Sossi and 8 co-authors reported in 2016 that komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton- Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios.
Following calculation of the primary magma composition of each suite, the authos indicated that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows.
The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, ~25% melting at 5 GPa (mantle potential temperature, TP¼1750°C) whereas the ADKs of Barberton segregated at the highest F and P (~40%, 9 GPa, TP¼1950°C).
The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2-7 Ga komatiites tend to have sources that are more depleted than those of older, 3-5 Ga komatiites. Komatiites are invaluable records of the mantle’s chemical and physical evolution during the Archean.
The authors concluded that the 38 predominantly spinifex-textured komatiites from five Archean cratons revealed that these magmas have diverse chemical characteristics that can be related to their source composition and P and T of origin. The primary magma MgO contents range from 23 to 30 wt %, corresponding to eruption temperatures of ~1470–1575°C.
Aluminium concentration is shown to be a robust indicator of pressure of melt extraction. Melts of ADK composition can be converted to AUK compositions with increasing degrees of melting. The degree of melting was calculated using KD Fe–Mg estimated from the Mg# of calculated parental komatiite melts, most of which were formed by ~30% partial melting, the point at which the peridotite matrix–melt system becomes a suspension and facilitates melt escape. However, a range from 25% melting at 5 GPa to ~40% melting at 9 GPa for the komatiites analysed in this study implies mantle potential temperatures between 1750 and 1950°C, 400–600°C hotter than present-day ambient mantle, requiring that melting in Archean plumes commenced in the mantle Transition Zone. The faithful retention of high-pressure melting chemistry in komatiites suggests minimal interaction with the lithospheric mantle as they ascended.
Incompatible trace elements show that the mantle sources of komatiites from Munro, Yilgarn, Belingwe and Regal–Ruth Well have experienced varying extents of prior melt extraction from ~4% to less than 1% (in that order) where the involvement of depleted upper mantle material in their plume sources is plausible. This is supported by the presence of positive Nb anomalies and radiogenic Nd isotope compositions in some komatiite suites, hinting at the presence in the plume of peridotite that had previously undergone continental crust extraction.
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