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

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

Overview of Perseverance’s Upper Fan Campaign

International audienceThe Mars 2020 Perseverance rover’s mission objective is to collect diverse suites of samples for return to Earth via Mars Sample Return and characterize their geologic and environmental context. Jezero crater was selected as the landing site for Perseverance partially due to the opportunity to explore the western fan deposits, which include water-lain sediments from a diverse ancient watershed that have been lithified and exposed by wind. The Upper Fan Campaign, described here, is the third campaign of the mission; it follows the crater floor campaign and the delta front campaign that explored ~40 m of sedimentary deposits at the base of two lobes of the fan. The Upper Fan Campaign includes the rover traverse from the top of the delta front campaign to the margin unit lining the inner crater rim.Prior to landing, two major geologic units were identified on the Upper Fan: the truncated curvilinear unit and the blocky unit. Our goals are to characterize the depositional context and major lithologies present in each unit and acquire samples. At time of writing, we have collected two samples and are planning 1-2 more sample acquisitions in the Upper Fan.The Melyn sample is a fine-grained sandstone from the light toned bands of the truncated curvilinear unit at Tenby, part of the Skrinkle Haven member of the Tenby formation. The sandstone lacks internal structure and decimeter-thick layers have steep (>20°) dip angles. It is interpreted to have formed in subaqueous downslope avalanches, likely on a downstream bar or delta foreset. Melyn contains detrital primary and altered silicates, minor sulfates, and Mg, Fe carbonate grains, coatings, and possible cement.The Otis_Peak sample is a cross-stratified conglomerate interpreted as part of a higher-energy delta plain braided river deposit in the Carew Castle member of the Tenby formation, overlying the Skrinkle Haven member. Proximity science and remote sensing indicate Otis_Peak contains coarse (up to 4 mm) detrital primary and altered silicates with sulfates and Fe, Mg carbonates.Perseverance is characterizing the most commonly observed boulder lithologies in the blocky unit, including boulders with dominant olivine and pyroxene spectral signatures. We plan to collect 1-2 more samples in the campaign, including a lithology representative of the latest phase of delta deposition

Overview of Perseverance’s Upper Fan Campaign

International audienceThe Mars 2020 Perseverance rover’s mission objective is to collect diverse suites of samples for return to Earth via Mars Sample Return and characterize their geologic and environmental context. Jezero crater was selected as the landing site for Perseverance partially due to the opportunity to explore the western fan deposits, which include water-lain sediments from a diverse ancient watershed that have been lithified and exposed by wind. The Upper Fan Campaign, described here, is the third campaign of the mission; it follows the crater floor campaign and the delta front campaign that explored ~40 m of sedimentary deposits at the base of two lobes of the fan. The Upper Fan Campaign includes the rover traverse from the top of the delta front campaign to the margin unit lining the inner crater rim.Prior to landing, two major geologic units were identified on the Upper Fan: the truncated curvilinear unit and the blocky unit. Our goals are to characterize the depositional context and major lithologies present in each unit and acquire samples. At time of writing, we have collected two samples and are planning 1-2 more sample acquisitions in the Upper Fan.The Melyn sample is a fine-grained sandstone from the light toned bands of the truncated curvilinear unit at Tenby, part of the Skrinkle Haven member of the Tenby formation. The sandstone lacks internal structure and decimeter-thick layers have steep (>20°) dip angles. It is interpreted to have formed in subaqueous downslope avalanches, likely on a downstream bar or delta foreset. Melyn contains detrital primary and altered silicates, minor sulfates, and Mg, Fe carbonate grains, coatings, and possible cement.The Otis_Peak sample is a cross-stratified conglomerate interpreted as part of a higher-energy delta plain braided river deposit in the Carew Castle member of the Tenby formation, overlying the Skrinkle Haven member. Proximity science and remote sensing indicate Otis_Peak contains coarse (up to 4 mm) detrital primary and altered silicates with sulfates and Fe, Mg carbonates.Perseverance is characterizing the most commonly observed boulder lithologies in the blocky unit, including boulders with dominant olivine and pyroxene spectral signatures. We plan to collect 1-2 more samples in the campaign, including a lithology representative of the latest phase of delta deposition