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Journal Geology v38, n10
Subject: Geology Type: Journal

Author: Kenneth G. Miller, Robert M. Sherrell, James V. Browning, M. Paul Field, W. Gallagher, Richard K. Olsson, Peter J. Sugarman, Steven Tuorto, and Hendra Wahyudi ,Stuart A. Robinson, Daniel P. Murphy, Derek Vance, and Deborah J. Thomas ,Daniel J. Lunt, Paul J. Valdes, Tom Dunkley Jones, Andy Ridgwell, Alan M. Haywood, Daniela N. Schmidt, Robert Marsh, and Mark Maslin ,Bernard W. Evans and et al.,

Publication Date: Octo 2010  Publication No: v38, n10Pages Number: 

-Relationship between mass extinction and iridium across the Cretaceous-Paleogene boundary in New Jersey
   We directly link iridium (Ir) anomalies in New Jersey to the mass extinction of marine plankton marking the Cretaceous-Paleogene (K-Pg) boundary. We confirm previous reports of an Ir anomaly 20 cm below the extinction of Cretaceous macrofauna (the “Pinna” bed) with new results from a muddy sand section from Tighe Park, Freehold, New Jersey (United States), but we also show that Ir anomalies correlate with marine mass extinctions at three other clay-rich New Jersey sections. Thus, we attribute the anomaly at Freehold to the downward movement of Ir and reaffirm the link between impact and mass extinction.
   -Formation of “Southern Component Water” in the Late Cretaceous: Evidence from Nd-isotopes
   Constraining deep-ocean circulation during past greenhouse climatic periods, such as the Cretaceous, is important for understanding meridional heat transfer processes, controls on ocean anoxia, and the relative roles of climate and tectonics in determining paleocirculation patterns. Ocean circulation models for the Late Cretaceous and early Paleogene suggest that significant deep-water production occurred in the Southern Ocean, but cannot constrain when this process commenced or what the temporal relationship was between opening tectonic gateways and Late Cretaceous climatic cooling. Nd-isotope data obtained from biogenic apatite (fish teeth and bones) are presented from lower bathyal and abyssal sites in the South Atlantic and Indian Oceans. During the mid-Cretaceous, relatively radiogenic Nd-isotope values suggest that deep-water circulation in these basins was sluggish with inputs likely dominated by seawater-particle exchange processes and, possibly, easily weathered volcanic terranes. In the Campanian–Maastrichtian the Nd-isotopic composition of proto-Indian and South Atlantic deep waters became less radiogenic, suggesting the onset of deep-water formation in the Southern Ocean (Southern Component Water, SCW), consistent with Paleogene reconstructions and ocean circulation models. A combination of Southern Hemisphere cooling and the opening of tectonic gateways during the Campanian likely drove the onset of SCW.
   -CO2-driven ocean circulation changes as an amplifier of Paleocene-Eocene thermal maximum hydrate destabilization
   Changes in ocean circulation have been proposed as a trigger mechanism for the large coupled climate and carbon cycle perturbations at the Paleocene-Eocene Thermal Maximum (PETM, ca. 55 Ma). An abrupt warming of oceanic intermediate waters could have initiated the thermal destabilization of sediment-hosted methane gas hydrates and potentially triggered sediment slumps and slides. In an ensemble of fully coupled atmosphere-ocean general circulation model (AOGCM) simulations of the late Paleocene and early Eocene, we identify such a circulation-driven enhanced intermediate-water warming. Critically, we find an approximate twofold amplification of Atlantic intermediate-water warming when CO2 levels are doubled from 2× to 4× preindustrial CO2 compared to when they are doubled from 1× to 2×. This warming is largely focused on the equatorial and South Atlantic and is driven by a significant reduction in deep-water formation from the Southern Ocean. This scenario is consistent with altered PETM circulation patterns inferred from benthic carbon isotope data and the intensity of deep-sea carbonate dissolution in the South Atlantic. The linkage between intermediate-water warming and gas hydrate destabilization could provide an important feedback in the establishment of peak PETM warmth.
