DISCOVERY OF PRIMITIVE ‘KOMATIITOID’ LAVAS FROM THE MARIANA ARC MAGMATIC FRONT

Samples of unusual, very primitive (Mg# ~ 76) lava have been dredged from a small seamount situated on the magmatic front NW of Rota in the southern Marianas (Cook 7 expedition, Spring 2001).  Samples include ankaramite and picrite, but all have identical bulk and mineral chemistries.  Whole rock compositions are 14.8-15.4 wt% MgO, 0.47-0.48 wt% TiO₂, and ~2% total alkalis and the lava can be classified chemically as a picrite. Abundant euhedral olivine and clinopyroxene phenocrysts indicate slow cooling and crystal accumulation at depth prior to eruption.  CPX have compositions of Wo_41.4-47.9, En_43.8-51.7 and Fs_2.7-9.1. Chromites have Cr#s (Cr/Cr+Al) of 0.95-0.74 and Mg#s (Mg/Mg+Fe²⁺) of 0.29-0.47. Olivine phenocrysts are Fo_87.2-92.0 with 0.09-0.26 wt% NiO.  Olivine Fo and FeO/MgO contents closely correlate with values predicted for olivine in equilibrium with whole rock, melt inclusion and glass compositions, indicating that these are cognate and not xenocrysts.  Isotopically the samples are similar to typical Mariana arc lavas (⁸⁷Sr/⁸⁶Sr ~ 0.7033,  eNd ~ +6.5), although Pb isotopic compositions are slightly more radiogenic.  Melt trapped within olivine phenocrysts have compositions similar to those of picrites and komatiites, with up to 21.7 wt% MgO, 0.4-0.8 wt% TiO₂, 43.2-47.0 wt% SiO₂, and 1.66-3.43% total alkalis. The major and trace element compositions of the melt inclusions within Fo₉₂ olivine indicate that these are samples of primary arc melts.  Trace element patterns display an arc signature, but at lower total abundances relative to most arc melt inclusions. Trace element ratios indicate depleted mantle and sediment components.  As much as 1346 ppm Cl is present in some inclusions and Cl systematics indicate the addition of a fluid component.

 

We propose that this lava represents a primary melt of the mantle wedge that underwent crystallization and accumulation prior to eruption.  While this lava is unusual, its existence is evidence that subduction zones can produce ‘komatiitoid’ lavas with high MgO and low TiO₂ and alkali contents along the magmatic front.  This has implications for the potential tectonic settings of ultramafic lavas such as picrites and komatiites found in the geologic record.  It remains to be determined whether this lava and these melt inclusions are representative of most primary arc melts.