We performed chemical analyses on eight peridotites and two gabbros from Site 1271 selected by the shipboard scientific party, using inductively coupled plasma–atomic emission spectrometry (ICP-AES) for determining major and trace element concentrations and gas chromatography for H2O and CO2. These 10 samples are representative of the rocks recovered from the two holes drilled at Site 1271. In Hole 1271A we sampled three dunites. In addition, from Hole 1271B we sampled two gabbros and one dunite from Unit I, one harzburgite from Unit II, and two peridotites (one harzburgite and one dunite with intergranular gabbroic material) from Unit III (see "Hole 1271B" in "Igneous and Mantle Petrology" and "Hole 1271B" in "Metamorphic Petrology" for the characterization of the lithologic units in Hole 1271B). Also in Unit III, Hole 1271B, one sample was taken from a thin chromite layer in order to quantify the spinel composition. However, only ~20% of the sample had dissolved after the LiBO4 digestion, with most spinels remaining undissolved. Discrete amphibole and plagioclase samples were taken from a BAG from Unit III, Hole 1271B, for analysis. The results for the major and trace elements, in both serpentinized peridotites and mafic rocks and in mineral separates are reported on a volatile-free basis in Tables T4 and T5, respectively.
Bulk rock analyses of serpentinized peridotites from Site 1271 show that the composition of all of these rocks was modified to different extents by alteration, leading to the addition of variable amounts of volatile constituents to the original composition. Site 1271 peridotites are characterized by high loss on ignition (LOI) values (8.6–14.9 wt%) and high H2O contents (10.5–17.5 wt%). These high LOI values and H2O concentrations are consistent with the visual core descriptions and X-ray diffraction results (see "Metamorphic Petrology") that show that Site 1271 peridotites are altered predominantly to serpentine and, to a lesser extent, brucite (mainly associated with veins). Both of these minerals are characterized by high water content and LOI values (Fig. F58). We note that the highest LOI values (>14 wt%) are found in samples in which brucite was observed (see "Metamorphic Petrology"). Sample 209-1271B-12R-1, 134–139 cm, described as a dunite containing a small fraction of altered interstitial material between olivine pseudomorphs and interpreted as an impregnated dunite (see "Igneous and Mantle Petrology"), displays the lowest LOI and water content. This is consistent with the thin section description, which suggests that part of the primary mineral content, in particular olivine (~40%) and small relics of clinopyroxene, were preserved.
There are no systematic relationships between the volatiles (carbon, hydrogen, and sulfur) and other elements in the Site 1271 peridotites. Sulfur in the analyzed peridotites ranges 760–1330 ppm (Table T4). One sample (Sample 209-1271B-10R-1, 109–112 cm) shows a distinctly higher CO2 content (1.5 wt%) compared to the other peridotites (0.11–0.30 wt%). This particular sample is crosscut by coarse-grained carbonate veins (see "Site 1271 Thin Sections"). The presence of these veins has apparently led to elevated calcium and strontium contents as well as high CO2.
Site 1271 harzburgites contain the major element oxides SiO2 (42.5–43.2 wt%), MgO (42.5–46.5 wt%), and Al2O3 (~0.4 wt%) (Fig. F58). Site 1271 dunites display wider ranges of SiO2 (38.3–44 wt%), MgO (41.3–46.2 wt%), and Al2O3 (0.05–0.99 wt%) than Site 1271 harzburgites. We note that the highest MgO values for dunites (45.8–46.2 wt%) and harzburgites (46.5 wt%) correspond to Samples 209-1271A-4R-2, 32–37 cm, and 106–111 cm, and 209-1271B-17R-1, 107–109 cm, in which significant amounts of brucite, a secondary mineral with as much as 69 wt% MgO, were described (see "Metamorphic Petrology").
