Oceanic peridotites

We shall hereafter follow the current terminology. It is convenient to distinguish three main types of mantle occurrences and to examine them in the following sequence (i) orogenic peridotite massifs, (ii) ophiolitic mantle rocks, and (iii) oceanic peridotites. 

In addition to the abyssal peridotites collected on the seafloor, the oceanic peridotites in a broad sense include mantle rocks that were exhumed above sea level by normal faults associated with rifting or by transcurrent movements along transform faults. Examples of emerged oceanic peridotites include ... 

During the last two decades, many studies have reported major- and trace-element data on orogenic, ophiolitic, and oceanic peridotites. Whole-rock compositions representative of the main peridotite occurrences are illustrated on element versus AI2O3 diagrams for major elements (Figure 5), minor transition elements (Figure 7),... 

While many refractory peridotites were likely produced by melt-rock reactions at nearly constant or increasing melt mass (Kelemen, 1990 Kelemen et al., 1990), some fertile peridotites were conversely refertUized as the result of the solidification of infiltrated melt (a process also referred to as percolative fractional crystallization by Harte et al., 1993). (Re)fertilization processes have been invoked for several types of orogenic, ophiolitic, and oceanic peridotites, such as ... 

Peridotite fertilization may also result from the fractional solidihcation of exotic (deep-seated) melts inhltrated in wall rocks of translithospheric magma conduits. This process was hrst described in composite mantle xenoliths (Wilshire and Shervais, 1975 Gurney and Harte, 1980 Irving, 1980 Wilshire et al., 1980 Boivin, 1982 Harte, 1983 Harte et al, 1993 Menzies et ah, 1987), where it is referred to as modal metasomatism when new (generally hydrous) minerals are precipitated (Dawson, 1984 Kempton, 1987), or Fe-Ti metasomatism (Menzies et al., 1987) when the attention is focused on chemical enrichment. In contrast with ultramafic xenoliths, the tectonically emplaced and oceanic peridotites contain only sparse rock types attributable to mantle metasomatism by deep-seated melts. Examples of wall-rock, modal, and Fe-Ti metasomatism were nevertheless described in IP orogenic Iherzolites, notably in the Pyrenees (Fabrics et al., 1989 Bodinier et al., 1988, 1990, 2003 McPherson et al., 1996 Woodland et al., 1996). 

However, a few examples of metasomatism and chemical enrichment by alkaline, deep-seated melts have been described, notably in the St. Paul s Island oceanic peridotites, in the Atlantic Ocean (Roden et al., 1984) and in the Lherz orogenic massif and neighboring massifs such as Caussou, in the Eastern Pyrenees (Fabries et al., 1989 Bodinier et al., 1988, 1990). 

Several Nd-Sr isotopic studies were also devoted to ophiolitic peridotites, and a more restricted number of studies to oceanic peridotites. The early studies have reported anomalously high Sr/ Sr values (e.g., Bonatti et al., 1970) in these rocks and concluded that they were genetically unrelated to oceanic cmst (see references in Menzies and Murthy, 1978). More recent studies... 

Stephens C. J. (1997) Heterogeneity of oceanic peridotite from the western canyon wall at MARK results from site 920. 

Apatite. Apatite is relatively common in noncratonic continental and oceanic peridotite xenoliths (Menzies and Wass, 1983 Ionov et al,... 

Alt J. C. and Shanks W. C. (1998) Sulfur in serpentinized oceanic peridotites serpentinization processes and microbial sulfate reduction. J. Geophys. Res. 103, 9917-9929. 

FIGURE 3.12 The composition of the subcontinental lithosphere, (a) Differences in composition between Archaean, Proterozoic, and Phanerozoic subcontinental lithosphere (after Griffin et al., 2003), compared to estimates of the BSE (from Table 3.1). The diamonds represent the average compositions given in Table 3.4. (b) Differences in composition between oceanic and continental lithosphere, as expressed in % modal olivine and olivine mg oceanic peridotites define the trend shown as the dashed line whereas garnet peri-dotites from the Kaapvaal Craton, South Africa - illustrative of Archaean subcontinental lithosphere - plot in the shaded field. The diamonds are the averages of the two fields (after Boyd, 1989). Also shown is the field for Proterozoic subcontinental lithosphere (ellipse) after Griffin et al. (2003). 

Alt JC, Shanks WC 111 (1998) SnUur in serpentinized oceanic peridotites serpentinization processes and microbial suUate rednction. J Geophys Res 103 9917-9929 Alt JC, Shanks WC 111 (2001) Uptake of snlfur dnring serpentinization of abyssal peridotite. Earth Planet Sci Lett (submitted)... 

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