We performed chemical analyses on ten harzburgites, two dunites, two basalts, a diabase, and two gabbronorites from Site 1272 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 samples are representative of the rocks recovered from Hole 1272A (see "Lithology and Stratigraphy" and "Lithologic Characterization" in "Igneous and Mantle Petrology" and "Metamorphic Petrology" for characterization of the lithologic units). The mafic samples, one harzburgite and one dunite, were sampled from Unit I. The remaining peridotites were sampled from Unit II. The results for the major and trace elements, for both ultramafic and mafic rocks, are reported on a volatile-free basis in Table T4.
Site 1272 peridotites are characterized by high volatile contents and loss on ignition (LOI) values (Fig. F53). Some of these peridotites display the highest LOI values measured during Leg 209. These high LOI values and H2O concentrations are consistent with the visual core descriptions and XRD results (see "Metamorphic Petrology") that show that Site 1272 harzburgites and dunites are completely altered to serpentine with minor brucite in some intervals. It should be noted that some sections of core were characterized by unusually soft, clayey serpentinites and serpentine mud (Sample 209-1272A-12R-1, 73–76 cm). Two samples from Unit I, a harzburgite (Sample 209-1272A-1R-1, 7–11 cm) and a dunite (Sample 2R-1, 61–63 cm) have the highest LOI (17.9 and 18.9 wt%, respectively) and H2O contents (14.9 and 14.0 wt%) among the Site 1272 peridotites. These samples also have the highest CO2 contents measured during Leg 209 (7.3 and 9.3 wt%, respectively). These high CO2 contents are consistent with the visual core descriptions and XRD results (see "Metamorphic Petrology"), indicating that aragonite veins are present as part of a low-temperature alteration assemblage in the Unit I peridotites. This is in addition to the higher-temperature serpentinization that is also observed in these peridotites. The high CO2 contents are correlated with high CaO (Fig. F54), suggesting metasomatic addition of CaCO3 (also see Figs. F45 and F46 with accompanying text in the "Leg 209 Summary" chapter).
Site 1272 peridotite bulk rock compositions plot in a restricted range of values compared to Site 1271 peridotites (Fig. F53). Site 1272 Unit II peridotites, on average, display higher SiO2 (42.5–43.2 wt%) and MgO (42.5–46.5 wt%) than the two Site 1272 Unit I peridotites, which are characterized by SiO2 (~40.3 wt%) and MgO (38.7–40.5 wt%). These variations in MgO and SiO2 content in the peridotite are not correlated with a difference in Fe2O3 concentration between Units I and II. The low Mg content in the Unit I peridotites might reflect a small amount of Mg loss that may occur during low-temperature alteration (e.g., Snow and Dick, 1995). It should be noted that despite the possible Mg loss, Site 1272 peridotites are characterized by high Mg# (100 x molar Mg/[Mg + Fe]) (90.3%–92.2%); the lowest values (90.3% and 91.3%) were found for the two samples from Unit I.
Site 1272 peridotites display lower Al2O3 contents than Leg 153 peridotites but are similar to Site 1268, 1270, and 1271 peridotites. In peridotites, most of the bulk rock Al2O3 content is concentrated in pyroxenes, and variations in Al2O3 content reflect the degree of fertility of peridotites. Site 1272 harzburgites have higher Al2O3 contents (0.57–0.78 wt%), whereas the Site 1272 dunites have lower Al2O3 contents (0.12 wt%). These low values suggest that prior to alteration Site 1272 peridotites were more refractory than Leg 153 peridotites. Except for the two Unit I peridotites, Site 1272 peridotites display low CaO contents (<0.03–0.19 wt%). CaO in peridotites is concentrated in clinopyroxene. The low and scattered CaO contents observed in Site 1272 peridotites suggests either preferential Ca loss during serpentinization and weathering and/or the presence of only minor amounts of clinopyroxene in these rocks. In contrast, the two samples from Site 1272 Unit I, a harzburgite (Sample 209-1272A-1R-1, 7–11 cm) and a dunite (Sample 2R-1, 61–63 cm), have high CaO values (9.65–12.2 wt%). These samples also have the highest CO2 and Sr (1790–2150 ppm) contents measured in Leg 209 peridotites. These values are consistent with the visual core descriptions and XRD results (see "Metamorphic Petrology"), indicating that aragonite veins are present as part of the low-temperature alteration assemblage in Unit I peridotites.
