The "Planetary Instrument for X-ray Lithochemistry" (PIXL) X-ray spectrometer conducts in situ geochemical analyses of martian rocks and regolith interrogated by the Mars 2020 rover, Perseverance. In addition to quantifying primary rock-forming elements, PIXL can quantify trace elements that in turn can provide additional constraints on the geologic history of Mars. Accurate quantifications of trace elements can require additional analytical techniques to mitigate experimental, background, and crystalline effects within PIXL spectra. In this study, we focus on reducing the impact of these effects and investigate the potential presence of rare earth elements (REEs). The study specifically investigates cerium given its typical relative abundance in many geologic materials compared to other REEs and its potential to mimic fluorescence features produced by organics under deep UV excitation. A detailed analysis of PIXL targets analyzed through the first 887 martian days of the Perseverance mission did not produce any conclusive Ce detections. Phosphorus-enriched materials analyzed by PIXL are estimated to contain sub-675 ppm Ce and sulfate-enriched materials sub-450 ppm Ce. The method presented can help constrain limits on the abundance of additional trace elements of interest that also face a similar analytical burden. PIXL's potential to assess REE abundances, outside of yttrium, is limited for expected concentrations in surface materials. Determining most REE concentrations in materials interrogated by Perseverance will therefore likely require terrestrial analyses.

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cooler Archaean ocean The widely held view that the Archaean climate around 3.5 billion years ago was remarkably warm — with ocean temperatures perhaps as high as 80 °C — has been questioned on the grounds that the established method of estimating ancient ocean temperature (from the oxygen isotope ratios of sedimentary deposits) is subject to significant uncertainty. Michael Hren et al . have adopted a different approach to estimating ocean temperature, based on an analysis of both oxygen and hydrogen isotopes of 3.4 billion-year-old Buck Reef Chert sediments from South Africa. The isotopes sampled are consistent with formation in waters no warmer than about 40 °C, suggesting that Earth's early oceans may have been far cooler than previously thought.

There is considerable debate about the mode and age of formation of large (up to ∼200m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite δ18O values range from −0.7 to +1.0‰ and δD from −125 to −146‰, which is consistent with formation from modern meteoric waters at 20 to 25°C. Hematite δ18O values range from −0.7 to −2.0‰, which is consistent with formation at low to moderate temperatures (40–55°C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs.

Thin carbonaceous laminations preserved in shallow-water facies of the 3416 Ma Buck Reef Chert, South Africa, have been interpreted to represent some of the oldest-known mats constructed by photosynthetic microbes. Preservation of these mats within a unit containing facies deposited at water depths ranging from 0 m to >200 m provides an opportunity to explore the electron donors employed in early microbial photosynthesis. The presence of siderite (FeCO3) as a primary sediment, lack of hematite (Fe2O3), and lack of cerium anomalies throughout the Buck Reef Chert imply that the entire water column was anoxic despite the presence of photosynthetic organisms. Authigenic uranium (Ua = U-Th/3) correlates inversely with siderite abundance, suggesting that variations in carbonate rather than oxygen activity controlled uranium mobility. The inferred lack of oxygen and ferric minerals and the presence of dissolved Fe2+ in the water column imply that H2O, Fe2+, and H2S could not have served as primary electron donors for carbon fixation. It is most likely that Buck Reef Chert bacteria utilized H-2 as the primary reductant for photosynthesis.

The 3.5-3.2 Ga sedimentary record shows evidence of surface temperatures of 70 +/- 15 degreesC, nahcolite (NaHCO3) as a primary evaporitic mineral, and an aggressive weathering regime even in the absence of land vegetation. These features are best explained by a mixed CH4 and CO2 atmospheric greenhouse in which CH4/CO2 ratios were much less than1 and pCO(2) was at least 100-1000 times the present value and perhaps as high as several bars. The formation of large areas of continental crust at 3.2-3.0 Ga, including the Kaapvaal and Pilbara cratons, resulted in the gradual depletion of atmospheric CO2 through weathering. By 2.9-2.7 Ga, declining pCO(2) was associated with climatic cooling and siderite-free soils. Transitory CH4/CO2 ratios of similar to1 may have resulted in the sporadic formation of organic haze from atmospheric CH4, reflected in one or more isotopic excursions involving global deposition of abnormally C-13-depleted organic C. Surface temperatures of <60 degreesC after 2.9 Ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes. Eventual lowering of new continental blocks by erosion, reduced loss of atmospheric CO2 due to weathering, and continued long-term tectonic recycling of CO2 resulted in rising pCO(2) and decreasing CH4/CO2 ratios in the later Archean and eventual reestablishment of a mainly CO2 greenhouse. Similar events may have been repeated in the latest Archean and earliest Proterozoic, but gradually rising production of O-2 effectively kept CH4/CO2 ratios to much less than1 at this time.

Raman spectra of carbonaceous material were collected in situ from samples of cherts of the Onverwacht and Fig Tree Groups in the central Barberton greenstone belt. The spectra feature two dominant peaks characteristic of disordered carbon: the D peak at similar to1310 cm(-1) and the O peak at 1580-1600 cm(-1). D peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above. No correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration, implying that metamorphism in the central Barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids. Samples from the Marble Bar Chert of the Pilbara block, Western Australia, have been heated to the same extent as samples from Barberton. This study demonstrates the use of Raman spectra of carbonaceous material as a sensitive geothermometer for low-temperature metamorphic facies. This application could also be used to establish the antiquity of putative microfossils from metamorphic terranes.