12 research outputs found
Lunar samples record an impact 4.2 billion years ago that may have formed the Serenitatis Basin
No full textThese files serve as publicly available data to follow the publication titled 'Lunar samples record an impact 4.2 billion years ago that may have formed the Serenitatis Basin' accepted in Communications Earth & Environment (Nature Springer).
Supplementary Data 1. A zip folder containing iSALE code modelling input parameters for 1) Serenitatis Basin formation at 4.5 and 4.0 Gyr; 2) Dawes crater and ejecta formation, and 3) Dawes ejecta landing. Details on the modelling are provided in Supplementary Methods 1.2 and Supplementary Discussion 2.3. of the published article.
Supplementary Data 2. An Excel spreadsheet containing U-Pb and Pb-Pb dating results presented in Supplementary Table 2.
Supplementary Movie 1. Mg distribution in the Atom Probe Tomography microtip M3 of shocked lunar apatite. Polygonal grains of lunar apatite can be recognized by the magnesium decoration of grain boundaries.
Supplementary Movie 2. Mg-decorated grain boundaries in the Atom Probe Tomography microtip M3 of shocked lunar apatite. Mg atoms (blue dots) and a 0.13 at. % magnesium isosurface (purple) in a 5 nm slice of the apatite data as it is moved through the needle from one side to the other, show the shape of the grain boundaries
Shock‐induced microtextures in lunar apatite and merrillite
No full textApatite and merrillite are the most common phosphate minerals in a wide range of planetary materials and are key accessory phases for in situ age dating, as well as for determination of the volatile abundances and their isotopic composition. Although most lunar and meteoritic samples show at least some evidence of impact metamorphism, relatively little is known about how these two phosphates respond to shock-loading. In this work, we analyzed a set of well-studied lunar highlands samples (Apollo 17 Mg-suite rocks 76535, 76335, 72255, 78235, and 78236), in order of displaying increasing shock deformation stages from S1 to S6. We determined the stage of shock deformation of the rock based on existing plagioclase shock-pressure barometry using optical microscopy, Raman spectroscopy, and SEM-based panchromatic cathodoluminescence (CL) imaging of plagioclase. We then inspected the microtexture of apatite and merrillite through an integrated study of Raman spectroscopy, SEM-CL imaging, and electron backscatter diffraction (EBSD). EBSD analyses revealed that microtextures in apatite and merrillite become progressively more complex and deformed with increasing levels of shock-loading.An early shock-stage fragmentation at S1 and S2 is followed by subgrain formation from S2 onward, showing consistent decrease in subgrain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2°–3° of intragrain crystal-plastic deformation in both phosphates at the lowest shock stage, apatite undergoes up to 25° and merrillite up to 30° of crystal-plastic deformation at the highest stage of shock deformation (S5). Merrillite displays lower shock impedance than apatite; hence, it is more deformed at the same level of shock-loading. We suggest that the microtexture of apatite and merrillite visualized by EBSD can be used to evaluate stages of shock deformation and should be taken into account when interpreting in situ geochemically relevant analyses of the phosphates, e.g., age or volatile content, as it has been shown in other accessory minerals that differently shocked domains can yield significantly different ages
Evidence for Sodium-Rich Alkaline Water in the Tagish Lake Parent Body and Implications for Amino Acid Synthesis and Racemization
No full textUnderstanding the timing and mechanisms of amino acid synthesis and racemization on asteroidal parent bodies is key to demonstrating how amino acids evolved to be mostly left-handed in living organisms on Earth. It has been postulated that racemization can occur rapidly dependent on several factors, including the pH of the aqueous solution. Here, we conduct nanoscale geochemical analysis of a framboidal magnetite grain within the Tagish Lake carbonaceous chondrite to demonstrate that the interlocking crystal arrangement formed within a sodium-rich, alkaline fluid environment. Notably, we report on the discovery of Na-enriched subgrain boundaries and nanometer-scale Ca and Mg layers surrounding individual framboids. These interstitial coatings would yield a surface charge state of zero in more-alkaline fluids and prevent assimilation of the individual framboids into a single grain. This basic solution would support rapid synthesis and racemization rates on the order of years, suggesting that the low abundances of amino acids in Tagish Lake cannot be ascribed to fluid chemistry
Microtextures in the Chelyabinsk impact breccia reveal the history of Phosphorus‐Olivine‐Assemblages in chondrites
No full textAbstract The geochemistry and textures of phosphate minerals can provide insights into the geological histories of parental asteroids, but the processes governing their formation and deformation remain poorly constrained. We assessed phosphorus‐bearing minerals in the three lithologies (light, dark, and melt) of the Chelyabinsk (LL5) ordinary chondrite using scanning electron microscope, electron microprobe, cathodoluminescence, and electron backscatter diffraction techniques. The majority of studied phosphate grains appear intergrown with olivine. However, microtextures of phosphates (apatite [Ca 5 (PO 4 ) 3 (OH,Cl,F)] and merrillite [Ca 9 NaMg(PO 4 ) 7 ]) are extremely variable within and between the differently shocked lithologies investigated. We observe continuously strained as well as recrystallized strain‐free merrillite populations. Grains with strain‐free subdomains are present only in the more intensely shocked dark lithology, indicating that phosphate growth predates the development of primary shock‐metamorphic features. Complete melting of portions of the meteorite is recorded by the shock‐melt lithology, which contains a population of phosphorus‐rich olivine grains. The response of phosphorus‐bearing minerals to shock is therefore hugely variable throughout this monomict impact breccia. We propose a paragenetic history for P‐bearing phases in Chelyabinsk involving initial phosphate growth via P‐rich olivine replacement, followed by phosphate deformation during an early impact event. This event was also responsible for the local development of shock melt that lacks phosphate grains and instead contains P‐enriched olivine. We generalize our findings to propose a new classification scheme for Phosphorus‐Olivine‐Assemblages (Type I–III POAs). We highlight how POAs can be used to trace radiogenic metamorphism and shock metamorphic events that together span the entire geological history of chondritic asteroids
