Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts)

Abstract

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57-58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a "hot line" related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113-115 Ma, and earlier magma chambers. The rocks were altered by low temperature hydrothermal solutions. The parental melts of the vol canics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of gar net lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volca nics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2 type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics cor responded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incor porated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

Insights into the petrogenesis of an intraplate volcanic province: Sr-Nd-Pb-Hf isotope geochemistry of the Bathymetrists Seamount Province, eastern equatorial Atlantic

“…Our data set does not support a significant contribution of an EM-like mantle end member as proposed by Skolotnev et al (2012Skolotnev et al ( , 2017. We note that those four previously published samples from the southern BSP (grey triangles in Figure 5) that show low 143 Nd/ 144 Nd and high 207 Pb/ 204 Pb, trending in the direction of EM II, also possess very high 87 Sr/ 86 Sr ratios (Figure 5a).…”

“…Although this seamount is offset ~350 km to the east of the BSP, Bertrand et al (1993) interpreted it to be formed from the same magma source as the BSP, due to its similar age and major and trace element composition to the Prince Albert Seamount. Considerably younger ages of 36-43 Ma, determined by whole rock K-Ar dating of two trachybasalts from Prince Albert Seamount (Kharin et al, 1988, see Figure 1b), are believed to be underestimated due to the altered nature of the samples (Skolotnev et al, 2012), although they could also represent late-stage volcanism as is commonly found on seamounts. A whole rock K-Ar age of 37 Ma derived in the same study (Kharin et al, 1988) for a trachyte from Arroyo Seamount (also known as Nephrite Seamount, see Figure 1b) at the southern end of the BSP is also questionable.…”

“…Geochemical data for volcanic samples were only reported from three northern (Annan, Carter and Prince Albert / Krause; Kharin et al, 1988;Jones et al, 1991;Peyve and Skolotnev, 2009;Skolotnev et al, 2012) and three southern (Gilg, Cigogne and Arroyo; Kharin et al, 1988;Skolotnev et al, 2017) seamounts, but systematic sampling of the BSP has not been carried out thus far. Previously recovered lavas have compositions ranging from trachybasalts to trachyandesites to trachytes (Kharin et al, 1988;Jones et al, 1991).…”

“…Previously recovered lavas have compositions ranging from trachybasalts to trachyandesites to trachytes (Kharin et al, 1988;Jones et al, 1991). The majority of the previously recovered lava samples are highly altered (with loss on ignition (LOI) values up to 14.3%) (Peyve and Skolotnev, 2009;Skolotnev et al, 2012Skolotnev et al, , 2017. Therefore, only immobile elements and isotope data from leached samples should be considered.…”

“…The strong enrichment of highly incompatible elements over less incompatible elements, coupled with the depletion of heavy rare earth elements (HREE), was interpreted to reflect an ocean island basalt (OIB) composition formed by relatively small degrees of partial melting at a high pressure with residual garnet (Peyve and Skolotnev, 2009;Skolotnev et al, 2012Skolotnev et al, , 2017. Samples from three different BSP seamounts (Carter, Gilg and Cigogno) have widely varying isotopic compositions, forming a weak trend from DM (Depleted Mantle) to HIMU (high µ = time-integrated 238 U/ 204 Pb) mantle end member compositions but with some samples deviating towards EM II (Enriched Mantle) composition (Skolotnev et al, 2012(Skolotnev et al, , 2017 (see section 4.3 for details).…”