Are spherulitic lacustrine carbonates an expression of large-scale mineral carbonation? A case study from the East Kirkton Limestone, Scotland

Abstract

Lacustrine carbonate deposits with spherulitic facies are poorly understood, but are key to understanding the economically important “Pre-Salt” Mesozoic strata of the South Atlantic. A major barrier to research into these unique and spectacular facies is the lack of good lacustrine spherulite-dominated deposits which are known in outcrop. Stratigraphy and petrography suggest one of the best analogue systems is found in the Carboniferous of Scotland: the East Kirkton Limestone. Here we propose a hydrogeochemical model that explains why the CaCO3, SiO2, Mg-Si-Al mineral suite associated with spherular radial calcite facies forms in alkaline lakes above basaltic bedrock. Demonstrating links between igneous bedrock chemistry, lake and spring water chemistry and mineral precipitation, this model has implications for studies of lacustrine sediments in rift basins of all ages. Using empirical and theoretical approaches, we analyze the relationship between metal mobilization from sub-surface volcaniclastic rocks and the potential for precipitation of carbonate minerals, various Mg-bearing minerals and chalcedony in a lacustrine spherulitic carbonate setting. This suite of minerals is most likely formed by in-gassing of CO2 to a carbon-limited alkaline spring water, consistent with the reaction of alkali igneous rocks in the subsurface with meteoric groundwater. We suggest that an analogous system to that at East Kirkton caused development of the ‘Pre-Salt’ spherulitic carbonate deposits

What Causes Carbonates to Form “Shrubby” Morphologies? An Anthropocene Limestone Case Study

“…Furthermore, the record of anthropogenically mediated carbonate deposits such as those described here or in an asbestos open pit pond in Yukon, Canada (Power et al, 2011) is increasingly demonstrating that spherulitic carbonate growth can easily arise from waters anomalously enriched in calcium and magnesium, apart from elevated alkalinities and pH. In these sites, the alkalinity engine is thought to be related to the progressive atmospheric CO 2 ingassing (Rogerson et al, 2017) or the dissolution of the carbonate bedrock (Power et al, 2011) rather than increasing evaporation as occurs in alkaline lakes (Eugster and Hardie, 1978).…”

“…Many recent EARS lakes (e.g., Lake Magadi or Lake Bogoria; Jones et al, 1977;Renaut et al, 2002) have been considered tectonically and hydrologically similar to the ancient Pre-Salt lakes. However, they do not show clear evidence of substantial accumulation of spherulitic carbonate sediments, questioning whether EARS usual Ca and Mg-poor chemistries are encouraging spherulitic carbonate precipitation (Rogerson et al, 2017).…”

“…Recent examples of shrubby carbonate formation are hard to find, reflecting that the microenvironment in which these growthforms develop are not well represented in the modern Earth system. One explanation for the origin of the high flux of metalliferous, alkaline water to the Pre-Salt lakes is hydrolysis of fresh alkaline extrusive igneous materials (Rogerson et al, 2017), and partial analogs of Pre-Salt mineralogies and fabrics do occur in ophiolite spring systems (from Oman and Liguria) (Leleu et al, 2016). Here, the alteration of mafic/ultramafic rocks by meteoric waters and seawater provides saline, hyperalkaline (pH ∼ 10) waters which in-gas carbon dioxide from the atmosphere.…”

“…In geothermal systems, CO 2 is available from the deep subsurface in surplus to the metals (Minissale et al, 1997), making carbonate production calcium-limited. In contrast, within non-geothermal systems driven by mineral hydrolysis, all CO 2 is provided from atmosphere and the carbonate system tends to be limited by the relatively slow kinetics of CO 2(g) dissolution and hydrolysis to CO 2− 3(aq) rather than the abundance of calcium (Andrews et al, 1997;Rogerson et al, 2017).…”

What do we really know about early diagenesis of non-marine carbonates?

“…2), but also in relation to strong physicochemical and biological gradients along flow paths (cooling/heating, evaporation, CO 2 degassing -or ingassing, respiration or photosynthesis) ( Fig. 8; Fouke et al, 2000;Pache et al, 2001;Andrews, 2006;Fairchild et al, 2006;Alonso-Zarza and Tanner, 2010b;Kele et al, 2011;Arp et al, 2013b;Frantz et al, 2014;Rogerson et al, 2017). The oxygen isotopic composition of carbonate minerals is controlled by the δ…”