High-pressure crystal chemistry of coesite-I and its transition to coesite-II
No full textThe high-pressure crystal chemistry of coesite was studied by means of single crystal X-ray diffraction in the pressure interval ∼2–34 GPa and at ambient temperature. We compressed the samples using diamond-anvil cells loaded with neon as pressure-transmitting medium and collected X-ray diffraction data using synchrotron radiation. The thermodynamically stable coesite – coesite-I – was observed up to ∼20 GPa, with the following unit-cell parameters: a = 6.6533(12) Å, b = 11.9018(10) Å, c = 6.9336(10) Å, β = 121.250(20)° and V = 469.38(15) Å3. The volume-pressure data of coesite-I are described by means of a third-order Birch-Murnhagan EoS with parameters V0 = 547.26(66) Å3, KT0 = 96(4) GPa, K′To = 4.1(4). Above such pressure we witness the formation of a well crystallized coesite-II, previously observed only by spectroscopic studies. The structure of the novel high-P polymorph was determined and refined at ∼28 and ∼31 GPa with final R indices of 8% and 12%, respectively. Coesite-II has P21/n symmetry and a unit cell that is “doubled” along the b-axis with respect to that of the initial coesite-I: a = 6.5591(10) Å, b = 23.2276(14) Å, c = 6.7953(9) Å, β = 121.062(19)° and V = 886.84(19) Å3 at ∼28 GPa. All Si atoms are in tetrahedral coordination. The displacive phase transition I->II is likely driven by the extreme shortening (0.05 Å or 3.2%) of the shortest and the most compressible Si1-O1 bond, related to the stiff 180° Si1-O1-Si1 angle. Under compression the linear angle bends, resulting in two independent angles, one of which, however, retains almost linear geometry (∼178°). The requirement of this angle to be close to linear likely causes further Si-O compression down to an extremely short distance of ∼1.52 Å which prompts subsequent structural changes, with the formation of a triclinic phase at ∼31 GPa, coesite-III
The shocking state of apatite and merrillite in shergottite Northwest Africa 5298 and extreme nanoscale chlorine isotope variability revealed by atom probe tomography
No full textThe elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet’s endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of δ37Cl values (-3 to 1 ‰; 2σ uncertainties 37Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative δ37Cl values (mean of -15 ± 8 ‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean δ37Cl value of -0.3 ± 0.6 ‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects
Compressional pathways of α-cristobalite, structure of cristobalite X-I, and towards the understanding of seifertite formation
No full textIn various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms—a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures
Reply to Bada: Acidity and fluid composition on the Tagish Lake parent body
No full textA comment from Bada (1) attempts to flag multiple issues with our recent study (2). We appreciate the opportunity to respond, as Bada raises some interesting points, but many of his comments appear to overinterpret our results in a manner well beyond the scope of our original study. We believe Bada’s comments (1) can be broadly grouped into two key points for discussion: 1) the calculation used to yield the racemization timeline and 2) issues with our final conclusion
Lunar samples record an impact 4.2 billion years ago that may have formed the Serenitatis Basin
No full textImpact cratering on the Moon and the derived size-frequency distribution functions of lunar impact craters are used to determine the ages of unsampled planetary surfaces across the Solar System. Radiometric dating of lunar samples provides an absolute age baseline, however, crater-chronology functions for the Moon remain poorly constrained for ages beyond 3.9 billion years. Here we present U–Pb geochronology of phosphate minerals within shocked lunar norites of a boulder from the Apollo 17 Station 8. These minerals record an older impact event around 4.2 billion years ago, and a younger disturbance at around 0.5 billion years ago. Based on nanoscale observations using atom probe tomography, lunar cratering records, and impact simulations, we ascribe the older event to the formation of the large Serenitatis Basin and the younger possibly to that of the Dawes crater. This suggests the Serenitatis Basin formed unrelated to or in the early stages of a protracted Late Heavy Bombardment
Preservation of primordial signatures of water in highly-shocked ancient lunar rocks
No full textSpurred by the discovery of water in lunar volcanic glasses about a decade ago, the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water. This is due to its ability to contain significant amounts of OH in its structure, along with the widespread presence of apatite in lunar rocks. There is a general understanding that crustal cumulate rocks of the lunar magnesian (Mg) suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks, such as mare basalts. Consequently, water-bearing minerals in Mg-suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior. Mg-suite rocks and most other Apollo samples that were collected at the lunar surface display variable degrees of shock-deformation. In this study, we have investigated seven Apollo 17 Mg-suite samples that include troctolite, gabbro and norite lithologies, in order to understand if shock processes affected the water abundances and/or H isotopic composition of apatite. The measured water contents in apatite grains range from 31 to 964 ppm, with associated δD values varying between −535 ±134‰ and +147 ±194‰(2σ). Considering the full dataset, there appears to be no correlation between H2O and δD of apatite and the level of shock each apatite grain has experienced. However, the lowest δD was recorded by individual, water-poor (∼100 ppm H2O), regardless of the complexity of the shock-induced nanostructures, there appears to be no evidence of water-loss or alteration in their δD. The weighted average δD value of 24 such water-rich apatites is −192 ±71‰, and, of all 36 analyzed spots is −209 ±47‰, indistinguishable from that of other KREEPy lunar lithologies or the Earth’s deep mantle. Despite experiencing variable degrees of shock-deformation at a later stage in lunar history, water-rich apatite in some of the earliest-formed lunar crustal material appears to retain the original isotopic signature of H in the Moon
