Mantle Depths Research Articles

Waveform tomography of the Antarctic Plate

Summary We present a new seismic shear wave velocity model of the upper mantle of the Antarctic Plate region, AP2024. It includes the lithosphere and underlying mantle down to 660 km depth beneath both the continental and oceanic portions of the plate. To augment the limited seismic station coverage of Antarctica, we assemble very large regional and global data sets, comprising all publicly available broadband seismic data. The model is built using 785 thousand seismograms from over 27 thousand events and 8.7 thousand stations. It is constrained by both body and Rayleigh surface waves, ensuring the dense data sampling of the entire upper mantle depth range. The tomographic inversion is global but focused on the Antarctic Plate, with the data sampling maximised in the Southern Hemisphere, with elaborate automated and manual outlier analysis and removal performed on the regional data, and with the regularisation tuned for the region. The upper mantle of the Antarctic continent exhibits a bimodal nature. The sharp boundary along the Transantarctic Mountains separates the cratonic eastern from tectonic western Antarctica and shows a shear-velocity contrast of up to 17% at ∼100 km depth. The bimodal pattern is also seen in the oceanic part of the plate, with the older oceanic lithosphere beneath the Indian sector of the Southern Ocean showing higher shear velocities. The continental lithosphere in East Antarctica shows high velocity anomalies similar to those beneath stable cratons elsewhere around the world. It is laterally heterogeneous and exhibits significant thinning in the near-coastal parts of Dronning Maud Land and Wilkes Land. A low velocity channel is observed along the southern front of the West Antarctic Rift System and is probably related to Cenozoic rifting. High seismic velocity anomalies are detected beneath the Antarctic Peninsula and are likely to indicate fragments of the recently subducted Phoenix Plate Slab. Low velocity anomalies beneath Marie Byrd Land extend into the deep upper mantle and are consistent with a deep mantle upwelling feeding West Antarctica intraplate magmatism.

Seismicity of a relic slab: space–time cluster analysis in the Vrancea Seismic Zone

The Vrancea Seismic Zone (VSZ) is an atypical intermediate-depth earthquake nest located in the South-East Carpathians in Romania, often regarded as a relic slab sinking into the mantle. The origin of the slab and the earthquakes it produces are debated because brittle failure is unlikely to occur at mantle depths. Cluster types and statistical properties of seismicity can vary with deformation regime, fluid content and temperature. Here we investigate the spatial–temporal properties of earthquakes recorded in the VSZ from a high-quality local catalogue, identify and classify earthquake clusters, placing constraints on the debated nature of the Vrancea slab, its coupling with the overlying crust, and its potential for triggering crustal earthquakes. We use the nearest-neighbour distance to estimate the correlation between pairs of earthquakes, considering the origin time, magnitude, b-value and fractal dimension of the observed seismicity with Mw ≥ 2.5 from the 1977–2023 period. With this approach, we find a small percentage of clustered seismicity (8%), suggesting that background seismicity dominates. We identified 12 conventional mainshock-dominated sequences and 17 swarms with relatively small (0.92) and large (1.73) b-values, respectively, that may reflect differences in stress or the presence of fluids in the crustal volume. The aftershock decays—p-values of 0.99 for the large-event clusters and 1.42 for the swarms—may reflect typical and, respectively, fast stress relaxations. Swarm clusters are predominantly localised within the crust, whereas conventional mainshock sequences lack foreshocks and prevail in the subcrustal domain, occasionally triggering aftershocks at shallower depths. This partitioning suggests higher heat flow and fluid-modulated seismicity in the crust, as opposed to brittle failure conditions, triggered by slab dynamics in the mantle, including abrupt stress release without preceding signals. The spread of aftershocks from subcrustal large mainshocks to shallower depths indicate a stress transfer process where the descending Vrancea slab influences seismic activity in the overlying crust.Graphical

Refining the Extent and Depth of the Shear Zone Surrounding the Alpine Fault Using Receiver Function Harmonics

