"Majorite" and "silicate perovskite" mineral compositions in xenoliths from Malaita.

Mineral Compositions in Xenoliths from Malaita The report by Collerson et al. (1) of transition zone and lower mantle phases in mantle-derived material from alnöite pipes on Malaita, Solomon Islands, is highly significant in view of the rarity of such natural assemblages. Malaita is part of the obducted southern margin of the Ontong Java Plateau, an area of overthickened oceanic crust (2–7) that was emplaced by two major magmatic events at ;122 Ma and ;90 Ma (8, 9) and two minor events at ;62 Ma and ;36 Ma (10, 11). Recent seismic evidence has demonstrated the presence of a “mantle root” beneath the Ontong Java Plateau, interpreted to represent the remains of a plume head (7, 12), that has been repeatedly tapped (10, 11). The alnöite magmas were emplaced in the OJP at ;34 Ma (13). Collerson et al. (1) used mineral chemistry, petrography, and infrared spectral data to identify majorite, silicate perovskite, and diamond in mantle xenoliths entrained by alnöite magmas from eastern Malaita. We urge extreme caution in interpreting those data in terms of ultrahigh pressure mineral phases, especially because definitive x-ray data are lacking. Mineral chemistry (Fig. 1 and Tables 1 to 3) previously reported for mantle xenoliths from the Malaitan alnöites (14–20) shows compositions akin to those reported by Collerson et al. (1); the strong compositional similarity between the two data sets suggests that the minerals have a common origin. The question is, is that origin deep in the mantle, or shallow? Mineral chemistry. It is not possible to definitively identify majorite on the basis of mineral chemistry alone (21–24). Diagrams of the type depicted here (Fig. 1) and by Collerson et al. [figure 2 of (1)] do not show direct evidence of Si in octahedral sites, although it can be inferred. The covariation between Si and Al 1 Cr can be interpreted either as majorite substitution in a garnet structure (Si-Al substitution in the octahedral site) or tschermak substitution in a pyroxene structure (Si-Al substitution in the tetrahedral site). The distinction between majorite (cubic) and pyroxene is best made by x-ray diffraction, but such data were not included in (1). The compositions defined as majorite by Collerson et al. (1) are consistent with pyroxene containing up to 30 mole percent tschermak (RAlAlSiO6) in solid solution with either diopside or enstatite. Substitution is 2Al for R 1 Si, for which the Malaita “majorites” and pyroxenes show an excellent linear correlation (Fig. 1). Practically all mineral compositions (and, as discussed below, textures as well) reported by Collerson et al. (1) as being of ultrahighpressure origin are also found in megacrysts and spinel and garnet-spinel peridotite xenoliths from the Malaitan alnöites (Tables 1 to 3). For example, the garnets postulated in (1) to have equilibrated at .6 GPa (equivalent to ;200 km depth) have essentially the same compositions as samples reported by other workers (14–20) as equilibrating at ,3.6 GPa, or ;120 km (Table 1). We believe that the phases described as majorite and perovskite in (1) are actually pyroxenes and amphiboles (Tables 1 to 3). For example, the Etype “majorites” of (1) have a composition similar to that of the bronzite megacrysts reported by Nixon and Boyd (15) and of an amphibole inclusion in a garnet megacryst (Table 2). The P-type majorite compositions (Table 2) are similar to amphibole and Alrich clinopyroxene of metasomatic origin (19). The E-type perovskite compositions (1) are remarkably similar to primary orthopyroxenes from both garnet-spinel and spinel peridotites (Mg-perovskite; Table 3) and clinopyroxene inclusions in garnet megacrysts (Ca-perovskite; Table 3). The P-type Ca-perovskites identified in (1) are compositionally similar to secondary, retrograde clinopyroxene found between garnet and spinel (16, 17). It is evident from photomicrographs (Fig. 2, A and B) that the secondary clinopyroxene and amphibole found between garnet and spinel are in a state of disequilibrium with the primary peridotite minerals (Fig. 2, A and B; 16–18). Ultra-high-pressure “majorite” [analyses 161 and 159, table 2 of (1); analysis KC-9816, table 3 of (1)] are clearly deficient in Al 1 Cr (1.85 to 1.55). According to (1), such a deficiency is an evidence for ultradeep origin. Actually, these three analyses have more than eight cations for 12 oxygen—that is, they are Fe-rich, which explains the Al 1 Cr deficiency in the octahedral sites. A simple Fe/Fe calculation allows the (Al 1 Cr 1 Fe) site to be filled to two cations per 12 oxygen (or one cation per six oxygens for pyroxenes). Moreover, the comparison of KC-98-16 with analysis 23 from ultrahigh pressure experiments of (25) at 2.3 GPa is disputable. The two analyses differ from each other with respect to their Al/Si ratio, Mg mole fraction, and calcium, which is three times higher in the Malaita analyses of (1). Taking into account precision now attainable by modern microprobe, an oxide total of 97.08 would generally be rejected, unless there were reasons to assume that Fe or that the mineral was hydrous, as in amphibole. Collerson et al. (1) note that high-Al orthopyroxene is found only in reaction products produced via the breakdown of olivine and garnet at temperatures greater than 1500°C (26, 27). The upper mantle temperature, based upon geothermometry (14–18), is ;900 to 1250°C. The plume head responsible for the generation of the Ontong Java Plateau, however, was likely hotter than ambient upper mantle by up to 400°C (28–31).