Abstract Taupō volcanic zone, the site of the 26 ka Oruanui supereruption, produced ~70 km3 of new rhyolites since 11 ka, culminating in 50 km3 Taupō eruption 1.8 ka. Major phenocrysts decrease from ...4 to 1 vol%, and Oruanui and post-Oruanui ignimbrites all have identical high-δ18Omelt values of 7.39 ± 0.1‰ and lack low-δ18O values despite overlapping calderas. The Δ’17O values are −0.07‰, lower than the mantle and indicate source contamination of high-δ18O, low-Δ’17O metasediments, and limited interaction with high-Δ’17O hydrothermally altered crust. Previously published U-Th-Pb zircon ages demonstrate their diversity spanning 104–105 years for each unit. Zircon crystal size distribution shows a decrease in abundance and the mean size, and some units lack small (<~10 um) zircons suggesting that zircons were both growing and dissolving in the coexisting magma generation areas. Isotope thermometry indicates heating of the system from ~812 ± 35°C to 874 ± 36°C past zircon saturation in 1.8 ka eruption. We advocate that a deep vertically continuous and laterally discontinuous silicic magma system at the base of the Taupō rift, rather than a shallow batholith or an evolving mush, drives volcanism at Taupō. To explain the post-Oruanui magma production, rift-base silicic magma origin and moderate (~2 km3/1000 years) rhyodacitic magma flux from a growing and heating liquid magma body creates a sufficient solution for the most recent magmatism.
Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics ...started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen.
Terrestrial weathering releases phosphorus and other essential nutrients that fuel life in Earth's surface environments and sustain oxygenic photosynthesis. Despite previous suggestions that major ...changes in terrestrial chemical weathering might have played a role in the global oxygen cycle in the geological past, little is known about the Earth's weathering history. To date, the cause-and-effect relationship between weathering and the long-term evolution of atmospheric oxygen, and whether chemical weathering became more efficient after the initial rise of atmospheric oxygen in the early Paleoproterozoic, remained largely elusive. Here we report a reconstruction of the intensity of terrestrial weathering for the last 2.7 billion years, based on coupled neodymium-hafnium isotope (ΔεHf(i)CLAY) and elemental analyses of the fine-grained clay-size fraction of shales. A pronounced shift towards higher ΔεHf(i)CLAY values and rubidium/aluminium (Rb/Al) ratios indicates that preferential dissolution of phosphate-bearing minerals and biotite intensified between ∼2.5 and 2.4 billion years ago (Ga), following the emergence of continental landmasses and coinciding with the initiation of the Great Oxidation Event. After a long time interval characterized by a constant degree of low-intensity chemical weathering, between ∼2.3 to 0.7 Ga, terrestrial weathering further accelerated after the Neoproterozoic glaciations at ∼0.6 Ga, as inferred from markedly decreased Rb/Al ratios, coincident with the second rise of atmospheric oxygen. These findings support a link between the long-term intensity of chemical weathering and atmospheric oxygen level since the late Archean. We further propose that the 100-million-year-long period of enhanced terrestrial weathering from ∼2.5 Ga played a major role, via crustal recycling of phosphorus and export to the surface ocean, in the early expansion of the aerobic biosphere that ultimately led to the Great Oxidation Event.
•Geochemical investigation of the clay-size fraction of shales for the past 2.7 Ga.•Hf-Nd isotopes and Rb/Al ratio as proxies for continental chemical weathering.•Enhanced crustal weathering of apatite and micas between ∼2.5 and 2.4 Ga.•Marked acceleration of feldspar weathering after ∼0.6 Ga.•Co-evolution of terrestrial weathering and atmospheric oxygenation since late Archean.
