Graphite found along contiguous horizons in the metamorphosed detrital sediments from the >= 3.7 Ga Isua Supracrustal Belt in Southwest Greenland may represent the oldest remains of life on Earth. Here, we examine the carbon isotope compositions of graphite occurring in situ as inclusions in minerals in both metasediment and secondary vein lithologies by secondary ion mass spectrometry. The carbon isotope composition of the metasediment-associated graphite displays a significantly narrower spread in delta 13C values (average -20.6 +/- 1.8 parts per thousand, 1 standard deviation) than vein-associated graphite does (-20.4 +/- 3.8 parts per thousand), which can be explained by different origins, respectively interpreted as detrital biogenic matter and abiotic fluid-precipitated graphite. Additionally, graphite within metasedimentary porphyroblasts displays lower delta 13C values (-20.9 +/- 1.6 parts per thousand, down to -26.4 +/- 0.5 parts per thousand) than graphite in the surrounding matrix (-18.1 +/- 1.7 parts per thousand), indicating the former to be less altered than the latter.

The dependency of Ti partitioning between quartz and zircon on the activity of TiO2 and Zr partitioning between zircon and rutile on the activity of ZrO2 suggest that an intercalibration among the three minerals (i.e., concentration information from all three phases in the same experiment) could reduce propagated errors when using multiple systems simultaneously. Experiments were undertaken to assess pressure effects in Ti and Zr partitioning in the zircon-quartz-rutile system and intercalibration of the three phases at low concentrations (down to similar to 20 ppm). Analysis of small crystals (down to similar to 8 mu m) was possible due to the high spatial resolution of the CAMECA ims 1290 ion microprobe. Regressions for pressure-temperature-phase domains for experiments between 800 and 1000 degrees C and 10 and 15 kbar array about the initial calibration data providing confidence in their merit despite recent criticisms. Ti and Zr partitioning into quartz, rutile, and zircon can be quantified as: log(Ti - in - quartz) + log a(SiO2)(alpha-quartz) = 6.71 (+/- 0.11) - 383(+/- 10)/T(K) - 0.122 (+/- 0.07)P - 0.00197 (+/- 0.00024)P-2 log(Zr - in - rutile) + loga(SiO2)(alpha-quartz) = 7.39 (+/- 0.21) - 4262 (+/- 220)/T(K) - 0.021 (+/- 0.0051)P and log(Ti - in - zircon) + log a(SiO2)(alpha- quartz) = -4147 (+/- 555)/T(K) - 5.30 (+/- 0.41) where T is temperature (Kelvin), P is pressure (kbar), concentrations have units of ppm, and a(SiO2)(alpha-quartz) is the activity of SiO2 referenced to alpha-quartz. Importantly, if alpha-quartz is present, a(SiO2)(alpha-quartz) is 1 and log (a(SiO2)(alpha-quartz)) is zero.

Investigations of Apollo-returned samples radically altered our understanding of lunar history which has important implications for terrestrial habitability and Solar System evolution. Radiometric dating of those samples inspired the hypothesis that Moon experienced a Late Heavy Bombardment (LHB) at ∼3.9 Ga. The LHB concept has come under several recent challenges, including the concern that 40Ar/39Ar step-heating dates of Apollo impactites had been misinterpreted. Ultraviolet laser ablation (UVLAMP) 40Ar/39Ar dates – with their capacity for much higher spatial resolution and thus potential to avoid dating near-ubiquitous clasts in impact melt rocks – should in principle provide more interpretable results. Here we compare new ion microprobe 207Pb/206Pb accessory mineral dates for two Apollo 17 impactites for which UVLAMP 40Ar/39Ar dates had been previously obtained. Our results are consistent with a single accessory phase growth event for each sample, though the two samples yielded statistically different mean ages of ca. 3.974±0.013 and 3.928±0.003 Ga. Both can reasonably be interpreted as dating an impact event, but the 207Pb/206Pb dates are older than the associated 40Ar/39Ar dates by several hundred million years. We interpret that the age differences result from subsequent thermal disturbances. The discordancy between impact ages inferred from lunar impactites using two different radiometric systems suggests caution in acceptance of the LHB hypothesis without the benefit of both larger lunar datasets and more multichronometric studies. Even with such information, our capacity to know the lunar bombardment history is likely limited by compositional and thermal effects which appear to restrict growth of impact-produced accessory minerals to a small fraction of the lunar surface. Using currently available datasets, the LHB hypothesis may be effectively untestable.