AbstractLarge continental transform faults are thought to cross‐cut the crust and extend into the lithospheric mantle. However, the strain distribution associated with these faults at mantle depths is not well understood. At the heart of this question is the rheology of the lithosphere: when stressed by displacement on a trans‐crustal fault, does the uppermost mantle localize strain in a continuing narrow fault zone, or is that strain instead distributed in a shear zone that widens with increasing depth? This study uses harmonic decomposition of receiver functions to measure the spatial and depth distribution of seismic anisotropy, a proxy for viscous deformation, around the Alpine Fault in Aotearoa New Zealand. Anisotropy aligned with the fault is present in the lithosphere at least 100 km away from the fault trace, suggesting that the Alpine Fault shear zone widens at depth.

Magnetotelluric evidence for the deep causes of different eruptive styles of Changbaishan Tianchi and Longgang volcanoes

Longgang Volcano (LGV) and Changbaishan Tianchi Volcano (CTV) share a common magmatic source at mantle depths. However, the two volcanoes have produced completely different types of eruptions. By performing 3D inversion of an MT dataset that completely covers the LGV and CTV, we have obtained high-resolution electrical resistivity images. The results reveal that the two volcanoes have distinct magmatic plumbing systems, and this is likely the reason for their different eruptive styles. Results from 3D modeling do not show a magma chamber in the shallow crust beneath LGV, interpreted as the rapid rise of the magma from the mantle is responsible for producing a series of densely distributed volcanic cones in the LGV field. In contrast, there is a magma chamber in the upper crust beneath the CTV, where the fractional crystallization and mixing of magma has occurred. This magma chamber has facilitated multiple centralized eruptions, and thereby has led to the formation of the large CTV volcanic cone. These results indicate that differences in their crustal structures may have controlled the different eruptive activities of the LGV and CTV in CVS, Northeast China.

3-D P-wave velocity structure of the upper mantle beneath eastern Indonesia from body wave tomography

Eastern Indonesia's tectonic setting is well known for its complexity and intense seismic activity. Controlled by several major and minor plates, including the Eurasian, Australian, and Pacific plates, this region is famous for its U-shaped subduction system beneath the Banda Arc. To better understand the architecture of the underlying structure in this region, we performed body-wave travel time tomography using ten years of catalog data provided by the Indonesian Agency for Meteorology, Climatology, and Geophysics. We utilize 9729 events in total, from which 46,446 P-wave arrival times were extracted. We used a double difference method to relocate the initial event catalog, which produced a pattern of seismicity consistent with a curved subduction system. Our tomographic model reveals a high velocity band between 90 and 240 km depth in the upper mantle, which is interpreted to be a concave dipping lithospheric slab that is parallel to the present-day Banda arc. Our results also show that lithosphere subducting from the north and south starts to collide at a depth of 300–350 km and becomes shallower further east. Apparent discontinuities in the high velocity band and a corresponding lack of seismicity supports the presence of a slab tear to the west of Seram. A dipping high velocity structure that is present from south to north beneath the island of Timor represents a subducting slab that dips more steeply beyond a depth of 150–200 km, which appears consistent with slab roll-back. Our tomographic model also shows evidence of back arc thrusting to the north of Sumbawa and Flores Islands in the form of a south-dipping higher velocity band at shallow depth. Furthermore, our tomographic models also reveal the possible presence of underthrust continental forearc in the form of a thin higher velocity anomaly that connects the backarc thrust and northward dipping lithosphere slab in the Timor area. Finally, a zone of low velocity above the higher velocity slab is clearly seen beneath Seram Island at a depth of ∼100 km and may represent a partial melting zone.

