GEOCHEMISTRY

We performed chemical analyses on 8 troctolites, 5 diabases, and 20 gabbros from Site 1275 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, CO₂, and sulfur. These 33 samples are representative of the rocks recovered from Holes 1275B and 1275D (see "Igneous and Mantle Petrology" and "Metamorphic Petrology" for the characterization of the lithologic units).

We sampled 4 diabases and 14 gabbros (see "Igneous and Mantle Petrology") from Units I and III in Hole 1275B. We also sampled 3 troctolites from Unit II. From Hole 1275D, we sampled 4 troctolites and 1 gabbro from Unit I, 3 gabbros from Unit II, 1 troctolite and 1 gabbro from Unit III, and 1 diabase and 1 granophyre from Unit IV. The lower part of Hole 1275D was not sampled because of time constraints for sample preparation and analyses. The results for the major and trace elements, for both ultramafic and mafic rocks, are reported on a volatile-free basis in Table T4.

Bulk rock analyses of troctolites from Site 1275 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. Site 1275 troctolites are characterized by high loss on ignition (LOI) values (9.7–15.9 wt%) and high H₂O concentrations (6.8–12.5 wt%). These high LOI values and H₂O concentrations are consistent with the visual core descriptions and X-ray diffraction results (see "Metamorphic Petrology"), which show that Site 1275 troctolites are altered predominantly to serpentine, a mineral that has a high water content (Fig. F69). We note that the highest LOI values (>12 wt%) are found in samples characterized by high CO₂ concentrations (1.1–9 wt%). In Site 1275 troctolites, variations in CO₂ are correlated with variations in CaO (Fig. F70). Hence, Sample 209-1275D-6R-1, 89–91 cm, displays the highest CO₂ content (9 wt%) and the highest CaO content (13.30 wt%) among the troctolites. This is consistent with core and thin section descriptions and XRD results that indicate variable but generally high proportions of calcite veins in these samples (see "Metamorphic Petrology"). The two other samples with CO₂ of ~3 wt% (Samples 209-1275B-6R-1, 61–64 cm, and 209-1275D-10R-2, 104–106 cm) also contain trace amounts of calcite (see XRD results in "Metamorphic Petrology;" Table T2). It is notable that the high CaO contents in these samples are consistent with high Al₂O₃ and with the high modal proportion of plagioclase in the troctolites. Thus, addition of CO₂ to the troctolites was probably not accompanied by metasomatic enrichment in CaO. In this way, carbonate metasomatism of Site 1275 troctolites probably differed from CaCO₃ addition inferred for many peridotites at other sites.

Site 1275 troctolites are characterized major element oxides SiO₂ (40.4–45.1 wt%), Fe₂O₃ (9.3–12.5 wt%), Al₂O₃ (1.3–5.1 wt%), TiO₂ (<1 wt%), and MgO (31.2–40.7 wt%) (Fig. F69). Site 1275 troctolites have Mg# (molar Mg/[Mg + Fe]) of 87%–89%. These values are high compared to those of cumulate rocks observed previously during Leg 209 (Fig. F71). Although they are lower in Mg# than average Leg 209 harzburgites (90%–92%), they are similar to Mg#s in dunite and peridotite with gabbroic impregnation from Site 1271 (85%–89%). However, compared to Site 1271 peridotites, Site 1275 troctolites have a lower MgO content, similar to fertile lherzolite values. They also have FeO contents that are higher than those commonly observed in mantle peridotites (Fig. F71). Such compositions may indicate that the Site 1275 troctolites are cumulate rocks. However, they also could have formed as a product of reaction between peridotite and a basaltic melt, as proposed for Site 1271 dunites.

Site 1275 troctolites contain high concentrations of compatible trace elements such as Cr (3390–4440 ppm) and Ni (1596–2048 ppm) and low concentrations of incompatible trace elements such as V (<70 ppm), Y (<6 ppm), Zr (<21 ppm), and Sc (<12 ppm) compared to Site 1275 gabbros. The troctolites have higher incompatible elements than most other Leg 209 peridotites (Figs. F72, F73), with the exception of the impregnated peridotite from Site 1271. The high Ni concentrations of the troctolites are consistent with their high olivine content, Ni being preferentially partitioned into olivine. The V and Sc concentrations in some of the troctolites can be attributed to the minor amounts of clinopyroxene. Troctolites show 1%–2% spinel in thin sections (see "Igneous and Mantle Petrology" and "Metamorphic Petrology"), which likely accounts for their high Cr concentrations. For additional discussion of troctolite compositions, please see Figure F54 and accompanying text in the "Leg 209 Summary" chapter.

We analyzed 18 gabbroic rocks from Hole 1275B and 8 from Hole 1275D. The selected samples include diabase, gabbro, gabbronorite, oxide gabbro, and granophyre. The samples can be divided into two textural groups (diabase/microgabbro and gabbro) based on the visual core and thin section descriptions (see "Igneous and Mantle Petrology," "Metamorphic Petrology," "Site 1275 Visual Core Descriptions" and "Site 1275 Thin Sections"). This division also groups the rocks in terms of their major and trace element chemistry, defining two groups of gabbroic rocks at Site 1275 with little or no overlap in the concentration of elements such as Mg, Ti, Al, Fe, Sc, V, Cr, and Ni. These chemical groups are found throughout Holes 1275B and 1275D with no apparent systematic spatial distribution.

