We performed chemical analyses of six harzburgites, three dunites, and two serpentinite mud samples from Site 1274 selected by the shipboard scientific party, by inductively coupled plasma–atomic emission spectrometry (ICP-AES) for determining major and trace element concentrations, and gas chromatography for H2O, CO2, and S. These samples are representative of the rocks recovered from Hole 1274A (see "Igneous and Mantle Petrology" and "Metamorphic Petrology" for characterization of the lithologic units). The harzburgites and dunites are from the different lithologic units. However, the two mud samples are both from Unit II. The results for the major and trace elements are reported on a volatile-free basis in Table T5.
The Site 1274 peridotites are relatively fresh compared to most other peridotites sampled during Leg 209. This is particularly true for samples from the upper section of the drill core at Site 1274 (Cores 209-1274A-3R to 7R) (see "Site 1274 Thin Sections" and "Metamorphic Petrology"). We note that two samples have the lowest H2O contents in peridotites from Leg 209 (Sample 209-1274A-3R-1, 87–89 cm, with 11 wt%, and Sample 7R-1, 48–54 cm, with 12 wt%). However, some of the Site 1274 peridotites have H2O contents of ~15 wt%, indicating that they are highly serpentinized (Fig. F49). The mud samples have elevated sulfur contents of 672 and 3268 ppm, compared to both the harzburgites and dunites which are below detection levels (<350 ppm). We note that one dunite from Unit II (Sample 209-1274A-20R-1, 141–144 cm) also contains 638 ppm sulfur, which may be related to the large proportion of black veins in this sample (see "Metamorphic Petrology"). Mud Sample 209-1274A-23R-2, 67–70 cm, also has a high sulfur content (>3200 ppm).
Site 1274 peridotite bulk rock compositions plot in a restricted range of compared to peridotites from previous Leg 209 sites. They are divided into three groups using thin section and visual core descriptions (see "Igneous and Mantle Petrology"): harzburgites, dunites, and mud. Major elements in the harzburgites are SiO2 (43–44 wt%), Fe2O3 (~8.5 wt%), Al2O3 (0.6–0.9 wt%), and MgO (44–46 wt%). In contrast, the dunites have lower SiO2 (40.8 wt%) and Al2O3 (0.16 wt%) and higher Fe2O3 (9–10.4 wt%) and MgO (47–48 wt%). Site 1274 peridotites are characterized by high and homogeneous Mg# (100 x molar Mg/[Mg + Fe]) (91.3%–91.6%) except for two samples (Fig. F50). Sample 209-1274A-20R-1, 141–144 cm, a dunite in Unit II, is characterized by slightly lower Mg# (90.2%). This sample has a higher Fe2O3 content (10.4 wt%) than other Site 1274 peridotites. This variation may result from melt-rock reactions involving Fe-Mg exchange between this peridotite and an olivine-saturated basaltic melt.
Site 1274 peridotites display lower Al2O3 contents than Leg 153 peridotites but are similar to Site 1268, 1270, 1271, and 1272 peridotites. In peridotites, Al2O3 is concentrated mainly in pyroxenes, so that variation in Al2O3 reflects their degree of fertility. Site 1274 harzburgites contain 0.6–0.9 wt% Al2O3, whereas the Site 1274 dunites have lower Al2O3 contents (0.3 wt%). These low values suggest that the Site 1274 peridotites were more refractory than Leg 153 peridotites.
On average, Site 1274 dunites have lower CaO (<0.06–0.45 wt%) contents than Site 1274 harzburgites (CaO = 0.4–0.7 wt%). Site 1274 peridotites have higher CaO contents than other Leg 209 peridotites (Fig. F51). The scattering of CaO relative to Al2O3 in Site 1274 harzburgites and dunites may by attributed either to the addition of small amounts of carbonates or to preferential Ca loss during serpentinization. Both Al2O3 and CaO are concentrated in pyroxenes and, more particularly, in clinopyroxene for CaO. The lower CaO concentrations observed in peridotites from previous sites has been attributed to either preferential Ca loss during serpentinization and weathering and/or the presence of only minor amounts of clinopyroxene in these rocks. However, the higher CaO content and the rough correlation observed between CaO and Al2O3 in Site 1274 peridotites suggests the presence of a larger proportion of clinopyroxene in these rocks. This is consistent with visual core description and petrographic analysis of thin sections indicating the presence of relics of a few percent clinopyroxene (see "Igneous and Mantle Petrology").
The two mud samples are similar in major element geochemistry to the harzburgites of Units I and III, apart from Sample 209-1274A-23R-2, 67–70 cm, which contains 0.15 wt% TiO2 compared to 0.01 wt% TiO2 in the harzburgite samples. In addition, this mud sample also has slightly elevated Al2O3 (1.54 wt%) and Na2O3 (0.23 wt%) and a low Mg# (90.5%) compared to the other Site 1274 peridotites. This sample is depleted in MgO when compared with Site 1274 peridotites. Its slightly lower Mg# may result from tectonic mixing of peridotite and gabbroic material, as also suggested by its slightly higher Al2O3 and TiO2 contents.
