James Darling

Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon

Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U–Pb (uranium–lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.

Impact melt sheet zircons and their implications for the Hadean crust

Impacts may have been important mechanisms of crustal redistribution and differentiation, particularly during intense postaccretionary bombardment between 4.5 Ga and 3.9 Ga ago. Evidence of crustal processes during this period is largely provided by detrital zircons from the Yilgarn craton, Australia. Trace element compositions, crystallization temperatures, and inclusion populations of these ancient zircons have been taken as evidence for predominantly granitic source magmas, implying widespread felsic continental crust on the early Earth. However, there is little knowledge of zircons formed in impact melt sheets, a potential source for the Hadean zircons. Here we present Ti thermometry, trace elements, and inclusion populations of zircons from the 1.85 Ga Sudbury impact melt sheet (Ontario, Canada). Our results demonstrate that large variations in zircon crystallization temperature and composition will be an inevitable consequence of the evolution of such magmatic systems. We also show that zircons in mafic rocks crystallize in residual liquids of granitic composition, producing inclusion assemblages that are remarkably similar to those reported for the ancient Yilgarn grains. Thus, we conclude that the trace element compositions and inclusion populations of the Hadean zircons are consistent with crystallization from more mafic melts than previously recognized, although high crystallization temperature distributions of Sudbury zircons indicate that impact melt sheets were not a dominant source for the grains older than 3.9 Ga.

Apatite trace element and isotope applications to petrogenesis and provenance

Abstract Apatite is an excellent tracer of petrogenetic processes as it can incorporate a large range of elements that are sensitive to melt evolution (LREE-MREE, Sr, Pb, Mn, halogens, Nd isotopes). Recent advances in the understanding of trace element concentrations and isotope ratios in apatite provide a novel tool to investigate magmatic petrogenesis and sediment provenance. Recent experimental work has better characterized trace element partition coefficients for apatite, which are sensitive to changes in magma composition (e.g., SiO2 and the aluminum saturation index value). The chemistry of apatites from granitoids has been suggested to reflect the composition of the host magma and yield information about petrogenetic processes that are invisible at the whole-rock scale (mixing, in situ crystal fractionation, metasomatism). Nd isotopes in apatite can now be analyzed by LA-MC-ICP-MS to constrain mantle and crustal contributions to the source(s) of the studied magma. These recent advances highlight exciting new horizons to understand igneous processes using accessory minerals. In this contribution, we use a compilation of recent data to show that apatite in the matrix and as inclusions within zircon and titanite is useful for providing insights into the nature and petrogenesis of the parental magma.Trace element modeling from in situ analyses of apatite and titanite can reliably estimate the original magma composition, using appropriate partition coefficients and careful imaging. This provides a new way to look at magmatic petrogenesis that have been overprinted by metamorphic processes. It also provides the rationale for new investigations of sedimentary provenance using detrital accessory minerals, and could provide a powerful new window into early Earth processes if applied to Archean or Hadean samples.

Redistribution of REE, Y, Th, and U at high pressure: Allanite-forming reactions in impure meta-quartzites (Sesia Zone, Western Italian Alps)

Abstract Accessory phases are important hosts of trace elements; allanite may contain >90% of the REE in a bulk rock. The mobility and redistribution of several trace elements, notably HREE, Th, U, and Y is thus controlled by reactions involving allanite and other REE phases, as well as several rock-forming minerals. As these elements are commonly concentrated in mature clastic sediments, a suite of impure quartzite was studied. Two eclogite facies samples from the Monometamorphic Cover Complex of the Sesia Zone (Western Italian Alps) are presented in some detail, as they reveal a remarkably rich spectrum of reaction relationships that involve REE phases. Two allanite-forming reactions were inferred from textures and phase compositions (1) monazite + Ca-silicate(?) + fluid → allanite + apatite + thorite; (2) monazite + thorite + Ca-silicate(?) + fluid → Th-rich allanite + auerlite ± apatite. Petrographic observations and thermodynamic models suggest that allanite entered the HP assemblage at ~530 °C and 17-18 kbar during prograde metamorphism. In one sample, allanite is rimmed by epidote rich in Y and HREE that grew at the expense of xenotime. Two net transfer reactions were derived (3) xenotime + allanite + fluid → Y-rich epidote + apatite + thorite; (4) xenotime + allanite + fluid → Y-rich epidote + aeschynite + thorite + (phosphate?). Textural relationships and trace element analyses of coexisting allanite/monazite and xenotime/ Y-rich epidote reveal systematic partitioning of the REE. Partition coefficients for the HREE are compatible with equilibrium fractionation, whereas those for the LREE show patterns that seem to be inherited from the precursor phases, in this case zircon with variable LREE composition.

Atomic-scale age resolution of planetary events

Resolving the timing of crustal processes and meteorite impact events is central to understanding the formation, evolution and habitability of planetary bodies. However, identifying multi-stage events from complex planetary materials is highly challenging at the length scales of current isotopic techniques. Here we show that accurate U-Pb isotopic analysis of nanoscale domains of baddeleyite can be achieved by atom probe tomography. Within individual crystals of highly shocked baddeleyite from the Sudbury impact structure, three discrete nanostructural domains have been isolated yielding average 206Pb/238U ages of 2,436±94 Ma (protolith crystallization) from homogenous-Fe domains, 1,852±45 Ma (impact) from clustered-Fe domains and 1,412±56 Ma (tectonic metamorphism) from planar and subgrain boundary structures. Baddeleyite is a common phase in terrestrial, Martian, Lunar and asteroidal materials, meaning this atomic-scale approach holds great potential in establishing a more accurate chronology of the formation and evolution of planetary crusts.

Discovery of mafic impact melt in the center of the Vredefort dome: Archetype for continental residua of early Earth cratering?

