To understand sulfur and carbon sequestration and release within the continental crust, scapolite minerals from a variety of granulite facies rocks were analyzed for their elemental composition and S isotope signatures. These high-grade scapolites host significant amounts of SO3 and CO2, up to approximately 5 wt% and 3 wt%, respectively, with delta 34SVCDT from -3 to +10 %o, and formed in relatively oxidizing environments characterized by low aH2O in which scapolite may form as a primary igneous mineral or via metamorphic reactions involving sulfides and silicates. The range of scapolite sulfur isotope compositions mirrors those observed in mantle xenoliths, suggesting transport of S from the mantle into the lower crust via fluids and melts. Although scapolite's contribution to the global S and C cycles may be modest, it is significant in the context of sulfur fluxing from the mantle to the lower crust, particularly in its oxidized form. We estimate that at least 10 % of lower crustal sulfur is sequestered within scapolite. The exhumation of scapolite-bearing lower crustal rocks can therefore liberate substantial quantities of sulfur species and CO2, which may serve to both supply and compositionally buffer retrograde metamorphic fluids. These fluids may exhibit a range of S isotope compositions from mantle-like (delta 34SVCDT approximate to 0 %o) to relatively 34S-enriched signatures. Consequently, retrograde fluids may have S isotope signatures indistinguishable from those of mantle fluids, even in the absence of direct mantle S input during fluid formation. Exhumation of scapolite-bearing lower crust may facilitate element mobilization through S and Cl complexing, particularly with respect to base metals, within exhumed lower crustal sections, thus providing sources of metals and fluids in mid- to high-grade metamorphic rocks. Globally, scapolite-bearing lower crust may help balance the global sulfur cycle through catch-and-release from scapolite.

At least 30% of preserved continental crust was generated in the late Archean (3.0-2.5 Ga). Constraints on how late Archean crust formed are therefore important to understanding continental growth. We present whole-rock elemental and Nd isotope data and zircon U-Pb-Hf isotope data for late Archean plutonic (mostly granitoid) rocks in an understudied portion of the southwestern Rae craton. Three plutonic suites with crystallization ages of ca. 2.71-2.69 Ga, 2.58-2.57 Ga, and 2.52-2.49 Ga are documented. The 2.5 Ga suite is characterized in detail because the tectonic setting of 2.5 Ga magmatism in the Rae craton is debated and magmatic rocks of this age are globally understudied. Data from the older suites provide constraints on the isotopic and chemical composition of the local basement. The 2.5 Ga rocks range from mafic to felsic (47-74 wt% SiO2), are high-K calc-alkaline and light rare earth element enriched, exhibit negative Nb anomalies, and yield restricted ranges of initial epsilon Nd (-0.5 to +0.6) and weighted mean initial epsilon Hf (-1.1 to +1.5) values. Mafic 2.5 Ga magmas derived from a subduction-enriched mantle source that had roughly chondritic isotopic compositions at the time of magma generation. The unradiogenic composition of the mantle source can be explained by: (1) metasomatism of depleted mantle by low Sm/Nd and Lu/Hf fluids or melts well before 2.5 Ga, such the source evolved from a suprachondritic to chondritic isotopic com position by 2.5 Ga, or (2) metasomatism of depleted mantle by melts of unradiogenic Nd2.5 Ga and epsilon Hf2.5 Ga <= 0) subducted sediments shortly before or during the 2.5 Ga magmatism. More evolved 2.5 Ga magmas were produced through some combination of fractional crystallization of mantle-derived magmas, assimilation of preexisting crust, and partial melting of juvenile 2.5 Ga crust. Rare ca. 2.57-2.56 Ga zircon xenocrysts in the 2.5 Ga suite are evidence of minor crustal contamination. Neodymium isotope modeling permits, but does not require, assimilation of the local 2.7-2.6 Ga crust. However, the data do require that all components of the suite are, at least in part, juvenile crustal additions. The 2.5 Ga rocks are geochemically similar to post-collisional I-type plutonic rocks from the Phanerozoic Caledonian and Variscan orogens, and their emplacement in post-collisional setting is consistent with existing constraints on the tectonic evolution of the Rae craton. This study exemplifies how the timing of some significant crustal growth events may not be accurately recorded in depleted mantle model ages, and how integrating isotopic and geochemical data with local geological context helps to identify such events. The results also suggest, along with other studies, that substantial late Archean continental growth may have occurred in post-collisional settings.

