Extreme End Members Research Articles

The spatial distributions of mineralisation

The concept of fractal spatial distributions of mineralisation has been widely proposed since Mandelbrot (1965) who emphasised the stable Pareto-Lévy distribution as the relevant distribution. The concept of a fractal is used as a basis for estimating endowment and for erecting exploration models based on self-organised criticality. This paper explores the proposition that the growth kinetics for a mineralising system are reflected in the probability distributions that describe the spatial patterns of mineralisation. We revisit the data sets and ask the question: What are the best fit probability distributions for the spatial distribution of mineralisation? The answer is: members of the Extreme Value Distribution family (Gumbel-, Fréchet- and Weibull-distributions) and not the Pareto distribution. Thus, the spatial distribution of mineralisation is not a fractal although the tails of the distributions can be or resemble power-laws. The standard box counting procedure for a spatial point distribution establishes a nearest neighbour distribution and hence, by definition, the resulting distribution is Weibull and not Pareto. The mass distributions are Fréchet and not Pareto. The extreme end members are Gumbel. We discuss the implications of these distributions for models that generate mineralisation sites within a system and for the underlying thermodynamics.

Granitoids and trachytes of Um Shager-Hamadat area, Central Eastern Desert, Egypt: Remote sensing characterization and petrology

The late Neoproterozoic granitoids of Gebel Um Shager and Gebel Hamadat, Central Eastern Desert of Egypt, in addition to Paleozoic trachytes of both areas were discriminated using Landsat-8, and Sentinel-2 dataset. Band rationing and Principal Component Analysis techniques were applied. The studied granitoids represent the extreme end-members of the subduction-related I-type calc-alkaline granitic magmas (primitive to more evolved varieties) that developed over a long period of time. These granites were extruded by Paleozoic intra-plate trachytes. Petrography and petrogenesis of these rocks were dealt with and discussed. The Um Shager Older Granitoids (calcic tonalite to quartz monzodiorite) represent poorly differentiated metaluminous magma characterized by very low Rb/Sr ratio ∼0.07. Evolutionary, they follow the calc-alkaline subtype trend and most probably were originated from parent gabbroic magma by fractional crystallization in a volcanic arc setting. On contrary, the Hamadat Younger Granites represent alkali calcic magma which was developed in post-collision setting. They comprise evolved subsolvus metaluminous syenogranite and alkali feldspar granite. Evolutionary, they follow the subalkaline subtype trend. Crystallization courses of both syenogranite and alkali feldspar granite were started by alkali feldspar, followed by cotectic crystallization of alkali feldspar and quartz and ended by eutectic crystallization of all quartzofeldspathic minerals. The trachytes represent intra-plate continental alkaline magmatism. The Um Shager trachytes were developed at local extension zones, while the Hamadat trachytes were originated during global continental rifting system. Syenogranite, alkali feldspar granite and trachytes are important sources of alkali feldspar, which has wide spectrum applications in industry and agriculture.

Flank margin caves can connect to regionally extensive touching vug networks before burial: Implications for cave formation and fluid flow

AbstractFlank margin caves are extreme endmembers of vuggy porosity that form as diagenesis drives the progressive coalescence of smaller solutional pore spaces. Due to their morphological isolation during formation, the prevailing hypothesis has been that fluid flow in and out of flank margin caves occurs via the matrix permeability and that adjacent chambers only become hydraulically connected through nonmatrix porosity during burial, collapse, and fracturing. To our knowledge, however, no studies have evaluated how flank margin caves are connected to regional flow systems in modern carbonate platforms. In this study, we evaluate the connectivity of wells, boreholes, blue holes, and flank margin caves in increasingly older bedrock on San Salvador Island, Bahamas, using tidal attenuation analysis. Phreatic karst features are not reported in Holocene bedrock, and permeability magnitudes from wells suggest Holocene deposits connect to the ocean along matrix‐dominated flow paths. Permeability magnitudes in bedrock surrounding wells, boreholes, and karst features deposited during Marine Isotope Substage (MIS) 5e suggest connection to the ocean through matrix and touching vug porosity. Boreholes, blue holes, and flank margin caves in pre‐MIS5 bedrock connect to the ocean via touching vugs. We suggest that increasing bedrock permeability, cave number, and cave size observed within progressively older bedrock on San Salvador is a function of the cumulative number of freshwater lenses emplaced over successive sea‐level stillstands. We suggest that the morphologies of the two largest caves are consistent with dissolution in multiple lenses occupying lower elevations, collapse resulting in breakout domes, and overprinting of collapse chambers during subsequent highstands. As a result, some caves may not reflect connectivity of the bedrock surrounding the main chambers but may reflect connectivity of more diagenetically mature bedrock at lower elevations where their antecedent chambers formed.