Melting by impact heating is thought to have been a significant process in the modification of early planetary crusts; however, crustally derived melt bodies in ancient terrestrial crust are frequently presumed to be absent due to erosion. Here we demonstrate that in the central basement uplift of the 2.020 Ga Vredefort impact basin (South Africa), components of mafic impact melt have survived amid Archean gneiss as decimeter-scale dikes and lenses of variably foliated gabbronorite. Zircon microstructural, trace element, and isotopic analyses (U-Pb, Lu-Hf) of the gabbronorite reveal a dominant population of 2.02 Ga unshocked igneous zircon with apparent Ti-in-zircon temperatures of 800–900 °C, similar to those from the mafic sublayer of the Sudbury impact melt sheet. Highly negative subchondritic e Hf values of −1.4 ± 1.1 to −7.9 ± 1.4 are consistent with a depleted mantle model age of ca. 3 Ga and gabbronorite derivation from the once superjacent Witwatersrand basin lithologies. The recrystallized igneous mineral textures and Archean felsic gneiss inclusions in the gabbronorite are attributable to the effects of emplacement and crater modification following ∼20 km elevation of the central uplift. Long mistaken as preimpact basement, the setting and characteristics of the Vredefort gabbronorite may provide new benchmarks in the search for remnants of large cratering events and melt residua on Earth’s cratons.

Baddeleyite as a widespread and sensitive indicator of meteorite bombardment in planetary crusts

Constraining the timing and intensity of Solar System bombardment is critical to understanding planetary formation, evolution and habitability. However, the identification and dating of shock-metamorphic events in the mafic igneous lithologies that dominate planetary materials remains highly challenging, particularly at relatively modest shock pressures. The accessory mineral baddeleyite (monoclinic-ZrO2) is

Linking shock microstructures and geochronology with zirconia (ZrO2)

Introduction: Baddeleyite (monoclinc-ZrO2) is a widely occurring accessory phase reported from an array of terrestrial mafic and ultra-mafic rocks [e.g. 1] as well as within shergottites [2,3], Lunar meteorites and Apollo samples [e.g. 4,5], asteroidal achondrites [6] and ordinary chondrites [7]. As an established U-Pb geochronometer [8], baddeleyite has the potential to resolve the timing of Solar System crystallization events for a number of low-Si lithologies where zircon (ZrSiO4) is absent. However, the exposure of these grains to shock metamorphism induces partial to complete loss of lead, resetting the U-Pb chronometer and complicating their interpretation. Recent work has focused on coupling isotopic analysis with microstructural observations, linking the extent of amorphisation and recrystallisation to the severity of lead-loss for the first time [i.e. 2, 9]. This approach allows for the targeting of pristine or deformed crystals in an attempt to differentiate the timings of igneous and impact events. However, given a discrepancy between the severity of lead diffusion within natural [2, 9] and experimental [10] shock conditions, our fundamental understanding of U-Pb age resetting in this potentially key planetary chronometer is poorly constrained. The application of atom probe tomography (APT) to zircon has proven exceptionally useful in distinguishing the response of Pb to both post-crystallization annealing [11] and deformation [12]. However, this approach has never been applied to heavily shock loaded material. Here we present the first insights into the atomic-scale shock response of lead cations within baddeleyite, coupling these observations with detailed EBSD analysis to produce a first order insight into the mechanisms of U-Pb age resetting in baddeleyite.

Atom probe insights into U-Pb age resetting in baddeleyite

Introduction: Baddeleyite (monoclinc-ZrO2) is a widely occurring accessory phase reported from an array of terrestrial mafic and ultra-mafic rocks [e.g. 1] as well as within shergottites [2,3], Lunar meteorites and Apollo samples [e.g. 4,5], asteroidal achondrites [6] and ordinary chondrites [7]. As an established U-Pb geochronometer [8], baddeleyite has the potential to resolve the timing of Solar System crystallization events for a number of low-Si lithologies where zircon (ZrSiO4) is absent. However, the exposure of these grains to shock metamorphism induces partial to complete loss of lead, resetting the U-Pb chronometer and complicating their interpretation. Recent work has focused on coupling isotopic analysis with microstructural observations, linking the extent of amorphisation and recrystallisation to the severity of lead-loss for the first time [i.e. 2, 9]. This approach allows for the targeting of pristine or deformed crystals in an attempt to differentiate the timings of igneous and impact events. However, given a discrepancy between the severity of lead diffusion within natural [2, 9] and experimental [10] shock conditions, our fundamental understanding of U-Pb age resetting in this potentially key planetary chronometer is poorly constrained. The application of atom probe tomography (APT) to zircon has proven exceptionally useful in distinguishing the response of Pb to both post-crystallization annealing [11] and deformation [12]. However, this approach has never been applied to heavily shock loaded material. Here we present the first insights into the atomic-scale shock response of lead cations within baddeleyite, coupling these observations with detailed EBSD analysis to produce a first order insight into the mechanisms of U-Pb age resetting in baddeleyite.

The shocking state of baddeleyite in basaltic shergottite Northwest Africa 5298

The nakhlite meteorites are samples of the Amazonian crust of Mars. They are olivine-bearing clinopyroxenites that contain veins of secondary minerals formed by pre-terrestrial water-mediated alteration. The precise origin of these veins is contentious, and a number of mechanisms have been suggested, e.g. fracture filling cementation [1, 2] and local dissolution-replacement [3]. It is also unclear whether the secondary minerals are the product of a single episode of alteration [2] or by multiple pulses of fluids [4]. In an attempt to answer these questions we have undertaken a petrographic and isotopic study of olivine-hosted secondary mineral veins in the Yamato 000593 and 000749 (Y593, Y749) nakhlites.