Constraints on the tectonic and magmatic processes that occurred during the 2.4-2.2 Ga tectono-magmatic lull are limited by the relative paucity of 2.4-2.2 Ga magmatic rocks available for study. One place with a substantial record of 2.4-2.3 Ga magmatism is the western Rae craton. We present whole-rock geochemical and Nd isotope data, and zircon U-Pb-Hf isotope and trace-element data for granitoids in an understudied portion of the southwestern Rae craton. Two groups of granitoids are documented, ca. 2.43-2.38 Ga leucogranitoids and ca. 2.32-2.31 Ga biotite granites. All of the granitoids are peraluminous and silica rich (>69 wt% SiO2), and have negative initial epsilon Nd and epsilon Hf values. The ca. 2.43-2.38 Ga leucogranitoids have comparatively low high-field strength element contents, zircon saturation temperatures (677-828 degrees C) and Ti-in-zircon temperatures (medians = 707-798 degrees C), and some of them contain abundant inherited zircon. Their generation was in part coeval with ca. 2.38 Ga migmatization of local metasedimentary and mafic metaigneous rocks. The ca. 2.32-2.31 Ga granites have elevated high-field strength and light rare earth element contents, relatively high zircon saturation (785-905 degrees C) and Ti-in-zircon (medians = 724-899 degrees C) temperatures, strongly negative initial epsilon Nd (-5 to -11) and epsilon Hf (-10 to -18) values, and little-to-no inherited zircon. These granites were produced via high-temperature (>900 degrees C) melting of tonalitic-granodioritic rocks, including a significant proportion > 3.0 Ga rocks that must be widespread at depth in the study area. Evidence of ca. 2.35-2.30 Ga mafic magmatism that could have supplied heat for this high-temperature melting has not been identified in the study area, but has been documented elsewhere in the western Rae craton. We propose that the ca. 2.43-2.38 Ga leucogranitoids reflect anatexis in response to one or more episodes of collisional orogenesis and the ca. 2.32-2.31 Ga granites formed in a post-orogenic setting. Comparisons with ca. 1.85-1.75 Ga syn- and post-orogenic granitoids associated with the Trans-Hudson orogen support these tectonic interpretations and indicate that the 2.4-2.3 Ga granitoids were produced through "modern-style" tectonic processes, similar to those that occurred during the Trans-Hudson orogeny.

Regional metamorphism is characterized by complicated mineral-mineral reactions and fluid-mineral interactions, which may promote elemental migration and ore genesis. A robust tracer of metamorphic reactions and fluid activities is a key for identifying material sources and quantifying mass flow along metamorphic pathways. Here, through examinations of not only bulk-rock nitrogen (N) isotopic variation along six prograde metamorphic zones but more importantly inter-mineral comparison of isotopic equilibration/disequilibration in the classic Barrovian metamorphic sequence in the Mica Creek area in the Canadian Cordillera, we show that N isotopes are a sensitive tracer for probing metamorphic devolatilization, mineral reaction and external fluid infiltration at various spatial scales from prograde to retrograde stages. In detail, our results show that, despite a relatively large variation in bulk-rock N/Al molar ratio, low-grade rocks (slates and phyllites) display a narrow δ15N range, which suggests relatively homogeneous N isotope compositions in their protoliths. Biotite-zone schists show reduced N/Al ratios with elevated δ15N values, which is consistent with devolatilization of 15N-depleted NH3 from rocks during prograde metamorphism at T < 500 °C. In contrast, samples from high-grade rocks (kyanite zone and sillimanite–K-feldspar zone) do not follow the metamorphic devolatilization trend but decrease in both N/Al ratios and δ15N values, indicating overprinting by a relatively large-scale flow (over at least tens of kilometers) of a 15N-depleted external fluid during the high-grade stage. The fluid was likely derived from the granitic magmas that intruded into these rocks during or slightly post peak metamorphism. The δ15N values of minerals in kyanite and sillimanite–K-feldspar zones displays negative correlations between biotite and both muscovite and plagioclase, with high δ15Nbiotite values close to isotope equilibrium but low δ15Nbiotite values at isotope disequilibrium. These negative correlations can be best explained by a kinetic isotopic effect associated with metamorphic reactions that consume muscovite and plagioclase to produce biotite. This suggests that plagioclase may be more involved in the metamorphic reactions in pelitic rocks than previously assumed. This kinetic isotopic effect can result in a Δ15Nplagioclase-biotite > 5 ‰, much larger than the theoretically predicted equilibrium N isotope fractionation factor (Δ15Nplagioclase-biotite < 0.5 ‰ at T > 350 °C) but close to the large N isotope fractionation (Δ15Nplagioclase-biotite ≈ 8 ‰) recently observed in field samples. Our data also imply that plagioclase is susceptible to retrograde alteration. Overall, combined N signatures of bulk rocks and coexisting minerals can provide multiple lines of constraints on mineral reactions and fluid activities from prograde to retrograde metamorphism.

