Simultaneous acquisition of U-Pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic, high pressure granulite using LA-ICP-MS split-stream analysis has enabled ...evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high-temperature, melt-present environments. Although the titanite studied here have a comparatively low initial Pb (Pb0) component (Pb0/Pb*), the Pb0 is highly radiogenic relative to model crustal values, indicating inheritance from U-bearing accessory minerals consumed in the melt/titanite-forming reactions. Additionally, titanite crystals typically exhibit core-rim decreases in Pb0/Pb*, as defined by 204Pb/206Pb, calculated 206Pb0/206PbT, and uncorrected 206Pb/238U spot date profiles. Near the margins this is clearly dominated by local U-enrichment, but in the uniformly low-U interiors outwardly decreasing Pb0/Pb* appears to reflect decreasing Pb0 concentrations during growth. The positive correlation among Pb0 and Sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation, Given the high crystallization temperatures (>800°C) and likely slow cooling rates (~5°C), our data support slow Pb diffusivity in titanite, even at high temperature conditions, as has been proposed in a number of recent studies.
Along the outer ~50–100μm, U, Th, Zr, and REE concentrations are variably elevated relative to the crystal interiors, with profiles taking one of two forms: 1) sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge, or 2) gradually increasing to highest concentrations nearest the crystal edge. High-contrast BSE imaging shows that the former profiles are associated with ~1–2μm wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is (or formerly was) in contact with matrix feldspar (i.e. crystallized melt), and are inferred to represent trace-element-enriched dissolution-precipitation reaction fronts. The latter profiles are associated with diffuse, locally wispy, brightness gradients adjacent to resorbed crystal boundaries, subgrain boundaries, or thicker bright bands formed in contact with matrix hornblende crystals, and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro-porosity pathways at some point in the recrystallization process.
Rheology of the continental lower crust plays an integral role in governing the style of continental extension. Temperature-dependent creep deformation in the lower crust decreases lithospheric ...strength and promotes coupling of deformation in the brittle crust and upper mantle; however, few constraints exist concerning the thermal evolution of extending lower crust. Here, we present a high-temperature thermochronological investigation of the Ivrea-Verbano Zone—archetypal continental lower crust that was attenuated during opening of the Alpine Tethys oceanic basin. Rutile U–Pb dates collected from three samples spanning the bottom ∼2-km of the crustal section are between ∼160 and ∼180 Ma, and exhibit near-rim zonations in Zr, Hf, Nb and Ta. Thermal-kinematic modeling of these data, combined with existing rutile U–Pb dates, show that the base of the Ivrea-Verbano Zone experienced heating on two timescales: conductive heating over ∼10 Myr, associated with thinning of the lithospheric mantle, and advective heating over <0.1 Myr, associated with high-temperature infiltration of fluids during crustal exhumation. These constraints match the thermal predictions of geodynamic models that predict high-magnitude thinning of lithospheric mantle during the early stages of extension. Conductive heating of lower crust directly preceded mantle exhumation and crustal excision, suggesting that thermal weakening of the lithosphere promotes focusing of extensional strain.
•Strength of continental lower crust critical for style of extension.•Mantle thinning resulted in conductive heating of archetypal lower crust.•Brief advective heating driven by influx of fluids prior to breakup.•Summation of rapid advective and slow conductive heating controls T in lower crust.
Sand‐shale mélanges from the Kodiak accretionary complex and Shimanto belt of Japan record deformation during underthrusting along a paleosubduction interface in the range 150 to 350 °C. We use ...observations from these mélanges to construct a simple kinetic model that estimates the maximum time required to seal a single fracture as a measure of the rate of fault zone healing. Crack sealing involves diffusive redistribution of Si from mudstones with scaly fabric to undersaturated fluid‐filled cracks in sandstone blocks. Two driving forces are considered for the chemical potential gradient that drives crack sealing: (1) a transient drop in fluid pressure ∆Pf, and (2) a difference in mean stress between scaly slip surfaces in mudstones and cracks in stronger sandstone blocks. Sealing times are more sensitive to mean stress than ∆Pf, with up to four orders of magnitude faster sealing. Sealing durations are dependent on crack spacing, silica diffusion kinetics, and magnitude of the strength contrast between block and matrix, each of which is loosely constrained for conditions relevant to the seismogenic zone. We apply the model to three active subduction zones and find that sealing rates are fastest along Cascadia and several orders of magnitude slower for a given depth along Nicaragua and Tohoku slab‐top geotherms. The model provides (1) a framework for geochemical processes that influence subduction mechanics via crack sealing and shear fabric development and (2) demonstration that kinetically driven mass redistribution during the interseismic period is a plausible mechanism for creating asperities along smooth, sediment‐dominated convergent margins.
