For years, Isua gneiss have been claimed as the oldest rock on Earth surface, until later discovery (Acasta gneiss 4.04Ga, Nuvvuagittuq "faux-amphibolite" 4.28Ga...). But after decades of study of old rocks in other cratons (an old and stable part of the continental lithosphere having often survived cycles of merging and rifting of continents), now we know that some banded ironstone from Isua Green Belt (IGB), with an 3.7-3.8Ga age, contains the oldest direct (and indirect) proofs of life froms in the early ocean : 

Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks

 
Discovery of boron-rich metasediments in Isua Supracrustal Belt :

Window to RNA genesis ?

TAKESHI KAKEGAWA (Graduate School of Science, Tohoku University, Japan;)

Selective stabilization of ribose is required among various sugars, in order to synthesize a basic unit of RNA (i.e., AMP). Recent study demonstrated that borate stabilized ribose selectively, and suggested the importance of borate-rich environments for prebiotic RNA genesis (1).

On the other hand, it has been uncertainif borate-rich environments were present on the ancient Earth. Here I report the discovery of boron-rich rocks in the >3.7 billion years old Isua Supracrustal Belt  of western Greenland. REE patterns and Al-rich characteristics of examined rocks indicate their marine sediment origin. These rocks belong most likely to the metasediment unit, which contain biogenic graphite(2).

Abundant tourmaline crystals (feruvite in chemical composition) were found in the examined rocks. Some tourmaline occuras inclusions in garnet and some are associated with biotite. Such mineralogical occurrence suggests that the precursor of tourmaline was present well before metamorphism.

Thus, pre-metamorphic Isua marine sediments were rich in borates and sedimentary clays were responsible to concentrate borates from ocean. This leads to a possibility that prebiotic RNA could be formed in Hadean marine sediments, which had abundant clays, borates, and prebiotic organic matter.

This model is consistent with the previously proposed chemical evolution model based on clay-rich marine
environments(3).

References(1)Furukawa et al. (2014OLEB on line; (2)Ohtomo et al. (2014) Nature Geoscience vol.