Alkaline magmas from Sal Island: host melanephelinites and intergranular melts in mantle xenoliths

 Ryabchikov I.D.*,Kogarko L.N.**

 *Institute for Geology of Ore Deposits (IGEM), Moscow, Russia; ** Vernadsky Institute of geochemistry and analytical chemistry, Moscow, Russia.

iryab@igem.ru

 

Mantle xenoliths of Sal Island, Cape Verde Archipelago were collected from volcanic neck Morrinho do Açurar in the northern part of the island. The volcanic rocks hosting the xenoliths consist of melanephelinite. The studied mantle xenoliths are anhydrous spinel-bearing peridotites, ranging from lherzolite to harzburgite. Interstitial glasses in these peridotites contain up to 8.6 % K₂O and  5.6 % Na₂O. These interstitial glasses contain sulphide globules with included Fe-Ni alloys and in some cases separate metallic phases. This implies, that oxygen fugacity of these interstitial melts were several log units below the f_O2 values of primary mineral assemblages in the investigated lherzolites and host melanephelinites (close to QMF buffer).

Other indications of extremely reducing conditions characteristic of infiltrating melts are very low Fe³⁺ contents in outer parts of spinels grown from intergranular melts and presence of stoichiometric FeS (troilite) in sulphide grains. The analysis of diffusion profiles in zoned spinels implies that highly reduced melts invaded the mantle rocks several months before the eruption of host lavas. It is suggested that these melts were generated in the part of Cape Verde plume enriched in metallic material during its residence in the lower mantle where disproportionation of Fe²⁺ into Fe⁰ and Fe³⁺ takes place.

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This study was financially supported by RFBR grant 08-05-00356-а and 08-05-00054-a.

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References:

Ryabchikov I.D., Kogarko L.N., Ntaflos T., Kurat G. Metallic phases in mantle xenoliths.// Doklady Akademii Nauk 1994. Vol. 338. P. 95-98.

Ryabchikov I.D., Ntaflos T., Kurat G., Kogarko L.N. Glass-bearing xenoliths from Cape Verde: Evidence for a hot rising mantle jet. // Mineralogy and Petrology. 1995. Vol. 55. P. 217-237.