From continent to intra-oceanic arc: zircon xenocrysts record the crustal evolution of the Solomon island arc

The first U-Pb ages from a ca. 26–24 Ma pluton on Guadalcanal, in the intra-oceanic Solomon island arc (southwest Pacific Ocean), reveal Eocene- to Archean-aged zircon xenocrysts. Xenocryst populations at ca. 39–33 Ma and ca. 71–63 Ma correlate with previously obtained ages of supra-subduction magmatism within the arc. A ca. 96 Ma zircon population may be derived from Cretaceous ophiolite basement crust or region-wide continental rift-related magmatism. Xenocryst age populations alternate with periods of oceanic basin formation that fragmented the East Gondwana margin. Early Cretaceous to Archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust; they may have been introduced by Eocene interaction of a continental fragment with the arc, and concealed by ophiolite obduction. The data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra-oceanic arc magmas, a process likely facilitated by cyclical subduction zone advance and retreat. The findings highlight the continuum of arcs that occurs between continental and oceanic end members, and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust

Duration and nature of the end-Cryogenian (Marinoan) glaciation

The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation

Four-photon correction in two-photon Bell experiments

Correlated photons produced by spontaneous parametric down-conversion are an essential tool for quantum communication, especially suited for long-distance connections. To have a reasonable count rate after all the losses in the propagation and the filters needed to improve the coherence, it is convenient to increase the intensity of the laser that pumps the non-linear crystal. By doing so, however, the importance of the four-photon component of the down-converted field increases, thus degrading the quality of two-photon interferences. In this paper, we present an easy derivation of this nuisance valid for any form of entanglement generated by down-conversion, followed by a full study of the problem for time-bin entanglement. We find that the visibility of two-photon interferences decreases as V=1-2\rho, where \rho is, in usual situations, the probability per pulse of creating a detectable photon pair. In particular, the decrease of V is independent of the coherence of the four-photon term. Thanks to the fact that \rho can be measured independently of V, the experimental verification of our prediction is provided for two different configuration of filters.Comment: 16 pages, 4 figures; published versio

Pluton Exhumation in the Precordillera of Northern Chile (17.8°–24.2°S): implications for the Formation, Enrichment, and Preservation of Porphyry Copper Deposits

Hypogene mineralization in porphyry Cu deposits is typically associated with crustal thickening and rapid exhumation, whereas supergene enrichment requires slow exhumation to allow sufficient time for leaching and downward transport of Cu before it is lost to surface erosion. Therefore, spatial and temporal patterns of exhumation within a metallogenic belt can highlight favorable locations for hypogene mineralization, supergene enrichment, and preservation. Here, we determine average pluton exhumation rates along an ~730-km segment of the middle Eocene-early Oligocene metallogenic belt in northern Chile (17.8°–24.2°S). By combining zircon U-Pb geochronology with Al-in-hornblende geobarometry, we pinpoint the time and depth at which each pluton was emplaced and use the age of overlying cover units or supergene minerals to date its arrival at the surface (or near-surface) environment. Uranium-lead zircon ages for 49 samples from plutons and porphyries range from Carboniferous to Eocene (~314–35 Ma). Al-in-hornblende emplacement depths for 19 plutons are ~4–7 km, with one Carboniferous pluton emplaced at ~12 km. Two phases of net exhumation are identified: early Permian-Middle Triassic and middle Eocene-late Oligocene, with an intervening period of net burial. The highest exhumation rates (>0.30 km/m.y.) derive from the second phase, coeval with the Incaic orogeny and the main phase of hypogene mineralization. Present-day preservation of plutons and porphyry Cu deposits required low post-Oligocene average exhumation rates of <~0.01 km/m.y.—favorable for the development of many world-class supergene blankets. However, spatial variability in exhumation and burial across the belt led to poor conditions for supergene development locally: enrichment was hampered in some places by rapid exhumation after hypogene mineralization (e.g., ≥~4 km at El Abra), by burial beneath significant cover (e.g., Ministro Hales, Queen Elizabeth), or, in the Inti region of northernmost Chile, by a combination of the two

Revisiting the discrimination and distribution of S-type granites from zircon trace element composition