   -Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life(?)
   The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50–300 °C), where lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich lizardite, magnetite, and dihydrogen gas (±brucite) are products of the reaction. At higher temperatures (400–600 °C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge.
   -Roveacrinids (Crinoidea, Echinodermata) survived the Cretaceous-Paleogene (K-Pg) extinction event
   Although crinoids appear not to have been involved in the great change in diversity at the Cretaceous-Paleogene (K-Pg) boundary extinction event, it has been assumed that representatives of order Roveacrinida became extinct during this time. Well-preserved fossils from the Danian (early Paleocene) of Poland demonstrate that these crinoids survived into the earliest Cenozoic. This find merits the qualification of this order as a “dead clade walking.”
   -ERRATUM: Equivalence of abrupt grain-size transitions in alluvial rivers and eolian sand seas: A hypothesis
   -Seismic anisotropy associated with continental lithosphere accretion beneath the CANOE array, northwestern Canada
   We examine upper-mantle seismic anisotropy beneath the cordillera and craton of northwestern Canada using the multi-event station averages of shear-wave splitting of SKS, SKKS, and sSKS phases recorded during the Canadian Northwest Experiment (CANOE). Splitting times derived from multi-event averaging at each station range from 0 to 1.4 s, with an array average of ~0.65 s. Over broad portions of the array, fast directions are coherent and roughly consistent with the direction of absolute plate motion in a hotspot reference frame, suggesting that coherent asthenospheric fabric underlies the plate in much of the region. Within this broad framework, fast directions and splitting times show marked variability over length scales of 50–200km, and are generally correlated with surface and/or crustal tectonics. Anomalous splitting is observed across an ancient suture within the cratonic lithosphere, apparently associated with complex dipping fabric produced during continental assembly. The deformation front of the Canadian Rockies correlates with a significant change in splitting behavior, consistent with the front range demarking the craton-cordillera transition within the mantle. Splitting times are small across much of the cordillera, indicating that lithospheric and/or asthenospheric fabric is weaker or less coherent than beneath the craton. In the western cordillera, fast directions rotate abruptly to parallel the plate boundary, implying that fabric associated with plate-boundary deformation extends ~200km into the North American continent.
   -Small, isolated glacial catchments as priority targets for cosmogenic surface exposure dating of Pleistocene climate fluctuations, southeastern Pyrenees
   A new 10Be data set from a small glacial catchment in the Pyrenees confirms almost full glacier extent during Marine Isotope Stage (MIS) 2, extends the oldest cosmogenic exposure ages of moraines in this mountain range to MIS 5, and constrains paleoclimate reconstructions in the Mediterranean region. It also illustrates the advantageous conditions for the long-term preservation of terminal moraines provided by small, low-gradient glacier catchments that did not merge with compound ice masses even during glacial maxima. These small, gently sloping, and isolated glacier catchments are proposed as priority targets for terrestrial cosmogenic nuclide dating of peak Pleistocene glacier advances, providing scope for greater accuracy in interregional paleoclimatic correlations.
   -The Yakutat terrane: Dramatic change in crustal thickness across the Transition fault, Alaska
   We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24–27 km thick overlain by sedimentary units that are 4.5–7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ~1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0–5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.
   -Astronomical tuning of the Aptian Stage from Italian reference sections
   A high-resolution grayscale series of the pelagic Fucoid Marls (Piobbico core, central Italy) shows strong, pervasive lithological rhythms throughout the Aptian interval. A hierarchy of centimeter- to meter-scale cycles characterizes the rhythms; when calibrating ~1 m cycles to Earth's 405 k.y. orbital eccentricity cycle, these rhythms correspond to the periods of the eccentricity, obliquity, and precession index. Tuning to orbital eccentricity cycles provides a high-resolution time scale for the Aptian. Correlation to the Cismon core (northern Italy) extends the tuning to the Aptian-Barremian boundary. The tuning indicates a minimum duration of 13.42 m.y. for the Aptian Stage, where previous estimates range from 6.4 to 13.8 m.y. The combined Aptian–Albian astronomical tuning of the entire 77-m-long Piobbico core (and part of the Cismon core) provides a 25.85-m.y.-long astronomically calibrated time scale for Earth history.
