A great thermal divergence in the mantle beginning 2.5 Ga： Geochemical constraints from greenstone basalts and komatiites

• Published in: Di xue qian yuan. Vol. 7; no. 4; pp. 543 - 553
• Main Authors: Condie, Kent C., Aster, Richard C., van Hunen, Jeroen
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.07.2016; Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA%Geosciences Department, Colorado State University, Fort Collins, CO 80523, USA%Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK; Elsevier
• Subjects: Archean greenstones; Basalt; Basalts; Cooling; Depletion; Divergence; Enrichment; Komatiites; Magma; Mantle; Mantle geochemistry; Mantle thermal evolution; Marine; 不相容元素; 亏损地幔; 地球化学; 富集地幔; 热发散; 玄武岩; 科马提岩; 绿岩; Mantle thermal evolution; Archean greenstones; Basalts; Mantle geochemistry; Komatiites
• ISSN: 1674-9871
• DOI: 10.1016/j.gsf.2016.01.006

Greenstone basalts and komatiites provide a means to track both mantle composition and magma generation temperature with time. Four types of mantle are characterized from incompatible element distributions in basalts and komatiites： depleted, hydrated, enriched and mantle from which komatiites are derived. Our most important observation is the recognition for the first time of what we refer to as a Great Thermal Divergence within the mantle beginning near the end of the Archean, which we ascribe to thermal and convective evolution. Prior to 2.5 Ga, depleted and enriched mantle have indistinguishable thermal histories, whereas at 2.5-2.0 Ga a divergence in mantle magma generation temperature begins between these two types of mantle. Major and incompatible element distributions and calculated magma generation temperatures suggest that Archean enriched mantle did not come from mantle plumes, but was part of an undifferentiated or well-mixed mantle similar in composition to calculated primitive mantle. During this time, however, high-temperature mantle plumes from dominantly depleted sources gave rise to komatiites and associated basalts. Recycling of oceanic crust into the deep mantle after the Archean may have contributed to enrichment ofTi, A1, Ca and Na in basalts derived from enriched mantle sources. After 2.5 Ga, increases in Mg# in basalts from depleted mantle and decreases in Fe and Mn reflect some combination of growing depletion and cooling of depleted mantle with time. A delay in cooling of depleted mantle until after the Archean probably reflects a combination of greater radiogenic heat sources in the Archean mantle and the propagation of plate tectonics after 3 Ga.