Magnetic and petrologic characterization of synthetic Martian basalts and implications for the surface magnetization of Mars

A suite of synthetic Martian basalts is generated with the objective of providing fundamental material properties data for use in modeling and interpretation of mission data. We systematically evaluate the effects of major element composition, oxygen fugacity (ƒO2), and cooling rate on phase chemistry and magnetic mineralogy, grain size, and intensity of remanent magnetization. The range of experimental compositions and ƒO2 are chosen to bracket the range expected in the Martian crust; our results should therefore span the range of possible mineralogies, textures, and magnetic properties in rapidly cooled Mars crustal materials. Two starting compositions are used for the sample synthesis: (1) an Fe-rich, Al-poor composition patterned after SNC basaltic meteorites and (2) a composition based on thermal emission spectrometer (TES) data with a much lower Fe/Al ratio. The resulting magnetic phase in samples generated at the quartz-fayalitemagnetite (QFM) buffer is a spinel-structured oxide with varying amounts of Cr, Ti, Mg, and Al. Compositional differences depend on bulk composition, cooling rate, differences in crystallization sequence, and the kinetics of silicate mineral nucleation and growth. Oxide abundance and magnetic intensity are most strongly influenced by fO2, with more subtle composition and cooling rate effects. Moderately oxidizing QFM conditions result in an intense magnetization (2.3 × 10-5 Am2 kg-1 to 1.4 × 10-2 Am2 kg-1), especially in the meteorite-derived basalts. However, an increase of magnetic grain size into the multidomain range (meteorite-type) and/or low unblocking temperatures resulting from increased Cr substitution (TES-type) may affect the long-term stability of the remanence in QFM samples

Multicomponent cubic oxide exsolution in synthetic basalts: Temperature dependence and implications for magnetic properties

Although the compositional unmixing of cubic-structured iron oxides has profound effects on the magnetic properties of rocks that contain them, a basic understanding of the kinetics and thermodynamics of this process has not been achieved in experimental studies due to sluggish reaction rates in binary oxide phases. Exploiting the fact that many natural Fe-oxides contain multiple additional cations, including Ti, Mg and Al, we perform novel “forward” laboratory experiments in which cubic-cubic phase exsolution proceeds from initially homogeneous multicomponent oxides. A variety of Fe-Ti-Mg-Al cubic iron oxides were nucleated and grown in synthetic, multicomponent basalt under different ƒO2 environments, and annealed at temperatures ranging from 590–790°C for up to 88 days. Fine-scale lamellar intergrowths of Fe-Ti-Al-Mg oxides, interpreted to represent cubic phase exsolution, were observed in seven samples, one that was synthesized and annealed at approximately constant ƒO2 (the quartz-fayalite-magnetite, or QFM, buffer) and six that were synthesized at very oxidizing conditions (~QFM + 6 log units) and then annealed at moderately oxidizing (~QFM) conditions. Results demonstrate that the consolute temperature of the multicomponent system is significantly higher than anneal temperatures and Curie temperatures, suggesting that samples that undergo this type of exsolution can carry a total thermal remanent magnetization. Exsolved samples are characterized by a dramatic increase in magnetization and coercivity, and a shift in Curie temperature(s), confirming predictions that this type of exsolution exerts strong control on the strength and stability of magnetization

Contribution of multidomain titanomagnetite to the intensity and stability of Mars crustal magnetic anomalies

Two basalts with compositions relevant to the crusts of Mars and Earth were synthesized at igneous temperatures and held at 650°C for 21 to 257 days under quartz‐fayalite‐magnetite fO2 buffer conditions. The run products are germane to slowly cooled igneous intrusions, which might be a significant volumetric fraction of the Martian crust and carriers of magnetic anomalies in the Southern Highlands. Both basalts acquired intense thermoremanent magnetizations and intense but easily demagnetized anhysteretic remanent magnetizations carried by homogeneous multidomain titanomagnetite. Hypothetical intrusions on Mars composed of these materials would be capable of acquiring intense remanences sufficient to generate the observed anomalies. However, the remanence would be easily demagnetized by impact events after the cessation of the Mars geodynamo. Coercivity enhancement by pressure or formation of single domain regions via exsolution within the multidomain grains is necessary for long‐term retention of a remanence carried exclusively by multidomain titanomagnetite grains

Contribution of multidomain titanomagnetite to the intensity and stability of Mars crustal magnetic anomalies

Two basalts with compositions relevant to the crusts of Mars and Earth were synthesized at igneous temperatures and held at 650°C for 21 to 257 days under quartz-fayalite-magnetite ƒO2 buffer conditions. The run products are germane to slowly cooled igneous intrusions, which might be a significant volumetric fraction of the Martian crust and carriers of magnetic anomalies in the Southern Highlands. Both basalts acquired intense thermoremanent magnetizations and intense but easily demagnetized anhysteretic remanent magnetizations carried by homogeneous multidomain titanomagnetite. Hypothetical intrusions on Mars composed of these materials would be capable of acquiring intense remanences sufficient to generate the observed anomalies. However, the remanence would be easily demagnetized by impact events after the cessation of the Mars geodynamo. Coercivity enhancement by pressure or formation of single domain regions via exsolution within the multidomain grains is necessary for long-term retention of a remanence carried exclusively by multidomain titanomagnetite grains