   -Timing the deposition of 17O-depleted barite at the aftermath of Nantuo glacial meltdown in South China
   Barite in basal Ediacaran cap carbonates in South China shows distinct, non-mass-dependent depletion in 17O, a signal most likely reflecting an extremely high pCO2 atmosphere during Marinoan glacial meltdown. The precise geological context of the barite within the cap carbonate was not defined, however, and such information is crucial to an accurate interpretation of the anomalous 17O signal. Based on an extensive field survey of Marinoan cap carbonates in South China and detailed sedimentological, petrographic, and isotope data, we propose here a unified sequence of events that followed the deposition of the Nantuo diamictite: the cap dolostone was first deposited and later uplifted due to isostatic rebound, undergoing karstic dissolution in both shallow platform and transitional facies of the Yangtze Block; subsequent transgression initiated a consistent sequence of mineral deposition on karstic surfaces or within cavities. The 17O-depleted barite, either visible on outcrops or identifiable in thin sections, is among the first minerals precipitated when the transgression flooded the karstified cap dolostone. The calcite with extremely negative d13C values occurs at the last stage of the cavity filling, well after the deposition of cap dolostone and the 17O-depleted barite, suggesting that the signal of methane hydrate was registered much later in the cap dolostone. A similar recognition of karstic dissolution and subsequent barite deposits in cap dolostones in northwest Africa and northwest Canada suggest that deposition of the 17O-depleted barites may be a global event, recording an atmospheric-biological-hydrological condition shortly (within 0–1.6 m.y.) after the initial meltdown of the Marinoan snowball Earth.
   -Poroelastic triggering in the 9–22 January 2008 Nima-Gaize (Tibet) earthquake sequence
   A sequence of three earthquakes of Ms = 6.4, 5.9, and 5.4 occurred in southern central Tibet on 9, 16, and 22 January 2008. Interferometric satellite aperture radar (InSAR) observations indicate that the two largest events occurred on two northeast-trending, northwest-dipping normal faults, separated by ~9.5 km, with slip reaching 1.2 m on the main fault and 0.75 m on the second. We develop a three-dimensional finite element model to calculate the Coulomb stress change due to poroelastic deformation in the crust after the first event. Using a uniform permeability of 8 × 10-15 m2 for faults and the host rock leads to negligible temporal pore pressure change in the two weeks after the first event. However, assuming higher permeability along faults (10-14 to 10-11 m2), the second fault channels the flow of pore fluids upward from the deeper region where the pore pressure increase was the largest immediately after the first event. In this case, the poroelastic Coulomb stress on the second fault increased suddenly by ~1 × 105 Pa after the first earthquake and continued to increase by as much as 70% in the subsequent week. Furthermore, the distribution of Coulomb stress change on the second fault correlates precisely with the slip distribution inferred for the second event. These findings suggest that the first earthquake not only triggered the second event, but the Coulomb stress change it induced on the second fault plane controlled the extent of the second rupture.
   -Aragonite in olivine from Calatrava, Spain—Evidence for mantle carbonatite melts from >100 km depth
   Aragonite, as an inclusion in olivine from a leucitite lava flow, provides evidence for high-pressure crystallization and carbonatitic activity beneath the geophysical lithosphere in Calatrava, Spain. The aragonite occurs as a single crystal within olivine (Fo87), interpreted to have crystallized from a carbonated silicate melt at mantle depths. Experimental data constrain the stability of aragonite to depths of >100 km at CO2-H2O-bearing mantle solidus temperatures. This is the first documented evidence of magmatic aragonite crystallized in the mantle. Entrained as xenocrysts, the olivines have not crystallized from the carrier melts, which must have formed deeper within the mantle. Lead isotope data of the leucitite and carbonate inclusions indicate that the source melts show isotopic enrichment relative to mid-oceanic ridge basalt and most ocean island basalt. Our evidence strengthens the argument for direct and deep mantle-derived volcanic carbonatite in alkaline volcanic provinces containing maar-type volcanism, such as Calatrava.
