Mauna Loan and Kilauean Tholeiites with Low ‘Ferromagnesian-Fractionated’ 100 Mg/(Mg + Fe2+) Ratios: Primary Liquids from the Upper Mantle?

• Published in: Journal of petrology 1991-10, Vol.32 (5), p.863-907
• Main Author: WILKINSON, J. F. G.
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/32.5.863-a

Many genetic models for mantle-derived mafic volcanic rocks depend on Fe-Mg partitioning relations between olivines in peridotitic mantle residua and presumed equilibrium liquid derivatives. On this basis, aphyric or glassy mafic volcanics with 100 Mg/(Mg + Fe2+) ratios (M-values) greater than 68–70 are often interpreted as primary liquids and those with lower M-values as differentiates of more Mg-rich primary or parental magmas (not necessarily primary), following (polybaric) olivine fractionation. The paper examines the relative merits of crystal fractionation and primary magma models for the voluminous shield-building low-M tholeiites of Mauna Loa and Kilauea volcanoes. The youngest lavas of each volcano are dominated by ol tholeiites and mildly qz-normative tholeiites [M≪75; 100 CIPW an/(ab + an), AN > 50] with low M-values (M ˜56–60). Picrites (M>75) are only relatively minor; low-Mg tholeiites (≪7% MgO) and, for Kilauea, tholeiitic andesites (AN ≤50, M˜50) are more important than Mg-rich eruptives. The tholeiites (< 7% MgO) generally have been interpreted as olivine-controlled differentiates of primary picrite magmas or parental magnesian ol tholeiite magmas (13–14% MgO). The low-Mg tholeiites and tholeiitic andesites are considered to be low-pressure derivatives of slightly more Mg-rich tholeiites, produced by olivine-augite-plagioclase cotectic fractionation. Evidence unfavourable to fractional crystallization models for lavas with M-values less than 68 includes the following: (1) MgO frequency histograms of the youngest lavas are positively skewed, with well-defined maxima at 7–8% MgO (M ˜56–60). Lavas with > 10% MgO are only minor components of the histograms. (2) The minor picrites are largely flow-differentiated olivine-enriched variants of olivine-liquid ‘mushes’ with up to 14–15% MgO and hence their compositions are not those of liquids. (3) There is no visible evidence of the very substantial complementary dunite crystal extracts implicit in olivine fractionation models employing Mg-rich parental magmas (> 14% MgO). (4) The TiO2, FeO1 and K2O contents of Kilauea lava lake differentiates (segregation veins and oozes) generally do not match those of Kilauean low-Mg tholeiites and tholeiitic andesites with comparable MgO contents. Compositions of Mauna Loa and Kilauea mantle sources have been calculated by combining appropriate melt fractions of primary or near-primary lava compositions with mantle residua compositions. The source compositions