Trace element compositions of zircon can be used to estimate the chemistry of their host magmas; as such they provide a useful tool in zircon provenance, and in the assessment of changing magma chemistries in time and space. Granites derived from the melting of sedimentary protoliths (S-types) have previously been discriminated by their P contents and P vs. REE+Y correlations, largely based on data from the Lachlan Fold Belt. Using a range of magmatic suites from different locations, we show that this discrimination commonly fails to discriminate S-type granite from others. We propose an alternative discrimination tool, based on a plot of Ce/U vs. Th/U, which makes use of low LREE/U and Th/U in metapelite-derived melts. Through coupled thermodynamic and accessory mineral saturation modelling, we demonstrate that these low ratios can be explained by monazite co-crystallisation. We demonstrate that Himalayan S-types, which are inferred to have formed from partial melting of metapelite, and thus can be classified as pure S-types, exhibit the lowest Ce/U and Th/U ratios, and overlap those of metapelitic zircon. Granites formed in oceanic arcs (I-types) and mantle-derived suites both have the highest Ce/U and Th/U ratios. Other S-types, such as those known to have mixed sedimentary and igneous protoliths, which we term Hybrid S-types, form a field overlapping pure I- and S-types. We use Ce/U versus Th/U to demonstrate the dominant I-type origin to early Earth (>3.6 Ga) zircon, and using a large detrital zircon database we assess the proportion of S-type zircon through Earth history. In contrast to previous findings, we find that the supercontinent Rodinia had a normal abundance of S-type zircon, as with other supercontinents, and that instead the period 1.7–1.2 Ga exhibits a marked low in S-type zircon, corresponding to fewer continental collisions

Tectonic and Crustal Processes Drive Multi-Million Year Arc Magma Evolution Leading up to Porphyry Copper Deposit Formation in Central Chile

Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu–(Mo–Au) deposits. Reading the magmatic record of individual arc segments and constraining the rates of magmatic changes are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over ~13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher meltwater content. Concomitant decreases in εHf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over ~3.5 Myr to form the world’s largest known Cu resource at Rio Blanco-Los Bronces

The Geodynamic Significance of Continental UHP Exhumation: new Constraints From the Tso Morari Complex, NW Himalaya

The burial and exhumation of continental crust to and from ultrahigh-pressure (UHP) is an important orogenic process, often interpreted with respect to the onset and/or subduction dynamics of continent-continent collision. Here, we investigate the timing and significance of UHP metamorphism and exhumation of the Tso Morari complex, North-West Himalaya. We present new petrochronological analyses of mafic eclogites and their host-rock gneisses, combining U-Pb zircon, rutile and xenotime geochronology (high-precision CA-ID-TIMS and high-spatial resolution LA-ICP-MS), garnet element maps, and petrographic observations. Zircon from mafic eclogite have a CA-ID-TIMS age of 46.91 ± 0.07 Ma, with REE profiles indicative of growth at eclogite facies conditions. Those ages overlap with zircon rim ages (48.9 ± 1.2 Ma, LA-ICP-MS) and xenotime ages (47.4 ± 1.4 Ma; LA-ICP-MS) from the hosting Puga gneiss, which grew during breakdown of UHP garnet rims. We argue that peak zircon growth at 47–46 Ma corresponds to the onset of exhumation from UHP conditions. Subsequent exhumation through the rutile closure temperature, is constrained by new dates of 40.4 ± 1.7 and 36.3 ± 3.8 Ma (LA-ICP-MS). Overlapping ages from Kaghan imply a coeval time-frame for the onset of UHP exhumation across the NW Himalaya. Furthermore, our regional synthesis demonstrates a causative link between changes in the subduction dynamics of the India-Asia collision zone at 47–46 Ma and the resulting mid-Eocene plate network reorganization. The onset of UHP exhumation therefore provides a tightly constrained time-stamp significant geodynamic shifts within the orogen and wider plate network

Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): is the Yellowstone hotspot waning?

Super-eruptions are amongst the most extreme events to affect Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km3, covering ≥12,000 km2. The ca. 8.72 Ma Grey’s Landing eruption was even larger, at magnitude of 8.8 and volume of ≥2800 km3. It covers ≥23,000 km2 and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning

Crystal mush dykes as conduits for mineralising fluids in the Yerington porphyry copper district, Nevada

Porphyry-type deposits are the world’s main source of copper and molybdenum and provide a large proportion of gold and other metals. However, the mechanism by which mineralising fluids are extracted from source magmas and transported upwards into the ore-forming environment is not clearly understood. Here we use field, micro-textural and geochemical techniques to investigate field relationships and samples from a circa 8 km deep cross-section through the archetypal Yerington porphyry district, Nevada. We identify an interconnected network of relatively low-temperature hydrothermal quartz that is connected to mineralised miarolitic cavities within aplite dykes. We propose that porphyry-deposit-forming fluids migrated from evolved, more water-rich internal regions of the underlying Luhr Hill granite via these aplite dykes which contained a permeable magmatic crystal mush of feldspar and quartz. The textures we describe provide petrographic evidence for the transport of fluids through crystal mush dykes. We suggest that this process should be considered in future models for the formation of porphyry- and similar-type deposits