Geochemistry and mineralogy of alkali basalts from Tropic Seamount, Central Atlantic Ocean

Abstract

Alkaline volcanic rocks dredged from the upper and central flanks of Tropic Seamount, central Atlantic Ocean, encompass a compositional spectrum ranging from basalt and trachybasalt (hawaiite, mugearite) to trachyte. Plagioclase and olivine form phenocrysts in partially vesicular basalt. Feldspar laths are occasionally Carlsbad and/or albite twinned with faint oscillatory zoning; compositions vary between An 80 and An 55. Minor biotite and kaersutitic amphibole are late magmatic in origin, attesting to higher concentrations of volatiles, and the hydrous nature of magmas at advanced stages of crystallization. The degree of crystallinity and the nature of fracturing govern alteration intensity. Low temperature mineral assemblages, formed under oxidizing conditions and high water rock ratios, predominate; smectite, celadonite, smectite/chlorite, carbonate, and zeolites are common vesicle filling. Hydration of extrusives is variable (0.3–5.7 wt.% H 2O); carbonation is generally sub-ordinate. Alteration-sensitive elements depleted during alteration include Sr and Mg, whereas Rb is enriched. The chemical features of Tropic Seamount alkali basaltic rocks correspond to the moderately alkaline group of ocean island basaltic (OIB) rocks. Alkali contents define a sodic suite sandwiched between a potassic series. The lithologies represent a single coherent magmatic sequence, initially generated by partial mantle melting. Incompatible elements are systematically enriched in alkali basaltic rocks relative to MORB. K Ba , sensitive to source heterogeneity, varies in the chemostratigraphic succession: typical OIB signatures of K Ba = 28 in lithologies from the upper slopes of the alkali volcanic pile contrast K/ Ba = 40–46 in the lower sampled portions of the cone, more typical for ocean island tholeiites (OIT). High valency LILE of basaltic rocks from the volcanic edifice are marked by characteristic elemental ratios: P Ce = 40, Zr Y = 17.9 , and Nb Y = 4.8 . Zr Nb is particularly low (3.7) relative to OIB from Atlantic islands. Chondritenormalized REE patterns, with LREE enrichment relative to OIT and particularly N-type MORB, is consistent with partial melting of a mantle source in which garnet remains a residual phase. In a spidergram, significant enrichment of the highly incompatible HFSE Nb and Ta in Tropic Seamount basaltic melts relative to LREE and LILE is apparent. Such a pattern, as well as La Ta ⪡ 17 , is in accord with OIB derivation from a dehydration residue of ancient recycled basaltic oceanic crust processed through a subduction zone, equilibrated with the mantle and accumulated in the mesosphere boundary layer. Consistently low LILE HFSE ratios of Tropic Seamount basaltic rocks point to a HIMU-type slab component, contaminated by only subordinate amounts of sediment. Whereas Nb U > 40 and Ta U > 2 are typical for HIMU end-member composition, minor admixture of an EMII component is suggested by somewhat variable Ba Th and Rb Nb ratios. Similarly, Ba Nb ⪡ 9 and La Nb ⪡ 1 suggest an input of an ancient pelagic component (EMI) not exceeding 1%. Elevated Ba Th and Rb Nb in basaltic rocks require minor mixing of HIMU source material with depleted upper mantle (N-MORB source).