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It is readily apparent from the fossil record that the first limestone was laid down hundreds of millions of years ago, but how long ago was the very first limestone formed? It could be approximately dated by its fossils, but what if it was in the Precambrian and contained no fossils - how could it be accurately dated then?

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Probably at the Hadean or at the beginning of the Archean, but there is no geological registry.

The mechanism that can lead to inorganic limestone formation is weathering of magmatic carbonatite rocks. I found Siilinjärvi carbonatite complex in Fennoscandia is dated 2.6Ga (Tichomirowa et al, 2013), so Archean. It is the oldest one I found.

Another mechanism that could produce limestones in ancient eons is described by user @BillDOe in this related answer:

Once life evolved in the world's oceans, the vast majority of limestone formation has been through the organic process. However, before life did evolve, limestone formed through an inorganic process where rain water falling through a CO2-rich atmosphere reacted to form a weak carbonic acid solution that then reacted with calcium-containing minerals to form calcites. This then washed into the world's oceans to form limestone deposits.

The premise comes from this stanford.edu work and is a bit controversial because there are no proofs limestones formed by this mechanism.


International Stratigraphic Chart image

M. Tichomirowa, M.J. Whitehouse, A. Gerdes, J. Götze, B. Schulz, B.V. Belyatsky (2013): "Different zircon recrystallization types in carbonatites caused by magma mixing: Evidence from U–Pb dating, trace element and isotope composition (Hf and O) of zircons from two Precambrian carbonatites from Fennoscandia", Chemical Geology, Volume 353, Pages 173-198, ISSN 0009-2541. https://doi.org/10.1016/j.chemgeo.2012.11.004.

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    $\begingroup$ Kudos for taking the time to write an answer when you know it will never be accepted! $\endgroup$ – Jean-Marie Prival Mar 2 at 9:36
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    $\begingroup$ @Jean-MariePrival Or upvoted by OP. Both $\endgroup$ – gansub Mar 3 at 1:40
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    $\begingroup$ @uhoh don't worry. Let's see if the new bounty attract more answers, if not I will need to bounty you 150 on the answer about Earth mass. I am happy earning some points. I will soon could edit tags on geology questions with no review. Thanks for the bounty. $\endgroup$ – Universal_learner Mar 9 at 8:15
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    $\begingroup$ The first limestones formed as chemical precipitation of initially dissolved calcium carbonate in the oceans. The carbonate (and bicarbonate anions) got there by mostly (1) dissolving directly from the CO2 in the atmosphere, and to a lesser degree (2) weathering of of carbonated subaerial mafic volcanic rocks. $\endgroup$ – Gimelist Mar 9 at 11:33
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    $\begingroup$ Time's up. btw please do not "bounty me back" as mentioned above. While courteous and well-meaning, there are "bounty scam detectors" in the software that may raise alarms. The reason each bounty one puts on the same question must increase by a factor of 2 is because of some bounty shenanigans years ago. This will just be a one-time lesson to me to keep better track of my bounties. $\endgroup$ – uhoh Mar 16 at 0:29
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Perhaps some of the oldest limestones are the banded Espanola Formation from 2.3 billion years ago. The Oxygenation of the environment happened 2.4 billion years ago. I'll find some references and chemistry info when my web connection works, here are some starter pages.

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