On the iron isotope composition of Mars and volatile depletion in the terrestrial planets

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have δ57Fe ≈ 0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, δ57Fe≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is δ57Fe = +0.04 ± 0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier δ57Fe(+0.05 ± 0.03‰), whereas MgO-rich rocks are lighter (δ57Fe≈−0.01 ±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the δ57Fe SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites

What Martian Meteorites Reveal About the Interior and Surface of Mars

Martian meteorites are the only direct samples from Mars, thus far. Currently, there are a total of 262 individual samples originating from at least 11 ejection events. Geochemical analyses, through techniques that are also used on terrestrial rocks, provide fundamental insights into the bulk composition, differentiation and evolution, mantle heterogeneity, and role of secondary processes, such as aqueous alteration and shock, on Mars. Martian meteorites display a wide range in mineralogy and chemistry, but are predominantly basaltic in composition. Over the past 6 years, the number of martian meteorites recovered has almost doubled allowing for studies that evaluate these meteorites as suites of igneous rocks. However, the martian meteorites represent a biased sampling of the surface of Mars with unknown ejection locations. The geology of Mars cannot be unraveled solely by analyzing these meteorites. Rocks analyzed by rovers on the surface of Mars are of distinct composition to the meteorites, highlighting the importance of Mars missions, especially sample return. The Mars 2020 Perseverance rover will collect and cache—for eventual return to Earth—over 30 diverse surface samples from Jezero crater. These returned samples will allow for Earth‐based state‐of‐the‐art analyses on diverse martian rocks with known field context. The complementary study of returned samples and meteorites will help to constrain the evolution of the martian interior and surface. Here, we review recent findings and advances in the study of martian meteorites and examine how returned samples would complement and enhance our knowledge of Mars

Shergottite Northwest Africa 6963: A Pyroxene‐Cumulate Martian Gabbro

Northwest Africa (NWA) 6963 was found in Guelmim‐Es‐Semara, Morocco, and based on its bulk chemistry and oxygen isotopes, it was classified as a Martian meteorite. On the basis of a preliminary study of the textures and crystal sizes, it was resubclassified as a gabbroic shergottite because of the similarity with terrestrial and lunar gabbros. However, the previous work was not a quantitative investigation of NWA 6963; to supplement the original resubclassification and enable full comparison between this and other Martian samples; here we investigate the mineralogy, petrology, geochemistry, quantitative textural analyses, and spectral properties of gabbroic shergottite NWA 6963 to constrain its petrogenesis, including the depth of emplacement (i.e., base of a flow versus crustal intrusion). NWA 6963 is an enriched shergottite with similar mineralogy to the basaltic shergottites but importantly does not contain any fine‐grained mesostasis. Consistent with the mineralogy, the reflectance (visible/near‐infrared and thermal infrared) spectrum of powdered NWA 6963 is similar to other shergottites because they are all dominated by pyroxene, but its reflectance is distinct in terms of albedo and spectral contrast due to its gabbroic texture. NWA 6963 represents a partial cumulate gabbro that is associated with the basaltic shergottites. Therefore, NWA 6963 could represent a hypabyssal intrusive feeder dike system for the basaltic shergottites that erupted on the surface

Role of carcinoscorpin, a haemolymph lectin of horseshoe crab Carcinoscorpius rotundacauda as humoral factor

This study demonstrated that a marine Indian horseshoe crab, Carcinoscorpious rotundacauda showed higher self defence in an experimental infection upon the induction of its circulatory lectin, carcinoscorpin. It resisted an infection with 10 7 live Escherichia coli per crab when the circulatory carcinoscorpin was 8-16-fold higher after administering 2-ketodeoxyoctonate (Kdo) into the live crab. The naive control with its natural level of circulatory lectin could tolerate a maximum infective dose of 106 live E. coli per crab. Bacterial kiliing and phagocytic uptake in association with the isolated crab amoebocytes in an ex vivo system was considerably higher for the lectin opsonized E. coli compared to unopsonized samples. Carcinoscorpin is thus functionally an opsonin in the defence of the primitive marine arthropod, C. rotundacauda, like vertebrate antibody, a humoral factor involved in the defence of the host. The natural capacity for defending an infection with 106 live E. coli per crab suggested that the crabs in the natural habitat hardly face such an infection and is possibly one of the reasons for its survival over millions of years as a living Ibssil

Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian Tectonic Zone, using P–T pseudosection modelling of mafic intrusions and host reworked granulites

In this study, we reconstruct the metamorphic pathways of reworking of a deep crustal granulite terrane in the Central Indian Tectonic Zone, using Archaean/Palaeoproterozoic (?) polycyclic felsic granulites and two groups of intrusive Mesoproterozoic mafic granulites, a coarse-grained noritic gabbro and a fine-grained gabbroic norite. The granulite terrane, locally referred to as the Bhandara–Balaghat granulite domain, is bounded by the South Indian Block and a Grenville-aged, younger tectonic domain of the Central Indian Tectonic Zone. The granulites and the mafic intrusions were multiply deformed and metamorphosed in the early Mesoproterozoic. Using P–T pseudosection modelling of host felsic granulites and the mafic intrusions, two distinct metamorphic events (BM2 and BM3) with contrasting P–T paths have been established.The P–T path of BM2 metamorphism has a clockwise sense, having an important prograde segment of heating of more than 250°C with pressure fall, followed by cooling. The peak BM2 metamorphism has been constrained at ∼6 kbar, ∼725°C. Emplacement of the mafic intrusions was broadly coincident with the low-pressure metamorphism, and they underwent a phase of subsolidus cooling. During subsequent metamorphism, BM3), the mafic intrusions and the host felsic granulites were re-metamorphosed along a counterclockwise P–T path. This led to tectonic burial of the mid-crust to ∼9.4 kbar, ∼760°C, which was followed by cooling accompanying pressure decline. Although, the clockwise P–T path is generally interpreted in terms of thermal relaxation of crust following thrusting, the scale of heating (∼250°C) during initial decompression, documented here, is much larger than modelled for tectonically thickened crust, but consistent with magmatically active extensional zones. Based on this evidence and also the syn-metamorphic mafic intrusions, we present an alternate interpretation in terms of mid-crustal extension for the BM2 metamorphism. In contrast, the counter clockwise P–T path during BM3), recording cooling following prograde burial is explained by tectonic thickening of a hot mid-crust. Collating available geochronological data, the two metamorphic events appear to indicate tectonic switching from lithospheric extension to contraction in the early Mesoproterozoic. The findings provide the first quantitative constraints on an early Mesoproterozoic hot orogen at the craton–mobile belt interface in the Central Indian Tectonic Zone

Mesoproterozoic reworking of palaeoproterozoic ultrahigh-temperature granulites in the Central Indian Tectonic Zone and its implications