Geological sampling forms the foundation for a vast array of preliminary and advanced geochemical data collection. The data generated after sample collection underpins many of the fundamental hypotheses and paradigms that have been developed in the earth sciences. Geological sampling can, at times, be an underappreciated aspect of a geochemist's workflow, particularly given the influence sample collection can have over resulting datasets. In this chapter, we provide examples of scenarios where biases can be imposed onto geochemical datasets during the sampling process. We also attempt to summarize aspects of geological sampling that tend to be under-discussed or are of a growing concern to the geoscience community. This chapter represents a somewhat personal account of geological sampling with anecdotes that serve to highlight various aspects of the important discussion surrounding geological sampling.

The first 500 million years of Earth history is thought to be a period of intense planetary bombardment, but the timing and flux of this meteorite bombardment is poorly understood. In particular, on the basis of an inferred lunar impact history, some workers have hypothesized a ∼3.9 Ga terminal cataclysm (TC) in which there was marked increase in the impact flux affecting the Moon, the Earth and possibly other terrestrial planets. Minerals that survived this enigmatic period offer a way to test early planetary bombardment models as they may contain telltale micro- to nanoscale shock features. Here, we present results from a numerical modeling calculation that assesses the probability that a zircon residing in the crust would escape shock melting or shock deformation during a TC bombardment event. Even with conservative pressure estimates for zircon shock deformation and intermediate bombardment intensities, we find that only ∼6% of ≥4.0 Ga crust would be expected to survive a 3.9 Ga cataclysm without experiencing either complete melting or zircon shock metamorphism. We couple this modeling with a search for shock effects in the oldest zircons from the Acasta Gneiss Complex, which would have been present in the Earth's crust during a putative 3.9 Ga TC. Spatially correlated electron and NanoSIMS ion microscopy of 4.02 Ga igneous zircons from Acasta reveals no evidence of ancient shock. These data, together with similar results from other Hadean zircon suites, confirm that a post-Hadean TC is unlikely to have occurred. We suggest that the dearth of pre-3.9 Ga terrestrial crust and zircons is instead best explained by endogenic processes related to the mechanisms of early crust formation. Our modeling allows us to evaluate bombardment scenarios from the terrestrial zircon record by applying probabilistic interpretations to zircon shock deformation data. This approach will be valuable for other planetary bodies, allowing broader conclusions to be drawn from geographically limited datasets.