The bulk rock composition of Sample 209-1272A-12R-1, 73–76 cm, a serpentinite mud, is similar to that of Site 1272 harzburgites, except for slight Na2O enrichment (0.18 wt%). This higher Na2O content suggests possible interaction with seawater or incorporation of gabbroic material during cataclastic deformation.
The trace element concentrations measured in Site 1272 peridotites fall in the range of the other peridotites analyzed during Leg 209. Cr and Ni behave as compatible elements, preferentially partitioning into the solid during partial melting. Site 1272 harzburgite Ni contents plot in the same range as those of Site 1268, 1270, and 1271 peridotites (2400–2900 ppm). Site 1272 dunites display slightly higher Ni concentrations (3000–3300 ppm), similar to the most Ni-enriched Site 1268 peridotites. The high Ni concentrations coupled with low Al2O3 contents (<1 wt%) in the Site 1272 peridotites suggests that olivine controls the Ni concentration. Cr content in Site 1272 peridotites (720–4885 ppm) is positively correlated with Al2O3 and plots in the same field as Site 1268 and 1270 peridotites except for the two Unit I peridotite samples (Fig. F55). Because nearly all of the Cr must be concentrated in spinel in these rocks, the correlation of Cr with Al2O3 suggests that spinel contributes a significant proportion to the bulk rock Al budget for the highly refractory, pyroxene-poor Site 1272 peridotites. The two Unit I peridotites display the highest Cr values (3400–4890 ppm) among Leg 209 peridotites. The lowest Cr is in the sample with the most depleted Al2O3 (Sample 209-1272A-2R-1, 61–63 cm; a dunite). Yet there are no variations in Al2O3 bulk rock content between Unit I and II dunites or between Unit I and II harzburgites. This difference in Cr content between Unit I and II peridotites may correspond to a variation in the spinel Cr/Al ratio.
Site 1272 peridotites are depleted in TiO2 (<0.01–0.02 wt%), Zr (1.2–4.8 ppm), and Sr (<2 ppm except for the two Unit I aragonite-rich samples) and, to a lesser extent, in Sc (5–9 ppm) and V (12–50 ppm) compared to Leg 153 peridotites (Fig. F56). These moderately to highly incompatible elements preferentially partition into the liquid during partial melting. Their concentration range suggests that Site 1272 peridotites underwent higher degrees of partial melting than Leg 153 peridotites. Variations in TiO2, Sr, and Zr in Site 1272 peridotites are within analytical error, but V and Sc are positively correlated with Al2O3. The dunites have lower concentrations of V (12 ppm) and Sc (4–5 ppm) compared to the harzburgites (20–50 and 6–9 ppm, respectively). V and Sc partition preferentially into clinopyroxene, and their correlation with Al2O3 content, a proxy for pyroxene content, suggests the presence of some minor amounts of clinopyroxene in these rocks before alteration. Alternatively, it should be noted that V and Sc may also be concentrated in spinel and magnetite. Therefore, as for Al2O3, spinel may also contribute to a significant amount to the bulk rock V and Sc budget for the highly refractory, pyroxene-poor Site 1272 peridotites.
The recovered mafic rocks, described as gabbronorite, diabase, and basalt, may be a single lithologic unit (termed diabase; Unit I) based on visual core and thin section descriptions (see "Igneous and Mantle Petrology"). We selected two gabbronorites, a diabase, and two basalts from this unit. Whereas these mafic rocks represent a range of magmas that may have cooled at different rates, each displays textural characteristics in thin section that suggest they cooled relatively quickly. This is especially true of Sample 209-1272A-7R-1, 109–113 cm, which comprises a matrix of devitrified glass containing plagioclase laths and needles with swallow-tail texture and a minor amount of olivine microphenocrysts, some of which display "hopperlike" texture (see "Igneous and Mantle Petrology"). Quench clinopyroxene and plagioclase are also present in the groundmass of this sample (see "Metamorphic Petrology;" Table T2).
