A depositional model for hydromagnesite–magnesite playas near Atlin, British Columbia, Canada

Abstract

This study formulates a comprehensive depositional model for hydromagnesite-magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metrescale) and amalgamated mounds composed primarily of hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 Á4H 2 O], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average % 2 m), CaMg-carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post-deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg-HCO 3 groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg-carbonate sediments (hydromagnesite mounds) overlies the Ca-Mg-carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments 'emerged' from the water surface. Capillary action and evaporation draw Mg-HCO 3 water up towards the ground surface, precipitating Mg-carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO 3 Á5H 2 O] which partially cements pre-existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca-Mg-carbonate boundary indicates that there has been ca 8000 years of continuous Mg-carbonate deposition at a rate of 0Á4 mm yr À1 . The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.

Changes in mineral reactivity driven by pore fluid mobility in partially wetted porous media

“…Brucite particles dissolved (SI Video S1) and drove pore fluid to chemical saturation with Mg-carbonate minerals. These observations are consistent with the replacement of brucite by carbonate minerals as has been documented in bench-scale experiments and inferred from weathering of industrial wastes under similar geochemical conditions (Assima et al, 2014a(Assima et al, , 2014b(Assima et al, , 2012Bea et al, 2012;Harrison et al, 2016Harrison et al, , 2015Harrison et al, , 2013Hövelmann et al, 2012;Wilson et al, 2014).…”

“…The pseudo-rhombohedral crystals were consistent with the morphology of lansfordite formed in our laboratory from natural water collected from a Mg-carbonate playa in Atlin, British Columbia, where lansfordite precipitation has previously been documented (Fig. 3;Power et al, 2014). The lansfordite precipitated from water collected at the Atlin site while in storage at 4°C.…”

The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

“…As such, hydrous magnesium carbonate minerals, such as hydromagnesite and nesquehonite are relatively common in natural environments, where they occur as weathering products of mafic or ultramafic rocks (Deelman, 2011), within evaporate deposits (Alderman and Von de Borch, 1960;Goto et al, 2003) and within speleotherms (Northup et al, 2001). Hydromagnesite has also been observed to precipitate in microbial mats or stromatolites in a number of alkaline lakes (Power et al, 2007(Power et al, , 2014Shirokova et al, 2011Shirokova et al, , 2013Mavromatis et al, 2012Mavromatis et al, , 2015.…”

“…The formation of these minerals can be catalyzed by bacterial activity (Thompson and Ferris, 1990;Power et al, 2007Power et al, , 2009Power et al, , 2014Shirokova et al, 2013).…”

Genesis of a giant Paleoproterozoic strata-bound magnesite deposit: Constraints from Mg isotopes

“…Thin beds of magnesite can precipitate from Mg-rich seawater in modern saline lakes, playas, or sabkhas (Power et al, 2014), but large-scale strata-bound magnesite deposits occur mainly in Precambrian strata (Frank and Fielding, 2003). Precipitation of evolved Mg-rich seawater in an evaporative marine setting may have triggered the formation of giant strata-bound magnesite deposits (Pohl, 1990).…”