Reply to “Comment on: Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin” by Schönig (2024)

Reply to “Comment on: Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin” by Schönig (2024)

The tectonic development of the Central African Plateau: evidence from shear-wave splitting

SUMMARY The Central African Plateau comprises a mosaic of numerous Archean terranes—the Congo, Bangweulu and Kalahari Cratons—sutured in a series of Proterozoic to early Cambrian orogenic events. Major upper-crustal deformation and complex craton margin fault zones reflect the region’s diverse tectonic history: rifting during the Neoproterozoic, collision during the Pan-African orogeny, and more recently, Permo-Triassic Karoo rifting and the Pliocene development of the Southwestern branch of the East African Rift. The tectonic evolution and extent to which the lithospheric mantle has been re-worked by each tectonic event is poorly understood. New seismograph networks across the Plateau provide fresh opportunity to place constraints on the plate-scale Precambrian-to-Phanerozoic processes that have acted across the region. Utilizing data from seismograph deployments across the Central African Plateau, including the new Copper Basin Exploration Science network—a NW–SE-trending, 750-km-long profile of 35 broad-band stations—we explore lithospheric deformation fabrics associated with past and present tectonic events via a shear-wave splitting study of mantle seismic anisotropy. Results reveal short length-scale variations in splitting parameters (fast direction: $\phi$, delay time: $\delta$t), suggestive of a fossil lithospheric fabric cause for the observed anisotropy. A lack of fault-parallel $\phi$ across the Mwembeshi shear zone, suggests it may be too narrow at mantle depths, a thin-skinned, crustal-scale feature, and/or did not experience sufficient fault parallel shear-strain during its last active phase to form a lithospheric deformation fabric discernible via teleseismic shear-wave splitting. In the heart of the Lufilian Arc, we observe abrupt changes in splitting parameters with NE–SW, N–S and NW–SE $\phi$ and 0.5 s $< $$\delta$t$< $ 1.2 s evident at short length-scales: no single, uniform, anisotropic lattice preferred orientation (LPO) fabric defines the entire region. This is consistent with the view that multiple episodes of deformation shaped the Lufilian Arc, or perhaps that pre-existing fabrics, relating to Neoproterozoic Katangan Basin development, have failed to be completely overprinted by the Pan-African orogeny. Near the Domes, where most intense crustal re-working is thought to have taken place during the Pan-African orogeny, there is a cluster of null and low $\delta$t splits which likely reflects the lack of organized LPO fabrics, perhaps due to the presence of depth-dependent anisotropy. The neighbouring Congo Craton margin is marked by consistently weak anisotropy ($\delta$t$\lt $ 0.7 s) indicating a weak horizontal alignment of olivine at mantle lithospheric depths, typical of several Archean terranes worldwide.

Detectable Continental Crust in the Earth's Deep Interior Inferred From Thermodynamic Modeling

AbstractCompelling evidence indicates that continental crust can subduct to >300 km and even enter the mantle transition zone (MTZ). However, detecting continental materials within the deep Earth is challenging due to our incomplete knowledge about their physical properties at mantle conditions. We use a newly compiled mineral‐physical database coupled with thermodynamic modeling to calculate seismic velocities of the subducted continental crust (SCC) beyond 150 km. Results show that the SCC has one seismically detectable window depth (300–390 km) with ∼4% VP anomaly. Besides, the upper crust has another two window depths (<250 km and 610–660 km) with anomalies of −6.4%–−1.6% and −7.6%–−2.2%, and 3.6%–7.9% and 3.9%–8.6% for VP and VS compared to those of the ambient mantle, respectively. These predicted SCC characteristics match seismic anomalies at mantle depths and suggest subducted upper crust potentially contributing to the high‐velocity anomalies in the MTZ.