Site 1271 harzburgites and dunites are depleted in Al2O3 (0.05–0.99 wt%) and CaO (<0.04 wt%), with the exception of Sample 209-1271B-10R-1, 109–112 cm. This sample displays significantly higher CaO content (2.1 wt%), which can be attributed to the addition of carbonates, revealed by visual core description and thin section observation (see "Metamorphic Petrology") and by high CO2 contents.
The most striking characteristic of Site 1271 peridotites is their high Fe2O3 content compared to peridotites previously analyzed during Leg 209. The Site 1271 dunites display the highest Fe2O3 values (11–14.15 wt%). As a result, Site 1271 dunites are characterized by low and variable Mg# (Mg/[Mg + Fe]), in the range of 85%–89%, whereas Site 1271 harzburgites display Mg#s of 90%–92%, comparable to those of massif peridotites, ophiolite mantle, and abyssal peridotites studied previously (Fig. F59). Although the bulk rock analyses of Site 1271 peridotites show that they have been modified to some extent by alteration, these low Mg#s appear to rule out MgO enrichment associated with the formation of brucite during alteration. The low Mg#s of Site 1271 dunites may therefore reflect enrichment in Fe relative to Mg before alteration. Sample 209-1271B-12R-1, 134–139 cm, interpreted as an impregnated dunite (see "Igneous and Mantle Petrology"), displays higher SiO2 (45.9 wt%), Al2O3 (3.35 wt%), and CaO (3.5 wt%) contents and lower MgO (35.6 wt%) than other Site 1271 peridotites. These values are consistent with the presence of gabbroic material interstitial to olivine, as suggested by thin section description (see "Igneous and Mantle Petrology").
Trace element compositions of Site 1271 harzburgites, in particular Ni (2500–2950 ppm), Cr (2500–2850 ppm), and V (24.3–33 ppm), plot in the same range as peridotites previously analyzed during Leg 209 (Fig. F60). The Ni contents (2100–2850 ppm) of Site 1271 dunites also plot in the same range of values. Site 1271 dunites are characterized by highly variable Cr (50–9100 ppm) and, to a lesser extent, V (6–36 ppm), probably due to high and variable proportions of spinel present as euhedral grains as large as 3 mm (see "Igneous and Mantle Petrology"). Spinel concentrates all of the Cr and a significant amount of V in these rocks. The presence or absence of spinel could explain the scattering in Al2O3 content observed in dunites, as aluminum in spinel contributes significantly to the bulk rock aluminum budget in these highly refractory peridotites.
Sample 209-1271B-12R-1, 134–139 cm, displays slightly lower Ni concentrations (1790 ppm) than the other Site 1271 peridotites. As Ni is mainly concentrated in olivine, the low Ni content in Sample 209-1271B-12R-1, 134–139 cm, coupled with its low MgO content relative to other Site 1271 dunites, results from the lower olivine proportion in this sample. The Cr content (3560 ppm) in the same sample (209-1271B-12R-1, 134–139 cm) plots in the same range as other Site 1271 dunites. The sample is enriched in V (88 ppm) compared to Site 1271 peridotites. As in the other 1271 dunites, the high Cr content is controlled by the amount of spinel sampled. However, the high V content of Sample 209-1271B-12R-1, 134–139 cm, does not correlate with its Cr content and probably results from the addition of clinopyroxene, a mineral into which V is preferentially partitioned.
With the exception of Sample 209-1271B-12R-1, 134–139 cm, Site 1271 peridotites are depleted in TiO2 (<0.02 wt%), Y (<2 ppm), Zr (<2–5.2 ppm) and, to a lesser extent, Sc (<4–8 ppm) compared to Leg 153 peridotites. They are also depleted in Sr (<5 ppm) except for Sample 209-1271B-10R-1, 110–112 cm, which displays higher Sr content (290 ppm), as a result of the presence of carbonates that led also to high Ca and CO2 values (see above). Site 1271 peridotites plot in the same range of Al2O3, TiO2, V, and Sc concentrations as Site 1268 and 1270 peridotites (Fig. F61). These are moderately incompatible elements, preferentially partitioning into the liquid during partial melting. Their concentration range suggests that, like the Site 1268 and 1270 peridotites, Site 1271 peridotites underwent higher degrees of partial melting than Leg 153 peridotites.
