Ethan Baxter

• Boston College

Multiple studies have applied zoned garnet geochronology to place temporal constraints on the rates of metamorphism and deformation during orogenesis. We report new high-resolution isotope dilution−thermal ionization mass spectrometry Sm-Nd isochron ages on concentric growth zones from microstructurally and thermodynamically characterized garnets f...

High‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation...

Early draft of article with the same title published in October 2022

Please ignore this preprint and go to the published article instead (Aerden et al. 2022, Tectonics).

Metasomatic reaction zones between mafic and ultramafic rocks exhumed from subduction zones provide a window into mass-transfer processes at high pressure. However, accurate interpretation of the rock record requires distinguishing high-pressure metasomatic processes from inherited oceanic signatures prior to subduction. We integrated constraints f...

Please ignore this item and go to the published article instead (Aerden et al. 2022, Tectonics).

Sedimentary records derived from detrital minerals provide valuable information about regional and global tectonic evolution. Detrital garnets are present within sedimentary deposits and can preserve in their major element and isotopic chemistry a direct record of tectonometamorphic processes not necessarily recorded by other detrital minerals. How...

Early un-reviewed manuscript. Please ignore and go to the published article instead (Aerden et al. 2022, Tectonics). I have been unable to definitely remove this item from my profile. It keeps coming back. Probably because the ms. was originally uploaded to The Earth and Space Science Open Archive (ESSOAr) where it cannot be removed either.

Constraining conditions and mechanisms of the early stages of exhumation from within subduction zones is challenging. Although pressure, temperature, and age can be inferred from the exhumed rock record, it is generally difficult to derive each of these parameters from any single rock, thus demanding assumptions that diverse data from multiple samp...

The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from...

The metamorphic devolatilization of serpentinites during subduction represents the largest potential source of fluids from the subducting slab and influences a range of important processes from subduction zone seismicity to arc magmatism. Obtaining a record of metamorphic dehydration directly from serpentinites, however, is challenging, as serpenti...

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 metamorphos...

Rare-earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy. Potentially economic deposits of these elements are typically hosted in minerals such as monazite, bastnäsite, and eudialyte (a complex Na-Ca-Fe-Zr silicate mineral with Cl), making these prime targets for geological research. Globa...

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in...

Nineteen single-grain detrital garnet ages from a tributary to the French Broad River (North Carolina, USA) establish a novel approach to Sm-Nd detrital garnet geochronology wherein the equilibrium bulk-rock composition lost during weathering and transport is replaced with the composition of inclusions leached from within each garnet grain. Detrita...

Phase equilibria modeling of the pressure–temperature (P–T) path of regional metamorphism and associated fluid expulsion, combined with constraints on the timescale of garnet growth by Sm–Nd geochronology, elucidates the fluid production rate and fluid flux during Barrovian metamorphism of pelitic rocks from Townshend Dam, VT, USA. This modeling bu...

A series of localized high-temperature granulite-facies domains (‘hot spots’) are present within the regional (10–100 km² scale) amphibolite-facies rocks of the Central Maine Terrane in central New Hampshire (NH), USA. Based on the spatial coincidence of a thermal anomaly and contours of depressed δ¹⁸O values centered on networks of quartz–graphite...

Compaction-driven fluid flow below the brittle-ductile transition may be a means of transporting fluids during metamorphism. In particular, when a decompaction weakening mechanism is introduced to account for the rock viscosity reduction due to fluid overpressures, channeling instabilities evolve into high porosity/permeability fluid conduits that...

We report the results of Sm-Nd garnet geochronology on a mafic granulite from the metamorphic sole of the Ballantrae Ophiolite, Scotland. The resultant age of 477.6 ± 1.9 Ma (2σ) is interpreted to represent the obduction of the Ballantrae Ophiolite and the onset of the Grampian phase of the Caledonian Orogeny in Scotland. Thermodynamic modelling in...