A subduction-dismembered Neoproterozoic large igneous province in the Qinling Orogenic Belt, China: Implications for 850 Ma–initiated mantle plume activity in the greater Yangtze Block

Large igneous provinces (LIP) provide invaluable clues for recognizing the growth, breakup, and cycles of supercontinents and constraining the activity of ancient mantle plumes (or superplumes). The widespread intraplate magmatism is considered as a possible fingerprint of ancient plume activities and LIPs. Here, we present the results of an integrated study on protoliths of eclogites and amphibolite-facies bimodal volcanic rocks from North Qinling Ultrahigh-pressure Metamorphic (UHPM) Terrane, Qinling Orogen, and identify a dismembered Neoproterozoic LIP. These rocks share similar protolith ages of ca. 850–800 Ma and experienced high- to ultrahigh-pressure metamorphism at ca. 500–480 Ma. The mafic rocks are mainly tholeiitic and display enrichment of LILEs and HFSEs, similar to the E-MORB/OIB characters. They also have typical features of continental flood basalts (CFBs) with low CaO/Al2O3 (0.68–0.87), Lu/Yb (0.14–0.15), Zr/Nb (8–13), high Nb/La (0.87–1.24, mean 1.07), La/Nb (0.80–1.15), La/Ta (11–20), Nb/Yb (3.0–6.2) ratios, and positive εNd(t) (+4.3 to + 5.4). Their occurrence, geochemical features, age data, and high mantle potential temperature (1500–1550 °C) suggest that they are remnants of Neoproterozoic CFBs with mantle plume origin. Based on the lateral distribution length (ca. 300 km) of meta-basaltic rocks and the maximum subduction depths (ca. 100–200 km) recorded by UHP eclogites, the identified ca. 850 Ma-initiated CFBs in the North Qinling terrane should represent an LIP, covering more than 100,000 km2. This Neoproterozoic LIP, together with the coeval volcanic sequences in the South Qinling terrane and the northern Yangtze, would have occupied a maximum landmass of 300,000 km2, which lends support for the onset of a mantle plume at ca. 850 Ma within the Rodinia supercontinent. We conclude that the Qinling Block is one of the fragments of the Rodinia supercontinent with a passive margin covered by CFBs, which was subducted to mantle depths in the Early Paleozoic.

Exhumation of ultra-high pressure (UHP) rocks modulated by rifted margin-subduction feedback: Implications for their preservation in old collisional orogens

In continental subduction, rifted margins can be carried to mantle depths (> 90 km) where ultra-high pressure (UHP) metamorphism above coesite stability is attained. Although different exhumation mechanisms for UHP rocks have been discussed, none of them integrate the recent understanding of rifted continental margins. Here, we perform high-resolution thermomechanical numerical experiments to demonstrate that segments of magma-poor rifted margins that reach UHP conditions can efficiently exhume back to shallower levels, while segments of magma-rich rifted margins cannot. This is because the thick layer of rocks with a basaltic composition in magma-rich margins becomes negatively buoyant during metamorphism, preventing their exhumation. This new concept might be pivotal for explaining why exhumed UHP rocks, a key feature of modern-style continental orogens, only appeared and became common late in Earth's history. We suggest that higher mantle potential temperatures and fertility in Earth's early history favored magma-rich rifted margins, making exhumation of UHP crust inefficient. Conditions for magma-poor rifted margins may have arisen during Earth's middle age (1.5–0.8 Ga) due to a colder, more refractory mantle that limited melting and magmatism. We argue that at the end of Neoproterozoic, these colder and positively buoyant magma-poor rifted margins were then subducted and efficiently exhumed to form the large collisional orogens of Gondwana where Earth's oldest coesite-bearing UHP rocks have been unequivocally found. Since then, UHP rocks have become a key and ubiquitous feature of continental orogens.

