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doi:10.2204/iodp.proc.310.202.2009

Results

Save for the highly calcareous volcanic sand Sample 310-M0021B-18R-1, 55–58 cm, anhydrous bulk rock analyses mainly plot in the basalt field (Table T1; Fig. F1) and overlap with the magmatic lineages identified in Tahiti (e.g., Duncan et al., 1987; Cheng et al., 1993). The majority of the volcaniclastic sediments are alkalic basalts that belong to the Series B magmatic lineage (0.6–1.3 Ma). It is important to note, however, that the samples are variably altered and therefore have low total weights of all major oxides (see the “Expedition 310 summary” chapter). Indeed, the samples have a large range of LOI values (Table T1), which are generally observed to positively correlate with degrees of alteration. However, although the fine-grained volcaniclastics unsurprisingly have higher LOI values (6–37 wt%) than the coarse-grained volcaniclasts (2–10 wt%), there appears to be no systematic compositional difference between the two groups in Figure F1 except that the fine-grained volcaniclastics span a larger range of SiO₂ values. Moreover, the anhydrous oxides of the samples generally show coherent trends when plotted against MgO or TiO₂ contents (not shown). Therefore, although samples with high LOI values are most probably the most altered, it is difficult to constrain the effect of alteration on their major element composition based on LOI values alone. It is only after looking at their trace element contents that a better picture of the alteration effect on the samples emerged, as described below.

Volcaniclastic sediments have variable trace element contents, but as a whole they are generally enriched in highly incompatible relative to less incompatible trace elements (Fig. F2) as indicated by their La/Sm_N values of >1 (Table T1). Some of the samples, though, have uncharacteristically low concentrations of some incompatible trace elements such as Rb, Th, and REE, particularly the heavy ones, some of which are below the detection limit of the ICP-MS used (indicated by dashes in Table T1). These samples also have a saw-toothed pattern in spider diagrams (not shown). This trace element characteristic is shown by both fine- and coarse-grained volcaniclastics and is most probably due to alteration, but available LOI data again show that coarse-grained volcaniclasts with unusually low incompatible trace element contents do not systematically have higher LOI values than volcaniclasts with higher incompatible trace element contents. Among the fine-grained volcaniclastics, however, the low incompatible trace element content is definitely due to the dilution of the volcaniclasts by calcareous components as illustrated by the aforementioned highly calcareous sand Sample 310-M0021B-18R-1, 55–58 cm. More importantly, most of the incompatible trace elements generally correlate inversely with inorganic carbon contents, except Sr, which generally shows positive correlation (not shown). New data for the less altered samples confirm the general increase in highly incompatible elements with depth in Hole M0008A (Fig. F3; see the “Tiarei marginal sites: Sites M0008, M0010–M0014, and M0022” chapter).

One of the objectives of carbon content analysis (Table T1) is for possible future paleoenvironmental reconstructions such as carbon storage on exposed shelves during the LGM (e.g., Sifeddine et al., 2004; Montenegro et al., 2006). In Tahiti, the presence of a shoreline is suggested by a gray to orange color transition in the volcaniclastic sediments in Sample 310-M0008A-8R-1, 100 cm (see the “Tiarei marginal sites: Sites M0008, M0010–M0014, and M0022” chapter). A deep brown paleosoil horizon containing fine plant roots occurs directly below the color transition. In Hole M0008A, total carbon contents of the fine-grained volcaniclastics range from ~0.1 to 1.4 wt% above the transition and are <0.1 wt% below it (Table T1). Thus it would be difficult to investigate in detail the elemental and isotopic composition of organic carbon in the purported shoreline (Fig. F3). Another, though less distinct, gray–orange color transition occurs in the bindstone unit in Section 310-M0021B-19R-1 (see the “Tiarei outer ridge: Sites M0009, M0021, and M0024–M0026” chapter), which suggests another possible shoreline farther offshore in the Tiarei area. Here total carbon contents of the fine-grained volcaniclastics range from 6.0 to 11.5 wt% above the color transition and from 3.8 to 4.2 wt% below it (Table T1). However, organic carbon contents of the brown fine-grained volcaniclasts are only ≤0.1 wt%, and thus it would also be difficult to investigate the elemental and isotopic composition of organic carbon in the volcaniclastic sediments below the color transition at this site.

The four coarse-grained volcaniclasts analyzed by the ⁴⁰Ar/³⁹Ar dating method range in age from 0.3 to 0.7 Ma (Table T2). Although the two boulders in Hole M0008A have the same age, they are relatively old (~0.7 Ma) to be a lava flow that covered the LGM shoreline. Moreover, although one of the boulders is highly altered (Table T1), available data suggest that they may have come from two different lava flows (see the “Tiarei marginal sites: Sites M0008, M0010–M0014, and M0022” chapter). In general, the ages of the samples combined with major element data (Fig. F1) suggest that the bulk of the volcaniclastic sediments drilled during Expedition 310 were derived from the younger (≤1.3 Ma) rock series in Tahiti.

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