The Eocene–Oligocene transition: a review of marine and terrestrial proxy data, models and model–data comparisons

Abstract. The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼34 million years ago (Ma) and lasting ∼790 kyr. The change is marked by a global shift in deep-sea δ18O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate-adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data–model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be under-sensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.

Grain assemblages and diagenesis in organic-rich mudrocks, Upper Pennsylvanian Cline shale (Wolfcamp D), Midland Basin, Texas

Grain assemblages in the organic-rich Cline shale in the Midland Basin are dominated by components of extrabasinal derivation (11.4 to 98.5 vol. %; average volume: 82.6%). Major extrabasinal components include K-rich clay minerals, detrital quartz, albite, K-feldspar, micas, and lithic fragments. Intrabasinal components include mainly biosiliceous allochems (sponge spicules and radiolarians), agglutinated foraminifera, Ca-phosphate peloids, clay-rich peloids, organomineralic aggregates, intraclasts, and other biocalcareous allochems. Authigenic minerals are most evident as grain replacements, euhedral ankerite, Ca-phosphate cement, and precipitates in large pores. A strongly localized spatial distribution of diagenetic products at micron to centimeter scales is observed in most siliciclastic samples, except in biosiliceous allochem-rich ones in which abundant intergranular pore-filling clay-size microquartz cement is observed. Compaction is evident in the Cline shale because of low porosity and generally low cement volumes. Neither textural variation nor bulk mineral composition alone is sufficient to confidently decipher the rock bulk property (e.g., total organic carbon) and reservoir quality variation (e.g., porosity and permeability). However, a good negative relationship between the ratio of extrabasinal to intrabasinal grains and favorable reservoir properties is observed in the Cline shale. Specifically, higher porosity, permeability, and total organic carbon are observed in samples representing the extreme end members of intrabasinal-derived biosilica-rich layers. Nickel, a recognized proxy for paleoproductivity, exhibits a positive relationship with intrabasinal grain content and reservoir properties. X-ray fluorescence–based analysis of nickel can be a rapid and cost-effective way to delineate favorable unconventional reservoir quality in the Cline shale.

Archean, highly unradiogenic lead in shallow cratonic mantle

Abstract Here, we present coupled geochemical and Sr-Nd-Pb-S isotopic data of Early Cretaceous primitive gabbros from the North China craton. Strikingly, these rocks have highly unradiogenic lead compositions (206Pb/204Pb = 16.58 ± 0.24) and anchor one extreme end member (low 206Pb/204Pb and 143Nd/144Nd) in the global array of oceanic-island volcanics. Our study shows that they originated from an Archean fluid-metasomatized refractory peridotite source, in which highly unradiogenic lead was preferentially released with subducted Archean seawater and sequestered into recrystallized sulfides at shallow mantle depths. Sulfide/silicate partition coefficients for lead show a negative pressure dependence: Lead is more enriched in sulfide with decreasing pressure. Sulfide-bearing and iron-poor harzburgite as well as dunite residues at shallow mantle are expected to develop low U/Pb (and thereby low time-integrated 206Pb/204Pb) relative to a deeper upper-mantle source. Our preferred interpretation is that an Archean, highly unradiogenic lead reservoir may be stored in the spinel-facies refractory cratonic mantle.

Pre- and syn-eruptive conditions of a basaltic Plinian eruption at Masaya Volcano, Nicaragua: The Masaya Triple Layer (2.1 ka)

The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style.By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080 and 1100 °C and 21–42 MPa, with a pre-eruptive volatile content of ~2 wt% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8–1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20 and 50 vol%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1–5 min, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt%, storage at moderate temperatures of 1080–1100 °C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.