New geochemical data including Sm/Nd isotopic data show evidence for an early Paleozoic arc/back-arc complex in the Nashoba terrane of southeastern New England. The Nashoba terrane lies between rocks of Ganderian affinity to the northwest and Avalonian affinity to the southeast. It consists of early Paleozoic mafic to felsic metavolcanic and metase...

The Norra Kärr alkaline complex is a small intrusion of peralkaline agpaitic nepheline syenites in southern Sweden. Eudialyte from the complex is rich in rare-earth elements [1] and constitutes mainland Europe's most promising potentially economic resource of heavy rare-earth elements. Textures and structures in Norra Kärr portray a convoluted hist...

Granulite-facies metamorphism of large tracts of continental crust is a characteristic feature of Archean cratons, where metamorphism ultimately led to crustal stabilization and chemical differentiation. Constraining the timing and duration of this metamorphism can provide insight into the processes behind Archean cratonic evolution, and here we pr...

In this study, we report the first discovery of microdiamond inclusions in kyanite–garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a meta-igneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further...

Phase equilibria modeling of metasedimentary granulites suggest that these rocks experienced a clockwise P-T path marked by rapid HT garnet growth during both heating and decompression by ~1 kbar at near-peak T (~780-800 °C). Diffusion modeling of major element zoning in garnet suggests that residence at peak T was brief (<2 Myrs), and that cooling...

Rock–fluid interactions in subduction zone settings have important implications for the metamorphic evolution of the subducting slab and overlying mantle. Hot fluids can dissolve and transport chemical species, and initiate and sustain metamorphic reactions. Sometimes only cryptic evidence of these fluids is recorded, but a small fraction of the fl...

A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40 cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precisio...

We use coupled zoned geochronology and thermodynamic modeling of garnet to elucidate the nature and scale of metamorphic dehydration during Eocene subduction of a quartzofeldspathic lithology from Sifnos, Greece. Two large garnet porphyroblasts were microdrilled to sample concentric growth zones, and these were dated using Sm–Nd geochronology. To p...

Studies of rocks exhumed from paleo-subduction zones (EXhumed TERRAnes) contribute greatly to our understanding of the materials and processes that are hidden beneath the surface in active subduction zones. The goal of this white paper and of ExTerra is to organize research on exhumed terranes in order to accomplish more as a collective than we can...

Garnet is a widespread mineral in crustal metamorphic rocks, a primary constituent of the mantle, a detrital mineral in clastic sediments, and an occasional guest in igneous rocks. Garnet occurs in ultramafi c to felsic bulk-rock compositions, and its growth and stability span from <300 to 2000 °C and from atmospheric pressure to 25 GPa. More than...

Garnet's potential as a chronometer of tectonometamorphic processes and conditions was first recognized over 30 years ago. The Sm–Nd and Lu–Hf systems have since emerged as the most effective chronometers, permitting age precision of better than ±1 My, even on tiny samples such as concentric growth zones within individual crystals. New, robust anal...

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO2 cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here...

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO2 cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here...

Microdiamond inclusions in garnet delineate the Rhodope Massif (RM) as a globally important UHP locality. To date there are 4 confirmed UHP localities across the RM, with UHP indicators being restricted to garnets from metapelites. All available protolith and metamorphic ages from the RM are based on accessory minerals such as zircon and monazite,...

We present a method to reconstruct the dehydration flux associated with garnet-forming reactions during subduction. Garnet-bearing blueschists from the island of Sifnos, Greece, in the Attic–Cycladic Blueschist Belt are used as a test case to extract information on the timescales of dehydration during subduction. We use garnet growth as a proxy for...

The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase...

We report the discovery of granulite facies gneisses that attained ultrahigh temperatures (UHT) above those predicted by typical models of conductive thermal relaxation of over-thickened crust during exhumation. The rocks, which form part of the Acadian (Devonian) metamorphic belt in Connecticut (United States), reached similar to 1000 degrees C an...

Garnet-forming reactions in subduction zones may be linked directly to dehydration. When coupled with precise zoned garnet geochronology, this permits quantification of H2O release rates and evaluation of models for progressive metamorphism (and thus dehydration) in subduction zones. Here, an integrated geochronologic and thermodynamic analysis see...

