In Tables T4 and T5 we report major and minor element abundances of the different metamorphic rocks recovered from the serpentinite muds of Legs 125 and 195, respectively. The major and trace element geochemistry of the metamorphic clasts is highly variable (Figs. F9, F10, F11). Some samples show highly variable SiO2 (30–65 wt%) and relatively uniform MgO (~40 wt%) (Fig. F9A), and others have uniform SiO2 contents (~45 wt%) and highly variable MgO contents (5–45 wt%) as well as elevated CaO (Fig. F9A, F9B). Some of the high-MgO samples could be derived from the metamorphism of ultramafic protoliths. This is because the same samples also show elevated LOI, high Ni (average = 2280 ppm), high Cr (1040–3950 ppm), and low TiO2 (0.04–0.4 wt%) abundances (see Table T5). However, we believe the majority of the samples are derived from mid-ocean-ridge basalt (MORB)-like protoliths because not only do they have low MgO, but they also have low LOI, Ni (average = 470 ppm), and Cr (14–340 ppm) as well as high TiO2 (0.97–3.58 wt%) (Figs. F9C, F10C) and Al2O3 (Figs. F9C, F10A). Samples from this same group possess MORB-like REE patterns (see below), as well as high-field-strength element and large-ion-lithophyle element abundances inconsistent with derivation from serpentinized peridotites. Moreover, the metabasic samples have Si/Mg ratios ranging from 2.1 to 8.9 (serpentinized peridotite Si/Mg ~ 1) (Figs. F10, F11). And finally, the Leg 195 metabasic samples show major and trace element sytematics similar to those of metabasalt and metadiabase samples recovered from Leg 125 (Johnson, 1992).
We analyzed 16 samples of serpentinite muds for B, Be, and Li abundances (Table T6) and several of the large schists (Fig. F2) for their trace element concentrations (Table T7). Both the serpentine muds and the metamorphic schists have higher B, Li, and Be contents relative to unaltered depleted mantle values (~0.25 and 1.6 ppm and 25 ppb, respectively) (Ryan and Langmuir, 1987, 1993; Salters and Stracke, 2004) (Figs. F11, F12, F13). When evaluated vs. the immobile TiO2 contents (as an approximate distinction between ultramafic and mafic protolith), boron seems to be enriched in both the high- and low-TiO2 samples (Fig. F12). The metamorphic clasts also have higher Li concentrations compared to the serpentinite muds and serpentinized peridotite clasts (Figs. F11A, F13). The metabasic rocks from both South Chamorro and Conical Seamounts have very high B/Be (8–2715; average = 358) and B/Li (average = 2.9) in respect to the depleted mantle B/Be and B/Li values (~2 and ~0.07, respectively) (Salters and Stracke, 2004). However, on plots such as Si/Mg ratios vs. Li concentrations (Fig. F11A) and Si/Mg ratios vs. B concentrations (Fig. F11B), the high B and Li of the clasts seems to be independent of Si/Mg. In the case of boron, it seems likely that processes of low-temperature fluid-rock exchange in the upwelling materials have resulted in relatively high B contents in any rock with appropriate host phases. The abundances of other fluid-mobile elements (FME), such as As and Cs, are also elevated by orders of magnitude relative to the depleted mantle values (Fig. F13). The extent of enrichment is very interesting because even though the host lithology is different (major phases in the metamorphic clasts are chlorite, amphiboles, and micas), the patterns of enrichment in FME resemble those found for serpentinite muds and serpentinized peridotites (i.e., B, As, and Cs) are orders of magnitude enriched, followed by moderate enrichments in Li and Sb and no enrichments in Pb, Rb, Ba, Th, and U (Savov et al., 2002, in press) (Fig. F13). Further microprobe studies of the FME abundances in metamorphic minerals is needed for better understanding of the mechanism of FME transfer from fluids to appropriate forearc host minerals.
All of the analyzed metamorphic schists exhibit flat chondrite-normalized REE patterns with ~10 x CI abundance levels, [La/Sm]N ~ 1.3, and [Sm/Yb]N ~ 1.15 (Fig. F14). These patterns are identical to earlier REE results on metabasalt samples recovered from Conical Seamount (Johnson, 1992). The Conical Seamount samples show slight light REE (LREE)-depleted patterns ([La/Sm]N = 0.6), and one sample from South Chamorro Seamount shows an LREE-enriched pattern ([La/Sm]N = 6.3) (Fig. F14). By comparison, the subducted sediment REE patterns from the Mariana convergent margin show LREE enrichments of up to 40 x CI (Plank and Langmuir, 1998). Thus, with the exception of one sample (195-1200D-8H-1, 119–121 cm), the REE chondrite-normalized patterns and abundances confirm the inferences from the major element geochemistry and show MORB-like schist protoliths resembling those of basalts and gabbros from mid-ocean-ridge (MOR) settings. The generally mafic nature of the schist protoliths is also supported by their very low Th/Nd ratios (average = 0.054) and their very high Nb/Th ratios (average = 11.2) (Th, Nd, and Nb abundances are listed in Table T7; average Mariana Trench sediments have Th/Nd = 0.12 and Nb/Th = 4.2) (Plank and Langmuir, 1998).