Assessment of shock effects on amphibole water contents and hydrogen isotope compositions: 2. Kaersutitic amphibole experiments

To constrain the influence of impact shock on water and hydrogen isotope signatures of Martian meteorite kaersutites, we conducted shock recovery experiments on three terrestrial kaersutite crystals. Homogeneous impact shock to 32 GPa, commensurate with shock levels experienced by Martian meteorite kaersutites, led to increases in kaersutite water contents (ΔH_2O = 0.25–0.89 wt.%), decreases in Fe^3+/ΣFe (4–20%), and enrichments in hydrogen isotope composition (ΔD = + 66 to + 87‰) relative to pre-shock values. The latter values represent the largest shock-induced hydrogen isotope fractionations measured to date. These observations are explained most completely by a two-step shock process. First, shock-induced devolatilization led to hydrogen isotope enrichment through preferential loss of H relative to D. Second, reaction of the kaersutite with the ambient atmosphere led to increased water contents and reduced Fe. Fe reduction and water addition via the reaction Fe^2+ + OH^− ↔ Fe^3+ + O_2− + ½H_2 explain the Fe^3+/ΣFe data and some of the water data. Further water addition mechanisms (irreversible adsorption, shock implantation) are necessary to fully explain the increased water contents. Addition of water from the terrestrial atmosphere, which is isotopically light relative to the experimental kaersutite compositions, means the measured hydrogen isotope enrichments are likely minima. The measured (minimum) levels of hydrogen isotope enrichment are relevant to the hydrogen isotope variability within and among Martian kaersutites, but are minor relative to their absolute δD values. Alternatively, addition of water from the enriched Martian atmosphere could explain both Martian kaersutite hydrogen isotope variability and absolute δD values. However, the low Martian kaersutite water contents leave little room for significant water addition. The importance of the ambient atmosphere to the outcome of the shock experiments makes it difficult to translate our results to Mars given the unknown influence of its more tenuous atmosphere on the processes observed in the experiments. Our results suggest that shock is a feasible mechanism for influencing Martian kaersutite water contents and hydrogen isotope compositions but that its complex signature precludes precise determination of that influence

LAMINATED MUDSTONES IN THE CLAY-SULFATE TRANSITION INTERVAL AT MT. SHARP -SIMILARITIES TO EVAPORITIC MUDSTONES FROM PAHRUMP HILLS AND POSSIBLE

International audienceMudstone-rich lacustrine strata at Gale Crater, Mars, dominate the Mt. Sharp sedimentary succession. The Gale crater strata provide the oppor-tunity to test hypotheses regarding past climate shifts early in the planets history [1, 2], a key motivator for selecting Gale Crater as a landing site for the Curiosity rover.At the base of the Mt. Sharp succession, at Pahrump Hills, mudstones with sedimentary features and geo-chemical attributes occur that suggest that these rocks accumulated in an underfilled lake basin. Lake waters were saline to hypersaline, and lake levels, shorelines, and salinities fluctuated greatly at various temporal scales [3, 4, 5].Laminated mudstones, characterized by alternating softer and harder (cemented) layers and apparent evaporite pseudomorphs [4, 5] associated with the lat-ter, dominate the Pahrump succession. They show multiple textural similarities to evaporitic lacustrine strata on Earth [3]. Although indications of evaporitic conditions have been reported from higher in the sec-tion [6], laminated evaporitic mudstones of the type seen at Pahrump Hills were not been observed for the next 400 m of Mt. Sharp stratigraphy since leaving Pahrump Hills.During the recent ascent from the clay trough to-wards the Sulfate bearing unit, rocks resembling the Pahrump Hills evaporitic mudstones were again en-countered at elevations above -4030 m. Examples of these laminated mudstones are compared to textural equivalents from Pahrump Hills

LAMINATED MUDSTONES IN THE CLAY-SULFATE TRANSITION INTERVAL AT MT. SHARP -SIMILARITIES TO EVAPORITIC MUDSTONES FROM PAHRUMP HILLS AND POSSIBLE

