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Day and night cycles make impossible possible: a case study of sedimentary magnesite formation through dis-equilibrium geochemical processes



Huifang Xu
Dept. of Geoscience, University of Wisconsin-Madison

ABSTRACT: Day and night cycles may cause oscillatory changes of temperature and even pH of surface water in presence of microbial biofilms. Such kind of process plays important role in many surficial geochemical reactions. We provide a case study of sedimentary magnesite formation through temperature and pH cycles. Incorporating magnesium into the carbonate structure is difficult at low temperatures. However, Veitsch-type anhydrous magnesites (MgCO3) are found in evaporative environments without the typical hydrothermal conditions associated with magnesite formation. To address the disconnect, this study refined a temperature and pH cycling method to achieve up to 91 mol% MgCO3 of (Mg,Ca)CO3 carbonates without exceeding 40˚C. Individual parameters were tested to develop a novel simultaneous growth and replacement mechanism for magnesite formation. The experimental conditions ideal for magnesite growth match very well with an evaporative/lagoonal/playa geologic setting: small thermal mass for daily temperature swings, algal mass for pH control, and a solution highly concentrated in Mg2+ with high Mg:Ca ratios. Finally, a high mol% MgCO3 carbonate precursor, such as the one produced in this study, is needed for the diagenetic alteration towards pure magnesite. This study proposes a non-equilibrium cyclic growth and replacement mechanism through precipitation and dissolution steps and a possible lagoonal/playa lake environment for the formation of sedimentary magnesium-rich carbonates. The proposed mechanism takes advantage of unique periods of increased carbonate activity that drive growth and a preferential dissolution of calcium-rich domains resulting in the concentration of magnesium within the carbonates. The conditions required for the mechanism include a high Mg2+ concentration, a high Mg:Ca ratio, and daily temperature and pH fluctuations. Such conditions are found in an evaporative lagoonal/playa lake setting and examples of this environment with magnesium-rich carbonates are observed in the modern to ancient sedimentary carbonate rock record.

BIO: Prof. Huifang Xu received his bachelor degree from Nanjing University and PhD degree from The John’s Hopkins University. He completed postdoctoral studies at Arizona State University in area of electron microscopy. Dr. Xu is a Faculty member in the Department of Geoscience at the University of Wisconsin – Madison. His research interests include studies of nano-phase structures and their reactivity, incommensurately modulated structures, and sedimentary carbonates using e-beam imaging, X-ray diffraction, and neutron scattering methods. Dr. Xu and his team members have recently discovered two strong magnetic minerals (luogufengite and valleyite) that commonly occur in basaltic rocks. 
Dr. Xu is an elected fellow of the Mineralogical Society of America.

Hobbs F. and Xu H. (2020) GCA 269:101–116

Figure: Schematic showing possible conditions resulting in the formation of magnesite. Both lagoons and evaporative lakes can have appropriate temperature fluctuations and cation concentrations that mimic conditions presented in this study. Not included in the schematic are microbial mats or other organics that can facilitate the cyclic pH changes required. See the following reference for details: Hobbs F. and Xu H. (2020) Geochimica et Cosmochimica Acta 269 (2020) 101–116.