Improved geochronological methods and in situ isotopic (O, Hf) and trace element studies of zircon require a new physical model that explains its behaviour during crustal melting. We present results ...of numerical modeling of zircon dissolution in melts of variable composition, water content, temperature, and thermal history. The model is implemented in spherical coordinates with two moving boundaries (for the crystal and the surrounding melt cell outer edge) using simplified mineral phase relationships, and accounting for melt proportion histories as a function of melting and crystallization of major minerals. We explore in detail the dissolution of variably sized zircons and zircon growth inside rock cells of different size, held at different temperatures and undersaturations, and provide an equation for zircon survivability. Similar modeling is performed for other accessory minerals: apatite and monazite. We observe the critical role of rock cell size surrounding zircons in their survivability. Diffusive fill away from a dissolving 100 mu m zircon into a large >3mm cell takes 10 super(2)-10 super(4) years at 750-950 degree C, but zircon cores may survive infinitely in smaller than 1mm cells. Heating followed by cooling for a similar amount of time leads to dissolution followed by nucleation and growth, but new zircon growth remains smaller than the original within the cell. The final zircon size is also investigated as a function of microzircons crystallizing on a front of major minerals, leading to shrinking cell sizes and bulldozing of Zr onto the growing zircon surface. We explore in detail the survivability and regrowth of zircon inside and outside dikes and sills of different sizes and temperatures, and in different rock compositions, on timescales of their conductive cooling and heating, respectively. For zircon-rich rocks, only the largest >200m igneous bodies are capable of complete dissolution-reprecipitation of typically sized zircons at significant distances from the intrusion. Smaller intrusions result in partial dissolution and rim overgrowth. Zircons captured near the contact of conductively cooling sills undergo more overgrowth than dissolution. In contrast, heat wave propagation from the sill will completely dissolve and reprecipitate zircons in Zr-poorer rocks many diameters of the sill away and often 10 super(3)-10 super(4) years after the sill intrusion. A single thermal spike and melting episode is capable of generating the observed complexity of isotopically diverse and geochronologically zoned zircons. A MATLAB program is presented for users to apply in their specific situations.
•Hydration of rhyolitic glass at 175–375 °C yields 3–5 wt.% H2O solubiliy.•H2O profiles have a “snowplow” form that resembles silicate melt DH2O experiments.•Molecular H2O diffusion is the primary ...mechanism for O isotope exchange in glass.•The δ18O of water-in-glass (wig) rapidly reaches local equilibrium within the glass.•The 103lnαglass-wig spans from ∼+14‰ at 100°C to ∼+10‰ at 375 °C.
In many volcanic settings, eruptive deposits experience prolonged cooling in the presence of water, such as in subglacial or submarine eruptions. Under these conditions, volcanic glass will rehydrate and record the isotopic composition of the water. This isotope exchange is moderated by H2O solubility and diffusivity in the glass. In this study, we report results from glass hydration experiments conducted at 175–375 °C to constrain H2O solubility and diffusivity under these hydrothermal conditions over timescales lasting hours to months. We use anhydrous high and low silica rhyolites as well as hydrous high silica rhyolite (perlites) with isotopically labeled water as starting materials. Measurements of bulk H2O by TC/EA of experimental glasses provide minimum H2O solubility estimates. High-Si rhyolitic glass has an H2O solubility between 2.75 wt.% (175 °C, 0.89 MPa) and 4.1 wt.% (375 °C, 21 MPa) while low-Si rhyolite H2O solubility is uniformly ∼0.5 wt.% higher at each temperature. We find a roughly linear relationship of solubility vs 1/T that is ∼1–2 wt.% greater than extrapolations from magmatic temperature solubility relationships. Furthermore, three independent methods of diffusion modeling – one in situ and two mass balance approaches – all produce H2O diffusivity (DH2O) values that up to 5.5 times greater than predicted by extrapolation of the 1/T – DH2O relationships above 400 °C to the experimental P-T-XH2O conditions. In situ H2O profiles in rhyolite particles measured by NanoSIMS have the characteristic “snowplow ” functional form that arises from the H2O concentration dependence of DH2O. We cannot detect diffusively driven kinetic fractionation of D relative to H with the NanoSIMS data. Diffusion and mass balance calculations that fit TC/EA time series of bulk H2O in particles of a single size distribution, and calculations that reconcile two sets of different sized particles at a single experimental duration, return similar DH2O constraints. We also present time series δ18O of bulk glass (δ18Obulk) and the δ18O of water-in-glass (δ18Owig) measurements, which indicate that molecular water (H2Om) dissolved in the glass is the primary driver of subsequent oxygen isotope exchange between glass and an external fluid. Local equilibrium between the δ18Owig and the δ18Obulk is rapidly established and ranges from approximately −14‰ at 175 °C to −10‰ at 375 °C. Both the δ18Obulk and δ18Owig then increase with time moving slowly towards estimated bulk glass δ18O equilibrium with the external experimental water. Oxygen isotope exchange between glass and a fluid is therefore strongly linked to – and is limited by – H2O diffusivity, which is slower at lower P-T conditions and lower H2O solubilities as H2Om diffusion is the main exchange mechanism.