The Site 1272 mafic rocks appear to form a coherent suite based on their major and trace element composition: MgO (6.1–8.8 wt%), Fe2O3 (9.5–10.3 wt%), TiO2 (1.2–1.8 wt%), K2O (0.15–0.64 wt%), Na2O (2.6–3.2 wt%), Al2O3 (15.0–15.7 wt%), and CaO (10.9–11.6 wt%) (Fig. F57). The two gabbronorites and the diabase collectively span the observed compositional range, whereas the two basalts analyzed are similar to each other and to the diabase: MgO (7.4 wt%), total alkalis (Na2O + K2O = 2.8–3.6 wt%), and SiO2 (50.3–52.6 wt%). All analyzed samples have tholeiitic basalt compositions.
TiO2 and K2O are anticorrelated with MgO in these rocks. TiO2 varies by a factor of ~1.5, whereas K2O varies by a factor of 4. In contrast, Fe2O3, Al2O3, and CaO are relatively constant, as is the CaO/Al2O3 ratio. Total iron, calculated as FeO, is ~9 wt% at 8 wt% MgO and the corresponding value for Na2O is ~2.6 wt% (Fig. F57). The Site 1272 mafic rocks generally show overlap with the compositions of basalts from the South Kane Fracture Zone (MARK) area (22°30´–22°50´N) (Meurer et al., 2001), although the Site 1272 rocks appear to be somewhat lower in TiO2 and Na2O (Fig. F57) (also see Fig. F39 and accompanying text in the "Leg 209 Summary" chapter).
There is a negative correlation of H2O with K2O in the mafic rocks analyzed from Site 1272 (Fig. F58). This is somewhat unusual because mid-ocean-ridge basalts usually show a positive correlation between potassium and water content (Michael, 1995). The high H2O contents of these rocks are probably a result of hydrothermal alteration. The negative trend seen in Figure F58 suggests the possibility of some K loss during alteration, which may suggest elevated temperatures (>150°C) of water-rock interaction, consistent with the presence of chlorite in some samples. Nonetheless, in comparison to the South MARK basalts, the Site 1272 mafic rocks are not obviously deficient in K (Fig. F57).
Site 1272 mafic rocks are depleted in Ni (<165–187 ppm), although they display slightly higher Ni content than the South MARK basalts (Meurer et al., 2001) with similar MgO content. Cr (41–338 ppm) shows a wide range of concentrations. Cr content is correlated with MgO in the Site 1270 mafic rocks and overlaps with the range of basalt data from the South MARK area (Meurer et al., 2001) (Fig. F59). Once again, Sample 209-1272A-4R-1, 82–85 cm, with the lowest MgO and Ni, is well outside the South MARK basalt range and is highly depleted in Cr (<41 ppm).
Incompatible trace element variations in the Site 1272 mafic rocks appear to be largely related to changes in MgO (Fig. F60), as observed previously for basalts from the South MARK area (Meurer et al., 2001). Sr and Zr in the Site 1272 mafic rocks each show a factor of 1.7 variation, similar to what is observed for Ti (1.5). In contrast, Ba varies by a factor of 7 (15–115 ppm) and is not simply related to MgO and Y content varies by only 20% (24–29 ppm).
Geochemical data on Site 1272 peridotites suggest that prior to alteration these rocks had chemical compositions similar to those of Site 1268 and 1270 peridotites and were more refractory than Leg 153 peridotites.