Native American Land-Use Impacts on a Temperate Forested Ecosystem, West Central New York State

Land survey records (LSRs) describing forest species composition prior to extensive European American settlement are critical sources of information on past environmental controls of forest dynamics in eastern North America. Embedded within these historical data sources is evidence of prior Native American land use. This study expands on previous LSR-based analyses of Seneca and Iroquoian populations’ impacts on the temperate forests of west central New York State. We use an enhanced array of geospatial LSR vegetation data beyond conventional bearing tree data and implement, for the first time, combined indirect ordination of vegetation data along major environmental gradients and numerical classification of discrete upland vegetation communities. Nonmetric multidimensional scaling revealed three main drivers of vegetation dynamics in the study area: (1) fire frequency (53.7 percent of total variance); (2) soil productivity (22.6 percent variance); and (3) Native American land use (15.9 percent variance). Agglomerative hierarchical clustering reinforced the primacy of these gradients by delineating two major forest types differentiated primarily by fire frequency and secondarily by soil productivity. Seneca and Iroquoian agricultural villages were preferentially concentrated within fire-tolerant dry upland forests on high-productivity soils within the interior portion of the Lake Ontario Lowland. Fire-tolerant, dry upland forests on low-productivity soils were situated on the adjacent Appalachian Plateau, which was likely used by indigenous populations for silvicultural land-use activities. Native American disturbance of temperate forested ecosystems likely varied across the diverse culture areas of eastern North America, with the Seneca and Iroquois representing an extreme end-member within a broad continuum of anthropogenic disturbance. Key Words: forest composition, land survey records, land-use history, Native Americans, vegetation disturbance.

Geochronology and petrogenesis of the Qibaoshan Cu-polymetallic deposit, northeastern Hunan Province: Implications for the metal source and metallogenic evolution of the intracontinental Qinhang Cu-polymetallic belt, South China

The recently recognized Qinhang metallogenic belt (QHMB) is an economically important intracontinental Mesozoic porphyry-skarn Cu-polymetallic metallogenic belt in South China. However, the origin of the ore-bearing magma and the major factors controlling the different metal assemblages in the QHMB are still unclear. The Qibaoshan deposit is a large Cu–Au–Pb–Zn–Ag–Fe deposit located at the juncture between the northern and central parts of the QHMB. In this study, new zircon U–Pb ages, Hf–O isotopic data, molybdenite Re–Os ages, and whole-rock geochemical data are combined to constrain the timing of the mineralization and the origin and petrogenesis of the ore-bearing porphyry in the Qibaoshan deposit. The ages obtained from both zircon U–Pb and molybdenite Re–Os dating fall in the Late Jurassic (between 152.7 and 148.3 Ma), revealing that this deposit is significantly younger than previously estimated (227–184 Ma). The Qibaoshan ore-bearing quartz porphyry shows variable negative zircon εHf(t) values (−14.8 to −5.5), high δ18O values (8.4 to 10.8‰), and high Mg# values (69.1 to 73.0), indicating that it formed via the partial melting of ancient crust triggered by the injection of mantle-derived magma. Zircon Hf–O isotopic modeling of the mixing of two extreme endmembers indicates that the magmatic source comprised ~70–80% reworked ancient crustal components and ~20–30% depleted mantle components. Based on comparisons with other ore-bearing porphyries in the QHMB, a magmatic source dominated by crust-derived material and relatively low oxygen fugacities (ΔFMQ −1.8 to ΔFMQ +0.8) was responsible for the high (Pb + Zn)/Cu ratio in the Qibaoshan deposit, and the Pb, Zn and Ag were mainly derived from the reworked ancient crust. Although four analyses of inherited Neoproterozoic zircons (~800 Ma) have variable positive εHf(t) values (0.72 to 11.21), indicating that Neoproterozoic juvenile crust was involved in the formation of the Qibaoshan ore-bearing quartz porphyry, the relatively low oxygen fugacities (ΔFMQ −1.2 to ΔFMQ +0.4) of the parent magma of these inherited zircons suggest that this parent magma could not have provided significant metallic Cu for mineralization in the Qibaoshan Cu-polymetallic deposit. Therefore, the metallic Cu in the Qibaoshan Cu-polymetallic deposit was probably provided by the injection of the mantle-derived magma. Because ore-forming magmas with relatively low oxygen fugacities have low Au solubility, the large Au mineralization in the Qibaoshan deposit may be related to high background values of Au in this area. Our data, integrated with regional petrogeochemical data, indicate that the magmatic source exerted a first-order control on the different metal assemblages in the deposits in the QHMB.