A biotic origin of isotopically light graphite in Hadean zircon remains contested, in part because it is unclear how biogenic carbon in sediments behaves during diagenesis, metamorphism and anatexis, and how it can be preserved as inclusions in zircon. Here, we report the discovery of graphitic inclusions in zircon from Rumburk granite in the Lusatian Block of eastern central Europe as the first example of such inclusion-bearing zircon in a well-constrained geological and petrological context. Most graphite inclusions are trapped at the interface between inherited zircon interiors of early Cambrian- Proterozoic age and late Cambrian (496.2 +/- 2.3 Ma; 95% confidence) zircon overgrowths that crystallized at-700 degrees C under highly reducing conditions (typically-4.6 log units relative to the fayalite-magnetite-quartz buffer). Zircon overgrowths are also enriched in xenotime component (up to 93 mu mol/g P) and 818O (average 818O = 7.7%0), both typical for S-type granitic melts. Raman microspectroscopy reveals crys-talline and disordered graphitic carbon, but because overgrowth temperatures reached >700 degrees C, sufficient for complete graphitization, disordering is presumably secondary and caused by in-situ irradiation from the zircon host. Because organic materials were avoided during sample preparation, and inclusions were excavated by ion beam sputtering, exposure to potential C-bearing contaminants can be dismissed. Inclusions range in 813C from-44.6 to-7.5%0 with a dominant mode at 813C =-34%0 that is positively skewed. Rayleigh-type graphite precipitation from a CO2-CH4 fluid with a starting composition equiva-lent to regional black shale (813C =-32%0) explains inclusion carbon isotopic range and distribution. Collectively, these observations suggest that graphite was initially trapped in voids formed via dissolution-reprecipitation of trace element enriched, and possibly metamict, detrital zircon in metased-imentary protoliths when exposed to C-O-H fluids during prograde metamorphism. Subsequently, graphite-filled voids in inherited zircon became enclosed when zircon rims crystallized from highly reduced S-type granitic melts. Besides partial disordering of graphitic carbon due to irradiation, there is no indication for post-entrapment alteration, demonstrating that graphitic inclusions in zircon can pre-serve isotopic biosignatures over hundreds of millions of years. (c) 2023 Elsevier Ltd. All rights reserved.

We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ94/90Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution.

Abstract Impact events modify and leave behind a complex history of rock metamorphism on terrestrial planets. Evidence for an impact event may be recorded in physical changes to minerals, such as mineral deformation and formation of high P-T polymorphs, but also in the form of chemical fingerprints, such as enhanced elemental diffusion and isotopic mixing. Here we explore laboratory shock-induced physical and chemical changes to zircon and feldspar, the former of which is of interest because its trace elements abundances and isotope ratios are used extensively in geochemistry and geochronology. To this end, a granular mixture of Bishop Tuff sanidine and Kuehl Lake zircon, both with well characterized Pb isotope compositions, was prepared and then shocked via a flat plate accelerator. The peak pressure of the experiment, as calculated by the impedance matching method, was ~24 GPa although a broader range of P-T conditions is anticipated due to starting sample porosity. Unshocked and shocked materials were characterized via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Raman spectroscopy. These methods show that the starting zircon material had abundant metamict regions, and the conversion of the feldspar to glass in the post-shock material. Analyses of the shocked product also yielded multiple occurrences of the high-pressure ZrSiO4 polymorph reidite, with some domains up to 300 μm across. The possibility of U-Pb system disturbance was evaluated via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS). The isotopic data reveal that disturbance of the U-Pb geochronometer in the reidite was minimal (<2% for the main U-Pb geochronometers). To better constrain the P-T conditions during the shock experiment, we complement impedance matching pressure calculations with iSALE2D impact simulations. The simulated results yield a range of P-T conditions experienced during the experiment and show that much of the sample may have reached >30 GPa, which is consistent with formation of reidite. In the recovered shocked material, we identified lamellae of reidite, some of which interlock with zircon lamellae. Reidite {112} twins were identified, which we interpret to have formed to reduce stress between the crystal structure of the host zircon and reidite. These two findings support the interpretation that shear transformation enabled the transition of zircon to reidite. The size and presence of reidite found here indicate that this phase is probably common in impact-shocked crustal rocks that experienced ~25 to ~35 GPa, especially when the target material has porosity. Additionally, shock loading of the zircon and transformation to reidite at these pressures in porous materials is unlikely to significantly disturb the U-Pb system in zircon and that the reidite inherits the primary U and Pb elemental and isotopic ratios from the zircon.