In the southern periphery of the Sausar Mobile Belt (SMB), the southern component of the Central Indian Tectonic Zone (CITZ), a suite of felsic and aluminous granulites, intruded by gabbro, noritic gabbro, norite and orthopyroxenite, records the polymetamorphic evolution of the CITZ. Using sequences of prograde, peak and retrograde reaction textures, mineral chemistry, geothermobarometric results and petrogenetic grid considerations from the felsic and the aluminous granulites and applying metamorphosed mafic dyke markers and geochronological constraints, two temporally unrelated granulite-facies tectonothermal events of Pre-Grenvillian age have been established. The first event caused Ultrahigh-temperature (UHT) metamorphism (M1) (T ∼950°C) at relatively deeper crustal levels (P ∼9 kbar) and a subsequent post-peak near-isobaric cooling P–T history (M2). M1 caused pervasive biotite-dehydration melting, producing garnet–orthopyroxene and garnet–rutile and sapphirine–spinel-bearing incongruent solid assemblages in felsic and aluminous granulites, respectively. During M2, garnet–corundum and later spinel–sillimanite–biotite assemblages were produced by reacting sapphirine–spinel–sillimanite and rehydration of garnet–corundum assemblages, respectively. Applying Electron Microprobe (EMP) dating techniques to monazites included in M1 garnet or occurring in low-strain domains in the felsic granulites, the UHT metamorphism is dated at 2040–2090 Ma. Based on the deep crustal heating–cooling P–T trajectory, the authors infer an overall counterclockwise P–T path for this UHT event. During the second granulite event, the Palaeoproterozoic granulites experienced crustal attenuation to ∼6•4 kbar at T ∼675°C during M3 and subsequent near-isothermal loading to ∼8 kbar during M4. In the felsic granulites, the former is marked by decomposition of M1 garnet to orthopyroxene–plagioclase symplectites. During M4, there was renewed growth of garnet–quartz symplectites in the felsic granulites, replacing the M3 mineral assemblage and also the appearance of coronal garnet–quartz–clinopyroxene assemblages in metamorphosed mafic dykes. Using monazites from metamorphic overgrowths and metamorphic recrystallization domains from the felsic granulite, the M4 metamorphism is dated at 1525–1450 Ma. Using geochronological and metamorphic constraints, the authors interpret the M3–M4 stages to be part of the same Mesoproterozoic tectonothermal event. The result provides the first documentation of UHT metamorphism and Palaeo- and Mesoproterozoic metamorphic processes in the CITZ. On a broader scale, the findings are also consistent with the current prediction that isobarically cooled granulites require a separate orogeny for their exhumation

Consolidated Chemical Provinces on Mars: Implications for Geologic Interpretations

International audienceChemical provinces were defined on Mars a decade ago using orbital nuclear spectroscopy of K, Th, Fe, Si, Ca, Cl, and H2O. However, past multivariate analyses yielded three sets of provinces, suggesting methodologic variability. Province-stability to the inclusion of Al and S is also unknown, presenting additional uncertainties for geologic insight. Here we consolidate key multivariate methods to define the first cross-validated provinces. In southern highlands, the highly incompatible K and Th show non-uniform distribution with higher values in mid Noachian and Hesperian than late Noachian – early Hesperian volcanic terrains. Silica- and Al-depletion trends from Noachian to Amazonian indicate highly differentiated mantle with variable degree of melting. Late Hesperian lowlands are highly depleted in Al and enriched in K and Th, consistent with volcanic resurfacing from a low-degree partially melted, garnet-rich mantle. Furthermore, older volatile-rich regions such as Medusae Fossae Formation exhibit igneous geochemistry, consistent with water-limited isochemical weathering throughout Mars's history

2001 Mars Odyssey Gamma Ray Spectrometer Element Concentration Maps

Chemical provinces were defined on Mars a decade ago using orbital nuclear spectroscopy of K, Th, Fe, Si, Ca, Cl, and H2O. However, past multivariate analyses yielded three sets of provinces, suggesting methodologic variability. Province-stability to the inclusion of Al and S is also unknown, presenting additional uncertainties for geologic insight. Here we consolidate key multivariate methods to define the first cross-validated provinces. In southern highlands, the highly incompatible K and Th show non-uniform distribution with higher values in mid Noachian and Hesperian than late Noachian – early Hesperian volcanic terrains. Silica- and Al-depletion trends from Noachian to Amazonian indicate highly differentiated mantle with variable degree of melting. Late Hesperian lowlands are highly depleted in Al and enriched in K and Th, consistent with volcanic resurfacing from a low-degree partially melted, garnet-rich mantle. Furthermore, older volatile-rich regions such as Medusae Fossae Formation exhibit igneous geochemistry, consistent with water-limited isochemical weathering throughout Mars\u27s history