In the ~20-m-thick Maiden Creek sill of the Henry Mountains (Utah, USA) intrusive complex, 2 magma sheets are locally separated by a 1.5-m-thick lens of sandstone. We studied the boundary between these sheets at the termination of this sandstone lens, where the upper sheet directly overlies the lower sheet, in order to test the reliability of using magnetic susceptibility in delineating internal magmatic contacts. The contact between these two sheets is along a cliff face and defined by a thin (<1 cm) brittle-ductile shear zone. Measurements of magnetic susceptibility (K) were collected within a grid every 20 cm across this contact. Drill cores (72) were also collected along four traverses across the shear zone. Mapping K across the cliff face reveals an abrupt decrease immediately below the shear zone contact. 1 m below the contact, K unexpectedly increases again to the same levels observed above the contact. This lower boundary coincides with a 1–2-mm-thick minor fracture zone. The 1-m-thick low-K zone (LKZ) is characterized by more intense microfracturing and is bleached compared to the surrounding igneous rock. Plotting the magnetic foliation from the drill cores reveals abrupt changes to the orientation across both the shear zone and fracture zone. We hypothesize that the LKZ was the original magma sheet that intruded the sandstone. The high-K zones above and below the LKZ represent later sheets that intruded above and below the original sheet, fracturing the partially or wholly crystallized original intrusion. These later sheets exsolved fluids that were injected into the original sheet, resulting in more advanced oxidation of magnetite and thus lowering the K. Alternatively, it is possible that the LKZ is simply the altered zone at the top of a thicker older sheet that was modified by the intrusion of a younger overlying sheet.
Thick dolerite sills show a range of vertical geochemical variation trends attributable to various processes during slow crystallization. We have identified chemical parameters in a 169-m-thick sill from the Karoo igneous province in South Africa that define three different lower crossover levels (maximum or minimum concentrations) creating S-shaped variation trends. The crossover level for whole-rock MgO is at 20 m height (due to mechanical sorting of olivine); the anorthite content of plagioclase is at 52 m (due to addition of primitive magma); and that of the incompatible trace elements is at 75 m (due to different proportions of early formed grains to trapped liquid). Each process can operate independently and concurrently, leading to their maximum effects occurring at different levels in the intrusion. The independence of these processes and the triple S-shaped geochemical profiles have not been recognized before in any mafic-ultramafic sills.
The term refuge describes, in both ecology and paleoecology, an ecosystem that acts as a sanctuary during times of environmental stress. This study tests the concept by examining the fate of a single community that lived ~50 k.y. after the end-Permian mass extinction (EPME). An assemblage of trace fossils, bivalves, and echinoids, living on a microbial mat in a slope environment, is preserved on a single bedding plane in the Shangsi section, south China. The microbial community was vital to the success of the refuge, acting as a stable substrate, food source, and oxygen supply. Shallow-water microbial communities have been interpreted as refugia, but this deeper site may have been critical to organisms with temperature sensitivities. Published paleotemperature calculations suggest sublethal surface temperatures of 34 °C at Shangsi. A species of cidaroid echinoid likely migrated to cooler deep waters to optimize development, suggesting that the success of this shallow-water clade is attributable to such refugia, when survival was most precarious after the EPME. The ecosystem was short lived, depending on low productivity and slow sedimentation. When conditions became suboptimal due to ash input and increasing productivity, the ecosystem quickly collapsed, allowing for colonization by opportunistic taxa including Claraia and microconchids. Elsewhere the ecosystem may have remained unchanged. Earliest Triassic refugia may have been restricted to these ephemeral environmental settings until organisms adapted to continuing harsh conditions.
The oxygen concentration of the atmosphere likely increased substantially in the late Neoproterozoic. Although several studies have presented compelling geochemical evidence for this stepwise oxygenation, few have addressed the mechanisms behind it. Recently it was hypothesized that the advent of eukaryotic life on land, and the associated increase in soil respiration, led to a transient reduction in the supply of oxygen for rock weathering, temporarily reducing oxidative weathering rates, allowing atmospheric oxygen levels to rise to restore the oxygen supply. To evaluate this hypothesis quantitatively, we developed a simple one-dimensional diffusion-reaction soil model that reduces the many oxygen weathering sinks to one, pyrite, given that it is the dominant sink at low oxygen concentrations. In simulations with no biological respiration, pyrite weathering rates become oxygen independent at an atmospheric oxygen concentration between 10–6x the present-day atmospheric level (PAL) and 1 PAL. On the other hand, when biological respiration is considered, pyrite weathering remains oxygen dependent even at modern oxygen levels. Constrained by modern weathering profiles and soil respiration rates, we find that the atmospheric oxygen level may have increased by up to two orders of magnitude as biotic soil respiration increased. This may be sufficient to explain the second rise in atmospheric oxygen inferred for the Neoproterozoic.