Mineral inclusion assemblages in zircon are almost certainly a complex function of crystallization sequence, magma chemistry, and physical proximity to other crystallizing phases. While the latter may be largely random, the former two factors should have more systematic controls. To better constrain the effects of crystallization sequence, we investigated a suite of granitoids from the Cretaceous arc of southern California and from the Miocene Colorado River Extensional Corridor, ranging from tonalites to leucogranites. In particular we examined inclusions in a variety of accessory minerals to elucidate crystallization order effects. Apatite is almost universally overrepresented as an inclusion in the studied minerals relative to its volumetric abundance in the rock as a whole. Where crystallization order can be inferred, earlier phases usually either have a higher proportion of apatite inclusions (11 out of 20 cases) or show no significant differences in apatite content from later phases (7 cases). An increase in the proportion of apatite with progressive crystallization is observed in four out of the 20 samples with clear crystallization orders (two rocks out of the 20 contain both an increasing and a decreasing case among different mineral pairs and are included in both groups). We interpret these observations as showing the widespread early crystallization of apatite inclusions – likely by pileup of the slow-diffusing phosphorus complex along growing mineral grains. The lower apatite contents in late-crystallizing modal phases are also reflected in the very low apatite contents within late-crystallizing zircon from some studied mafic-intermediate units, contrary to earlier suggestions that apatite decreases as a proportion of the inclusion assemblage with increasing whole rock silica. Plagioclase inclusions in zircon are on average more albitic than matrix plagioclase. In some samples, sequences of plagioclase crystallization can be identified by albite content among 1) magmatic cores of matrix grains, 2) inclusions in mafic accessory minerals, 3) inclusions in zircon, and 4) post-magmatic alteration veins and exsolution lamellae. However, the relative proportions of quartz, K-feldspar, and plagioclase inclusions do not systematically vary in accessory minerals with crystallization order, suggesting other origins of the often-recognized modal shifts toward more quartz- and K-feldspar-rich inclusion assemblage compared to the matrix. Unlike Himalayan leucogranites, when muscovite is present in the matrix in our sample suite, it is a rare inclusion phase in later-crystallizing accessories and may, in some cases, even be absent in zircon even when present in other phases. It is possible to mistake non-magmatic inclusions in detrital grains for primary magmatic inclusions, and examination of internal host grain texture and geochronology (when applicable) is especially important in a detrital setting. These results should allow an improved framework for interpreting inclusion assemblages in detrital magmatic minerals and in interpreting the phase petrology of the host igneous rock. Given the remaining ambiguities in linking a number of observations (i.e., apatite contents, position on the quartz-alkali feldspar-plagioclase ternary, and the ratio of felsic to mafic silicate phases) to specific magmatic provenance, the identification of rare but petrologically significant inclusion phases such as muscovite, halides, Mn and Nb minerals, sulfides, or baddeleyite, may be more helpful in pointing to either highly evolved or more mafic provenances.