Coupled geodynamic-thermodynamic models make predictions about the progressive dehydration flux from subducted lithologies. However, it has been difficult to test or confirm these predictions through direct petrologic assessment of natural systems. We have developed a method that may be used to reconstruct the rate, timing, and flux of dehydration...

Metamorphic rocks are usually modeled as equilibrium Pressure-Temperature-Chemical-Mineralogical (P-T-X-M) systems; however, we know that chemical disequilibrium drives reactions and produces mineral compositional zoning and complex textures. Mineral growth produces changes in effective bulk-rock compositions because certain elements are fractionat...

Zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes, with growth histories spanning many millions of years. Well known challenges exist in the physical preparation, purification, and geochronologic analysis of zoned garnets, which have limited the progress and application of this potentially powerful geoc...

In EARTHTIME's quest for

The diffusive behavior of argon in quartz was investigated with three analytical depth profiling methods: Rutherford Backscattering Spectroscopy (RBS), 213 nm laser ablation, and 193 nm (Excimer) laser ablation on the same set of experimental samples. The integration of multiple depth profiling methods, each with different spatial resolution and se...

Chemically zoned garnets provide a record of the changing conditions within the tectonically evolving crust where metamorphic mineral growth occurs. Recent advances in Sm–Nd geochronology of zoned garnets now permit the measurement of a higher resolution temporal record than previously attainable. Here, we present precise (< ± 0.7 million years) Sm...

Whether quartzofeldspathic rocks transform to (U)HP minerals during subduction – and back to low-pressure minerals upon exhumation – remains one of the more profound questions pertaining to collisional orogenesis. Garnet-bearing quartzofelds-pathic gneisses from the Western Gneiss Region, Norway provide an opportunity to answer this question. High-...

Technological and scientific developments have demonstrated both the attainability and the utility of very high precision (i.e. b10 ppm 2σ RSE) neodymium (Nd) isotopic measurements with thermal ionisation mass spectrometry (TIMS). However, such high precision has been limited to relatively large aliquots of Nd, on the order of several hundred nanog...

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over-thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at dif...

Garnets provide one of the Earth Science community's most useful tools for studying rates, duration and timing of crustal processes. In this study we describe new techniques for fine sampling of multiple growth zones of garnet and Sm/Nd dating of each individual zone. We test these techniques on large (>5cm) garnets from a shear zone in the Tauern...

Technological and scientific developments have demonstrated both the attainability and the utility of very high precision (i.e. 5-20ppm 2 sigma) Nd isotopic measurements with TIMS. However such high precision has been limited to relatively large aliquots of Nd, on the order of several hundred nanograms. Several potential applications of precise Nd...

We use the records of Sr diffusion preserved in apatite and corroborating multicomponent Fe-Mg–Ca–Mn diffusion preserved in garnet from the classic Barrovian metamorphic zones (Scotland) to quantify the timescale for the thermal peak of crustal heating c. 465 Ma. We show that Sr diffusion in apatite is a powerful means to determine thermal timescal...

Growing evidence is emerging to support the idea that metamorphism, even in a regional context, may be punctuated – or dominated – by relatively short pulses of heating, fluid flow, and/or mineral growth. Here, we describe data from two Barrovian metamorphic terranes which test this idea. In the Barrovian zones of Scotland, garnet Sm/Nd geochronolo...

Recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid Earth (crystals, melts and fluids). Although far from complete, the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length- and time scales of many natu...

An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized un...

Determining the extent to which ultrahigh-pressure (UHP) terranes transform to high-pressure (HP) minerals during subduction is central to understanding the processes attending the subduction of continental margins. The Western Gneiss Region (WGR) of Norway is one of two giant UHP terranes on Earth, and as such constitutes an important natural labo...

Metamorphic reactions in subduction zones have implications for the timing and depth over which water is released from subducting lithologies. Water release is related to the densification of the slab, the fluxing of arc magmatism, and seismogenesis. Garnet-forming reactions frequently involve the release of water. In order to quantify the rate at...