Isotope dilution is a highly accurate and precise technique for measuring element concentrations in a wide array of samples in the natural sciences. With isotope dilution, the element concentration is directly determined from the masses of the sample and an added element with non-natural isotope composition, the so-called spike, as well as the measured isotope ratios of the sample, spike, and sample-spike mixture. Isotope dilution is therefore a primary method of analysis, is directly traceable to SI units, and considered superior to methods that rely on calibration against external reference materials. Isotope dilution is, however, not applicable to some mono-isotopic elements. Another disadvantage is its destructive nature: It generally requires dissolution of the sample for homogenization with the spike, often limiting analysis of other elements in the dissolved sample aliquot. With the so-called 'double-spike' technique, isotope dilution can also be used to distinguish between the mass-fractionation that occurs before spike-sample equilibration from that which occurs afterwards. This allows the removal of isotope fractionation effects induced during analysis to reveal the stable isotope fractionation inherent in the sample.

We present the first Lu-Hf dating on high-pressure/ultra-high-pressure mineral assemblages in eclogitic rocks from Puerto Cabello, northern central Venezuela. The rocks characterize the Carayaca terrane of the Cordillera de la Costa (CdlC) along the coast and faults bounding the valley of Caracas City. The new late Eocene peak metamorphic ages (38-36 Ma) pertain to Eocene subduction of Proto-Caribbean crust to 80-100 km depth beneath the Caribbean forearc as the latter collided with Venezuela. The ages break the formerly assumed correlation with Margarita Island eclogites and Villa de Cura blueschists of southern CdlC. The distinct histories of these high-pressure/low-temperature metamorphic suites and the Etpana terrane in Guajira, Colombia, constrain depth geometries in regional evolutionary models. By integrating our results with an updated view of synorogenic sedimentation in the Piemontine belt, southern CdlC, we propose crustal scale present and evolutionary cross-sections and maps for the development and exhumation history of the Carayaca terrane and the greater CdlC. In addition, this synthesis concludes that Caribbean collision was preceded by Paleocene incipient subduction and rift fault inversion along Venezuela's Proto-Caribbean margin, exposing Caucagua-El Tinaco belt basement to erosion as recorded in Los Cajones Formation flysch of the Piemontine belt.

The time at which the Neotethys Ocean started to subduct along the southern margin of Eurasia within Iran has been debated in recent years. This margin, referred to as the Sanandaj-Sirjan Zone (SaSZ), features a series of Jurassic calc-alkaline igneous formations which were emplaced at c. 170 Ma. However, an alternative model suggests that subduction initiation occurred during the Cretaceous period. To address this age controversy, we utilized the Lu-Hf isochron method and employed SIMS U-Pb zircon and rutile geochronology to determine the metamorphic history of the earliest known Neotethyan eclogite outcrop along the upper Zayanderud River section of the SaSZ. Two eclogites separated by similar to 25 km yielded Lu-Hf garnet dates of 175.1 +/- 1.0 Ma and 172.5 +/- 0.6 Ma. Rare <20 mu m eclogitic zircon crystals in one of the eclogites are strongly zoned and yielded U-Pb dates for which a 177 +/- 10 Ma rim age was deconvolved. The relatively low Dy/Yb of these zircon rims suggests that they crystallized prior to garnet formation, or retrograde in disequilibrium with garnet. Homogeneous U-Pb rutile ages averaging 162 +/- 7 Ma are only marginally younger than the Lu-Hf garnet ages, consistent with the petrographic observation that rutile is predominantly found in the matrix or within garnet rims, and open-system behavior of radiogenic Pb during cooling. The newly obtained ages are close to previously published Ar-40/Ar-39 phengite dates and indicate narrow time window during the Middle Jurassic for deep burial within a subduction channel, followed by eclogitization and rapid exhumation. These new constraints on the metamorphic history support subduction initiation along the SaSZ margin in the Jurassic period.