Trace elements in magmatic zircon exhibit characteristic trends during magma compositional evolution. Zircon geochemistry presents the advantage over whole rock geochemistry of showing progressive ‘snapshots’ of accompanying melt chemistry during magma crystallization, and in situ methods such as LA-ICPMS and SIMS can provide multiple analyses in zoned zircons for higher resolution of magmatic processes. However, one limitation is that zircon is typically a late-crystallizing phase, leaving much of the magmatic history unrecorded. Using the CAMECA ims 1290 ion microprobe with Hyperion-II ion source , we explore whether micro-zircon (ca. 5–40 µm) captured by relatively early-crystallizing magmatic accessory minerals can meaningfully enhance the interpretations of zircon trace element records of magmatic evolution in two granites from Southern California: the La Posta 2-mica granite and the granite of Butler Peak (Big Bear Lake Intrusive Suite). We find that contamination of the inclusion measurement by major structural constituents of the host phase is a problem for most measurements, but that species which are not structural components of the host phases follow typical magmatic zircon trace element evolution trends. Compared to free grains from a mineral separate, zircon inclusions in ilmenite and apatite from the La Posta 2-mica granite are on average lower in Hf, recording plausible extensions of these magmatic evolution trends to earlier periods in magma evolution. Free zircons from the granite of Butler Peak’s mineral separate form a more complex story, recording both mixing between Cretaceous magmas derived from different depths (based on their U/Yb and Eu/Eu* behavior) and inheritance of older Mesozoic to Proterozoic zircon. Most zircon inclusions in Butler Peak magnetite and ilmenite are chemically like the apparently shallow magma-derived Cretaceous zircons in the free matrix, while a small proportion of the included zircon resemble more closely the apparently deeper magma or the inherited grains.

In paleogeographic reconstructions of the Columbia and Rodinia Supercontinents, the position of the Greater India landmass is ambiguous. This, coupled with a limited understanding of the tectonic evolution of the mobile belts along which the mosaic of crustal domains in India accreted, impedes precise correlation among the dispersed crustal fragments in supercontinent reconstructions. Using structural, metamorphic phase equilibria, chronological and geochemical investigations, this study aims to reconstruct the tectonic evolution of the Chottanagpur Gneiss Complex (CGC) as a distinct crustal block at the eastern end of the Greater Indian Proterozoic Fold Belt (GIPFOB) along which the North India Block (NIB) and the South India Block (SIB) accreted. The study focuses on two issues, e.g. dating the Early Neoproterozoic (0.92 Ga) accretion of the CGC with the NIB contemporaneous with the assembly of Rodinia, and documenting the widespread (>24,000 km2) plutonism of 1.5–1.4 Ga weakly peraluminous, calc-alkalic to alkali-calcic and ferroan A-type granitoids (± garnet) devoid of mafic microgrannular enclaves and coeval mafic emplacements in the crustal block. These dominantly within-plate granitoids arguably formed by asthenospheric upwelling induced partial melting of garnet-bearing anatectic quartzofeldspathic gneisses that dominate the Early Mesoproterozoic basement of the block. The major and trace element chemistry of the granitoids is similar to the 1.35–1.45 Ga A-type granitoids in Laurentia/Amazonia emplaced contemporaneous with the 1.5–1.3 Ga breakup of the Columbia Supercontinent. This study suggests the Chottanagpur Gneiss Complex occured as a fragmented crustal block following the breakup of the Columbia Supercontinent; the crustal block was subsequently integrated within India during the Early Neoproterozoic oblique accretion between the NIB and SIB contemporaneous with the Rodinia Supercontinent assembly.

Double-Wien filter-selection-aperture and hexapole-collision-cell technologies coupled to laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS/MS) enables in situ analysis of 87Sr variations produced by 87Rb decay.

Figures and tables.