Lithospheric structure of the Paraná, Chaco-Paraná, and Pantanal basins: Insights from ambient noise and earthquake-based surface wave tomography

To investigate the lithospheric seismic structure of southern South America beneath the Paraná, Chaco-Paraná, and Pantanal basins, we measure two distinct sets of dispersion curves of Rayleigh waves: the first corresponds to 25,986 phase velocity dispersion curves in the period range 5–50 s, extracted from cross-correlation of the ambient noise between pairs of stations and thus sensitive mainly to crustal structure; the second set is derived from group velocities extracted from earthquake recordings, resulting in 33,888 dispersion curves with periods ranging from 8 to 200 s, allowing us to probe deeper Earth structure. From these data sets, we derive two independent S-wave velocity models following a two-step inversion procedure, resulting in a crustal and a lithospheric mantle model with depths extending up to 50 and 200 km, respectively. Features imaged in our crustal model include low velocity anomalies in agreement with the surface position of the major sedimentary basins and high velocity anomalies within fold-thrust provinces and the basement of the Pantanal basin. In the uppermost mantle, we identify high velocities in the region of the Paraná basin, São Francisco and Amazonian cratons, and Luiz Alves block, and a low velocity feature beneath the Pantanal basin, which suggests a potentially thinner lithosphere under the basin. We also illuminate a strong velocity gradient boundary between the eastern border of the Pantanal basin with the Paraná basin, from crustal to lithospheric mantle depths, which seems to delineate the western and southern borders of the Paraná basin, coinciding with the Western Paraná Suture. Additionally, we construct a Moho depth proxy map for the study area, which has an overall good agreement with previous results, except for a thicker crust beneath the southern Chaco-Paraná basin. This area, together with a thicker crust counterpart in the Paraná basin, has a remarkable correlation with the position of a volcanic layer related to the Paraná Magmatic Province, which leads us to suggest that magmatic processes played an important role in the south Chaco-Paraná basin as well.

40Ar diffusion in phlogopite: [formula omitted] atomistic calibration and implications for Ar–melt–solid kinetic interactions and ascent dynamics of mantle xenoliths and kimberlites

40Ar/39Ar ages of phlogopite from kimberlite-hosted mantle xenoliths are commonly older than the kimberlite eruption, despite the fact that argon is supposed not to be retained by phlogopite at temperatures above hydrothermally-derived Ar closure temperatures (<500°C). Combining Molecular Dynamics (MD) with Nudged Elastic Band (NEB) and Transition State Theory (TST), we investigate 40Ar−PVT (Pressure−Volume−Temperature) relationships in pristine, defect-free, phlogopite and show that 40Ar diffusivity is several orders of magnitude slower than existing estimates with a strong effect of pressure on diffusion rates and retention of 40Ar at mantle conditions. These results imply to fundamentally revise residence- and transit-time estimates based on Ar kinetics in phlogopite assuming simple diffusive relaxation during upwelling. When accounting for pressure, 40Ar retention trends in phlogopite predict substantially slower kimberlite ascent rates than documented by independent chronometers, indicating that 40Ar resetting during ascent in phlogopite does not result from simple decompression-driven diffusive relaxation. We argue that 40Ar remobilization probably involves secondary structural–textural modifications induced by reaction-driven recrystallization or rim overgrowth. These findings have far-reaching consequences in terms of argon isotopic mobility at mantle depths as well as for dating and tracing metasomatic events during crust−mantle interactions in the evolution of the subcontinental mantle.

Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin

In this contribution, we investigate the ultra-high pressure metamorphism (UHPM), and subsequent pressure–temperature-time (P-T-t) exhumation path recorded by granulites (rutile-kyanite-garnet-Kfeldspar and rutile-titanite-hornblende-garnet gneisses) of a coherent thrust package (Três Pontas-Varginha Nappe) of the southernmost edge of the Brasília Orogen. The Brasília Orogen, along with Northern Borborema Province, Dahomey, and Hoggar Belts, comprise a widespread linear system of orogens developed during the West Gondwana assembly, recording the diachronic closure of the Tonian-Ediacaran long-lived Goiás-Pharusian Ocean. Optical petrography, Raman spectroscopy, mineral chemistry, Ti-in-zircon and Zr-in-rutile thermometry, and U-Pb dating (LA-ICP-MS) of zircon, monazite, and rutile were combined to identify UHP minerals and textures and to track the decompression/exhumation path. Garnet porphyroblasts preserve UHPM chemical domains and are often radially fractured around rounded quartz inclusions. Remnants of microcoesite were detected by Raman spectroscopy within these inclusions, identified by the diagnostic bands at approximately 170 cm−1, 270 cm−1, and 520 cm−1. Coesite remnants were identified across the nappe stack along with 60° angle-oriented rutile needle inclusions in garnet, which suggests exsolution from Ti-bearing UHP garnets. Soccer ball zircon crystals of Ky-bearing granulites, with flat HREE pattern and negative Eu anomaly, yielded a U-Pb age of around 620 Ma. This age is interpreted as the record of the granulite facies metamorphism (750–805 °C and 15 kbar) post-dating the UHPM from the subduction channel. Zircon and monazite record exhumation under high temperatures around 595–590 Ma, and rutile crystals record the final stages of the decompression path from 590 to 585 Ma. Monazite crystallization occurred until 570 Ma. The new data combined with previously reported metamorphic paths, record a time interval of about 25 m.y. for the exhumation of the Tres Pontas-Varginha Nappe under subsolidus conditions. Our findings represent one of the oldest reports of Neoproterozoic coesite-forming UHPM recorded in metasedimentary rocks buried to mantle depths in the subduction of the passive continental margin.

New P-T path of Huwan high-pressure unit from Hong'an orogen, Central China Orogenic Belt: Implications for oceanic to continental subduction and exhumation in the subduction zones

Oceanic to continental subduction is a pivotal process in plate tectonics, yet characterizing this tectonic transition and subsequent exhumation remains a challenging task. This study integrates our novel pressure-temperature (P-T) estimates on a metabasite lens and its hosting metasedimentary rock in the Xiongdian area of the Huwan high-pressure unit of the Hong'an orogen, Central China Orogenic Belt, with existing geochronological data from the same and neighboring outcrops. Our preliminary but comprehensive findings reveal the evolution of this subduction transition process. After the opening at ca. 430–400 Ma the Paleotethyan ocean subducted northward and formed a warm oceanic subduction zone at ca. 400–385 Ma, marked by relatively low P and high T conditions (∼610–740 °C and ∼1.5–1.8 GPa; M1 stage). Subsequently, the Paleotethyan oceanic plate continued its subduction and cooled the overlying mantle until its final closure during ca. 385–300 Ma. This phase led to the subduction of South China Block (SCB) and the formation of a cold continental subduction zone at ca. 300–270 Ma, featured by relatively high P and low T conditions (∼570–610 °C and ∼2.1–2.7 GPa; M2 stage). Following the delamination from the subducted SCB continental lithospheric mantle, the detached high-pressure rocks experienced a two-stage accelerated exhumation process, i.e., early slow exhumation (∼0.7 ± 0.4 mm/a) to shallower mantle depth (∼610–730 °C and ∼1.5–2.1 GPa; M3 stage) at ca. 270–243 Ma and later rapid exhumation (>2.5 ± 0.9 mm/a) to approximately the Moho depth (∼550–600 °C and ∼0.8–1.3 GPa; M4 stage) at ca. 243–233? Ma. To facilitate the buoyancy-driven exhumation of eclogite blocks from their maximum depth (M2 stage), a minimum of ∼16–25 vol% of low-density metasedimentary rock is required. The increasing buoyancy caused by decreasing densities of the high-pressure rocks with exhumation (from M2 to M4 stage), slab break-off induced rebound forces, and other tectonic factors may jointly explain the accelerated exhumation pattern. Our density results also emphasize that, partly owing to a stronger contribution from a buoyancy-driven mechanism, the high-pressure rocks would be preferentially exhumed in warm to extremely warm oceanic subduction zones compared to their intermediate to cold counterparts.