Incorporation of rare earth elements (REE) in garnet enables garnet chronology (Sm-Nd, Lu-Hf), and imparts a garnet-stable signature on cogenetic phases, which allows petrochronology and general petrogenetic tracing of garnet stability in minerals and melts. Constraints on the uptake and redistribution mechanisms, as well as on the diffusive behaviour of REE in garnet are required for allowing accurate interpretation of REE signatures and ages. Garnet REE profiles are often measured to gain insight into the nature and cause of REE zoning. Interpretation of such profiles is nevertheless complicated by poor constraints on the extent of diffusive relaxation. This is especially relevant for Lu, which, according to experiments, has a relatively high diffusivity and thus may re-equilibrate with possible consequences for Lu-Hf chronology. To provide new insight into the REE systematics of garnet, we applied quantitative trace-element mapping of garnet grains from metamorphic rocks that record peak temperatures above 750 degrees C and cooling rates as low as 1.5 degrees C Ma-1. Garnet in all samples preserves Rayleigh-type or oscillatory growth zoning with sharply defined interfacial angles that match the garnet habit. Re-equilibration of REE compositions appears restricted to domains with nebulous and patchy zoning, which likely form by interface-coupled dissolution and re-precipitation reactions mediated by fluids or melts, rather than REE volume diffusion. The possible effect of Lu diffusion in the analysed grains was investigated by comparing the observations to the results from 2D numerical modelling using Lu diffusivities from recent diffusion experiments. This test indicates that Lu diffuses significantly slower in natural garnet than experiments predict. The retentiveness of REE in garnet demonstrates the reliability of REE signatures in magmatic tracing and petrochronology and establishes Lu-Hf chronology as a robust means of dating garnet growth and recrystallization in metamorphic rocks, including those that underwent high- or ultrahigh-temperature conditions.

Reconstructing the evolution and composition of the Hadean crust remains a major challenge because of the scarcity and incomplete preservation of rocks older than 3.8 Ga.Whole-rock 146,143 Nd (TIMS), major (XRF) and trace element

A prominent occurrence of Cretaceous metamorphic rocks in the central Indonesian region is the Bantimala Complex, SW Sulawesi, where an eclogite-and blueschist-bearing sequence is part of an accretionary wedge with a complex internal structure. Zircon U-Pb dating of metamafic blocks indicate a significant protolith age range (c. 369-185 Ma) for the HP/LT rock suite. The HP/LT rocks include lawsonite-free and lawsonite-bearing samples. Lawsonite in eclogite is of retrograde origin. Assuming fast exhumation, multipoint Rb-Sr age data (c. 132-125 Ma) are interpreted to broadly approximate the time of peak HP/LT metamorphism. Existing and new K-Ar and Rb-Sr dates can be subdivided into two groups (c. 137-130 Ma and c. 126-113 Ma). Samples with post-eclogitic lawsonite growth belong to the older age group and all lawsonite-free samples are in the younger age group. The mineralogical and age diversity of various blocks indicates detachment of individual fragments from the subducting plate at different depths, followed by return flow in the subduction channel along different P-T-t trajectories. The role of serpentinite during exhumation remains ambiguous. Field observations offer little support for the suggestion that most HP/LT blocks were originally transported in a serpentinite matrix. Subduction-related magmatism is recorded by a U-Pb zircon age of 109.2 & PLUSMN; 1.1 Ma for a quartz dioritic me & PRIME;lange block. Detrital zircon U-Pb data from a sandstone indicate that tectonic stacking of the Bantimala Complex occurred only after partial erosion of already exposed Cretaceous basement.