International audienceMudstone-rich lacustrine strata at Gale Crater, Mars, dominate the Mt. Sharp sedimentary succession. The Gale crater strata provide the oppor-tunity to test hypotheses regarding past climate shifts early in the planets history [1, 2], a key motivator for selecting Gale Crater as a landing site for the Curiosity rover.At the base of the Mt. Sharp succession, at Pahrump Hills, mudstones with sedimentary features and geo-chemical attributes occur that suggest that these rocks accumulated in an underfilled lake basin. Lake waters were saline to hypersaline, and lake levels, shorelines, and salinities fluctuated greatly at various temporal scales [3, 4, 5].Laminated mudstones, characterized by alternating softer and harder (cemented) layers and apparent evaporite pseudomorphs [4, 5] associated with the lat-ter, dominate the Pahrump succession. They show multiple textural similarities to evaporitic lacustrine strata on Earth [3]. Although indications of evaporitic conditions have been reported from higher in the sec-tion [6], laminated evaporitic mudstones of the type seen at Pahrump Hills were not been observed for the next 400 m of Mt. Sharp stratigraphy since leaving Pahrump Hills.During the recent ascent from the clay trough to-wards the Sulfate bearing unit, rocks resembling the Pahrump Hills evaporitic mudstones were again en-countered at elevations above -4030 m. Examples of these laminated mudstones are compared to textural equivalents from Pahrump Hills

Encounters with an Unearthly Mudstone: Understanding the First Mudstone Found on Mars

The Sheepbed mudstone forms the base of the strata examined by the Curiosity rover in Gale Crater on Mars, and is the first bona fide mudstone known on another planet. From images and associated data, this contribution proposes a holistic interpretation of depositional regime, diagenesis and burial history. A lake basin probably received sediment pulses from alluvial fans. Bed cross‐sections show millimetre to centimetre‐scale layering due to distal pulses of fluvial sediment injections (fine‐grained hyperpycnites), fall‐out from river plumes, and some aeolian supply. Diagenetic features include mineralized synaeresis cracks and millimetre‐scale nodules, as well as stratiform cementation. Clay minerals were initially considered due to in situ alteration, but bulk rock chemistry and mineralogy suggests that sediments were derived from variably weathered source rocks that probably contained pre‐existing clay minerals. X‐ray diffraction analyses show contrasting clay mineralogy in closely spaced samples, consistent with at least partial detrital supply of clay minerals. A significant (ca 30 wt%) amorphous component is consistent with little post‐depositional alteration. Theoretical modelling of diagenetic reactions, as well as kinetic considerations, suggest that the bulk of diagenetic clay mineral formation occurred comparatively late in diagenesis. Diagenetic features (synaeresis cracks and nodules) were previously thought to reflect early diagenetic gas formation, but an alternative scenario of synaeresis crack formation via fabric collapse of flocculated clays appears more likely. The observed diagenetic features, such as solid nodules, hollow nodules, matrix cement and ‘raised ridges’ (synaeresis cracks) can be explained with progressive alteration of olivine/glass in conjunction with centrifugal and counter diffusion of reactive species. Anhydrite‐filled fractures in the Sheepbed mudstone occurred late in diagenesis when fluid pressures built up to exceed lithostatic pressure. Generating fluid overpressure by burial to facilitate hydraulic fracturing suggests a burial depth of at least 1000 m for the underlying strata that supplied these fluids

INVESTIGATION OF SEDIMENTARY FAN DEPOSITS ALONG THE BEAGLE GAP TRAVERSE AT JEZERO CRATER, MARS

International audienceIn February 2023, the Perseverance rover ascended the Jezero sedimentary fan deposit that has been interpreted as an ancient river delta (~3.6-3.8 Ga). The ~1.2 km traverse up “Beagle Gap” climbed ~35 m in elevation over 10 sols (martian days). The route passed outcrop exposures up to 20 m higher than had been observed along the delta front drive. Image and compositional data add new information on fan sedimentology and stratigraphy, and indicate a complex history of depositional processes and paleoenvironments.At five mounds along the route, 5-10 m thick, thin-bedded horizontal sandstone strata are present in the uppermost exposures. Locally, conglomerate beds are observed within these strata. This unit is interpreted as fluvial deposits in a delta top environment punctuated by episodic high discharge floods that transported cobble to boulder size clasts. Multiple outcrops of inclined strata are present, although these vary significantly in clast caliber, bedding thickness, and dip direction, which may indicate a range of deposit types. With the diversity in observed sedimentary attributes, a variety of depositional models are being evaluated, including deltaic foresets or lobes, and fluvial deposits.Proximity science using the arm instruments was conducted at one location, “Jenkins Gap.” The poorly sorted, pebbly coarse sandstone has extensive porosity evident in WATSON images. Carbonate composition was identified in the matrix by both arm spectrometers. A candidate lake level (-2490 m) identified in the Kodiak outcrop passes through Jenkins Gap, supporting the interpretation of carbonate formation in a shore-marginal environment. Additionally, this outcrop is texturally similar to lacustrine shoreline ‘tufa’ deposits documented at the Provo level of Pleistocene Lake Bonneville in northeast Utah. Further investigation of the fan stratal geometry through correlation with Beagle Gap units will provide constraints on the relative timing and duration of lakes levels at Jezero crater