Large-volume caldera-forming eruptions of silicic magmas are an important feature of continental volcanism. The timescales and mechanisms of assembly of the magma reservoirs that feed such eruptions ...as well as the durations and physical conditions of upper-crustal storage remain highly debated topics in volcanology. Here we explore a comprehensive data set of isotopic (O, Hf) and chemical proxies in precisely U-Pb dated zircon crystals from all caldera-forming eruptions of Yellowstone supervolcano. Analysed zircons record rapid assembly of multiple magma reservoirs by repeated injections of isotopically heterogeneous magma batches and short pre-eruption storage times of 10(3) to 10(4) years. Decoupled oxygen-hafnium isotope systematics suggest a complex source for these magmas involving variable amounts of differentiated mantle-derived melt, Archean crust and hydrothermally altered shallow-crustal rocks. These data demonstrate that complex magma reservoirs with multiple sub-chambers are a common feature of rift- and hotspot related supervolcanoes. The short duration of reservoir assembly documents rapid crustal remelting and two to three orders of magnitude higher magma production rates beneath Yellowstone compared to continental arc volcanoes. The short pre-eruption storage times further suggest that the detection of voluminous reservoirs of eruptible magma beneath active supervolcanoes may only be possible prior to an impending eruption.
To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations ...of similar to 500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1-50 mu m) silicates (with rare sulphides) entrapped in high cr-number 100Cr/(Cr + Al) up to 80 chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f sub(O2), of similar to 3 log units below the quartz-fayalite-magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950 degree C at subsolidus conditions, were overgrown by more oxidized host chromite (f sub(O2) approximately QFM) in association with microcrystal inclusions of silicates (plagioclase An sub(86), clinopyroxene, and pargasite) that were formed between 950 and 1050 degree C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to similar to 0.1 degree C a super(-) super(1)). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.
Abstract
The Early Jurassic Butcher Ridge Igneous Complex (BRIC) in the Transantarctic Mountains contains abundant and variably hydrated silicic glass which has the potential to preserve a rich ...paleoclimate record. Here we present Fourier Transform Infrared Spectroscopic data that indicates BRIC glasses contain up to ~8 wt.% molecular water (H
2
O
m
), and low (<0.8 wt.%) hydroxyl (OH) component, interpreted as evidence for secondary hydration by meteoric water. BRIC glasses contain the most depleted hydrogen isotopes yet measured in terrestrial rocks, down to δD = −325 ‰. In situ
40
Ar/
39
Ar geochronology of hydrated glasses with ultra-depleted δD values yield ages from 105 Ma to 72 Ma with a peak at c. 91.4 Ma. Combined, these data suggest hydration of BRIC glasses by polar glacial ice and melt water during the Late Cretaceous, contradicting paleoclimate reconstructions of this period that suggest Antarctica was ice-free and part of a global hot greenhouse.
Abstract
Oxygen isotopic ratios are largely homogenous in the bulk of Earth’s mantle but are strongly fractionated near the Earth’s surface, thus these are robust indicators of recycling of surface ...materials to the mantle. Here we document a subtle but significant ~0.2‰ temporal decrease in δ
18
O in the shallowest continental lithospheric mantle since the Archean, no change in Δ′
17
O is observed. Younger samples document a decrease and greater heterogeneity of δ
18
O due to the development and progression of plate tectonics and subduction. We posit that δ
18
O in the oldest Archean samples provides the best δ
18
O estimate for the Earth of 5.37‰ for olivine and 5.57‰ for bulk peridotite, values that are comparable to lunar rocks as the moon did not have plate tectonics. Given the large volume of the continental lithospheric mantle, even small decreases in its δ
18
O may explain the increasing δ
18
O of the continental crust since oxygen is progressively redistributed by fluids between these reservoirs via high-δ
18
O sediment accretion and low-δ
18
O mantle in subduction zones.
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