Weathering of quartz as an Archean climatic indicator

Chert and other hard monomineralic quartz grains weather mostly by mechanical processes in modem environments. Their clasts are overrepresented in conglomerates and sands relative to their sources regions. Conversely, macroscopic dissolution features, including quartzite karst, are rare but not nonexistent. The similar rarity of quartz dissolution in Archean deposits provides a paleothermometer for climate on the early Earth. For example, chert is overrepresented in conglomerates and sands of the similar to 3.2 Ga Moodies Group (South Africa) relative to the source region. Features related to the far-from-equilibrium dissolution rate are particularly diagnostic as it increases an order of magnitude over 25 degrees C, much more than solubility. Extrapolating from observed dissolution rates in modem environments that weather at -25 degrees C, we expect obvious dissolution features in ancient climates above -50 degrees C. Polycrystalline quartz and chert would readily disaggregate by solution along grain boundaries, yielding silt and clay. Quartz grains within slowly weathering granite would become friable, yielding silt and clay, rather than sand. At still higher temperatures, Al2O3-rich clays from weathered granite would stand above solution-weathered chert on low-relief surfaces. The observed lack of these features is evidence that the Archean climate was not especially hot. (c) 2005 Elsevier B.V. All rights reserved.