Uncertainty in Future Summer Precipitation in the Laurentian Great Lakes Basin: Dynamical Downscaling and the Influence of Continental-Scale Processes on Regional Climate Change

Physics-based miniensembles of Weather Research and Forecasting (WRF) Model configurations have been employed to investigate future precipitation changes over the Great Lakes basin of eastern North America. All physics configurations have been employed to downscale multiple distinct Community Earth System Model, version 1 (CESM1), simulations driven by the representative concentration pathway 8.5 (RCP8.5) radiative forcing scenario, spanning a range from moderate (2045–60) to considerable (2085–2100) climate change. Independent of the physics configuration employed, all projected future precipitation changes are characterized by a general increase and a fattening of the tail of the daily rainfall distribution by the end of the century. The fattening of the tail can however be masked by natural variability in the case of the moderate warming expected by midcentury. The heavy-rainfall-derived precipitation increase is projected to be larger than or equal to the Clausius–Clapeyron thermodynamic reference of 7% increase per degree Celsius of surface warming, whereas the increase of average-rainfall-based precipitation becomes limited only for the largest global warming projections. This limitation is dramatically illustrated in one physics configuration at the end of the century. By downscaling the results obtained from the initial-condition ensemble, it is demonstrated that the extreme drying of the Great Lakes basin region characteristic of the most extreme end member of the CESM1 ensemble is significantly modified by downscaling with the version of WRF coupled to the Freshwater Lake model (FLake) of lake processes. This result does, however, depend upon the physics configuration employed in WRF for the parameterization of processes that cannot be explicitly resolved.

3D textural evidence for the formation of ultra-high tenor precious metal bearing sulphide microdroplets in offset reefs: An extreme example from the Platinova Reef, Skaergaard Intrusion, Greenland

The Platinova Reef in the Skaergaard Intrusion, east Greenland, is an example of a type of layered-intrusion-hosted, precious metal-enriched, stratiform, disseminated sulphide deposit referred to as “offset reefs”. These typically show platinum-group element (PGE) enrichment immediately below a major increase in the abundance of Cu-rich sulphides, with a prominent peak in Au enrichment exactly at that transition between the PGE-rich and the Cu-sulphide-rich zones. The reasons for the relative sequence of offsets in metal peaks, and the occasionally very high metal tenors have been subject to great debate. Here we use an integrated approach of high-resolution X-ray computed tomography (HRXCT), SEM, synchrotron and desktop microbeam XRF mapping, and thin section petrography to comprehensively classify the textural relations of the precious metal-bearing sulphides of the Platinova Reef as an extreme end member example of an exceedingly high tenor offset deposit. Our results show that in the zones of PGE enrichment, precious metal minerals (PMMs) are intimately associated with Cu sulphide globules, mostly located at, or close to, silicate and oxide boundaries. The textures are identical in zones enriched in Pd and Au, and thus we do not see any evidence for different processes forming the different zones. The PMM:Cu sulphide ratio in each globule varies significantly but overall the size of the globules increases from the Pd-rich, through the Au-rich, and into the Cu zone, with a significant corresponding decrease in PM tenor. As such, this records a progression of exceedingly high tenor, microdroplets of sulphide, which progressively get larger up through the section, and decrease in tenor proportionally to their size. Cumulus droplets of Cu sulphide became enriched in metals, and were trapped in situ without significant transport from their point of nucleation. The transition to larger sulphides represents a change from sulphides nucleated and trapped in situ, to larger ones that liquated from magma devoid of crystals, and that were able to grow and sink. This feature is common in all offset reef deposits, and is marked by the major enrichment in Au. Although the metal ratios of PGE to Au in the Pd- and Au-rich offset zones differ, the identical textures and comparable mineralogy show the physical mechanisms of concentration are the same, indicating a similar physical method of concentration. The relative position of the Pd, Au and Cu peaks in the Platinova Reef is essentially the same as that in numerous other offset reefs, suggesting that common overarching processes are responsible for the enrichment in metals, and relative offsets in peak metal concentrations in all such deposits. The most important of these processes are their relative Dsul/sil values and the diffusivities of the metals, which determine the order of offsets and the high tenors of the smallest sulphide droplets. The Platinova Reef therefore records the extreme enrichment via equilibrium and diffusive partitioning into sulphide liquid microdroplets very close to their point of nucleation.