Abstract This study corroborates interpretations suggesting that the Makrotantalon Unit on Andros represents a tectonic slice with Pelagonian affinity in the nappe stack of the Cycladic Blueschist Unit. Previously reported Cretaceous 40Ar–39Ar dates of a garnet-glaucophane schist from the Makrotantalon Unit could not be reproduced by Rb–Sr geochronology, but this is not an indication of contamination with excess Ar. Instead, the newly dated samples record disturbance of the Rb–Sr isotope system by partial recrystallization. Subsets of these phengite populations, representing the smaller grain-size fractions, yielded low-precision dates ranging from c. 21 to c. 15 Ma that document deformation-related resetting and recrystallization of a presumably Cretaceous white mica population. Although these Miocene dates cannot be linked with blueschist-facies metamorphism, they provide time constraints on the formation of shear zones that overprinted the original thrust contact during exhumation. The geological relevance of a Cretaceous high-pressure event is confirmed by a Rb–Sr date of c. 121 Ma for an epidote-glaucophane schist collected further away from the tectonic contact. The occurrence of a second blueschist-facies event in the Eocene is verified by Rb–Sr dates of two epidote-glaucophane schists (c. 40 Ma and c. 44 Ma) that can unambiguously be assigned to the Makrotantalon Unit.

The Sistan suture zone, eastern Iran, exposes a well-preserved accretionary complex that formed by the closure of a Neotethyan ocean basin between the Central Iranian Lut block and the Afghan microcontinent. Me ' lange sequences include tectonic slices of adakitic plagiogranitic rocks and rare jadeitite. Adakitic whole rock characteristics, compositional zoning of phengites, and matching U-Pb zircon and Rb-Sr isochron ages (c. 87-85 Ma) suggest that the plagiogranites formed before or during the high-pressure/low-temperature (HP/LT) metamorphism that affected other parts of the subduction zone complex. Meta-plagiogranites, eclogites, and blueschists only shared the same P-T evolution after blocks of the felsic intrusions had been tectonically incorporated into the me ' lange. The suture zone has also been intruded by felsic melts that have largely escaped metamorphic overprinting. The weighted mean U-Pb zircon age of 81.0 +/- 1.2 Ma for a newly dated sample of this group is consistent with the 86-71 Ma range previously reported for such rocks. Melt formation is not limited to the forearc region and can be reconciled with a model suggesting subduction of a spreading ridge. A newly dated gabbro yielded a weighted mean U-Pb zircon age of 104.2 +/- 0.5 Ma, which is interpreted to represent magmatic crystallization and the youngest protolith age reported so far for the Birjand ophiolite. The geological relevance of an even younger intercept age (91.0 +/- 2.7 Ma) of a small zircon subset remains elusive.

The Sanandaj-Sirjan zone (SaSZ) on the northern edge of the Arabia-Eurasia suture in Iran includes a significant high-pressure (HP) metamorphic suite exposed along the upper Zayanderud River north of Shahrekord. Phengitic micas from eclogite in the Zayanderud metamorphic complex (ZMC) yielded 40Ar/39Ar dates ranging from 184 to 173 Ma [1], whereas zircon from an associated anatectic pegmatite gave an average U-Pb age of 176 ± 3 Ma [2]. These data are consistent with a subduction channel metamorphism and rapid exhumation during the Early to Middle Jurassic. To constrain the timing of high-pressure conditions, we have conducted Lu-Hf mineral-whole rock dating on two eclogite samples. The resulting garnet-controlled isochron dates of 171.4 ± 0.4 (MSWD = 1.2) and 175 ± 1 (MSWD = 0.43) Ma have important geodynamic implications as the Jurassic initiation of the Neotethyan subduction in Iran has recently been disputed [3][4]. The metamorphic ages of the ZMC eclogite now leave no doubt that subduction was ongoing along the SaSZ peri-Tethyan margin during the Middle Jurassic. [1] Davoudian et al., 2016 Gondwana Research 37: 216-240; [2] Jamali Ashtiani et al., 2020 Gondwana Research 82: 354-366; [3] Azizi & Stern, 2019 Terra Nova 31: 415-423; [4] Lechmann et al., 2018 Contrib. Mineral. Petrol. 173 (12): 102

