OIB-type Rocks

Rocks akin to the Ocean Island Basalts have been found at the Vavilov and Managhi seamounts, off the eastern Sardinian coast (Quirra and Etruschi seamounts and rocks drilled at ODP Site 654), and north and northwest of the western Sicily coast (Aceste seamount, Promoteo lava field and Ustica island Fig. 9.1). Ustica has been described in Chap. 8. 

Etruschi seamount. Basaltic fragments about 0.1 Ma old have been recovered from this site (Keller 1981). The rock is mostly glassy with a few small grains of olivine and acicular plagioclase crystals. 

A Site 665 Vavilov Smt. + Enarete X Southern Marsili Basin  

It is undersaturated in silica and displays a hawaiitic composition, similar to the analogous rocks from Sardinia. 

Magnaghi and Vavilov seamounts. Magnaghi (3.0 to 2.7 Ma) and Vavilov (Late Pliocene to 0.1 Ma) seamounts are two N-S elongated volcanoes. The few rocks recovered to date have a basaltic composition with moderate to high TiC 2 (1.9 to 3.2 wt %), variable Ni (about 30 to 180 ppm) and Cr (about 30 to 350 ppm Selli et al. 1977 Beccaluva et al. 1982 Robin et al. 1987 Kastens et al. 1988, 1990 Savelli 1988). Incompatible elements are moderately enriched and mantle-normalised patterns show a small upward convexity, a moderate spike of Nb, and no LILE spikes or HFSE depletion (Fig. 9.12). 

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The OIB-type rocks have variable isotopic and trace element characteristics, which appear to be related to different processes and/or sources. 

Rock type MORB-type basalt MORB-type basalt OIB-type basalt OIB-type basalt Mugearite Lamproite... 

MORB, OIB and island-arc geochemical signatures, respectively. The distribution of various rock types are schematically shown in Fig. 9.1. Compositional characteristics and ages are given in Tables 9.2. Compositions for selected rocks are reported in Table 9.4. The TAS diagram based on oxide concentrations recalculated on a water-free basis is shown in Fig. 9.10. Variation diagrams of some major and trace elements and mantle-normalised incompatible element patterns for selected samples are shown in Figs. 9.11 and 9.12. 

Fig. 23. Lanthanide abundance patterns for Mid-Ocean Ridge Basalts (MORE) and Ocean-Island Basalts (OIB) (data are from table 16). Note the depletion in the light lanthanides (La-Sm) in MORE derived from a depleted mtmtle source, and the enriehment of light lanthanides in OIB. Nd isotopic evidence indicates that the source of both these rock types was characterized by long term depletion of Nd relative to Sm, indicating the observed light lanthanide enrichment in OIB is a recent event.

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