Sample 209-1271B-12R-1, 134–139 cm, which is interpreted as an impregnated dunite (see "Igneous and Mantle Petrology"), displays higher TiO2 (0.09 wt%), Y (16 ppm), and Zr (35 ppm) than the other Site 1271 peridotites. These high values are consistent with the addition of small amounts of gabbroic material as indicated by thin section description (see "Igneous and Mantle Petrology").
We analyzed two gabbroic rocks from Site 1271, one of which was described as a BAG (see "Igneous and Mantle Petrology" for more details). We also analyzed plagioclase and amphibole separates from one BAG from Unit III (Sample 209-1271B-11R-1, 66–67 cm). Both the gabbro and the BAG from Site 1271 have relatively low LOI values (1.8–2.5 wt%) and H2O contents (2.3–3.5 wt%), which are similar to the gabbroic rocks analyzed from Site 1268 (Fig. F62).
Major element geochemistry of the gabbro and BAG shows that they are similar in composition for SiO2 (~48 wt%), Al2O3 (~16 wt%), and CaO (~12 wt%). However, the gabbro and BAG have very different concentrations of TiO2 and Fe2O3. The gabbro has a lower concentration of TiO2 (0.77 wt%) and higher Fe2O3 (10 wt%), whereas the BAG has higher TiO2 (3.3 wt%) and lower Fe2O3 (4.7 wt%) (Fig. F63).
Trace element geochemistry also discriminates between the gabbro and BAG. The gabbro has Cr (640 ppm), Ni (223 ppm), Sr (84 ppm), Ba (65 ppm), and low concentrations of incompatible elements. The BAG has higher Ni (598 ppm) and Ba (498 ppm) but lower Cr (69 ppm) (Fig. F64) and higher concentrations of incompatible elements such as V, Y, Zr, and Sc compared to the gabbro.
The plagioclase separate taken from a BAG (Sample 209-1271B-11R-1, 66–67 cm) was also analyzed for major and trace element geochemistry: SiO2 (46.8 wt%), Al2O3 (32.8 wt%), CaO (15.5 wt%), and Na2O (2.1 wt%). The plagioclase is close to the detection limit for most trace elements apart from Sr (687 ppm) and Ba (337 ppm). An anorthite content of 81 mol% was calculated for the plagioclase. The amphibole sample, taken from the same interval as the plagioclase, has also undergone some alteration to chlorite (see "Site 1271 Thin Sections"). The amphibole contains SiO2 (51.5 wt%), TiO2 (1.6 wt%), Al2O3 (5.5 wt%), MgO (19.9 wt%), and CaO (12.6 wt%). This composition represents a mixture of tremolite/actinolite, pargasite, and hastingite end-members, with tremolite/actinolite composing about two-thirds of the solid solution. Apart from Sr, Y, and Zr, the amphibole contains high concentrations of trace elements, for example, Ni (887 ppm), V (588 ppm), and Sc (135 ppm). This may be because the trace element composition of the sample represents the original clinopyroxene and not the secondary amphibole. Unfortunately, the plagioclase and amphibole separates that were analyzed are >28 m away from the whole-rock BAG sample and are from a separate igneous unit. Because the original mineral compositions have been altered and the mineral separates were taken at a distance from the whole-rock sample, it is not certain that the whole-rock composition should correspond to a mixture of the analyzed plagioclase and amphibole. However, the BAG sample does appear to lie along a simple mixing line between the plagioclase and amphibole compositions for most elements, apart from Fe2O3, Cr, and TiO2 (Figs. F63, F64). The differences are probably due to the presence of minor amounts of rutile and/or olivine in the BAG whole-rock sample, as seen in "Site 1271 Thin Sections" and reported in "Igneous and Mantle Petrology".
