Data suggest that early continents formed over mantle sinks and grew rapidly between 2.7 and 3.0 Ga ago. Nd isotopic results indicate that Archean tonalites, which are largely responsible for early continental growth, are derived chiefly from ancient depleted mantle sources. Trace element and oxygen isotope data, however, indicate enriched sources for such tonalites. These constraints can be explained if the mantle is fractionated into depleted (DMR) and enriched (EMR) reservoirs during early degassing of the Earth (≥4.4 Ga). Blocks of EMR are recycled back into the mantle and the EMR-DMR mixtures serve as sources for Archean basalts. These basalts are enriched in incompatible elements by deep-sea alteration, metasomatism in sinks, or by other mantle processes, and then undergo wet partial melting to produce tonalite magmas. Enrichment must occur not more than 150 Ma prior to tonalite production. Basalt plateaus accumulate at sinks and as cooling continues, geotherms at sinks pass into the eclogite stability field. Dense eclogite roots form on the basalt plateaus which pulls them into the mantle between 2.7 and 3.0 Ga ago, resulting in rapid tonalite production and, thus in rapid continental growth.
|Number of pages||18|
|Publication status||Published - 1 Sep 1986|