If the entirety of Earth history is considered, no other mineral yields more information about crust—from its primordial formation to present-day volcanic eruptions—than zircon (ZrSiO4). This information trove is borne out of a combination of key zircon properties, such as high physical hardness and a structure that can accommodate radioactive actinide isotopes like 235,238U and 232Th that decay to different isotopes of Pb. Crystalline zircon has slow diffusion rates for U and Pb, which means that ages will not be perturbed by volume diffusion under most geologic conditions. The robust age data obtained from zircon anchors most of the geologic time scale in use today. For example, zircon geochronology obtained from a variety of techniques has been used to constrain the timing of igneous and metamorphic activity, to correlate biostratigraphic units and mass extinctions with absolute time though investigations of zircon-bearing volcanic ash beds, and to explore the assembly of supercontinents and the detrital record to almost 4.4 Ga. Zircon also has two well-recognized crystallization thermometers, based on broadly different principles, which have found widespread use in the geological sciences. Chemical impurities (e.g., Li, Al, P, Ti, rare earth elements, Th, U), stable isotope ratios (e.g., Li-, O-, Si-, and Zr-isotopes), and mineral inclusions have constrained the source characteristics for in- and ex situ zircon. Evidence for assimilation of water-altered supracrustal material into silicate melts, melt evolution of plutons and volcanoes during fractional crystallization, the source of melt generation and crust-mantle interactions, and earliest evidence for biological activity on Earth have been explored though zircon chemistry and their primary inclusions.

Magmatic zircon tends to exhibit characteristic trends in trace element contents in response to magma cooling and fractionation, such that zircon may provide a window into melt evolution not accessible by whole rock geochemistry. The Peninsular Ranges Batholith of southern California and Baja California consists of older rocks of oceanic arc affinity in the west. Younger rocks to the east been interpreted via whole rock chemistry to reflect (Transition Zone) deeper origins and (Eastern Zone) deeper origins with significant crustal assimilation compared to the Western Zone. The youngest magmas in the Eastern Zone form large, typically zoned plutons and are high in Sr/Y. We present zircon trace element and oxygen isotope measurements in selected Western and Transition Zone samples and in many units from the high-Sr/Y La Posta and San Jacinto plutons. Most Western and Transition Zone populations exhibit typical shallow fractionation-related trace element trends, while one Eastern Transition Zone sample and most samples from the high-Sr/Y Eastern Zone lack Eu/Eu* evidence for plagioclase fractionation. Oxygen isotopes, zircon P contents, and Ce-based redox estimates suggest that the most evolved unit (2-mica granodiorite) of the La Posta pluton is significantly contaminated by sediments, with other high-Sr/Y units displaying zircon oxygen isotope and redox estimates falling between the 2-mica granodiorite and the relatively uncontaminated Transition Zone in these parameters. However, most of these zircons lack unambiguous markers for supracrustal contamination that could be interpreted even in a detrital context. The contrasting behavior of zircon U/Yb and Eu/Eu* during shallow fractionation and their similar response to melting depth variations can be used to distinguish the effects of shallow fractional crystallization, depth of melting, and introduction of exotic materials by either crustal assimilation or magma mixing. On the basis of U/Yb vs Eu/Eu* relationships, we interpret the behavior of Western Zone and most Transition Zone zircons as reflecting mainly shallow fractionation. One Eastern Transition Zone tonalite appears to result from mixing of magma from different depths. We interpret the high-Sr/Y Eastern Zone magmas as resulting from mixing of magmas that have assimilated varying amounts of metasediment. Moderate levels of sediment assimilation may complicate magma depth interpretations which are based on zircon (Th, U)/Yb plots or detrital zircon Eu/Eu* – without yielding clear zircon geochemical evidence for supracrustal assimilants. Magma mixing is less problematic for regional-scale detrital zircon Eu/Eu* studies and may be easier to detect based on the relationships among (Th, U)/Yb and Eu/Eu*.