Basaltic rocks from ophiolitic mélanges provide information on geodynamic setting, origin, and later tectonometamorphic conditions. This paper resolves the P–T conditions and timing of high-pressure metamorphism in an accretionary wedge that formed during the Late Jurassic closure of the Neotethyan Meliata Basin. Blueschist-facies metabasites of the Meliatic Bôrka Nappe and the Albian conglomerate pebbles of the Fatric Klape Nappe contain rare assemblages of garnet in association with glaucophane, phengite, rutile, and epidote (±albite). Here, we compare a Lu–Hf garnet age from the Meliatic Bôrka Nappe in the southern margin of the Inner Western Carpathians (IWC) with a garnet age from inferred Meliatic blueschists of the Klape Nappe overlying the IWC northern margin. The Hačava type locality of the Bôrka Nappe hosts calc-alkaline type metabasite (∼VAB-C; εNd(245) = +0.9) embedded within Middle to Upper Triassic marbles of the Neotethyan Meliata Basin northern continental margin. The investigated Klape Nappe island arc tholeiite (∼VAB-T; εNd(240) = +5.9) blueschist pebble, and associated deepwater metasedimentary rock pebbles were found in a conglomerate layer of unmetamorphosed Albian flysch. The garnet ages of 153.95±0.69 Ma and 152.1±1.5 Ma correspond to closure of the Meliata Basin during southward intraoceanic and continental margin subduction. This was followed by the exhumation of HP blocks in serpentinite mélange and the formation of an accretionary wedge with included anchimetamorphosed Jurassic flysch. The P–T conditions of the blueschists were constrained by Perple_X modelling to be 520°C and 1.55 GPa for the Bôrka Nappe, and 490–510°C and 1.68–1.72 GPa for a pebble from the Klape Nappe conglomerate. The similarity of garnet dates and metamorphic conditions between the two samples suggest that the blueschists formed during the Late Jurassic Meliatic subduction. The north-vergent Meliatic nappes are the inferred source of the Albian flysch conglomerates deposited in the foreland Fatric Basin. This material was subsequently transported by the Fatric Klape Nappe to the IWC orogenic front during the Turonian, following the closure of the Fatric Basin.

The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent "Perseverance" and "Tianwen-1" missions, "follow the water," and, more broadly, "volatiles," has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average "bulk silicate" δ41K values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na "volatility" ratio, and most notably, bulk planet H2O abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo)planets to retain enough H2O to enable habitability and plate tectonics, with mass exceeding that of Mars.

Combined geochronological and (isotope) geochemical investigations of tonalite-trondhjemite-granodiorite (TTG) suites, which dominate Archean cratons, provide constraints on the sources and petrogenetic processes that gave rise to the Earth's early continental crust. In situ U-Pb and bulk Lu-Hf analyses of single zircon grains from ITGs of the Paleoarchean Bastar Craton in central India date the emplacement of the igneous rock suite and trace the chemical evolution of its source rocks. Complementary whole rock Lu-Hf and major and trace element data constrain the petrogenesis of the TTGs. The rocks are variably enriched in fluid mobile elements, particularly K and Pb, and are characteristically depleted in heavy rare earth elements. Calculated initial Hf isotopic compositions of zircon and whole rocks cluster around the chondritic value at ca. 3.55 to 3.45 Ga. The high precision Hf-isotope data reveal a trend of increasing radiogenic Hf-176/Hf-177 with age, the slope of which implies a Lu-176/Hf-177 = 0.02, and thus derivation from a mafic protolith. Geochemical modeling indicates that the granitoids can be directly produced by melting of a hydrous basalt in the garnet stability field, followed by minor fractional crystallization. Variations in the modal abundance of garnet suggest the source rocks melted at different depths. Rocks with moderately to strongly depleted HREE patterns require >10% garnet in the residue, consistent with derivation from a garnet-amphibolite or garnet-pyroxenite source. Modeling of the Lu-Hf systematics further highlights the important role of melting as opposed to fractional crystallization for reproducing the primary chemical characteristics of TTGs. The global Hf-isotope record of Archean mantle-derived rocks is considerably more radiogenic than that of the bulk silicate Earth, providing unambiguous evidence that parts of the mantle were significantly depleted in incompatible elements since at least the early Archean. The differences between Hf isotopic compositions of the mafic (greenstone belts) and felsic (gneissic terranes) rocks can be accounted for by crustal residence times of several 100 million years for the protoliths of the felsic magmatic suites. Consequently, TTGs integrate the geologic history of their precursor(s), complicating their isotopic record. Therefore, the near-chondritic Hf isotopic composition of TTGs is not direct evidence for their derivation from a primitive mantle, but rather a consequence of their specific petrogenesis involving an aged, mafic precursor. This relationship introduces uncertainties into models of crustal growth and geodynamics that are based on the felsic record, especially where the whole rock context is absent. In contrast, high-precision Hf-isotope analyses of single zircon grains, combined with whole rock isotope and elemental constraints, can be used to reliably identify the sources and processes involved in the generation of Earth's oldest continental crust.

