The analyses were obtained at the microprobe laboratories at Bonn and Cologne Universities (Germany) (Tables T2, T3, T4). At Bonn, 536 individual analyses were performed using a Cameca Camebax Microbeam under operating conditions of 15 kV and 15 nA beam current and a beam diameter of 1.5–2 µm. Counting times were 40 s for Cl (detection limit = 0.01 wt%) and 20 s for all other elements analyzed (Na, Mg, Al, Si, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Zn). Data were corrected using the ZAF correction (Z = atomic number, A = adsorption, and F = fluorescence) from Pouchou and Pichoir (1984).
A JOEL EMP8900 microprobe was used for 364 individual analyses at Cologne. The operating conditions for olivines were 20 kV and 50 nA. All other minerals were analyzed applying 15 kV and 20 nA (beam diameter focused to ~1.5–2 µm). Counting times were 10 s for Na and K, 50 s for Ti, 50 s for Ni in olivine, and 20 s for the other elements. The ZAF data correction was also applied.
The standards used in Bonn were jadeite-diopsideSS (Na, Ca, and Si), sanidine (K), corundum (Al), periclase (Mg), rutile (Ti), apatite (Cl), sphalerite (Zn), and elemental Fe, Cr, Mn, Ni, and Cu. In Cologne, diopside (Mg, Ca, and Si), Cr2O3 (Cr), NiO (Ni), FeO (Fe), corundum (Al), orthoclase (K), albite (Na), rutile (Ti), and rhodonite (Mn) were employed as standard materials. The mineral formulas calculated are based on the following oxygen bases: olivine, 4; pyroxene, 6; spinel, 4; serpentine, 7; talc, 22; and iowaite, 5. The sulfides analyses were recalculated based on four cations.
Many samples show a comparatively low degree of alteration (<80 vol% alteration minerals), and variable proportions of orthopyroxene, clinopyroxene, olivine, and spinel are present in samples of altered harzburgite and dunite. Orthopyroxene and olivine are altered to serpentine, and brucite (after olivine) is locally present. Brucite was also detected in Hole 1274A by shipboard X-ray diffraction (XRD) measurements (Shipboard Scientific Party, 2004).
Clinopyroxene compositions were determined in 12 samples and compositional variation is limited (average = En51Fs3Wo45). The clinopyroxenes are diopsides with low FeO (1.7–2.9 wt%) and a limited range in MgO (17.0–21.3 wt%) (Fig. F1A) content and Mg# (92.3–95.1) (Fig. F1B). Orthopyroxenes are enstatitic in composition with somewhat higher contents of FeO (4.6–5.8 wt%) (Fig. F1A) and lower Mg# (91.2–93.0) (Fig. F1B) than clinopyroxene. The olivines are forsterite rich with 7.3–9.0 wt% FeO and 48.2–52.5 wt% MgO (Fig. F1A). Olivines in harzburgites tend to lower FeO contents and higher Mg# than olivines in dunite (Fig. F1A, F1B). In Figure F1C, NiO from olivines, orthopyroxenes, and clinopyroxenes is plotted against Cr2O3. NiO contents in olivines range between 0.35 and 0.41 wt% and correlate with low Cr2O3 content (up to 0.12 wt%). In contrast, orthopyroxenes and clinopyroxenes show higher Cr2O3 (0.6–1.9 wt%) and lower NiO (up to 0.16 wt%) contents.
Serpentines show considerable compositional range, which is in part related to variability in precursor mineral composition. In general, serpentine pseudomorphs after orthopyroxene have lower MgO contents (25.8–38.3 wt%) than serpentines formed after olivine (MgO = 34.8–49.8 wt%); however, some overlap is apparent (Fig. F1D). In a similar fashion, most of the serpentine formed after orthopyroxene has considerably higher Al2O3 and Cr2O3 concentrations than serpentine formed after olivine (Fig. F1E). However, many of the analyses of serpentine replacing olivine have higher Mg contents than what could be accommodated in the serpentine mineral structure (Fig. F1F). These are most likely mixed analyses of finely intergrown serpentine and brucite formed during hydration reactions of olivine under low aSiO2 conditions (Bach et al., 2006). In a similar fashion, some serpentine analyses with particularly elevated Cl concentrations (0.7–1.6 wt%) (Fig. F1G) could be attributable to finely intergrown iowaite.
