Channelization of transformation in the ikaite to calcite system

Research output: Contribution to conferenceAbstractAcademic

Abstract

Carbonate bearing mineral phases are key targets for paleoclimate research due to the isotopic signatures that they can retain. Particularly, calcite that has pseudomorphically transformed from the metastable phase ikaite (CaCO3*6H2O), termed a glendonite, is thought to be critical for understanding past cold-water environments (Morales et al. 2017). Natural glendonites can reach metres in size and have areas with high porosity due to the large volume deficit between ikaite and calcite (-21%). Interestingly, these pores often contain oil and methane that may be related to the formation environment of the ikaite itself. Retention of signatures from ikaite formation, both isotopic and trapped within the original crystal, will depend on the phase transformation mechanism and connectivity of the porosity. Here we have grown ikaite crystals in a climate controlled room set at 4 °C using a method similar to that described in Tollefsen et al. 2018. Nucleation in solution was extremely fast after which the precipitate was allowed to age in the solution at 4 °C for several months producing several single crystals up to 300 µm in size. These crystals were extracted from the solution and placed at room temperature under the microscope of a Raman spectrometer, after which they began to transform into calcite. We observed that the transformation quickly develops into channel structures that extend over 10s of µm between the crystal interior and surface. Within the size of the Raman spot, the channels have an increasingly more calcite-like signature towards their centre. With time, the width of the channels slowly increases until the entire crystal has been pseudomorphically transformed to calcite. After 20 hours of transformation no ikaite signature is left. However, at this point in the reaction there is evidence for an as yet unidentified ‘intermediate’ phase that retains a channel structure and spectral similarities with both calcite and ikaite, but, cannot be explained as a mixture of these two phases. Although a dissolution-reprecipitation reaction has been suggested for the transformation (Sánchez-Pastor et al. 2016), the intermediate phase indicates that it may actually be solid-state. This would be consistent with the observation of retain clumped isotope signatures in experiments (Vickers et al. 2022), despite our evidence of channel extension to the surface of the crystals, thus a higher potential for chemical reservoir mixing.

Morales, Chloé, et al 2017 Geology 45.6, 503-506.

Sanchez-Pastor et al. 2016 Geochimica et Cosmochimica Acta 175, 271-281.

Tollefsen, Elin, et al. 2018 Mineralogical Magazine 82.5, 1119-1129.

Vickers et al. 2022 Geochimica et Cosmochimica Acta, 334, 201-216.
Original languageEnglish
Publication statusPublished - 2022
EventGranada-Münster Discussion Meeting 9 - University of Granada, Granada, Spain
Duration: 30 Nov 20222 Dec 2022

Workshop

WorkshopGranada-Münster Discussion Meeting 9
Country/TerritorySpain
CityGranada
Period30/11/222/12/22

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