Pedogenic hematitic concretions from the Triassic New Haven Arkose, Connecticut: Implications for understanding Martian diagenetic processes

Abstract

The Triassic New Haven Arkose locally contains mm-scaled pedogenic hematitic concretions, within red arkosic sandy mudstones, that provide insights into concretion-forming processes on Earth and Mars. Concretions represent ~8% of the sediment by mass (~7% by volume), are irregularly distributed and have a near-normal size distribution with a mean diameter of 1.53mm and graphic standard deviation of 0.64mm. X-ray diffraction (XRD) and attenuated total reflectance (ATR) spectroscopy indicate that the concretions are composed of ambient sediments: quartz, montmorillonite, and feldspar, and a minor phosphatic phase that is also implied by geochemistry; however, the concretions preferentially exclude coarser grain sizes, and are cemented by hematite and goethite. Optical observations and synchrotron X-ray fluorescence (XRF) chemical mapping indicate that concretions are massive to weakly zoned with respect to hematitic cements. Compared to the surrounding sediments, concretion rare earth elements (REE) are elevated in total abundances, exhibit light rare earth element (LREE)-enrichment, and possess negative Ce-anomalies; Th/U ratios are lower due to elevated U. Mass balance calculations indicate that ~20% of the concretions are composed of iron oxides and that ~30mm3 of ambient sediment is required to provide the hematitic cement in a single 1.5mm diameter concretion. Element mobility during concretion formation was tested assuming different immobile elements (Al, Ti, Zr). Assuming Zr immobility provides intermediate results, and indicates the following gains in concretions: Pb (572%), Fe (322%), Mn (142%), REE (438–116%), V (138%), U (124%), Ni (84%), and Nb (58%); other elements show either gains or losses of less than ±50%. Ce-anomalies, low Th/U, and elevated V and U abundances point to a significant redox influence on element distributions in the concretions. High iron content, crude internal concentric banding, and redox controls suggest that the concretions are formed in seasonally variable, but generally moist, soil conditions with annual precipitation >130cm. Trace element enrichment patterns are broadly consistent with derivation from downward percolating weathering fluids. Although the New Haven Arkose pedogenic concretions have significant differences and were clearly formed by different mechanisms than hematitic spherules discovered on Mars, they nevertheless exhibit a number of textural (e.g., sphericity, size distributions) and geochemical (e.g., Ni enrichments) similarities that support models suggesting that the Martian spherules are formed as sedimentary concretions.