Colloquium: Erik Melchiorre

Professor, California State University, San Bernardino

Abstract - Ore deposits provide a record of past climate and biological activity. Weathering products of primary metal deposits, such as native gold and the copper-, iron-, and lead-carbonates have been used for such purposes and can provide a detailed temporal record (e.g., Carothers et al., 1988; Melchiorre et al., 1999; Melchiorre et al., 2000; Melchiorre et al., 2001; Melchiorre and Williams, 2001; Melchiorre and Enders, 2003; Melchiorre and Gilg, 2011; Kamenov et al., 2011; Melchiorre et al., 2016).

Stichtite, a rare member of the hydrotalcite group, is found in association with Archean to Phanerozoic chromite-rich serpentinite breccia-pipe rocks around the world. Natural stichtite shows a range of Cr-Fe-Al compositions within the hydrotalcite group, and a range of reaction completion textures, with unique trends reflecting history and evolution of each discrete serpentinite host body. Paragenetic analysis clearly reveals that stichtite forms during active serpentinization, not as a surface weathering product of chromium deposits. The chemistry of the chromite associated with stichtite indicates formation in fore-arc rocks by chromite reacting with methane- or H2-rich serpentinizing fluids. The degree to which chromite is replaced by stichtite correlates with the depth that the host rocks experienced during serpentinization. The most advanced reaction completion textures are associated with the stichtite 2H polytype (nee barbertonite) + aragonite ± antigorite, suggesting these samples experienced higher pressures and/or temperatures. Carbon stable isotope analyses of stichtite suggest carbon sourcing from marine kerogen with a minor marine carbonate component in some samples (Melchiorre et al., 2014). The combined carbon and hydrogen stable isotope profile of stichtite ranges from the field of methane from active serpentinizing zones, to deep organic thermogenic methane. The combined evidence suggests that stichtite forms during serpentinization of fore-arc setting rocks in a methane/H2-rich environment within fluid conduits, ranging from near the surface to depths experiencing 1.2 to 0.8 GPa and ~300°C. The unique chemical, isotopic, and textural properties of stichtite provide a window into the conditions associated with a potentially habitable environment on early Earth and other bodies of the solar system.

My recent work on hydrotalcite minerals suggests a plausible pathway for the origin of life, while our extraction and characterization of relict nitrogen biomarkers from ancient hydrotalcite minerals may record the effect of the 2.35 Ga Great Oxidation Event and address the RNA First vs. Metabolism First question. Minerals of the hydrotalcite group have a unique ability to store specific molecules such as peptides and amino acids within the variably hydrated interlayer between their brucite-like layers (e.g., Newman et al., 2002; Gerstel et al., 2006). Natural hydrotalcite from early-Earth rocks are found in intimate association with elevated levels of known biocatalysts of the iridium subgroup. Thus, this mineral system may serve as a molecular sieve to accumulate simple organic materials in the presence of catalysts. As catalysts assemble larger molecules within the interlayer, the clay-like structure of the hydrotalcite may serve as a genograph or repeating template into which these trapped organic molecules are stored. These repeating and complex organic polymers, or their macromolecular descendants, could be the self-replicating molecules on which life is based (RNA First). Alternately, this organic accumulation could have been hosted by clay-like hydrotalcite minerals capable of catalyzing primitive metabolic reactions (Metabolism First). Hydrotalcite minerals occur within ancient, warm, and nutrient-rich serpentinizing environments which are similar to the proposed environment in which LUCA lived (Weiss et al., 2016). Serpentinizing systems are known to occur on both Earth and Mars, providing a possible mechanism for origins of life in the Solar System.

Dr. Melchiorre received his PhD in Earth and Planetary Science from Washington University in St. Louis in 1998. He holds a BS in Geology from the University of Southern California, and an MS degree in Geology from Arizona State University. He is presently a tenured full professor in the geology department at California State University, San Bernardino.

REFERENCES

Carothers, W. W., Adami, L. H., & Rosenbauer, R. J. (1988) Experimental oxygen isotope fractionation between siderite-water and phosphoric acid liberated CO2-siderite. Geochimica et Cosmochimica Acta 52(10), 2445-2450.

Gerstel, P., Hoffmann, R. C., Lipowsky, P., Jeurgens, L. P., Bill, J., Aldinger, F. (2006) Mineralization from aqueous solutions of zinc salts directed by amino acids and peptides. Chemistry of materials, 18(1), 179-186.

Kamenov, G.D., Melchiorre, E.B., Ricker, F.N., and DeWitt, E., 2013, Insights from Pb isotopes for native gold formation during hypogene and supergene processes at Rich Hill, Arizona: Economic Geology, 108, 1577-1589.

Melchiorre, E.B., Criss, R.E., and Rose, T.P. (1999) Oxygen and carbon isotope study of natural and synthetic malachite. Econ. Geol. 94, 245–259.

Melchiorre, E.B., Criss, R.E., and Rose, T.P. (2000) Oxygen and carbon isotope study of natural and synthetic azurite. Econ. Geol. 95, 621–628.

Melchiorre, E. B., Williams, P. A., & Bevins, R. E. (2001) A low temperature oxygen isotope thermometer for cerussite, with applications at Broken Hill, New South Wales, Australia. Geochimica et Cosmochimica Acta 65(15), 2527-2533.

Melchiorre, E.B., and Williams, P.A. (2001) Stable isotope characterization of the thermal profile and subsurface biological activity during oxidation of the Great Australia deposit, Cloncurry, Queensland, Australia. Economic Geology 96(7), 1685-1693.

Melchiorre, E.B., and Enders, M.S. (2003) Stable isotope geochemistry of copper carbonates at the Northwest Extension Deposit, Morenci District, Arizona: implications for conditions of supergene oxidation and related mineralization. Economic Geology 98(3), 607-621.

Melchiorre, E.B., and H.A. Gilg, 2011, Oxygen stable isotope fractionation behavior of cerussite and hydrocerussite: New results and reconciliation of the recent literature: Geochimica et Cosmochimica Acta 75, 3191-3195.

Melchiorre, E.B., Huss, G.R., and Lopez, A. (2014) Carbon and hydrogen stable isotope microanalysis and data correction for rare carbonate minerals: Case studies for stichtite (Mg6Cr2[(OH)16|CO3]∙H2O) and malachite (Cu2CO3(OH)2). Chemical Geology 367, 63-69.

Melchiorre, E.B., Kamenov, G.D., Sheets-Harris, C., Andronikov, A., Leatham, W.B., Yahn, J., Lauretta, D.S. (2016) Climate-induced geochemical and morphological evolution of placer gold deposits at Rich Hill, Arizona, USA. Geological Society of America Bulletin (in press).

Newman, S.P., Cristina, T.D., Coveney, P.V., Jones, W. (2002) Molecular dynamics simulation of cationic and anionic clays containing amino acids. Langmuir 18, 2933–2939.

Weiss, M. C., Sousa, F. L., Mrnjavac, N., Neukirchen, S., Roettger, M., Nelson-Sathi, S., Martin, W. F. (2016) The physiology and habitat of the last universal common ancestor. Nature Microbiology, 1, 16116.