Basal erosion during the initiation of continental deep subduction in the North Qaidam ultrahigh-pressure metamorphic belt (NW China): Constraints from geochemistry and geochronology on eclogites and gneisses in the Chachahe unit

Abstract Subduction erosion is thought to be a common process in active continental margins that removes upper-plate material and transfers it to the subduction channel. The North Qaidam ultrahigh-pressure metamorphic belt of NW China was formed by subduction of the Qaidam Block beneath the Quanji Block in the early Paleozoic. In this study, we found gneisses and eclogites in the Chachahe unit of the North Qaidam ultrahigh-pressure metamorphic belt that recorded 2.39–2.28 Ga magmatism and 1.93–1.87 Ga amphibolite-facies metamorphism prior to the early Paleozoic (452–439 Ma) eclogite-facies metamorphism. The Paleoproterozoic tectono-thermal history recorded by these gneisses and eclogites is distinct from that of the Qaidam Block but similar to that of the Quanji Block. The rock assemblages, field occurrences, geochemical characteristics, and zircon Lu-Hf isotopic compositions of these rocks closely resemble those of gneisses and enclosed mafic enclaves in the Delingha Complex in the basement of the Quanji Block and the mafic dikes intruded within it. This evidence clearly illustrates that the protoliths of gneisses and eclogites in the Chachahe unit were from the basement of the upper Quanji Block rather than the subducted Qaidam Block. Further considering the spatial location of the Chachahe unit, as well as similarities in early Paleozoic metamorphic ages, peak metamorphic conditions, and clockwise P-T paths between rocks in the Chachahe unit and those that originated from the Qaidam Block, we propose that the bottom basement of the Quanji Block was scraped off by basal erosion during the initiation of continental subduction, transported to mantle depth, and then exhumed with other slices from the subducted slab.

Onset of Plate Motion in the Presence of Chemical Heterogeneities in the Mantle and the Effect of Mantle Temperature

AbstractChemical heterogeneities of various origins have been observed in the Earth's mantle. Their assumed higher density acts on the style of mantle convection and therefore, as tectonic plates form the highly viscous upper boundary layer of mantle convection, the onset of surface motion is also affected by chemical heterogeneities. We perform 2D basally heated thermochemical mantle convection models which initially include layers of dense material at different mantle depths. In comparison to purely thermal convection models, the onset of first plate motion is delayed. This delay is even more pronounced, the deeper the location of dense material, the larger its volume and the higher its density. Additionally, we varied the starting temperature of our models. In an initially hot mantle, a strong temperature‐dependent viscosity contrast between the mantle and cold surface leads to a cold, highly viscous plate (stagnant lid). For an initially cold mantle, rising plumes first need to transport basal heat upwards to create a sufficient viscosity contrast. Consequently, the formation and subsequent breaking of the stagnant lid is delayed. Considering a hotter mantle after Earth's magma ocean phase, we find that plate motion can occur within approximately the first 0.5 Gyr of solid‐state convection if no chemical structures are present or for dense material situated at the surface. Deep chemical heterogeneities delay the onset by at least one order of magnitude. Furthermore, the early surface motion will have been more erratic than todays stable plate motion, probably with a few single subduction events.

Mineralogical and petrological constraints and tectonic implications of a new coesite-bearing unit from the Alpine Tethys oceanic slab (Susa Valley, Western Alps)