We report the discovery of one of the largest ancient (>3.0 Ga) crustal terranes on Earth. Granitoids with crystallization ages >3.0 Ga and/or Sm-Nd depleted mantle model ages ≥3.2 Ga define a ~1000 × 100 km belt on the western margin of the Rae craton, Canada, referred to herein as the Perry River terrane (PRT). Zircon U-Pb-Hf-O isotope and whole-rock geochemical data from granitoids show that the PRT is a predominantly juvenile 3.3–3.2 Ga terrane that was partially reworked by more evolved ca. 3.1 Ga magmatism. These findings call for a reassessment of the timing and extent of ancient continental growth on Earth. A global compilation of zircon Hf isotope data from 3.6 to 3.0 Ga igneous rocks reveals clusters of relatively juvenile (initial εHf −2 to +3) rocks at ca. 3.31 and ca. 3.23 Ga, which include samples from the PRT and 13 other terranes worldwide. Other global zircon data sets also document age peaks between 3.3 and 3.2 Ga, and a cluster of broadly chondritic initial εHf values around 3.2 Ga. The 3.3–3.2 Ga period may therefore have been a time of enhanced net continental growth on Earth, and the PRT is one of the largest terranes preserved from that time. Furthermore, zircon Hf isotope data from 3.3–3.1 Ga PRT granitoids and 3.5–3.0 Ga igneous rocks worldwide yield little evidence for parent magmas that interacted with or derived from pre–3.6 Ga continental material. Contrary to some continental-growth models, this latter observation suggests that the volume of continental crust established by 3.6 Ga was relatively small.

Earth's crustal Re-Os budget has conventionally been divided into three primary reservoirs (organic-rich sedimentary rocks, hydrocarbons, and sulfides) that collectively defined the field of crustal Re-Os geochronology.Recent research by Toma et al. (2022) has expanded this definition to include natural graphite, which previously had no geochronological potential.Instead, graphite-forming events were inferred from the age dating of adjacent host-rock materials (e.g.white mica Ar-Ar dating).Here we discuss the state of the art of graphite Re-Os research.We begin by testing graphite Re abundances in relation to those found in the upper continental crustal (UCC), terrestrial sulfides, organic-rich sedimentary rocks, and hydrocarbons and show that graphite Re concentrations exceed UCC values but are comparable to other terrestrial reservoirs.This is followed with comparisons of the inter-group Re variations for graphites formed in metamorphic, hydrothermal, and meteoritic environments.Our findings indicate that Re contents trend highest in metamorphic graphite, followed by meteoritic, and then hydrothermal graphite.Applications of graphite Re-Os dating are presented in the context of three (two published [1] and one unpublished) Re-Os graphite datasets.The first two samples comprise hydrothermal graphite formed in low-crustal shear zones (Wollaston-Mudjatik Transition [Gr WMT ], Canada]) and tanzanite-tsavorite gemstone deposits (Merelani Hills [Gr MH ] Tanzania), whereas the last sample consists of metamorphic graphite formed in the Franciscan subduction zone (Laytonville Quarry [Gr LQ ]; USA).In all three cases, high-precision Re-Os graphite dates were obtained (Gr WMT = 1731.52± 7.43; Gr MH = 586.89± 2.39 [MSWD = 1.2];Gr LQ = 161 Ma]) that match the inferred formation ages of graphite mineralization in each locality determined by other radiometric methods or geological constraints.When coupled with SIMS C isotopes and Raman and XRD thermometry, Re-Os graphite dating is a powerful tool capable of expanding our understanding of ore genesis, graphitization, and carbon cycling in the crust-mantle system.

The volume, nature and origin of continental crust that existed on Earth before 3 Ga is poorly understood.One of the greatest obstacles in addressing these topics is the apparent paucity of preserved ancient (>3 Ga) crust.Despite being one of the largest Archean nuclei on Earth, the record of ancient crust in the Rae craton is virtually unexplored.Previous work has hinted at the presence of pre-3 Ga crust at the southern [1] and western [2] margins of the craton, but the true aerial extent, age, nature and origin of that ancient crust is unknown.In this study, we present whole-rock Sm-Nd isotope and elemental data, and zircon U-Pb, Hf and O isotope data, for granitoids from the western margin of the Rae craton.Samples with crystallization ages >3.0 Ga and/ or Sm-Nd depleted mantle model ages >3.2 Ga define an ~1000 x 100 km belt that stretches from central Canada to the Arctic coast, which we call the Kugyoak terrane.Pre-3.0Ga granitoids from this terrane are broadly similar in composition to typical Archean tonalitetrondhjemite-granodiorite [3].These granitoids yield crystallization ages between 3.25 and 3.07 Ga, initial zircon εHf values between +3 and -2, mantle-like zircon O isotope compositions and rare ca.3.3 Ga inherited zircon.Collectively, these data show that one of the largest Paleo to Mesoarchean terranes on Earth was previously unrecognized, and that this terrane represents a volumetrically significant addition of juvenile ca.3.3-3.1 Ga continental crust.Granitoids from this giant terrane yield little-to-no evidence for interaction with the large volumes of pre-3.5 Ga continental crust (up to 45% of present-day continental volume) that are inferred from proxy records, such as detrital zircon U-Pb-Hf isotope data, to have been present at the time of its formation [4].