In terms of trace element geochemistry, the dunites and harzburgites are subtly different (Fig. F52, F53). Mud samples plot in the same range as harzburgites, except for Sample 209-1274A-23R-2, 67–70 cm, which displays slightly higher incompatible element contents. The trace elements in dunites are V (14–18.5 ppm), Sc (3–6 ppm), Ni (2500–2700 ppm), and Co (~123 ppm). In contrast, the harzburgites and mud samples are slightly enriched in the incompatible elements V (29–35 ppm) and Sc (7–9 ppm) but lower in Ni (2200–2500 ppm) and Co (107–116 ppm). Ni and Co are preferentially partitioned into olivine, whereas V and Sc are in clinopyroxene. Compatible trace element variations in Site 1274 peridotites mainly reflect the modal proportion of olivine and clinopyroxene. Incompatible trace element concentrations observed in Site 1274 peridotites fall in the range of the peridotites analyzed during Leg 209. Site 1274 peridotites are depleted in TiO2 (~0.01 wt%), Zr (<2–3.8 ppm), and Sr (<5 ppm) compared to Leg 153 peridotites (Fig. F53). These moderately to highly incompatible elements preferentially partition into the liquid during partial melting. Their concentration range suggests that Site 1274 peridotites underwent higher degrees of partial melting compared to Leg 153 peridotites. Variations in TiO2, Sr, and Zr in Site 1274 peridotites are within analytical error, but V and Sc are positively correlated with CaO and Al2O3. V and Sc partition preferentially into clinopyroxene, and their correlation with CaO and Al2O3, both proxies for the proportion of pyroxene present, suggests the presence of a small amount of clinopyroxene preserved in these rocks.
Loss on ignition values and volatile contents are high in Site 1274 peridotites (>10 wt%). All these rocks have been modified by alteration, predominantly to serpentine, leading to the addition of significant amounts of volatile constituents including the addition of a S-enriched component in Unit II peridotites. The Unit II mud samples analyzed from Site 1274 are compositionally the same as the peridotites from the same site, except for a slight enrichment in TiO2 and Al2O3 in one sample that can be attributed to alteration or mechanical mixing of peridotite and gabbroic material in the fault gouge.
Geochemical data on Site 1274 peridotites suggest that prior to alteration these rocks had chemical compositions similar to those of Site 1268, 1270, and 1272 peridotites, apart from higher CaO at Site 1274. In contrast to other Leg 209 peridotites, CaO variations in Site 1274 peridotites are correlated with Sc variations. Both elements are preferentially partitioned into clinopyroxene in peridotites. It should be noted that a few percent clinopyroxenes were observed in Site 1274 samples. This suggests that the apparent CaO enrichment of Site 1274 peridotites is due, in part, to the presence of less altered clinopyroxenes in these rocks compared to peridotites at Sites 1268, 1270, and 1272. At the other sites, CaO may have been mobile in peridotites during alteration. For additional discussion of CaO variation in these peridotites, particularly the possibility of CaCO3 metasomatism, please see Figures F45 and F46 with accompanying text, in the "Leg 209 Summary" chapter.
Site 1274 is the first site drilled during Leg 209 that recovered peridotite from north of the 15°20´N Fracture Zone. Previous geochemical studies on dredge samples from the Mid-Atlantic Ridge between 14° and 16°N revealed that the mantle source of basalts south of the 15°20´N Fracture Zone is geochemically "enriched" and similar to the source of hotspot-related mid-ocean-ridge basalt as observed elsewhere along the Mid-Atlantic Ridge (fig. F3 in Kelemen et al., 2002). This may be related to the fact that mantle peridotites seem to have undergone unusually high degrees of melting. North of the fracture zone, however, basalts and peridotites have compositions typical for the Mid-Atlantic Ridge away from hotspots (Fig. F4 in the "Leg 209 Summary" chapter). Our initial geochemical results on Site 1274 peridotites are not consistent with these other data. Instead, in their major and moderately incompatible element contents, Site 1274 peridotites are similar to the peridotites sampled south of the fracture zone during this leg. All Leg 209 peridotites were more refractory than Leg 153 peridotites.
The average composition of Leg 209 peridotites plots at the most depleted end of the mantle fractionation array on a MgO/SiO2 vs. Al2O3/SiO2 diagram (Fig. F54). Leg 209 peridotites display low Al2O3/SiO2 (<0.01–0.03), similar to those of the most refractory peridotites such as the Izu-Bonin Mariana forearc peridotites (Parkinson and Pearce, 1998), the fast-spreading East Pacific Rise peridotites (Niu and Hekinian, 1997), and the Oman peridotites (Godard et al., 2000). A trend toward low MgO/SiO2 values observed in Site 1268 peridotites as well as in some samples from the other sites drilled during Leg 209 is due to MgO loss and/or SiO2 addition during hydrothermal alteration and weathering. The high MgO/SiO2 ratio in some Leg 209 peridotites mainly reflects the variations of the orthopyroxene/olivine modal ratio, from harzburgites to the dunites. When samples that are most affected by alteration are excluded (Site 1268 and 1270 peridotites), Leg 209 peridotite compositions are consistent with an evolutionary trend parallel to the "mantle array," similar to that shown by Southwest Indian Ocean Ridge and Western Alps peridotites. Leg 209 peridotites represent the most refractory peridotites yet sampled at a slow-spreading ridge.