A cone on Mercury: Analysis of a residual central peak encircled by an explosive volcanic vent

We analyse a seemingly-unique landform on Mercury: a conical structure, encircled by a trough, and surrounded by a 23,000km2 relatively bright and red anomaly of a type interpreted elsewhere on the planet as a pyroclastic deposit. At first glance, this could be interpreted as a volcanically-constructed cone, but if so, it would be the only example of such a landform on Mercury. We make and test the alternative hypothesis that the cone is the intrinsic central peak of an impact crater, the rim crest of which is visible beyond the cone-encircling trough, and that the trough is a vent formed through explosive volcanism that also produced the surrounding bright, red spectral anomaly. We test this hypothesis by comparing the morphology of the cone and the associated landform assemblage with morphologically-fresh impact craters of the same diameter as the putative host crater, and additionally, by modelling the original morphology of such a crater using a hydrocode model. We show that the present topography can be explained by formation of a vent completely encircling the crater’s central peak and also make the observation that explosive volcanic vents frequently occur circumferential to the central peaks of impact craters on Mercury. This indicates that, although this cone initially appears unique, it is in fact an unusually well-developed example of a common process by which impact-related faults localize magma ascent near the centre of impact craters on Mercury, and represents an extreme end-member of the resulting landforms.

Gulf of Alaska continental slope morphology: Evidence for recent trough mouth fan formation

AbstractGlaciated continental shelves are host to numerous morphologic features that help understand past glacier dynamics. Southeastern Alaska is home to the St. Elias mountains, an active orogen that also hosts temperate marine glaciers. During glacial periods ice streams advance across the continental shelf, carving shelf‐crossing troughs that reach the shelf edge. We use high‐resolution multibeam data to develop the relationship between the Yakutat and Alsek Sea Valleys and the resulting continental slope morphology. The shelf and slope geomorphology can be divided into statistical groupings that relate to the relative balance of erosion and deposition. Our analysis indicates that only the Yakutat system has been able to build an incipient trough‐mouth fan. The extreme sediment supply from this region was able to overwhelm the steep initial topography of the transform margin while further to the east sediment slope‐bypass dominates. This analysis provides an extreme end member to existing studies of temperate glaciation along continental margins. The unique interplay between rapid uplift due to ongoing collision and the massive erosion caused by temperate glaciers provides for sedimentary flux far above most other systems.

Melt stagnation in peridotites from the Godzilla Megamullion Oceanic Core Complex, Parece Vela Basin, Philippine Sea

The Godzilla Megamullion, located in the Parece Vela Backarc Basin of the Izu–Bonin–Mariana (IBM) system, is the largest known example of an Oceanic Core Complex (OCC). Peridotites recovered from the megamullion are divided petrographically into fertile (e.g. lherzolites), depleted (e.g. harzburgites), and plagioclase-bearing groups (Ohara et al., 2003a). A total of 151 thin sections were studied from the Kairei KR03-01, Hakuho Maru KH07-02, and Yokosuka YK09-05 cruises. Melt stagnation is studied via the incidence of plagioclase-bearing peridotites and the major element chemistry of Cr-spinels in the plag-bearing samples. A distinct trend in melt stagnation is evident along the length of the megamullion representing a secular evolution in the entrapment of melts rising through the lithosphere. The distal (furthest from the termination of spreading), depleted portion of the mullion represents a robust mantle section that was still producing abundant melt and can be compared to typical oceanic spreading with its relatively “normal” percentage of plagioclase peridotites and average spinel Cr# of 0.35. The medial, fertile portion of the mullion represents a steep falloff in melt productivity represented by fertile spinel compositions (i.e. Cr# <0.25) and the presence of plagioclase-free lherzolites. The proximal (closest to termination of spreading), heavily plagioclase impregnated portion (with spinel Cr#s covering nearly the entire range of abyssal peridotite spinel compositions) of the mullion then represents a period of increasing stagnation of melt into a lithosphere that was undergoing progressive thickening. We infer that the processes of mantle evolution through melt stagnation and impregnation, as evidenced by the systematic variations in plag–peridotites along Godzilla Megamullion, represent a possibly common way in which the mantle reacts to OCC formation. In this case, Godzilla Megamullion may represent an extreme endmember in OCC formation.