We report new petrological and geochronological data for eclogitic rocks of the Pouebo Eclogite Melange (PEM) in the New Caledonian high-pressure/low-temperature (HP/LT) belt. Pseudosections were calculated to link newly determined Lu-Hf dates with P-T conditions corresponding to garnet growth, which started on the pro grade path and continued into the eclogite facies to approximately 560 degrees C and 2.05 GPa, coinciding with omphacite formation. The Lu-Hf garnet dates of 38.27 +/- 0.14, 36.5 +/- 0.7, and 38.42 +/- 0.15 Ma are weighted towards the early part of the garnet growth interval, and thus represent maximum ages of eclogite-facies conditions. These dates are indistinguishable from those reported earlier from Ar-Ar in white mica, which had been interpreted as cooling ages, and they are about 6 Myr younger than those from U-Pb in metamorphic zircon, which had been previously considered to date peak eclogite-facies metamorphism at 44 Ma. The new Lu-Hf dates overlap with Rb-Sr white mica ages of nearby metasomatic veins related to the blueschist-to-eclogite transition. The Lu-Hf and Rb-Sr data imply that eclogite-facies metamorphism in parts of the PEM lasted at least until 38 Ma and is thus younger than previously suggested. This prograde, near-peak event cannot be reconciled with concurrent (41-36 Ma) cooling previously inferred from the Ar-Ar dates. We conclude that both the 38.4-36.5 Ma Lu-Hf and Rb-Sr dates as well as similar Ar-Ar results record near-peak conditions, while the 44 Ma U-Pb zircon rims probably are related to prograde mineral reactions. The HP stage is followed by rapid exhumation and cooling at minimum rates of similar to 0.7 cm/yr and similar to 50 degrees C/Myr. (C) 2021 Elsevier B.V. All rights reserved.

Garnet chronology (Lu-Hf, Sm-Nd), and REE-based linkage between garnet and chronometric accessory phases, are powerful means of placing garnet's rich metamorphic record in time. The interpretation of REE compositions and zoning of garnet is crucial in either approach. Nevertheless, it can be difficult to distinguish between primary growth features and the effects of diffusive re-equilibration, especially because there are substantial uncertainties regarding the effects of REE diffusion on zoning and chronology [1]. Taking advantage of new capabilities in trace-element mapping, we investigated the REE composition and zoning of garnet in high-grade rocks that underwent (ultra)high temperature metamorphism on different time scales. Garnet grains were mapped for trace elements using an ArF excimer laser ablation system with a small-cell geometry and rectangular-spot ablation capabilities, coupled to a sector-field ICP-MS. The grains typically show step-wise concentric zoning, with large chemical gradients on the scale of micrometers and angular geometries that mimic crystal faces. Some grains preserve oscillatory zoning, locally with a very short (10-μ m) wave length. Cloudy and smooth zoning coexists with sharper features; the general interpretation of such zoning as resulting from volume diffusion is non-unique at best. The retention of zoning provides important constraints on the interpretation of the Lu-Hf ages obtained for these samples. The

