The petrology of some picrites from Mauna Loa and Kilauea volcanoes, Hawaii

TLDR: In this paper, the authors investigated the mineralogy and chemistry of picrites from Mauna Loa and Kilauea and evaluated, for Hawaiian tholeiitic picrites, the contrasting genetic models which have been proposed for these Mg-rich volcanics, namely products of direct crystallization of high-Mg melts (20-25% MgO) or the result of accumulation of olivine phenocrysts into less Mgrich melts.

Abstract: The mineralogy and chemistry of picrites from Mauna Loa and Kilauea have been investigated to evaluate, for Hawaiian tholeiitic picrites, the contrasting genetic models which have been proposed for these Mg-rich volcanics, namely products of direct crystallization of high-Mg melts (20–25% MgO) or the result of accumulation of olivine phenocrysts into less Mg-rich melts. Genetic interpretations rely heavily on Mg-Fe partitioning relations between olivines and their picrite hosts. Although the 100 Mg/(Mg + Fe2+) ratios (M) of picrites are wide-ranging (M=73.6–82.9 for Fe2O3/FeO=0.15), with MgO as high as 27.8%, the average 100 Mg/(Mg+Fe) ratios (mg) of the cores of olivine phenocrysts (megacrysts) show only restricted compositional variation (mg=87.2–89.0). Successive olivine generations are progressively more Fe-rich. Olivine/liquid Mg-Fe partitioning data and the Mn and Ni abundances in olivine phenocrysts collectively indicate that they were precipitated by Mg-rich basaltic melts with 12–14% MgO. Spinel compositions range from liquidus magnesiochromites, occurring mainly as inclusions in olivine phenocrysts, to groundmass titanomagnetites which crystallized at nearsolidus temperatures. The Cr2O3 contents and M values of liquidus magnesiochromites suggest that their parent melts were neither Mg-rich picritic (MgO>20%) nor relatively Mg-poor basaltic types. On MgO variation diagrams (extending from approximately 7% to more than 25% MgO), Mauna Loa and Kilauea picrites and their respective microcrystalline/glassy groundmasses (the major component of quickly-cooled picrites) plot on linear regression lines (‘olivine control lines’). At a given MgO content, Kilauean picrites and tholeiites (M<75) generally contain more TiO2 FeO t , CaO, K2O and P2O5, and less SiO2 and Na2O than Mauna Loan types. The compositions of the groundmasses in picrites and Mg-rich ol-tholeiites equate closely with those of the Mg-poor tholeiites (7–9% MgO) which dominate the petrology of each shield. Low-pressure closed system differentiation of Hawaiian tholeiitic magmas (10–15% MgO) can yield picritic derivatives which differ, however, from the extrusive picrites by virtue of distinctly higher FeO t contents and correspondingly more Fe-rich olivines and Cr-spinels. The calculated Mg-Fe olivine megacryst-‘liquid’ partition coefficient K D for individual picrites indicate that lowpressure equilibria (K D =0.30–0.34) are defined only by melts with approximately 12–14% MgO (M∼ 71–74). Assessed in conjunction with Ni-MgO modeling, these data indicate that the more Mg-rich picrites (MgO> 14–15%) are indeed olivine-enriched and do not represent melt compositions. Olivine enrichment resulted from post-eruptive mechanical (flow) differentiation of extruded ‘mushes’ of intratelluric cognate olivine phenocrysts (mg∼88) and tholeiitic melts (M∼60), which are ‘residua’ of the parental magmas (12–14% MgO), following the crystallization of the olivine phenocrysts. The ‘parental’ magmas of both picrite suites were generated by 35–40% melting of relatively Fe-rich spinel lherzolites (mg∼84) containing kaersutitic amphibole as a major primary constituent.