Peridotites major elements

Table 1.13. Major-element data (%) on clinopyroxenes from the mid-ocean ridge peridotites...

A quite serious problem, however, still obscures most applications of the percolation theory to the transport of magmas. Most major elements, such as Si, Mg, Ca,... can be considered as compatible since their concentration in the peridotite source and the basaltic melt are similar within a factor of 3. Equation (9.4.37) indicates, as would equations (8.3.17) and (8.3.19) in the most general case, that major elements are slower than the liquid, especially for small porosities. But, what is the liquid made of, then The velocity of a medium is the weighted average velocity of its constituents [see equation (8.1.4)]. The basalt velocity is that of Si, Mg, Ca,... weighted by their... 

Table 5.4 Olivine major element compositions (in weight %). Samples occur in different types of rocks (1) = forsterite from a metamorphosed limestone (2) = hortonolite from an olivine gabbro (3) = fayalite from a pantelleritic obsidian (4) = fayalite from an Fe-gabbro (5) = forsterite from a cumulitic peridotite (6) = forsterite from a tectonitic peridotite. Samples (1) to (4) from Deer et al. (1983) sample (5) from Ottonello et al. (1979) sample (6) from Piccardo and Ottonello (1978). ...

Major-element compositions (weight ratios of Mg/Si and Al/Si) for mantle rocks (peridotites) and estimates of the primitive mantle composition of the Earth compared with various groups of chondrites and the Sun. No mixture of chondrite types provides an exact match to the primitive mantle composition, although some carbonaceous chondrites provide the closest match. Modified from Righter et al. (2006). 

In addition, many peridotites bear the obvious signatures of metasomatism, which re-enriches the rock in incompatible components subsequent to depletion by melt extraction. Where this is obvious (e.g., in reaction zones adjacent to later dikes) it may be avoided easily but often the metasomatism is cryptic, in that it has enriched the peridotite in incompatible trace elements without significantly affecting major-element chemistry (Frey and Green, 1974). Peridotites thus have very variable contents of highly incompatible trace... 

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),... 

In spite of their variable provenance (subcontinental lithosphere, supra-subduction mantle wedge, or oceanic mantle), most of the tectonically emplaced and abyssal peridotites show coherent covariation trends for major elements (Eigure 5). These variations reflect their variable modal compositions between a fertile end-member— comparable to proposed estimates for pristine... 

Ytterbium versus AI2O3 covariation trend and fertile orogenic Iherzolites. Similar to the correlations observed between major (and minor transition) elements, the good correlation between ytterbium and AI2O3 in tectonically emplaced and abyssal peridotites (Figure 10) is classically ascribed to variable degrees of melt extraction. However, any of the alternatives envisioned for the major elements (melt-rock reactions, melt... 

Positive slopes at moderate angles may be related to silicate melt-metasomatism, whereas subvertical trends of increasing AI2O3 and (Pd/Ir) are indicative of sulhde addition (Rehkamper et al, 1999). Hence, the bulk rock PGE systematic s of cratonic peridotites are indicative of their major-element depleted characters, but with metasomatic effects superimposed on this signature to varying degrees. 

In contrast, peridotites metasomatized by small melt fractions show enrichment in platinum and palladium and elevated (Pd/Ir) . Bulk mineral separate PGE-Re analyses of two fertile xenoliths from southeastern Australia indicate less than 6% of the whole-rock PGE budget resides in either silicate or oxide phases and further implicates sulfides and alloys as the main controls of PGE-Re abundance. Comparison of sulfide versus whole-rock budgets by Lorand and Alard (2001) demonstrates the dominance of sulfide as the main PGE host in relatively fertile peridotites. This confirms the results of earlier studies of xenolith PGE mass balance (Hart and Ravizza, 1996 Mitchell and Keays, 1981) plus xenolith-derived and diamond inclusion sulfide studies (Jagoutz et al, 1979 Pearson et al, 1998b). As with cratonic xenoliths, sulfur-PGE and major-element-PGE correlations in more depleted noncratonic peridotites indicate that I-PGEs are probably not hosted entirely by sulfide (Lee, 2002). 

Boyd F. R. (1997) Origins of peridotite xenoliths major element considerations. In Short Course on High P and T Research on the Lithosphere. U. Siena, pp. 89-106. 

Figure 6 Major-element oxides (wt.%) versus FeO as a function of pressure (GPa) and degree of batch melt extraction (sources the 1 GPa and 2 GPa trends are based on the Kinzler and Grove (1992a, 1993) model for melting of primitive mantle of McDonough and Sun (1995) (composition 1, Table 1), and the trends at higher pressures are based on the data of Walter (1998) for melting of fertile peridotite KR4003).

Figure 10 Major-element oxides versus Mg for the off-craton mantle subset (shaded circles) and reconstructed abyssal peridotite compositions (open circles). Primitive mantle compositions from Table 2 are also shown with symbols as in Figure 9.

Press S., Witt G., Seek H. A., Eonov D., and Kovalenko V. 1. (1986) Spinel peridotite xenoliths from the Tariat depression, Mongolia 1. Major element chemistry and mineralogy of a primitive mantle xenolith suite. Geochim. Cosmochim. Acta 50, 2587-2599. 

Figure 2 Major element variations of eclogites and associated ultramafic rocks from the Su-Lu and Dabie terranes of east-central China (Jahn, 1998). Eclogites are generally basaltic and quartz-normative, with some showing the cumulate nature of their protoliths. Type 1 is gneiss-hosted, type II, marble-associated. Type III are members of layered intrusions or associated with ultramafic rocks. Also plotted are data from peridotites (Perid.),...

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