The earliest differentiation of the silicate Earth remains poorly constrained due to the scarcity and imperfect preservation of Earth's oldest rocks.These limitations can be circumvented, however, using the short-lived 146 Sm-142 Nd system, which provides selective chronological information on Hadean crustmantle differentiation processes [1] .The oldest rock record on Earth is represented by a chemically diverse suite from the 3.6-4.0Ga Acasta Gneiss Complex (AGC) in northwest Canada [2,3] .Previous 146 Sm-142 Nd studies of the AGC revealed the presence of negative 142 Nd anomalies, inherited from a so-far elusive Hadean crustal component [4,5,6] .However, the majority of AGC samples define a 147 Sm-143 Nd regression age of 3.37 ± 0.14 Ga [7] , suggesting late disturbance of their Sm-Nd systematics.This issue limits the potential of combined 146,147 143 Nd investigations to reveal the timing of Hadean crust-mantle differentiation.We will present coupled 146,147 Sm- 142,143 Nd systematics of layered gneisses from the AGC in combination with zircon U-Pb age constraints.The results for the short-lived 146 Sm-142 Nd system show µ 142 Nd values ranging from +2.7 to -8.5 ppm (±2.5 ppm, 2 S.D.) and document the existence of both mantle and Hadean crustal components within the sources of the AGC.The data for the long-lived 147 Sm-143 Nd system deviate from the ~3.4 Ga regional array, defining an apparent age of 4.01 Ga [8] , which may be geologically meaningless.Although the actual sample ages still need to be further constrained by forthcoming zircon data, our results suggest that the studied samples preserve a record of primary 142,143 Nd heterogeneities in the AGC and shed some light on the petrogenetic history of Earth's oldest rocks.

The continental crust grew and matured compositionally during the Paleo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogs of arc magmas. However, partial melting of deep-residing mafic reservoirs, e.g., during density-driven crustal overturning, delamination, or plume activity, would likewise produce TTGs without requiring active-margin processes. Boron analysis could allow discriminating between these models and, so, provide new insight into the formation and petrogenetic setting of Archean juvenile crust. We analyzed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. Low δ 11 B values (down to -16‰)

The timing of metamorphism in the Karagwe-Ankole Belt (KAB, Central Africa) has never been investigated in detail. Although its metamorphic history and geochronology would provide a unique and crucial contribution, these aspects have largely been overlooked in most published geodynamic reconstructions of this belt. In this study, well-constrained Lu-Hf garnet dates obtained on four samples (three metasiltstones, one schist) can be directly linked to pre- and post-deformational greenschist metamorphism and broadly constrain the timing of Neoproterozoic deformation in the Karagwe-Ankole Belt. Depending on the host rock composition, garnet porphyroblasts grew over intervals of < 2 Myr up to 9 Myr. Three Neoproterozoic garnet growth episodes were identified in the Western Domain of the Karagwe-Ankole Belt. In the Satinsyi Complex, just west of Gatumba, two prograde, pre-deformational garnet generations of Tonian age were identified. The first generation, mostly preserved as relict cores, grew before 920 Ma. Around 880 Ma, a second garnet generation formed new crystals, and overgrowths on first generation porphyroblasts. In the Kibuye area, garnet growth occurred between 618.9 +/- 1.6 and 602.3 +/- 1.1 Ma (Ediacaran period) during post-deformational greenschist metamorphism. In the western part of the Karagwe-Ankole Belt, the youngest deformation, probably of Pan-African origin, occurred after 880 Ma (2nd garnet growth episode), and had ceased by 620 Ma (3rd garnet growth episode) at the latest. The latter age is at least 70 Myr older than the Pan-African culmination and deformation observed in the northeastern part of the KAB (NW Tanzania), and the southern Ubendian Belt but corresponds to the timing of the West Gondwana Orogeny in the Congo Craton west of the study area. The Lu-Hf dates additionally indicate a limited timespan between Rodinia break-up (705-748 Ma) and Gondwana amalgamation ( > 620 Ma).