   -Evidence from zircon U-Pb age spectra for crustal structure and felsic magma genesis at Taupo volcano, New Zealand
   U-Pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at Taupo volcano, as well as unexpected crustal structure beneath the Taupo Volcanic Zone. Ages from the xenolith in a 28 ka rhyolite dome match those from Early Cretaceous Pahau Terrane graywackes that crop out >75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process. Boundaries between graywacke terranes in the North Island are thus more complex than implied from map views. In contrast, ages from zircons in the 20 ka Omega dacite match the nearby surficial graywacke of the Jurassic Kaweka Terrane (55% of grains), Quaternary plutonic sources (39%), and Pahau Terrane graywacke (6%). Five of 72 grains analyzed contain graywacke cores and Pleistocene rims. Survival of euhedral xenocrystic zircons through the generation processes for Omega dacite magma reflects wholesale melting of crustal protoliths (including varieties not previously considered for petrogenetic modeling), open-system behavior, and crustal melts being generated and erupted within ~1–10 yr.
   -Pre-eruptive reheating during magma mixing at Quizapu volcano and the implications for the explosiveness of silicic arc volcanoes
   Effusive-explosive transitions are observed in volcanoes erupting water-rich magmas that lack mineralogical evidence of degassing, suggesting that gas loss occurred rapidly and ascent was fast despite the high viscosity of rhyodacitic to rhyolitic melts. Here, we show that pre-eruptive heating of water-rich, silicic magma by mixing with hot recharge in subvolcanic chambers controls the effusive-explosive transition. Volcán Quizapu (Chile) emitted a 5 km3 mingled dacite-andesite lava flow in A.D. 1846–1847 that was ~130 °C hotter than the 5 km3 of the same dacite that lead to a Plinian eruption in A.D. 1932. We explain the suppression of explosive fragmentation in the 1846–1847 lavas by enhanced syneruptive magma degassing as a consequence of late reheating following mixing with andesite recharge. Higher magma temperatures led to the transitioning into effusive eruptive behavior by significantly accelerating volatile diffusion and lowering melt viscosity, which facilitated bubble nucleation, growth, and coalescence, while also inhibiting brittle fragmentation. Thus, recharge by mafic magmas in subvolcanic magma chambers may reduce the risk of explosive eruptions in cases where heat can be efficiently transferred to cold, volatile-rich silicic magmas.
   -Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite
   The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra–marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an “infinite reservoir” such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced.
   -Paleogeographic implications of non–North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA
   A non-North American provenance for the lower Belt Supergroup of North America has been used to support various pre-Rodinian paleogeographic reconstructions. Unlike the lower Belt Supergroup, most upper Belt Supergroup provenance studies have inferred Laurentian sediment sources. We test this hypothesis by analyzing U-Pb and Lu-Hf isotopes on detrital zircons, and whole-rock Nd isotopes from the Missoula (upper Belt Supergroup) and Lemhi Groups, and comparing to possible Laurentian sources. Detrital zircons from 11 sandstones analyzed show dominant ages between 1680 and 1820 Ma. These zircons are predominantly magmatic in paragenesis. Belt Supergroup–aged (1400–1470 Ma) and 2400–2700 Ma populations represent minor components. Lu-Hf isotopic analyses for 1675–1780 Ma Missoula Group and Lemhi Group detrital zircons range from eHf(i) +9 to –12 and +8 and –7, respectively. Belt Supergroup–aged grains from the Bonner Formation, Missoula Group, have eHf(i) values between +5 and –9, exceeding coeval ranges from the Mojave and Yavapai terranes [eHf(i) between +5 and 0]. Whole-rock Nd isotopes from Lemhi Group argillites yield a range in eNd(1400) between +1.1 and –5.9. Immature feldspathic sediment, nearly unimodal detrital zircon spectra, and dissimilar Belt Supergroup–aged zircon Hf signatures suggest that distal portions of the Yavapai and Mojave terranes intruded by A-type magmas were not the source for the Missoula and Lemhi Groups. Instead, a slightly modified Mesoproterozoic proto-SWEAT (southwestern United States and East Antarctica) model can best account for the sedimentologic and isotopic characteristics of the Missoula and Lemhi Groups. An alternative model with a source from southeastern Siberia and the Okhotsk Massif is less preferred.