Natural graphite forms in a range of metamorphic and hydrothermal environments across timelines spanning from the birth of the solar system, to the evolution of early Precambrian life, and the development of contemporary geotectonic cycles. A precise timeline of these and other graphite-forming events, however, has hitherto been obscured by a lack of radiometric ages and as such, chronologies are inferred from host-rock or hydrothermal mineral ages. Herein we examine the Re-Os systematics and chronology of graphite formed in a suite of terrestrial and extraterrestrial environments (n = 17) with the principal aim of establishing the viability of Re-Os geochronology of natural graphite. Graphite Re and Os contents and isotopic ratios exhibit a wide range of values that extend up to 1520 ppb Re, 19,577 ppt Os, and 4101 and 42.18 for Re-187/Os-188 and Os-187/Os-188 ratios, respectively. These values are broadly comparable to those reported for crustal sulfides, organic-rich sedimentary rocks, and hydrocarbons. X-ray diffraction crystallinity data reveals that graphite Re abundances show a broadly inverse correlation with graphite formation temperature and crystallinity (d(002) and L-c(002)) with interplanar spacing (d(002)) having the strongest anti-correlation with graphite Re contents. Graphite Re-Os geochronology is demonstrated with two independent case studies (Wollaston-Mudjatik Transition shear zones, Saskatchewan, Canada and Merelani Hills, Tanzania) yielding precise (< 1%) Re-Os isochron dates of 1731.52 +/- 7.43 Ma (2r; MSWD = 1.3) and 586.89 +/- 2.39 Ma (2r; MSWD = 1.2) that are consistent, within uncertainty, to their mineralization ages constrained by other radiometric methods. These data confirm that graphite mineralization was synchronous with Trans-Hudsonian exhumation and tsavorite-tanzanite gemstone mineralization, respectively. Method accuracy, however, appears contingent on the analytical protocols used to isolate graphite, e.g. handpicking vs. heavy liquids (SPT) and water, with the latter perturbing graphite Re-Os systematics by as much as 20%. We, therefore, recommend handpicking paired with magnetic separation and grain mount examination. Our Re-Os age results are then coupled with new SIMS carbon isotope data (Wollaston-Mudjatik Transition graphite: delta C-13 =-21.64 to-15.28 parts per thousand; Merelani Hills graphite: delta C-13 =-25.90 to-24.36 parts per thousand) and( 187)Os/(188)Osi isotope data (Wollaston-Mudjatik Transition graphite = 0.3119 +/- 0.0037; Merelani Hills graphite = 1.680 +/- 0.038) to constrain graphitic carbon to sedimentary carbonate/organic (Wollaston-Mudjatik Transition graphite) and organic (Merelani Hills graphite) carbon sources. This unique pairing of isotope systems in graphite provides the first detailed chronology of localized carbon mobility in the Earth's crust. Re-Os graphite geochronology likely has wide applications in ore-deposit and metamorphic geology with the potential toreshape our understanding of carbon cycling in the crust-mantle system, and for graphite exploration initiatives that are critical for a global transition to a green economy. (C) 2022 Elsevier Ltd. All rights reserved.

This chapter is devoted to the carbon and nitrogen stable isotope compositions of terrestrial diamonds with a strong focus on monocrystalline diamonds formed in Earth’s mantle. The wealth of Cand N-stable isotope studies forces us to make some choices to keep this chapter within an acceptable length. Here, we focus on both the ground-breaking early diamond stable isotope studies as well as the latest developments in the field. Using a comprehensive database of diamond stable isotope data compiled from literature, we examine key constraints on diamond-forming processes, the origins of diamond growth media and the cycling of C and N through shallow and deep Earth reservoirs.