Plain Language Summary
Geophysical monitoring of active subduction zones has revealed plate boundary slip behaviors such as creep, slow slip events, and earthquakes that vary spatially and temporally for different plate boundaries and downdip along a given boundary. Fault rocks exposed on land from paleosubduction plate boundaries provide a record of the deformation processes that likely occur during slip along active boundaries, so we review the characteristics of these ancient rocks to develop insight into slip behavior in subduction zones. We find that plate boundary deformation in these cases occurs within a wide fault zone through processes that involve redistribution of silica from shearing mudstones to cracking sandstone blocks. We use a geochemical model to calculate how long it would take to seal a crack by this process and conclude that cracks seal at rates that could influence the earthquake cycle, with rates of crack healing dependent on the temperature structure and the depth where slip occurs. Our results suggest that processes of frictional failure and geochemical healing in downgoing sediments may influence the slip stability along the subduction interface.
Key Points
Microstructural observations of subduction mélange record dissolution along a scaly fabric and diffusion to cracks in sandstone blocks
A silica kinetics model depicts crack healing that is highly dependent on temperature and slab‐top geotherm
Kinetic healing of cracks could modulate fault zone strength during the interseismic period
Allanite has the potential to be a useful chronometer of crustal evolution, forming in response to a wide spectrum of metamorphic and magmatic conditions and incorporating weight-percent ...concentrations of LREE, Th and U. Despite its growing use in in situ U–Th–Pb geochronology, allanite reference materials lack sufficient U–Th–Pb isotopic characterisation and little is known concerning the response of U–Th–Pb systematics of allanite to hydrothermal alteration and self-irradiation. This contribution presents the results of a combined ID-TIMS and LA-ICPMS U–Th–Pb study on a suite of five allanite crystals, spanning ∼2.6Ga and including three commonly-used allanite reference materials: the Siss, Bona and Tara allanites. Siss and Bona allanites preserve an inherited ca. 1Ga Pb component, consistent with the presence of xenocrystic allanite cores or the presence of zircon micro-inclusions. Tara allanite yields concordant U–Pb ages (407–430Ma), but is affected by Th/U fractionation, likely caused by hydrothermal alteration. Additionally, the tendency for Th to become mobilised post-crystallisation is further evidenced by two Precambrian allanite megacrysts, LE40010 (ca. 2.8Ga) and LE2808 (ca. 1.1Ga), that both exhibit discordant Th/Pb analyses, linked to the formation of thorite micro-inclusions along hydration pathways. Self-irradiation dose versus discordance relationships show that a percolation threshold is present in allanite at cumulative dose values close to 3×1017α-decayg−1, an order of magnitude smaller than zircon. Collectively, the presence of common-Pb and excess-206Pb, its susceptibility to incur Th/U fractionation and hydrothermal Pb-loss complicates the use of allanite as a geochronometer. These factors explain dispersion of ∼4% in the isotopic compositions of Siss and Tara allanites measured by LA-ICPMS, providing a fundamental limit on the accuracy of the allanite chronometer using these reference materials.
We combine monazite petrochronology with thermal modeling to evaluate the relative roles of crustal melting, surface denudation, and tectonics in facilitating ultrafast exhumation of the Nanga Parbat ...Massif in the western Himalayan syntaxis. Our results reveal diachronous melting histories between samples and a pulse of ultrafast exhumation (9 to 13 mm/year) that began ~1 Ma and was preceded by several million years of slower, but still rapid, exhumation (2 to 5 mm/year). Recent studies show that an exhumation pulse of similar timing and magnitude occurred in the eastern Himalayan syntaxis. A synchronous exhumation pulse in both Himalayan syntaxes suggests that neither erosion by rivers and/or glaciers nor a pulse of crustal melting was a primary trigger for accelerated exhumation. Rather, our results, combined with those of recent studies in the eastern syntaxis, imply that larger-scale tectonic processes impose the dominant control on the current tempo of rapid exhumation in the Himalayan syntaxes.
Synchronous acceleration of exhumation in the Himalayan syntaxes implies a tectonic control on rapid exhumation in mountain belts.
•Garnet–whole-rock Lu-Hf dates span from 311 to 264 Ma across the Ivrea Zone.•Isothermal loading of 1–2 kbar c. 311 Ma implies post-Variscan magmatic overplating.•Garnet P-T-t consistent with ...lower-crust assembly by tectonic underplating.•Temperatures >650 °C for >50 Myr weakened lower crust prior to rifting.