Eruption-triggered mixing of extra-caldera basalt and rhyolite complexes along the East Gallatin–Washburn fault zone, Yellowstone National Park, WY, USA

Though mixing and commingling of magmas is common, mixing between rhyolite and basalt magmas is not commonly preserved in volcanic rocks. The presence of at least four mixed magma complexes at Yellowstone National Park suggests that mingling is not due to random intersections of feeder dikes, geochemical analyses also show that though these magmas appear to be commingled, there is mixing between the two disparate end members. Our model combines previous work on the Grizzly Lake, Gardner River, Crystal Spring, and Appolinaris Spring mixed magma complexes with results from new analyses, recent mixing experiments, and regional structural geology. Coeval extensional tectonism, as seen in the East Gallatin–Washburn fault zone, is also present in other areas of basalt and rhyolite mixing/mingling (e.g. Iceland). The central portions, or core, of the complexes contain increased concentrations of emulsion rock, occasional basaltic pillows in a rhyolite matrix, net veining, and mixed magma with highly variable geochemistry (SiO2 ranges from 50 to 78wt.%). Phenocrysts have been transferred between mafic and felsic portions of the complexes and suggest that these mixed magmas did not have enough time, or energy (e.g. heat), to thoroughly mix into complete hybrid intermediate magmas. This implies that mixing occurred during eruption. Furthermore, analyses at the micron-scale suggest that zones of chaotic mixing between basalt and high-silica rhyolites may be more complete than previously thought during mixing of high silica rhyolites and basalts with greater than 4wt.% MgO. The temperature of the rhyolitic magmas was approximately 850°C with a viscosity between 1×106 and 3×106Pas. The basalt was approximately 1070°C with a viscosity of 2×102 to 9×103Pas prior to mixing. Mixing of these two extreme end members may have required decompression of the lower basaltic magma chamber during eruption of the overlying rhyolitic magma chamber into through structurally weakened zones of the upper crust. The decompression induced mixing was facilitated by coeval extensional tectonics and structures in the Norris–Mammoth Corridor. Extra-caldera rhyolites are some of the first volcanics following the collapse of the current caldera and also some of the youngest volcanics in the Yellowstone Plateau and require further understanding of volcanism in the Yellowstone Plateau.

Grain assemblages and strong diagenetic overprinting in siliceous mudrocks, Barnett Shale (Mississippian), Fort Worth Basin, Texas

Porosity, permeability, and total organic carbon (TOC) in a heterogeneous suite of 21 high-maturity samples (vitrinite reflectance 1.52–2.15%) from the Barnett Shale in the eastern Fort Worth Basin display few correlations with parameters of rock texture, fabric, and composition, these factors being mostly obscured by the effects of a protracted history of diagenesis. Diagenesis in these rocks includes mechanical and chemical modifications that occurred across a wide range of burial conditions. Compaction and cementation have mostly destroyed primary intergranular porosity. The porosity (average 5 vol. % by Gas Research Institute helium porosimetry) and pore size (8 nm median pore-throat diameter) are reduced to a degree such that pores are difficult to detect even by imaging Ar ion–milled surfaces with a field-emission scanning electron microscope. The existing porosity that can be imaged is mostly secondary and is localized dominantly within organic particulate debris and solid bitumen. The grain assemblage is highly modified by replacement. A weak pattern of correlation survives between bulk rock properties and the ratio of extrabasinal to intrabasinal sources of siliciclastic debris. Higher porosity, permeability, and TOC are observed in samples representing the extreme end members of mixing between extrabasinal siliciclastic sediment and intrabasinal-derived biosiliceous debris. Reservoir quality in these rocks is neither more strongly nor more simply related to variations in primary texture and composition because the interrelationships between texture and composition are complex and, importantly, the diagenetic overprint is too strong.

Paleoglaciological reconstructions for the Tibetan Plateau during the last glacial cycle: evaluating numerical ice sheet simulations driven by GCM-ensembles

The Tibetan Plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate. However, the glacial history of the Tibetan Plateau is more poorly constrained than that of most other formerly glaciated regions such as in North America and Eurasia. On the basis of some field evidence it has been hypothesized that the Tibetan Plateau was covered by an ice sheet during the Last Glacial Maximum (LGM). Abundant field- and chronological evidence for a predominance of local valley glaciation during the past 300,000 calendar years (that is, 300 ka), coupled to an absence of glacial landforms and sediments in extensive areas of the plateau, now refute this concept. This, furthermore, calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence. Surprisingly, the robustness of such numerical ice sheet model results has not been widely queried, despite potentially important climate ramifications. We simulated the growth and decay of ice on the Tibetan Plateau during the last 125 ka in response to a large ensemble of climate forcings (90 members) derived from Global Circulation Models (GCMs), using a similar 3D thermomechanical ice sheet model as employed in previous studies. The numerical results include as extreme end members as an ice-free Tibetan Plateau and a plateau-scale ice sheet comparable, in volume, to the contemporary Greenland ice sheet. We further demonstrate that numerical simulations that acceptably conform to published reconstructions of Quaternary ice extent on the Tibetan Plateau cannot be achieved with the employed stand-alone ice sheet model when merely forced by paleoclimates derived from currently available GCMs. Progress is, however, expected if future investigations employ ice sheet models with higher resolution, bidirectional ice sheet-atmosphere feedbacks, improved treatment of the surface mass balance, and regional climate data and climate reconstructions.