When compared to gabbros recovered from previous drill sites, the gabbro from Site 1271 lies within the major element range of Al2O3, TiO2, and Fe2O3 observed for the Site 1270 microgabbros. This is also true for the moderately incompatible trace elements V and Sc. The Site 1270 microgabbros and the Site 1271 gabbro are, in general, more enriched than the Site 1268 gabbros but less enriched than the Site 1270 oxide gabbros for those elements that are concentrated in the oxide phases (e.g., TiO2, V, and Fe2O3). The BAG also falls in the range of the Site 1270 microgabbros for most other elements, apart from Cr, Fe2O3, and TiO2. The BAG is enriched in TiO2 and depleted in Cr and Fe2O3 compared to the Site 1271 gabbro and the Site 1270 microgabbros; indeed, it is more depleted in Fe2O3 than the Site 1268 gabbros and has a comparably low Cr content. At Site 1268, low Cr contents in gabbros were interpreted to indicate that the original gabbronorite protolith was more evolved than the Leg 153 gabbros.
The composition of serpentinized peridotite and gabbroic rocks from Site 1271 was modified to some extent by alteration, leading to the addition of significant amounts of volatile constituents to the original composition and the modification of the protolith Ca and Sr content in at least one of the studied peridotites.
Compared to Site 1270 peridotites, Site 1271 peridotites display a wider range of compositions for major elements. This reflects mainly the variability of lithology sampled at Site 1271. The bulk rock composition of Site 1271 harzburgites is similar to that of Site 1270 peridotites for major and trace elements. Site 1271 dunites are distinguished from Site 1271 harzburgites and Site 1270 peridotites by Fe enrichment (Mg# = 86–89), which is interpreted as a primary feature. However, Site 1271 dunites have Ni contents in the same range as Site 1271 harzburgites and Site 1270 and 1268 peridotites. Ni partitions preferentially into olivine. As a result, fractional crystallization of olivine leads to low and variable Ni contents in olivine cumulates. Therefore, the high Ni content of Site 1271 dunites precludes a cumulate origin for these rocks despite their low Mg#. Like Site 1271 dunites, abyssal peridotites sampled at fast-spreading ridges (e.g., Niu and Hekinian, 1997) and peridotites from the mantle transition zone in ophiolites (e.g., Godard et al., 2000; Korenaga and Kelemen, 1997; Koga et al., 2001) display low Mg# and often have impregnation textures. Such impregnated dunites are thought to be the product of porous melt flow involving high melt fluxes at near-solidus conditions. In these rocks, relatively low Mg# compared to residual peridotites does not indicate a cumulate origin, but instead it is evidence for widespread melt-rock reactions between peridotite and olivine-saturated basaltic melt at shallow levels (e.g., Niu and Hekinian, 1997). These melt-rock reactions involve Fe-Mg exchange and, potentially, pyroxene dissolution and olivine precipitation (e.g., Kelemen, 1990) at high melt/rock ratios. We suggest that a similar process accounts for Site 1271 dunite composition.
As with other gabbros recovered during Leg 209, the major and trace element composition of these rocks is consistent with their primary mineralogical constitution as described in thin section descriptions. The low Cr contents of the BAG (<100 ppm), comparable to the Cr concentration in Site 1268 gabbros, indicates that the initial BAG was more evolved than the other Site 1271 gabbros. The amphibole analysis from the BAG has a major element composition close to 28 mol% pargasite and 72 mol% tremolite/actinolite with minor hastingite. Its low Al2O3 content suggests that it crystallized at relatively low temperature, suggesting a low-temperature igneous or high-temperature metamorphic origin (replacement of the initial gabbroic mineralogy). However, the trace element concentrations in the amphibole point reflect to an original clinopyroxene composition (e.g., high V and Sc).