Holocene glacial activity in Barilari Bay, west Antarctic Peninsula, tracked by magnetic mineral assemblages: Linking ice, ocean, and atmosphere

We investigate the origin and fate of lithogenic sediments using magnetic mineral assemblages in Barilari Bay, west Antarctic Peninsula (AP) from sediment cores recovered during the Larsen Ice Shelf System, Antarctica (LARISSA) NBP10-01 cruise. To quantify and reconstruct Holocene changes in covarying magnetic mineral assemblages, we adopt an unsupervised mathematical unmixing strategy and apply it to measurements of magnetic susceptibility as a function of increasing temperature. Comparisons of the unmixed end-members with magnetic observations of northwestern AP bedrock and the spatial distribution of magnetic mineral assemblages within the fjord, allow us to identify source regions, including signatures for ‘‘inner bay,’’ ‘‘outer bay,’’ and ‘‘northwestern AP’’ sources. We find strong evidence that supports the establishment of a late Holocene ice shelf in the fjord coeval with the Little Ice Age. Additionally, we present new evidence for late Holocene sensitivity to conditions akin to positive mean Southern Annual Mode states for western AP glaciers at their advanced Neoglacial positions

Geochemical fingerprints of glacially eroded bedrock from West Antarctica: Detrital thermochronology, radiogenic isotope systematics and trace element geochemistry in Late Holocene glacial-marine sediments

Geochemical provenance studies of glacial-marine sediments provide a powerful approach to describe subglacial geology, sediment transport pathways, and past ice sheet dynamics. The marine-based West Antarctic Ice Sheet (WAIS) is considered highly vulnerable to ocean warming and sea level rise that is likely to cause its rapid and irreversible retreat. Studies of its past response to climate change are hence essential for projecting its future behaviour. The application of radiogenic and trace element provenance studies for past ice sheet reconstructions requires surveying the geographic variability of geochemical compositions of glaciomarine sediments. In this study, we characterize the provenance of the detrital fraction of 67 Late Holocene marine sediment samples collected off the Pacific margin of West Antarctica (60°W to 160°W), including 40Ar/39Ar ages of individual hornblende and biotite grains (> 150 μm), as well as Sr and Nd isotope and trace element composition of the fine-grained (40Ar/39Ar ages of iceberg-rafted hornblende and biotite grains record primarily Carboniferous to Lates Quaternary ages (~0 to 380 Ma), with a notable age peak of ~100 Ma, associated with plutonic intrusions or deformation events during the mid-Cretaceous. Permian-Jurassic 40Ar/39Ar ages are widespread in the Amundsen Sea sector, marking episodes of large-volume magmatism along the long-lived continental margin. Metasedimentary rocks and Late Cenozoic alkali basalts in West Antarctica cannot be detected using detrital hornblende and biotite 40Ar/39Ar ages due to the absence or small grain-size (i.e. < 150 μm) of these minerals in such rocks. These sources can however be readily recognized by their fine-grained geochemical composition. In addition, geographic trends in the provenance from proximal to distal sites provide insights into major sediment transport pathways. While the transport of fine-grained detritus follows bathymetric cross-shelf troughs, the distribution of iceberg-rafted grains shows influence by transport in the Antarctic Coastal Current. Our study provides the first systematic geochemical characterisation of sediment provenance off West Antarctica, and highlights the importance of combining multiple provenance approaches in different size fractions of glacial-marine sediments, and paves the way to investigate past WAIS dynamics

Ancient marine sediment DNA reveals diatom transition in Antarctica

Antarctica is one of the most vulnerable regions to climate change on Earth and studying the past and present responses of this polar marine ecosystem to environmental change is a matter of urgency. Sedimentary ancient DNA (sedaDNA) analysis can provide such insights into past ecosystem-wide changes. Here we present authenticated (through extensive contamination control and sedaDNA damage analysis) metagenomic marine eukaryote sedaDNA from the Scotia Sea region acquired during IODP Expedition 382. We also provide a marine eukaryote sedaDNA record of ~1 Mio. years and diatom and chlorophyte sedaDNA dating back to ~540 ka (using taxonomic marker genes SSU, LSU, psbO). We find evidence of warm phases being associated with high relative diatom abundance, and a marked transition from diatoms comprising <10% of all eukaryotes prior to ~14.5 ka, to ~50% after this time, i.e., following Meltwater Pulse 1A, alongside a composition change from sea-ice to open-ocean species. Our study demonstrates that sedaDNA tools can be expanded to hundreds of thousands of years, opening the pathway to the study of ecosystem-wide marine shifts and paleo-productivity phases throughout multiple glacial-interglacial cycles