Worldwide, Ultrahigh Pressure (UHP) oceanic units are rare and to date only three were recognized: Tianshan (China), and Lago di Cignana and Lago Superiore Unit (Western Alps, Italy). The UHP oceanic units represent the only geological object directly exhumed from mantle depth and they record fundamental information about processes occurring in the deepest portions of the subduction interface.In this work, we describe the occurrence of a new UHP oceanic unit within the Western Alps (mid Susa Valley, Internal Piedmont Zone). We here report the finding of a UHP index mineral, i.e., coesite and we provide a detailed study of garnet inclusions in metapelites and metabasites part of the meta-sedimentary cover of the meta-ophiolites cropping out in the mid Susa Valley. The samples were investigated via optical microscope, Raman spectroscopy, Elastic Geothermometry and classic thermometry (Zr-in-rutile) in order to constrain the P-T evolution of the area.The finding of a new UHP unit in the Internal Piedmont Zone, together with the already described Lago di Cignana and Lago Superiore units, points towards the possible existence of a UHP oceanic slice that reached a similar peak-P (i.e., the return point) along the same subduction gradient. This slice was then removed from the slab and dismembered during exhumation, and it is now exposed as coesite-bearing units juxtaposed with lower pressure eclogite-facies ophiolites. Moreover, the large occurrence of coesite along the entire Internal Piedmont Zone significantly increases the extension of the UHP oceanic units. Hence, the model of a localized non-lithostatic pressure is at the state of the art difficult to apply to the oceanic units of the Western Alps.

Fluid‐Driven Mass Transfer During Retrograde Metamorphism and Exhumation of the UHP Western Gneiss Region Terrane, Norway

AbstractDehydration reactions within subducted oceanic crust are important for fluid‐mediated element transfer within the subducting plate and potentially to the mantle wedge. The effects of metamorphic reactions and fluid flow on element recycling that occur during retrogression and exhumation of subducted continental crust from mantle depths are poorly understood. We study two metabasite pods with fresh eclogite cores and retrogressed amphibolite‐facies rims and surrounding host gneiss within the Western Gneiss Region (WGR), Norway, to better understand element mobility and mass transfer during exhumation of subducted continental crust. Bulk‐rock data were collected from samples taken across the pod and into the host gneiss. Phengite breakdown in eclogite and epidote recrystallization in veins and/or gneiss within pod cores contributed large ion lithophile elements and REE to retrogressed eclogite closest to the pod cores. In gneiss hosting the pods, phengite and epidote breakdown provided fluid that mediated elemental transfer and redistribution to the pod rim or tail. Compared to the studied pod in the southern WGR, the pod in the northern WGR underwent higher P‐T conditions, partial melting and higher strain rates. This resulted in the infiltration of external fluid farther into the pod interior from the rim and facilitated larger mass gain in trace elements in the amphibolite tail of the pod relative to fresh eclogite in the core. The results show clear evidence for retrogression dehydration reactions driving significant fluid‐mediated element redistribution as observed on the outcrop scale during exhumation following ultrahigh‐pressure metamorphism, which directly impacts element signatures within the exhuming crust.

Location and polarity of Variscan sutures based on petrological and seismological data from the Bohemian Massif and the implications for the European Variscides

The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Teplá-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle lithosphere domain beneath the Teplá-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Teplá-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (∼340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Teplá-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their granulite-amphibolite facies thermal overprint.

Survivability of amorphous ice in comets depends on the latent heat of crystallization of impure water ice

Abstract Comets are believed to have amorphous rather than crystalline ice at the epoch of their accretion. Cometary ice contains some impurities that govern the latent heat of ice crystallization, Lcry. However, it is still controversial whether the crystallization process is exothermic or endothermic. In this study, we perform one-dimensional simulations of the thermal evolution of kilometer-sized comets and investigate the effect of the latent heat. We find that the depth at which amorphous ice can survive significantly depends on the latent heat of ice crystallization. Assuming the cometary radius of 2 km, the depth of the amorphous ice mantle is approximately 100 m when the latent heat is positive (i.e., the exothermic case with Lcry = +9 × 104 J kg−1). In contrast, when we consider the impure ice representing the endothermic case with Lcry = −9 × 104 J kg−1, the depth of the amorphous ice mantle could exceed 1 km. Although our numerical results indicate that these depths depend on the size and the accretion age of comets, the depth in a comet with the negative latent heat is a few to several times larger than in the positive case for a given comet size. This work suggests that the spatial distribution of the ice crystallinity in a comet nucleus depends on the latent heat, which can be different from the previous estimates assuming pure water ice.