Analyses of magnetite show that magnetites are close to the end-member composition (FeOtot = 76.6–93.3 wt%). The spinels show a negative correlation of Cr# (41.4–57.8) and Mg# (48.5–70.3) (Fig. F2A). Variations in FeOtot (12.7–20.7 wt%) and MgO (10.9–17.0 wt%) concentrations are correlated (Fig. F2B).
Serpentinization is generally complete in Hole 1272A; however, relics of olivine and orthopyroxene could be analyzed in six samples. A total of 21 analyses of olivine and 9 of orthopyroxene show similar compositional characteristics to olivine and orthopyroxene in Hole 1274A. Enstatitic orthopyroxene (En86 to En90) contains ~5 wt% FeO and 1.5–2.6 wt% Al2O3, and the forsteritic olivine (Mg# = 91–93) contains 0.36–0.4 wt% NiO (Fig. F3A, F3B, F3C).
The serpentines pseudomorphing orthopyroxene and serpentine after olivine are similar in MgO and FeO contents (Fig. F3D). Only a few analyses of serpentine after olivine with elevated Mg + Fe contents could be interpreted as an indication of intergrown brucite (Fig. F3F). However, there are systematic differences in Cr and Al contents (Fig. F3D, F3E). Serpentine formed after orthopyroxene has considerably higher Al2O3 and Cr2O3 concentrations than serpentine formed after olivine.
The serpentinites of Hole 1272A are notable for the occurrence of the Cl-rich mineral iowaite (Shipboard Scientific Party, 2004). The presence of iowaite is reflected in the high Cl content of serpentine analyses, ranging mainly from 0.2 to 3.0 wt% (Fig. F3G), whereas the Cl content in serpentines from Hole 1274A, ranges between 0.01 and 1.24 wt% (Fig. F1G). The iowaite mix analyses show wide variations in Cl (0.6–6.2 wt%) and MgO (26–46.3 wt%) contents (Fig. F3H).
The few measured spinels are Cr rich (41.3–42.9 wt%) and correlate in FeOtot and MgO contents (Fig. F2B).
Peridotite in Hole 1268A is completely altered, including serpentinite and static talc alteration following serpentinization (Bach et al., 2004). Here, fresh pyroxene or olivine are absent and the only primary mineral is spinel.
Serpentine is often intimately associated with talc so that some analyses represent mixtures of these two compositions. However, differences in SiO2 content clearly separate the bulk of the data (Fig. F4A). Serpentines formed after orthopyroxene generally contain higher concentrations of FeOtot, Al2O3, and Cr2O3 and lower concentrations of MgO than serpentine formed after olivine (Fig. F4B, F4C). However, these differences, related to the primary precursor mineral, are apparently obliterated during talc alteration (Fig. F4B, F4C).
Consistent with results from XRD analyses (Shipboard Scientific Party, 2004), the microprobe data provide no indication for the presence of brucite in the serpentinites of Hole 1268A (Fig. F4D). This lack of brucite is consistent with the interpretation that serpentinization reached a mature stage in Hole 1268A (Bach et al., 2004). The serpentines show comparatively low chlorine contents (generally well below 0.4 wt%) (Fig. F4E) compared to serpentine analyses of Holes 1272A and 1274A. Talc is also poor in chlorine (<0.05 wt%), and most analyses are close to or below the detection limit.
Spinel is the only primary mineral in Hole 1268A and it has a chromitic composition. Overall, the data occupy the FeOtot and Cr2O3-rich end of the compositional spectrum compared to the spinel in Hole 1274A, but they show a positive correlation and lower Cr# (44.9–55.8) and Mg# (45.4–63.4) than found in Hole 1274A (Fig. F2A). The analyzed magnetites show near end-member compositions with high FeO (88.6–91.6 wt%) and low Al2O3 (up to 0.21 wt%) contents.
Sulfide minerals were analyzed in 11 samples of Hole 1268A, and the data show that pyrite and pentlandite dominate. The pyrites have Fe as the single cation and an Fe/S ratio of ~0.5. The pentlandite shows high Ni contents and a (Ni + Fe)/S ratio of ~1 (Fig. F4F, F4G). In one sample (209-1268A-3R-1, 29–38 cm) a chalcopyrite crystal was analyzed that contains about equal proportions of Cu and Fe.