Abstract The Acasta Gneiss Complex (AGC) is a ∼2,400 km2 Hadean‐Mesoarchean terrane that contains the oldest known zircon‐bearing rocks on Earth. Despite its importance for early Earth geology, only a small fraction (∼50 km2) of the AGC has been mapped in detail. We use detrital zircon grains from late Pleistocene eskers that transect the Complex to approximate the lateral extent and relative proportions of diverse‐aged ancient rock units within the vast, little‐studied parts of the AGC. The esker sediment was derived from glacially eroded bedrock and therefore zircon grains can serve as a proxy for the ages of exposed bedrock in the study area. U‐Pb dates on ∼2400 detrital zircons from coarse and fine grain‐size fractions along the esker transect yield age distributions that coincide with ages of regionally mapped AGC bedrock, the adjacent Wopmay Orogen, and granitoids of the Slave craton. Based on detrital zircon age distributions and new reconnaissance‐scale mapping, we infer that 3.37 Ga granitoids are a volumetrically significant component of the unmapped AGC. Esker zircons older than 3.7 Ga are present in most esker samples but at low abundance, which suggests that Eoarchean and Hadean rocks are a volumetrically subordinate component of the AGC. However, the data also suggest that unmapped rocks at least as old as 3.95 Ga are present toward the inferred eastern limit of the AGC, a location where Eoarchean rocks have not been recognized previously.

The largely inaccessible lower continental crust (LCC) in cratons is sporadically sampled by kimberlites, allowing insights into its composition and evolution. We report mineral and bulk trace-element compositions of rare Archaean, dominantly mafic, granulite xenoliths from the Diavik kimberlite in the central Slave craton, focussing on first-row transition elements and high field-strength elements (HFSE) for new insights into their petrogenesis and implications. Quantitatively recovered HFSE abundances and ratios confirm an OIB-type source for the samples’ igneous protoliths, and differ from those in eclogite xenoliths from the same locality, indicating a different origin. A single metagabbroic granulite (MGG) has the lowest V, Ti, Zr and Nb contents, whereas metabasaltic granulites (MBG) show higher abundances of these elements, qualitatively indicating incompatible behaviour during differentiation, possibly in a magma chamber near the palaeo-crust-mantle boundary. Rutile controls Nb and Ta abundances in the bulk rock, but in its absence, ilmenite dominates the budget, containing up to 100s ppm of Nb. If the intermediate-composition CC formed by partial melting of mafic LCC similar to Diavik mafic granulites, its V abundance can be satisfied if melting involved mixtures of basaltic and gabbroic granulites at ~FMQ-1 to FMQ-0.4. This is higher than the fO2 estimated for Archaean spreading ridges, but lower than modern oceanic arc mantle. High median Nb abundances (6.8 ppm) and Nb/Ta (27) recorded in the granulites indicate that, despite its low mass fraction (<0.3% of Silicate Earth), the LCC may be a significant Nb reservoir with suprachondritic Nb/Ta and, combined with the amount of LCC recycled through time (total ~2%), may help resolve the “missing Nb” paradox.

The formation and preservation of cratons—the oldest parts of the continents, comprising over 60 per cent of the continental landmass—remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth’s early continents and central to defining the cratons, although we extend the definition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses—the cratons.

Ambient Moho temperatures and lower crustal heat production are surprisingly poorly constrained in cratons. Here we address these problems using 15 lower crustal xenoliths from the Diavik A-154 kimberlite, Slave craton, Canada. Iron-magnesium exchange geothermometry on small biotite and amphibole inclusions in garnet indicates that the Slave craton lower crust was at a temperature of <= 500 degrees C at the time of kimberlite eruption (-55 Ma). The ambient lower crustal temperature was likely lower than 500 degrees C because the thermometers record the closure temperature of diffusional Fe2+-Mg exchange between touching mineral pairs. New measurements of K, U and Th concentrations in the constituent minerals, together with xenolith modes, allow reconstruction of the heat-producing element (HPE) K, U, and Th budget of the Slave craton lower crust. Metasedimentary granulites have an average heat production of 0.29 0.01 mu W/m(3) (n = 3) whereas mafic granulites have an average heat production of 0.13 +/- 0.03 mu W/m(3) (n = 12). Our new data clearly show that plagioclase abundance in both lithologies has a major influence on overall lower crustal heat production, being an important reservoir of all three HPE. Combining the heat production of mafic and metasedimentary granulites in their observed 80:20 proportions results in an average heat production value for the Slave craton lower crust of 0.16 +/- 0.03 mu W/m3. Using these heat production estimates, modeled Moho temperatures beneath Diavik of -450-470 degrees C are broadly consistent with maximum lower crustal temperatures indicated by geothermometry. The low HPE contents predicted for cratonic lower crust must result in lower temperatures in the deep crust and mantle lithosphere, and in turn higher estimates for the thickness of mantle lithosphere. This effect becomes larger as the thickness of the low-HPE lower crustal layer increases. In the specific case of the central Slave craton, we find that model estimates of the diamond potential of the mantle lithosphere, as judged by the proportion of lithospheric mantle in the diamond stability field, are not strongly affected by small variations in lower crustal heat production and Moho temperature. (C) 2020 Elsevier B.V. All rights reserved.