LOI values and volatile contents are particularly high in Site 1272 peridotites. All these rocks have been modified by alteration, predominantly to serpentine, leading to the addition of significant amounts of volatile constituents. In spite of their high volatile content, the major and trace element compositions show little evidence of compositional variation resulting from alteration, except for the two samples from Unit I and one harzburgite from Unit II, which was altered to a serpentinite mud. The two samples from Unit I show evidence of weathering and addition of carbonate (aragonite), which led to a strong enrichment in CaO, Sr, and CO2 compared to other peridotites analyzed from Site 1272. The serpentinite mud, a former harzburgite, is slightly Na enriched compared to other Site 1272 harzburgites.
With the exception of the above-mentioned chemical changes attributed to alteration in some samples, the composition of Site 1272 harzburgites is remarkably homogeneous. Only a slight variation in Cr content between Unit and Unit II peridotites with similar Al2O3 content suggests a significant difference in composition between the two units, maybe associated with variations in the spinel Cr/Al ratio. Differences in Mg# between Unit I peridotites (90.3%–91.3%) and Unit II peridotites (91.6%–92.2%) could reflect differences in original composition or, possibly, Mg loss during seafloor weathering of the two Unit I samples.
It is most reasonable to view the chemical results for the mafic rocks as defining three groups rather than a continuum: (1) one gabbronorite with high MgO (Sample 209-1272A-1R-1, 42–45 cm), (2) the diabase from Core 3R and the two basalt samples from Core 7R with intermediate MgO, and (3) a second gabbronorite with low MgO (Sample 4R-1, 82–85 cm). Although they consist of abundant plagioclase and variable amounts of pyroxene with lesser amounts of olivine (see "Site 1272 Visual Core Descriptions"), their compositions do not show appreciable effects of accumulation of either of these phases (Fig. F57). For example, plagioclase accumulation would lead to elevated Al2O3 at lower MgO, whereas olivine accumulation would lead to high MgO at low CaO. Instead, the mafic rocks approximate trends expected for fractional crystallization of olivine and plagioclase from an ocean-ridge basaltic magma.
The Site 1272 mafic rocks are chemically similar to basalts from elsewhere along the Mid-Atlantic Ridge. The major element variation indicates that crystal fractionation played an important role in producing their chemical composition. However, certain features indicate that these mafic rocks are not simply related by crystal fractionation from a common parental magma. Most significantly, incompatible elements show variable enrichment trends with decreasing MgO. The most incompatible elements, such as Ba and K, display a large range in concentration (enrichment factors = 7 and 4, respectively), whereas moderately incompatible elements such as Ti and Zr show a smaller variation (enrichment factors = 1.5–1.7). In our samples, Ba and K may have been affected by alteration and weathering. However, Meurer et al. (2001) found that basalt glasses from South MARK displayed similar variation, with overenrichment in the highly incompatible elements but underenrichment in moderately incompatible elements, relative to what could be produced by crystal fractionation. If Ba and K concentrations in the Site 1272 mafic rocks reflect original magmatic compositions, their variable enrichment might be due to a varying mantle source composition or to variation in the extent of mixing with small volumes of highly fractionated magma. The extremely low Cr and Ni contents of Sample 209-1272A-4R-1, 82–85 cm, suggest that if mixing with evolved magma plays a role, the end-member could be similar in composition to this gabbronorite.
The dispersion in chemical composition of Fe and Al with decreasing MgO is also not readily explained by crystal fractionation at a single depth. Crystallization of olivine and plagioclase at a fixed pressure would drive samples to higher FeO and lower Al2O3 with decreasing MgO. This is not observed in the Site 1272 mafic rocks that were analyzed. Their dispersion in major element oxides such as FeO and Al2O3 suggests that some of the chemical variability arises from polybaric crystal fractionation. This is also consistent with the detailed modeling of the South MARK basalts (Meurer et al., 2001), where it was demonstrated that crystallization over a range of pressures (ranging to 8 kbar, or 25 km depth below the seafloor) adequately accounts for the major element variability observed in lavas from that region. Similar conditions of magmatic differentiation may be present in the Site 1272 region.