ALBERT

Description: 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.

Description: 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 –6 x 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.

Description: 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 10 4 –10 5 yr, with a best estimate of ~5 x 10 4 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 〉10 4 yr, which is consistent with both local sea-level records and the preservation of magnetic excursions and reversals.

Description: Although most natural streams have concave-upward longitudinal profiles, there are conflicting perspectives about whether an alluvial channel is less concave than a bedrock channel. Alluvial channels can be classified as coarse-bed and fine-bed channels, depending on the bed grain size. Both are transport limited, but the threshold of motion differs greatly. Whereas a coarse-bed alluvial channel can be claimed to be as concave as a bedrock channel, we claim that a fine-bed channel is distinguishably less concave. We derive the concavity index of a fine-bed alluvial channel using the power-law relationships emergent at a steady-state river network. For known ranges of the scaling parameters, our formulation informs a range of concavity index as 0.07 ± 0.09 for a fine-bed alluvial channel. Our analyses of previous laboratory experiments and real fine-bed alluvial channels in the midwestern U.S. and northern Europe also support our conclusions, i.e., small profile concavity of steady-state fine-bed alluvial channels.

Description: 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.

Description: 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.

Description: 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.

Description: The presence of Paleoproterozoic glacial diamictites deposited at low latitudes on different continents indicates that three or four worldwide glaciations occurred between 2.45 and 2.22 Ga. During that time period, the first atmospheric oxygen rise, known as the Great Oxidation Event (GOE), occurred, implying a potential connection between these events. Herein we combine triple oxygen isotope systematics and in situ and high-precision U-Pb zircon ages of mafic intrusions to date two episodes of snowball Earth glaciations. Subglacial hydrothermal alteration was induced by intrusions of high-Mg and high-Fe gabbros during the early Paleoproterozoic rifting on the Baltic Shield, which at the time was located at low latitudes. The low 18 O values of hydrothermally altered rocks associated with these intrusions are attributed to high-temperature isotopic exchange between hot rock and glacial meltwater, indicating the presence of glacial ice globally. The triple oxygen isotope approach is used here to show that the 18 O of glacial meltwaters during the dated episodes of snowball Earth glaciation was approximately –40 VSMOW (Vienna standard mean ocean water). High-Mg gabbro intrusions and associated low- 18 O hydrothermally altered rocks formed during the earliest episode of snowball Earth glaciation between 2.43 and 2.41 Ga. High-Fe gabbro from the Khitoostrov locality (Karelia, Russia) hosts a 18 O value of –27.3 and is dated here at 2291 ± 8 Ma. This age is interpreted to reflect the interaction between the intrusion and glacial meltwaters during the third Paleoproterozoic glaciation, which occurred after the GOE.

Description: Tectonic tremors in Alaska (USA) are associated with subduction of the Yakutat plateau, but their origins are unclear due to lack of depth constraints. We have processed tremor recordings to extract low-frequency earthquakes (LFEs), and generated a set of six LFE waveform templates via iterative network matched filtering and stacking. The timing of impulsive P (compressional) wave and S (shear) wave arrivals on template waveforms places LFEs at 40–58 km depth, near the upper envelope of intraslab seismicity and immediately updip of increased levels of intraslab seismicity. S waves at near-epicentral distances display polarities consistent with shear slip on the plate boundary. We compare characteristics of LFEs, seismicity, and tectonic structures in central Alaska with those in warm subduction zones, and propose a new model for the region’s unusual intraslab seismicity and the enigmatic Denali volcanic gap (i.e., an area of no volcanism where expected). We argue that fluids in the Yakutat plate are confined to its upper crust, and that shallow subduction leads to hydromechanical conditions at the slab interface in central Alaska akin to those in warm subduction zones where similar LFEs and tremor occur. These conditions lead to fluid expulsion at shallow depths, explaining strike-parallel alignment of tremor occurrence with the Denali volcanic gap. Moreover, the lack of double seismic zone and restriction of deep intraslab seismicity to a persistent low-velocity zone are simple consequences of anhydrous conditions prevailing in the lower crust and upper mantle of the Yakutat plate.