Garnet from five metapelitic samples that span the archetypal Ivrea-Verbano Zone (IVZ) lower continental crust section were dated with the lutetium-hafnium (Lu-Hf) method. Dates systematically decrease from 311.27 ± 1.90 Ma in the sub-solidus amphibolite-facies domain to 263.81 ± 1.29 Ma in the granulite-facies section. Core-to-rim zonation in Lu and Ca in sub-solidus amphibolite-facies garnet grains constrains a phase of isothermal burial of 1–2 kbar to ∼311 Ma. Garnet trace-element zonation and Lu-Hf systematics of a sample adjacent to the Mafic Complex preserves evidence for garnet core growth at or prior to 300 Ma and garnet rim growth concomitant with emplacement of the Mafic Complex (∼286–282 Ma). Garnet grains in a high-grade amphibolite facies sample and granulite-facies metapelites exhibit no evidence of Lu remobilization, indicating that their Lu-Hf dates of between 275.30 ± 2.47 Ma and 263.81 ± 1.29 Ma record the timing of garnet growth prior to cooling of the section during exhumation. The >50 Myr period of garnet growth recorded in this dataset implies that the IVZ lower continental crust remained at elevated temperatures, >650 °C, over a protracted period prior to the initiation of Tethyan rifting. When combined with constraints on the maximum depositional age for the IVZ metasediments, the P-T-t data presented here are consistent with entrainment of sedimentary rocks into the lower crust by tectonic underplating between ∼440 and ∼311 Ma, prior to the onset of regional Variscan metamorphism.
Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear ...whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole – a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5–35% and 60–80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior.
•We model the transport of noble gases during subduction of oceanic crust.•Fractionation of noble gases is controlled by the availability of free fluid.•Hot subduction zones fractionate slab-bound noble gases.•Mantle heavy noble gas signature is consistent with recent increase in recycling efficiency of noble gases and water.
The problem of how dense high- (HP) and ultrahigh-pressure (UHP) metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep within subduction zones is crucial to understanding processes ...occurring at the interface between the subducting slab and the overlying plate and mantle wedge. In this study, we use the P-T-t evolution of metamorphosed FeTi gabbros hosted in meta-serpentinite from the Voltri Ophiolite (Ligurian Alps, Italy) to evaluate potential exhumation mechanisms and assess whether HP mafic rocks were exhumed as blocks within a complex serpentinite-hosted mélange or as coherent sections of oceanic lithosphere. A number of observations provide evidence for subduction and exhumation of the central portion of Voltri Ophiolite as a coherent package of ultramafic and mafic material: (1) a similar evolution in mineral assemblages, textures and compositions suggesting peak metamorphism in the eclogite-facies, followed by a retrograde evolution through blueschist-, amphibolite- and greenschist-facies conditions; (2) uniform P-T conditions of both initial garnet growth (~475–480 °C and 2.0–2.2 GPa) and peak metamorphism (~500–525 °C and 2.3–2.5 GPa); and (3) bulk SmNd garnet ages that reveal a very tight grouping of ages for four well-equilibrated foliated FeTi meta-gabbros clustering at 39.98 ± 0.84 Ma. A massive undeformed gabbro also from the central Voltri area gave a slightly younger age of 38.15 ± 0.89 Ma, which is interpreted to reflect kinetic overstepping and delayed growth of garnet. By contrast, an SmNd garnet age from the NW portion of the Voltri Ophiolite indicates peak eclogite-facies metamorphism occurring ~10–12 Ma earlier at 49.68 ± 0.35 Ma. A compilation of the current geochronological data suggests that the NW portion of the Voltri Ophiolite underwent peak eclogite-facies metamorphism first at ~50 Ma, the sedimentary portion of the Voltri Unit (the Voltri-Rossiglione meta-sediments) experienced peak metamorphism around ~47–44 Ma, whilst subduction and peak metamorphism of the central part of the ophiolite occurred later at 41–38 Ma. Alongside recent studies of other Alpine ophiolites (Zermatt Saas, Monviso, Alpine Corsica), this study suggests that detachment and buoyant exhumation of large kilometric-scale coherent sections of slab material, rather than exhumation within a chaotic serpentinite mélange, is the dominant mechanism of preserving and returning eclogite-facies metamorphic rocks to the surface in the Western Alps. In the case of the Voltri Ophiolite, which lacks any associated continental massif, this exhumation was likely aided by the buoyancy of the serpentinite-dominated oceanic lithosphere, rather than by coupling to continental crust.