There is a lot of debate surrounding the geodynamic environment of the Hadean Earth, and the only definitively known fragments from the first 500 Ma are detrital zircons. This makes the combination of natural and experimentally obtained data a powerful tool to unravel the chemistry of the early Earth. We have used an incompatible marker element, B, to partially derive the chemistry of the parent melts of the Hadean and modern zircons. We report an experimental calibration for temperature dependent B partitioning between zircon and a hydrous weakly peraluminous granitic melt. log(10) D-B(zrc/melt) = -(1027 +/- 372)/T (K) - (2.011 +/- 0.257) where D-B(zrc/melt) is the zircon-melt partition coefficient for B and T is temperature in K. This calibration has been applied to natural samples, viz., Hadean zircons from the Jack Hills (JH), Australia, Phanerozoic zircons from the Lachlan Fold Belt (LFB), Australia and three pegmatitic zircons from Seiland Igneous province, Norway, Paicoma Canyon, California and Freeman Mine, North Carolina. Our results present direct evidence of B being present in the Hadean crust. The zircons from JH and LFB are rather poor in B (8-80 ppb), but comparable to each other, while the pegmatites have as much as ten times the [B] (similar to 0.35-0.45 ppm). Application of our experimental calibration yields calculated B melt concentrations of 10-90 ppm for the JH and LFB zircons. Such values for melt [B] (concentrations) are similar to the modern upper continental crust (17 ppm) and volcanic arcs, but are high when to compared to OIBs (0.6-1.8 ppm) and MORBs (1.3 ppm). The Lachlan zircons have [B] values that are broadly similar to the detrital Hadean and Archean zircons, and these have been presented as a point of comparison between Hadean and modern zircons. The pegmatite zircons return calculated melt values (244-701 ppm) that are much higher than the parent melts of the Australian zircons. One of the pegmatite zircons show a variation in calculated Ti-in-zircon crystallization T from the core to the rim (716 degrees C - 916 degrees C). Two of the other zircons crystallized at 664 +/- 14 degrees C and 598 +/- 13 degrees C (2 s.e.; not taking into consideration a similar to 50 degrees T uncertainty due to imperfectly constrained silica/titania activities) and show no intracrystalline variation in [B] or T. Finally, B has been proposed to have been a possible stabilizing agent for ribose aqueous solutions and could have played a part in the formation of carbohydrates and proteins which were building blocks for RNA. Our documented presence of B in the Hadean crust at calculated concentrations similar to a modern volcanic arc setting, makes the role of B in ribose stabilization at least possible on the primordial Earth.

The magmato-tectonic environment(s) of origin for Earth’s earliest crust are enigmatic and fiercely debated. Revealing the composition of the melts from which Hadean (>4.02 Ga) zircons crystallized might clarify conditions of initial crust construction. We calculate model melts using Ti-calibrated zircon/melt partition coefficients (KdZrc(Ti)) and published trace element data for Hadean and Archean zircons. The same treatment is applied to zircons from possible analogue environments (MORB, Iceland, arcs, lunar), to constrain potential petrogenetic similarities and distinctions between the early and modern world. Model melts from oceanic environments (MORB, oceanic arc, Iceland) have higher heavy rare earth element (HREE) contents and shallower middle REE (MREE) to HREE/chondrite (ch) slopes than those from continental arcs and tonalite-trondhjemite-granodiorite suites (TTGs). Hadean and Archean model melts are nearly indistinguishable from one another, both resembling TTGs and continental arcs, with pronounced depletion of HREE and slope reversal in heaviest REE. A limited number of samples > 4.25 Ga yield model melts with broadly similar characteristics to those from younger Hadean and Archean zircons, but with relatively elevated REE (~half order of magnitude) and higher LREE and MREE relative to HREE. Rare earth element patterns of early Earth model melts suggest a common petrogenetic history in the Hadean and Archean, involving garnet +/-amphibole in relatively low-temperature, high-pressure, environments.

The time of origin of the geodynamo has important implications for the thermal evolution of the planetary interior and the habitability of early Earth. It has been proposed that detrital zircon grains from Jack Hills, Western Australia, provide evidence for an active geodynamo as early as 4.2 billion years (Ga) ago. However, our combined paleomagnetic, geochemical, and mineralogical studies on Jack Hills zircons indicate that most have poor magnetic recording properties and secondary magnetization carriers that postdate the formation of the zircons. Therefore, the existence of the geodynamo before 3.5 Ga ago remains unknown.