Geochemical data from cap carbonates deposited above Cryogenian glacial deposits have been widely used to infer the conditions that prevailed in the aftermath of snowball Earth. However, the time scale over which these carbonates were deposited and the degree to which they record the chemistry of a globally well-mixed ocean have remained poorly constrained. During deglaciation, a large volume of meltwater entered the ocean, creating two distinct layers: the fresh, hot, and light upper layer, and the salty, cold, and dense lower layer. Here we estimate the ocean mixing time scale based on energetic constraints. We find that the mixing time scale is 104–105 yr, with a best estimate of ~5 x 104 yr, or up to 100 times longer than that of the modern ocean. Mixing of the surface temperature anomaly implies a delayed sea-level rise of 40–50 m associated with pure thermal expansion. This result reconciles geological, geochemical, and paleomagnetic data from basal Ediacaran cap carbonates with physical oceanographic theory. In particular, our model suggests that (1) the cap dolostones formed predominantly in a freshwater environment; (2) the waters in which the dolostones formed were not well mixed with saline deep water, allowing for large geochemical differences between the cap dolostones and the deep ocean; and (3) the cap carbonate sequences formed in a two-phase transgression that lasted >104 yr, which is consistent with both local sea-level records and the preservation of magnetic excursions and reversals.
Eocene paleoclimate reconstructions are rarely accompanied by parallel estimates of CO2 from the same locality, complicating assessment of the equilibrium climate response to elevated CO2. We reconstruct temperature, precipitation, and CO2 from latest middle Eocene (ca. 38 Ma) terrestrial sediments in the posteruptive sediment fill of the Giraffe kimberlite in subarctic Canada. Mutual climatic range and oxygen isotope analyses of botanical fossils reveal a humid-temperate forest ecosystem with mean annual temperatures (MATs) more than 17 °C warmer than present and mean annual precipitation ~4x present. Metasequoia stomatal indices and gas-exchange modeling produce median CO2 concentrations of ~630 and ~430 ppm, respectively, with a combined median estimate of ~490 ppm. Reconstructed MATs are more than 6 °C warmer than those produced by Eocene climate models forced at 560 ppm CO2. Estimates of regional climate sensitivity, expressed as MAT per CO2 doubling above preindustrial levels, converge on a value of ~13 °C, underscoring the capacity for exceptional polar amplification of warming and hydrological intensification under modest CO2 concentrations once both fast and slow feedbacks become expressed.
Bridging the gap between the plutonic and volcanic realms is essential for understanding a variety of magmatic processes from caldera-forming eruptions to the formation of magmatic-hydrothermal ore deposits. Porphyry copper deposits are commonly associated with large and long-lived volcanic centers, but the temporal and dynamic link between mineralized intrusions and volcanic eruptions has remained controversial. Based on the combination of (1) high-precision zircon U-Pb geochronology and trace element geochemistry with (2) plagioclase textures, we discovered an intimate connection between an ignimbrite eruption and a nearby world-class porphyry deposit (Bajo de la Alumbrera in the late Miocene Farallón Negro Volcanic Complex of Argentina). Our results indicate that the magmatic-hydrothermal deposit and explosive volcanism were derived from a common magma reservoir that evolved over a minimum duration of 217 ± 25 k.y. before the final eruption. We show that the volcanic pile represents the inverted magma reservoir, recording systematic differences in plagioclase textures and juvenile clast content from bottom to top. This tight temporal and geochemical link suggests that deposit formation and volcanic eruption were both triggered by the same injection of a volatile-saturated primitive magma into the base of the magma chamber. A time gap of 19 ± 12 k.y. between porphyry mineralization and the onset of explosive volcanism indicates a minimum duration of magma reservoir rejuvenation that led to the explosive eruptive event. Catastrophic loss of volatiles by explosive volcanism terminated the ore-forming capacity of the upper-crustal magma chamber, as evidenced by the intrusion of a syn-eruptive barren quartz-feldspar porphyry. Our results demonstrate that porphyry copper deposits provide critical information to understand how volatiles control the fate of hydrous magmas between pluton formation and explosive volcanism.
High levels of radioactive cesium remain in the soil near the Fukushima Daiichi nuclear power plant and these radionuclides have migrated at least 5 centimeters down into the ground at several areas since the nuclear accident five years ago, according to preliminary results of a massive sampling project being presented at the JpGU-AGU joint meeting in Chiba, Japan.
The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490–595 °C, which is 250–450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8–9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000–1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ~48% calcite, ~12% apatite, and ~2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.
Zircon grains from the Kiruna iron oxide–apatite (IOA) ore bodies in northern Sweden are distinct in their hafnium and oxygen isotopic ratios compared to zircon grains from adjacent metavolcanic host rocks and related intrusions. Here, we combine these two isotopic systems on previously dated zircon grains to improve our understanding of these ore deposits with a long-debated origin. Contrasting theories for the formation of the Kiruna iron ores suggest either (1) emplacement through immiscible silicate–iron oxide melts or (2) transportation and deposition of iron by hydrothermal fluids. Zircon from the metavolcanic host rocks and intrusions have oxygen isotopic ratios (18O ~3) that lie below typical magmatic compositions, which is evidence that roof rocks altered by meteoric water were digested into the magma. In contrast, the ores show an influence of a fluid that is higher in 18O (~7). Based on these findings, we propose the involvement of episodic magmatic-hydrothermal fluids in the ore genesis of the Kiruna iron ore deposits: (1) the first episode related to a deep-seated magmatism and to regional-scale metasomatic alteration, and (2) a later fluid event related to shallow intrusions and responsible for the ore formation. Distinct differences in the Hf isotopic ratios for host rocks and intrusions (Hfi = –6 to –10, Archean crust) and ore (Hfi = –5 to +3, depleted mantle) further allow us to screen possible fluid sources for their involvement in the ore process.