Australia: The Land Where Time Began

A biography of the Australian continent 

The Earliest Evolved Crust on Earth Generated in Setting like Iceland

It is not certain how the earliest continental crust was formed on Earth at a time when it probably had a surface of oceanic crust. The authors¹ suggest the first continental crust may have formed in a setting such as an oceanic island where mantle upwelling generates magmas that crystallise to form new crust. The character of Iceland, of the oceanic plateaux, is closest to continental crust as it is anomalously thick (Allen et al., 2002) and contains a relatively high proportion of rocks (Jakobsson, Jonasson et al. 2008) that are sialic (silica-rich). Therefore, Iceland has been considered to be a suitable analogue for the earliest continental crust to be generated (Kroner, 1982). According to the authors¹ the sialic rocks from Iceland has a geochemical signature  (Carmichael, 1964; Muehlenbachs, Anderson & Sigvaldason, 1974; Nicholson et al., 1991; Bindeman et al., 2012) that is distinct from that of the typical Archaean rocks that have been found so far that date from about 3.9-2.5 Ga (Martin, Martin & Sigmarsson, 2008). Reimink et al. report in this paper the discovery of a tonalitic gneiss rock unit that is exceptionally well preserved that dates to 4.02 Ga within the Acasta Gneiss Complex in Canada. It was shown by geochemical analysis that iron enrichment, negative Europium anomalies, patterns of unfractionated rare-earth-elements, and magmatic zircons with low oxygen isotope ratios. These geochemical characteristics are not like typical igneous rocks from the Archaean, rather, they are strikingly similar to the sialic rocks from Iceland, which implies that the formation of this ancient rock unit was formed by magmatic processes at a shallow level that include the assimilation of rocks that had previously been altered by surface waters. Direct evidence for the earliest continental crust of the Earth being formed in a tectonic setting comparable to that of Iceland of the present has been provided by the data obtained by the authors¹.

Sources & Further reading

1. Reimink, J. R., et al. (2014). "Earth[rsquor]s earliest evolved crust generated in an Iceland-like setting." Nature Geosci 7(7): 529-533.