABS-PGE phases. This work describes an example of thin PGE-rich sill-like injections that could be of importance to PGE exploration elsewhere.

New specimens of Cyclocystoides scammaphoris Smith and Paul, 1982, are here reported from the Upper Ordovician Platteville Formation of northern Illinois, Plattin and Decorah groups of east-central Missouri, and Lebanon Limestone of central Tennessee. These fossils reveal skeletal details that provide insight into the anatomy of cyclocystoids. Of particular significance is a network of channels that likely originate near the center of the central disk and extend along the oral side of the radial plates, bifurcating distally two or three times before entering the radial facets on the proximal surface of each marginal ossicle. From here, the network enters a series of facet canals that extend upward through each marginal ossicle, exiting in a linear row of pores. The canals are very similar in size and distribution to the nerve canals in living echinoderms. The axes of the canals, which number up to 500 in some specimens, and those of the radial ducts project proximally away from the oral surface at an elevation angle of about 25°, apparently forming a network that could have converged within the upper part of the body cavity. This origin and function are made clear by the connection between the channel on each radial plate and the radial facet canal pores within each marginal ossicle.

The 1.1 Ga Mesoproterozoic Midcontinent rift hosts the Eagle, Eagle East, and Tamarack Ni-Cu-PGE deposits and Embayment Prospect. These deposits are hosted by ultramafic igneous rocks and have some of the highest Ni-Cu grades on Earth. We use new bulk-rock data and published datasets (bulk-rock, mineral chemistry, and isotopic analyses) to examine major, minor, and trace element trends of both Midcontinent rift-related alkaline and tholeiitic intrusions. In addition, we compare the geochemical data to local kimberlite-hosted lower-crustal xenoliths and local igneous (Archean) and sedimentary (Paleoproterozoic) country rocks. We found the peridotite magma compositions dominantly consist of primitive mantle compositions with varying abundances of subduction-related components, alkaline-transitional melts, and local country rock contaminates (e.g., Baraga and Animikie Basin sediments). The subduction-related components are interpreted to be derived from previous Archean and Paleoproterozoic subduction events and likely hosted within the sub-continental lithospheric mantle. Importantly, these subduction-related components are also interpreted to have acted as oxidizing agents within the melt, stabilizing sulfate (+2 FMQ (fayalite–magnetite–quartz) to FMQ) while inhibiting sulfide crystallization as the magma ascended through ~50 km of the Superior craton. This study largely corroborates the previous findings with respect to the contribution of local country rock contamination to the Eagle–Tamarack peridotite host rocks, which is estimated to be minimal (<5%). However, the incorporation of <5% reductive pelitic siltstone contamination results in strong shifts in the oxygen fugacity of the peridotite melt, from +2 FMQ to slightly below FMQ, as determined from spinel Fe3+/∑Fe ratios. This shift in oxygen fugacity resulted in the transition from total sulfate (+2 FMQ) to sulfate + sulfide (<+2 FMQ to FMQ) to total sulfide (

Journal Article

Share this book

Add to My Shelf