   -The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons
   U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Late Cretaceous McCoy Mountains Formation of southwestern Arizona (United States) are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south–trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region flowed northeast ~700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green River–Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau.
   -Crinoids for lunch? An unexpected biotic interaction from the Upper Ordovician of Scotland
   Specimens of the polyplacophoran mollusk ‘Helminthochiton’ thraivensis Reed from the Upper Ordovician of southwest Scotland provide rare examples of complete valve series preserved in near life position, albeit as external molds. Application of high-resolution X-ray microtomography to one such specimen has revealed the exceptional preservation of its last meal, which included elements of a crinoid column, in its intestine. The interaction was either predatory or scavenging; extant chitons are not known to be crinoidivorous. This is the earliest direct record of predation or scavenging on crinoids in the fossil record. It is also the first indication that the broad axial canal of primitive crinoids may have contained nutritious tissues. The predatory or scavenging habit of H. thraivensis is consistent with its inferred phylogenetic position as a stem-group aplacophoran and provides new data suggesting an origin of carnivory early in the evolution of this clade.
   -Submarine fans at all sea-level stands: Tectono-morphologic and climatic controls on terrigenous sediment delivery to the deep sea
   Early models of stratigraphic sequence development across continental margins predicted terrigenous sediment delivery to the deep sea principally during periods of sea-level fall and lowstand. However, subsequent models acknowledged caveats to these early predictions. We employ a global database of millennial-scale continental-margin deep-sea deposition rates, activities of canyon-channel systems, and episodes of submarine-fan growth since 35 ka to assess the timing of terrigenous sediment delivery to the deep sea. As predicted by the early models, the majority of continental-margin deep-sea deposition occurred during periods of marine transgression and lowstand of sea level. However, deep-sea deposition is shown to have occurred at all sea-level states, and some deep-sea fans predominantly grew during periods of sea-level highstand. Terrigenous sediment delivery to the deep sea depends on factors such as tectono-morphologic character of the margin (e.g., narrow shelves) and climatic forcings (e.g., subglacial meltwater and monsoonal pulses). A fundamental understanding of the tectonic and climatic forcings inherent to a continental margin and its terrestrial sediment source area is essential to accurately predict timing and magnitude of deep-sea deposition and place it in the context of sequence stratigraphy.
   -Active drumlin field revealed at the margin of Múlajökull, Iceland: A surge-type glacier
   Recent marginal retreat of Múlajökull, a surge-type outlet glacier of Hofsjökull, Iceland, has revealed a drumlin field consisting of more than 50 drumlins. The drumlins are 90–320 m long, 30–105 m wide, 5–10 m in relief, and composed of multiple beds of till deposited by lodgment and bed deformation. The youngest till layer truncates the older units with an erosion surface that parallels the drumlin form. Thus, the drumlins are built up and formed by a combination of subglacial depositional and erosional processes. Field evidence suggests each till bed to be associated with individual recent surges. We consider the field to be active in the sense that the drumlins are shaped by the current glacial regime. The Múlajökull field is the only known active drumlin field, and is therefore a unique analogue to Pleistocene drumlin fields.
