Off-craton mantle

Modern oceanic mantle is defined solely by abyssal peridotites, which are samples of harzbur-gite and Iherzolite collected from fracture zones at oceanic spreading centers, and these samples are representative of shallow oceanic lithosphere that has been processed at mid-ocean ridges (see Chapter 2.04). Two types of continental mantle lithosphere are considered (i) cratonic mantle, which refers to xenoliths collected from kimberlites that sample portions of mantle beneath stable, Archean cratons and (ii) off-craton mantle, which refers to xenoliths collected from alkalic basalts that have sampled portions of the subcontinental mantle adjacent to ancient cratonic mantle (see Chapter 2.05). Also included with off-craton lithosphere are orogenic Iherzolites and ophiolites, which are slices of mantle tectonically emplaced typically at convergent margins. 

In this section the normative modal and major-element compositions of a large number of samples collected from both oceanic and continental mantle lithosphere are presented. Compositional trends exhibited by oceanic and off-craton mantle are used to develop a model fertile upper-mantle composition. The reader is referred to Chapters 2.04 and 2.05 in this volume, and to reviews by McDonough (1990), McDonough and Rudnick (1998), and Griffin et al. (1999b) for alternative and detailed perspectives of the mantle sample. 

Figure 9 Normative spinel Iherzolite mineral abundances (wt.%) versus rock Mg for a subset of 292 off-craton mantle compositions (shaded circles). Data from a given locality were included if compositions had a correlation between Mg and normative olivine with a correlation coefficient (i ) of 0.6 or better. Data sources for xenoliths are Beccaluva et al. (2001a,h), Reid and Woods (1978), Rivalenti et al. (2000), Stem et al. (1999), Vaselli et al. (1995), Wiechert et al. (1997), Xu et al. (1988), and Zangana et al. (1999). Data sources for orogenic Iherzolites are Bodinier et al. (1988), Frey et al. (1985, 1991), Hartmann and Wedepohl (1993), and Lugovic et al. (1991). Open circles are the reconstmcted abyssal peridotite compositions from Figure 7. Also shown are estimates of primitive mantle from Table 2 white square = 1, circle = 2, triangle = 3, diamond = 5, inverted triangle = 6, ex = 7, and shaded star = 8.

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.

The off-craton mantle subset is shown relative to isobaric batch melt extraction curves on plots of normative olivine and major-element oxides versus Mg in Figure 17. Generally speaking, off-craton mantle compositions are consistent with effectively 0-30% melt extraction from fertile upper mantle in the range of 1-5 GPa. The chemical signature recorded in off-craton mantle mimics closely the maximum degree of melt extraction recorded in oceanic mantle, but in contrast there are many samples that show little or... 

Experimental studies show that volatile components such as H2O and CO2 can have profound effects on melting temperature and melt composition (e.g., Kushiro, 1972 Hirose and Kawamoto, 1995 Kawamoto and Holloway, 1997 Dalton and Presnall, 1998 Gaetani and Grove, 1998 Hirschmann et al., 1999a Lee et al., 2000 Asahara and Ohtani, 2001). It has been implicitly assumed in the assessment above that melt extraction occurred in nominally anhydrous mantle. This assumption is most robust in the case of MORE mantle, which has been shown to have a low volatile content (e.g., Michael, 1988 Saal et al., 2002 Chapter 2.07). Inasmuch as off-craton mantle has isotopic characteristics that indicate similar long-term incompatible element depletion to the MORE source, and considering that volatiles are very incompatible elements, a low volatile content at the time of melt extraction from off-craton mantle is implied. Indeed, off-craton mantle may be genetically related to modern MORE mantle. 

In order to focus more closely on the melt extraction component in off-craton mantle, a subset of data has been selected. A single criterion... 

Here, the fertile upper-mantle composition derived in the previous section is assumed for oceanic and off-craton mantle, and the model of Kinzler and Grove (1992a, 1993) is used to model melt extraction at pressures of s 2.5 GPa. 

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