The earliest evidence for terrestrial contienntal crust on Earth comes from the Jack Hills zircon population, some of which crystalized >4.3 Ga. The isotopic and elemental compositions of these rare detrital grains have been instrumental in developing our understanding of the early crust on Earth. However, there is no general consensus on the rock compositions parental to these valuable zircon grains. Here we compare the chemistry of the Jack Hills zircons to a new zircon dataset from the 4.0—2.95 Ga Acasta Gneiss complex in the western margin of the Slave craton in Northern Canada. The AGC has previously been studied in detail, with a wide variety of existing whole-rock and zircon datasets. By comparing whole-rock and zircon analyses, we show that zircon REE patterns are, at best, tenuous indicators of primary magmatic REE patterns, particularly for ancient zircon grains. However, we document a systematic co-variation in whole-rock La/Yb and zircon U/Yb, which shows potential for tracing a residual garnet signature in the magma source rocks and in turn providing constraints on the depth of melting. We show that the ACG zircon crystals have very similar Ti concentrations to the Jack Hills zircons, potentially indicating similar crystallization temperatures. These low Ti concentrations are distinct from

Kimberlite-borne granulite xenoliths provide rare insights into the age, chemical composition and tectonothermal evolution of the otherwise largely inaccessible deep cratonic crust. The formation and transformation of the lower continental crust (LCC) beneath the central Slave craton (Canada) is here illuminated using whole-rock trace-element and Sr-Nd isotope compositions of nine metabasaltic (MBG), one gabbroic (MGG) and two metasedimentary/hybrid (MSG) granulite xenoliths. On the one hand, published sulphide Re-Os and a few zircon U-Pb data indicate that at least a portion of the LCC beneath the central Slave craton has a Palaeoarchaean origin (similar to 3.3 Ga), which apparently coincides with a period of juvenile crust and deep lithospheric mantle formation during plume impingement beneath the pre-existing cratonic nucleus. On the other hand, enrichment in Li, Sr, LREE, Pb and Th, but relative depletion in Ti, Hf and HREE, suggest formation of (picro)basaltic protoliths by partial melting of a subduction-modified garnet-bearing source, Crystallisation in the crust after fractionation of plagioclase is inidicated by their Sr and Eu negative anomalies, which are complementary to the positive anomalies in the MGG. Samarium-Nd isotopes in MBG and MGG show large scatter, but fall on Neo- or Mesoarchaean age arrays. These elemental systematics are suggested to fingerprint deserpentinisation fluids plus small amounts of sedimentary melt as the main contaminants of the mantle source, supporting the operation of at least regional and transient subduction at 3.3 Ga. Evidence for quasi-coeval plume impingement and subduction beneath the central Slave craton in the Mesoarchaean is reconcilable in a dynamic regime where vertical tectonics, though waning, was still active and plate interactions became increasingly important. Unradiogenic Sr-87/Sr-86 (down to 0.7017) is consistent with significant loss of Rb and probably other heat-producing elements (K, Th, U) plus H2O during Neoarchaean metamorphism, which helped to enhance LCC viscosity and stabilise the cratonic lithosphere. (C) 2019 Elsevier Ltd. All rights reserved.