GEOCHEMISTRY AND MINERALOGY OF ALKALI BASALTS FROM TROPIC SEAMOUNT, CENTRAL ATLANTIC-OCEAN

Alkaline volcanic rocks dredged from the upper and central flanks of T ropic Seamount, central Atlantic Ocean, encompass a compositional spec trum ranging from basalt and trachybasalt (hawaiite, mugearite) to tra chyte. Plagioclase and olivine form phenocrysts in partially vesicular basalt. Feldspar laths are occasionally Carlsbad and/or albite twinne d with faint oscillatory zoning; compositions vary between An(80) and An(55). Minor biotite and kaersutitic amphibole are late magmatic in o rigin, attesting to higher concentrations of volatiles, and the hydrou s nature of magmas at advanced stages of crystallization. The degree o f crystallinity and the nature of fracturing govern alteration intensi ty. Low temperature mineral assemblages, formed under oxidizing condit ions and high water/rock ratios, predominate; smectite, celadonite, sm ectite/chlorite, carbonate, and zeolites are common vesicle filling. H ydration of extrusives is variable (0.3-5.7 wt.% H2O); carbonation is generally sub-ordinate. Alteration-sensitive elements depleted during alteration include Sr and Mg, whereas Rb is enriched. The chemical fea tures of Tropic Seamount alkali basaltic rocks correspond to the moder ately alkaline group of ocean island basaltic (GIB) rocks. Alkali cont ents define a sodic suite sandwiched between a potassic series. The li thologies represent a single coherent magmatic sequence, initially gen erated by partial mantle melting. Incompatible elements are systematic ally enriched in alkali basaltic rocks relative to MORE. K/Ba, sensiti ve to source heterogeneity, varies in the chemostratigraphic successio n: typical OIB signatures of K/Ba = 28 in lithologies from the upper s lopes of the alkali volcanic pile contrast K/Ba = 40-46 in the lower s ampled portions of the cone, more typical for ocean island tholeiites (OIT). High valency LILE of basaltic rocks from the volcanic edifice a re 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 A tlantic islands. Chondrite-normalized REE patterns, with LREE enrichme nt relative to OIT and particularly N-type MORE, is consistent with pa rtial melting of a mantle source in which garnet remains a residual ph ase. In a spidergram, significant enrichment of the highly incompatibl e HFSE Nh and Ta in Tropic Seamount basaltic melts relative to LREE an d LILE is apparent. Such a pattern, as well as La/Ta much less than 17 , is in accord with OIB derivation from a dehydration residue of ancie nt recycled basaltic oceanic crust processed through a subduction zone , equilibrated with the mantle and accumulated in the mesosphere bound ary layer. Consistently low LILE/HFSE ratios of Tropic Seamount basalt ic rocks point to a HIMU-type slab component, contaminated by only sub ordinate 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 much less than 9 and La/Nb much less than 1 suggest an input of an ancient pelagic component (EMI) not exceeding 1%. Elevated Ba/Th an d Rb/Nb in basaltic rocks require minor mixing of HIMU source material with depleted upper mantle (N-MORB source).