   -Wave-enhanced sediment-gravity flows and mud dispersal across continental shelves: Reappraising sediment transport processes operating in ancient mudstone successions
   Recent studies of marine shelf sediment dispersal show that wave-enhanced sediment-gravity flows are widespread phenomena and can transport large volumes of fluid mud rapidly across low-gradient shelves. Flow evolution is controlled by sediment supply, seabed gradient, and spatial distribution of wave energy at the seabed. Using existing flow models, we predict that such flows in mud-dominated sediments will develop a three-part microstratigraphy produced by changing flow conditions, beginning with wave-induced turbulent resuspension, then development of a wave-enhanced sediment-gravity flow, prior to lutocline collapse and suspension settling. Petrographic examination of modern flow deposits collected from the Eel Shelf reveals that resultant beds possess a microstratigraphy consistent with our hypothesis: a silt-rich basal subunit with curved ripple laminae, abruptly overlain by a subunit composed of continuous intercalated silt/clay laminae, and an upper clay-rich drape. Analyses of beds from ancient mud-rich outer-shelf and basinal successions (Cleveland Ironstone, Jurassic, UK, and Mowry Shale, Cretaceous, United States) show that they too contain beds with this three-part organization, suggesting that such flows were active in these ancient settings too. Recognition of these microstructures in these ancient mud-dominated successions demonstrates that sediment in these settings was commonly reworked and transported advectively downslope by high-energy events, contrasting with previous interpretations of these units that deposition was dominated by quiescent suspension settling. Identification of these recognition criteria now allows the products of this newly recognized sediment dispersal mechanism to be identified in other shale-dominated successions.
   -Wetting facilitates late-stage segregation of precious metal–enriched sulfosalt melt in magmatic sulfide systems
   The wetting behavior of As-rich sulfosalt melts against monosulfide solid solution (MSS) is investigated using experiments to elucidate late-stage fractionation processes in magmatic sulfide systems, which may control the distribution of platinum group elements. A range of As-rich melt compositions is found to wet MSS, including those that contain significant proportions (1%–45%) of precious metals (Pt, Pd, and Au). However, extremely Au rich or Pt rich sulfosalt melts (>~40% Au; >~50% Pt + 5% Au) do not wet MSS. These results imply that if magma contamination and/or fractionation processes were able to produce a late-stage As-rich melt (sulfosalt melts crystallize cooler than MSS) that exceeds ~0.2% of the rock volume, then an interconnected melt drainage network would be able to form along MSS crystal triple junctions. The dense sulfosalt melt could thereby drain downward, progressively sequestering incompatible Bi, Sb, Te, Pt, Pd, and Au to form sulfosalt melt accumulations that continue to fractionate to form the platinum group minerals. This late-stage fractionation model is consistent with the observed mineral distribution in numerous magmatic sulfide deposits.
   -Quaternary origin of the inverse latitudinal diversity gradient among southern Chilean mollusks
   Mollusks along the Pacific coast of South America provide one of the main exceptions from the latitudinal diversity gradient of decreasing species numbers toward the poles. From 42°S southward, coincident with the onset of the mosaic coastline of the southern Chilean fiordlands, species numbers show a twofold to threefold increase compared to the central Chilean coast. Here we use the Neogene fossil record of central and southern Chile to test hypotheses on the origin of this unusual pattern. Contrary to previous suggestions, we did not find evidence for a buildup of biodiversity since the Eocene, or for the northward spread of Antarctic taxa into southern mid-latitudes. Rather, our data suggest that the mosaic coastline south of 42°S was colonized after the retreat of the glaciers from their marine termini since the later Pleistocene, by taxa that were already present along the Chilean coast. The majority of the most species-rich genera in this area are shallow-water hardground inhabitants, suggesting a rapid adaptive radiation of these taxa. The unusually low present-day diversity to the north of 42°S most likely results from the severe late Neogene extinction and only moderate subsequent origination in this area.

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