•Eclogite-facies metamorphism in the central Voltri Ophiolite occurred at 41–38 Ma.•HP metamorphism occurred earlier at ~50 Ma in the NW part of the ophiolite.•The central Voltri area was subducted and exhumed as a coherent lithospheric slice.•The ophiolite comprises at least two slices with variable P-T-t histories.•Exhumation of the ophiolite achieved via buoyancy of serpentinite-dominated crust.
Abstract Ultrahigh‐temperature (UHT; >900°C) metamorphism drives crustal differentiation and is widely recognized in the rock record, but its geodynamic causes are debated. Previous work on ...granulite‐facies metapelite xenoliths from San Luis Potosí, Mexico suggests the lower crust experienced a protracted UHT metamorphic event that coincided with the onset of regional extension. To determine the duration, conditions, and heat sources of UHT metamorphism recorded by these xenoliths, this study characterizes the major‐element, trace‐element, and U‐Pb isotopic systematics of quartz, rutile, feldspar, garnet, and zircon by in situ electron microprobe (EPMA) and laser‐ablation inductively coupled‐plasma mass spectrometry (LA‐ICP‐MS), and augments these data with detailed petrography, thermobarometry, phase equilibria modeling, and diffusion modeling. Thermobarometry and phase equilibria modeling suggest peak metamorphic conditions exceeded 0.7 GPa and 900°C. Zircon petrochronology confirms >15 Myr of UHT conditions since its onset at ∼30 Ma. A small population of zircon record elevated temperatures following transition from backarc compression to extension during the waning stages of orogenesis (60–37 Ma). Garnet preserves trace‐element zoning and mineral inclusions consistent with suprasolidus garnet growth and subsequent compositional modification by intracrystalline rare‐earth element diffusion during protracted heating, with diffusion chronometry timescales in agreement with zircon data, followed by fluid‐driven remobilization of trace elements along now‐healed fractures within ∼1 Myr of eruption. In sum, these data are most compatible with lithospheric mantle attenuation or removal as the dominant heat transport mechanism driving synextensional UHT metamorphism and crustal melting, which has bearing on models for crustal differentiation and formation of modern and ancient granulite terranes globally.
Plain Language Summary The production of melt in the deep crust has important implications for how continents are physically and chemically modified, which in turn controls the distribution of critical mineral resources and the expression of plate tectonics at Earth's surface. One setting where significant partial melting can occur is in a high‐temperature granulite terrane, such as the lower crust beneath the Basin and Range province of southwestern North America. The Basin and Range province is a region where the continent is actively experiencing tectonic extension and, in several places, has been interpreted to have lower crust that was (or is currently) heated to above 900°C. To understand the causality between continental extension in North America and these extreme metamorphic conditions, we study exotic rock fragments of the deep crust beneath San Luis Potosí, Mexico that were brought up by a geologically recent volcanic eruption. These rocks record chemical information that tracks the temperature of the geologically modern lower crust prior to eruption and the duration of extreme temperatures and melting in this crust over the last 60 million years. Using this chemical information, we further our understanding of how tectonic extension is actively driving deep crustal melting beneath southwestern North America.
Key Points Lower crustal metapelite xenoliths from San Luis Potosi, Mexico record ultrahigh (>900°C) temperature (UHT) metamorphism Zircon petrochronology suggests protracted (>15 Myr) UHT conditions since the onset of regional extension Garnet preserves complex trace‐element zoning despite extreme temperatures; garnet diffusion chronometry timescales agree with zircon data
The diffusion kinetics of He and Ne in four amphibole specimens have been experimentally determined using stepwise degassing analysis of samples previously irradiated with energetic protons, and ...Arrhenius relationships have been fit to these data. The primary finding is that He and Ne diffusivities are systematically lower in amphiboles that have higher concentrations of unoccupied ring sites, suggesting that unoccupied ring sites act as traps for migrating noble gases. Ring site influence of noble gas diffusivity in amphiboles has substantial implications for 40Ar/39Ar thermochronology applied to these phases and the efficiency of noble gas recycling in subduction zones. These findings are consistent with the correlation between noble gas solubility and the concentration of unoccupied ring sites in amphibole (Jackson et al., 2013a, 2015) but are inconsistent with the ionic porosity model for noble gas diffusion (Fortier and Giletti, 1989; Dahl, 1996). Rather, these findings suggest that the topology of ionic porosity and absolute volume of ionic porosity compete in determining the rate at which noble gases diffuse.
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