Structural analysis of fault populations along the oblique, ultra-slow spreading Knipovich Ridge, North Atlantic Ocean, 74°30′N-77°50′N

The Knipovich Ridge (73°30′–78°40′N) is an extreme end-member of the mid-ocean ridge spreading system, both in terms of spreading rate (<1.5 cm/yr) and angle between the ridge and the spreading direction (40°–53°). Structural analysis of side-scan sonar images obtained along ∼400 km of the ridge axis reveals systematic relationships between fault population parameters, obliquity, and axial segmentation along the ridge. Fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems. For the ridge as a whole, faults in the axial region are short, straight, isolated (not linked into complex fault zones), and exhibit length-scaling relationships characteristic of young and active fault systems. Faults are generally oblique to both the ridge axis and the spreading direction, and orientations vary systematically with angle between ridge and spreading direction. Along-axis analysis reveals the influence of axial segmentation on fault population characteristics. Segment centers are dominated by faults perpendicular and sub-perpendicular to plate motion with longer characteristic length and generally lower fracture density. Conversely, segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density. We infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion, while faulting in segment ends is controlled by the mechanics of oblique rifting, non-transform discontinuities, and/or accommodation zones. The contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra-slow spreading rate of this spreading system.

Cosmogenic nuclide concentrations in episodically eroding surfaces: Theoretical results

The buildup of cosmogenic nuclides in an eroding surface can be used to infer erosion rates and exposure ages. This situation is often modelled by assuming that the erosion rate is constant in time. In many cases, however, the erosion is episodic: surface denudation occurs by the spalling off of slabs of rock at discrete times. We consider a stochastic model of such exfoliation processes, and compute the expected behavior of the cosmogenic nuclide concentration. We also consider a nonstochastic model, in which the exfoliation events occur at regularly spaced intervals. These two models represent extreme end members of the episodic spallation scenarios: in the first, the spallation events are uncorrelated in time, while in the second they are tightly correlated. Understanding how the nuclide concentration is related to the timing and the magnitude of these events is important in making geologic inferences.

New constraints on the petrogenesis of the Nuanetsi picrite basalts from Pb and Hf isotope data

Previous studies established strong linear correlations between 143Nd/ 144Nd and a variety of incompatible element abundances and inter-element ratios for a suite of picrite basalts from Nuanetsi in the Karoo CFB province. While a heterogeneous source was clearly indicated, the extent to which mantle heterogeneity exceeds that sampled by magmatism was unconstrained because the linear correlations can be extrapolated to extreme end-members well beyond the picrite basalt compositions. The Nuanetsi samples showed a small variation in Pb isotope composition and any coherent Pb–Nd isotope relationship was obscured by the likely analytical uncertainty. New Pb isotope data obtained by MC-ICP-MS reveal coherent Pb–Nd isotope arrays that are suggestive of hyperbolic mixing curves. New Hf isotope data reveal very similar Pb–Hf relationships. Least-squares hyperbolic fits are used to constrain the asymptotes of these putative mixing hyperbolae, thereby defining end-member isotopic compositions. The low ε Nd, low ε Hf end-member equates with the enriched lithospheric mantle component previously postulated, and it is now possible to define the Pb isotope characteristics of this end-member more precisely. The higher ε Nd and ε Hf end-member is constrained to have sub-chondritic 143Nd/ 144Nd and 176Hf/ 177Hf. These characteristics are not readily reconciled with an asthenospheric source because there are few oceanic basalts with sub-chondritic ε Nd and it seems unlikely that the Karoo would fortuitously have sampled an extreme asthenospheric composition. Either, the Nuanetsi picrite basalts are derived largely from lithospheric mantle, or they sample a more complex mixture of source materials than previously inferred.