The Site 1275 gabbroic rocks are similar to the diabase and gabbroic rocks from Sites 1270 and 1272, but they range to slightly more evolved compositions, with higher Fe₂O₃ and lower MgO (Fig. F74). The fine-grained diabase/microgabbro at Site 1275 (Group 1) contain MgO (8.3–9.5 wt%), TiO₂ (0.8–1.3 wt%), Al₂O₃ (15.0–17.6 wt%), and Fe₂O₃ (7.9–12.8 wt%). In contrast, the coarse-grained gabbroic rocks (Group 2) contain MgO (4.7–6.9 wt%), TiO₂ (2.0–8.9 wt%), Al₂O₃ (10.9–13.8 wt%), and Fe₂O₃ (11.2–22.0 wt%) (Fig. F74). The single granophyre analyzed (Sample 209-1275D-31R-2, 69–71 cm) clearly stands out in comparison to the gabbroic groups, having lower MgO (3.2 wt%) and TiO₂ (0.6 wt%) and much higher SiO₂ (69.5 wt%). In Group 2 there are two samples that were initially considered to be an intermediate group on the basis of their texture (Samples 209-1275B-3R-1, 103–107 cm, and 20R-2, 105–107 cm). These two samples are finer grained than the rest of the Group 2 rocks and also have the lowest TiO₂ and Fe₂O₃ and highest Al₂O₃. However, we include them in Group 2 because their concentrations for the other elements are similar to the Group 2 rocks. Nevertheless, their intermediate concentrations of Ti, Fe, and Al, along with their intermediate texture, suggest that grain size may be useful proxy for composition in Site 1275 gabbroic rocks.

The correlation between Fe₂O₃ and TiO₂ (Fig. F75) suggests that their concentrations are governed by the oxide abundance, with Group 1 having only a minor oxide content and therefore low concentrations of Fe₂O₃ and TiO₂. Overall, TiO₂ increases with increasing Fe₂O₃ in the gabbroic rocks from Site 1275. However, with the exception of the most iron-enriched sample (Sample 209-1275B-21R-1, 90–92 cm; Fe₂O₃ = 21.2 wt%) and the intermediate samples, the coarser-grained Group 2 rocks (those with TiO₂ > 4 wt%) show a trend of decreasing TiO₂ with increasing Fe₂O₃ (Fig. F75).

All the gabbroic rocks from Site 1275 have relatively low CO₂ (0.05–0.50 wt%) (Fig. F70; Table T4). The Group 1 rocks have slightly higher H₂O (1.3–2.3 wt%) compared to the Group 2 rocks (0.85–1.7 wt%) except for Sample 209-1275D-1R-1, 113–115 cm, which contains 2.3 wt% H₂O. Site 1275 gabbroic rocks typically contain 10%–40% secondary amphibole (see "Site 1275 Thin Sections"). Overall, there appears to be no systematic relationship between volatile species and other elements in gabbroic rocks from Site 1275.

The compatible trace elements of the gabbroic rocks show the same groupings defined by the major elements. Most Group 1 rocks have Ni and Cr concentrations of 170 ppm and 240–410 ppm, respectively. In contrast, most of the Group 2 rocks have Ni and Cr below the detection limit (Ni < 165 ppm and Cr < 41 ppm) (Table T4; Fig. F72). The moderately incompatible elements V and Sc show more complex relationships in the gabbroic rocks from Site 1275 related to the proportion of oxides and clinopyroxene. Collectively, the gabbroic rocks from Site 1275 show an increase in V content with increasing TiO₂, although there is considerable scatter (Fig. F76). In the coarsest-grained Group 2 gabbroic rocks (those having TiO₂ > 4 wt%), there is a rough negative correlation between Fe₂O₃ and V with TiO₂ (Figs. F75, F76). The Group 2 gabbroic rocks from Site 1275 have higher Ti/V ratios (20–80) compared to Group 1 rocks (25–35). Collectively, the oxide gabbros recovered from Hole 1270B plus the coarse-grained gabbroic rocks from Site 1275 delineate a broad anticorrelation between V and Ti. This anticorrelation leads to an increase in Ti/V ratio from ~10 to ~80 as TiO₂ increases from ~2.5 to ~7.0 wt% in these rock suites (Fig. F76).

Sc shows a good anticorrelation with Al₂O₃ in the Site 1275 gabbroic rocks (Fig. F77). There is no overlap in Sc and Al₂O₃ contents in Group 1 and Group 2 gabbroic rocks from Site 1275, and Sc and Al₂O₃ form a similar negative trend within each of these groups. Sc also shows a rough positive covariation with TiO₂ (Fig. F78). In the Group 1 gabbroic rocks, Sc and Ti are positively correlated, varying between 32 and 38 ppm (variation = 20%), whereas TiO₂ varies between 0.8 and 1.3 wt% (variation = 60%). The narrow range and lower concentration of Sc in the Group 1 gabbros from Site 1275 also overlaps with the Sc concentrations measured for microbgabbro and diabase from Sites 1270 and 1272 (Fig. F78). In the Group 2 gabbros, Sc shows a restricted range of variation (44–65 ppm), whereas TiO₂ shows a wide range of concentrations (2–9 wt%). There also appears to be an upper limit of ~65–70 ppm for Sc concentration that is common to both the Group 2 gabbroic rocks from Site 1275 and the oxide gabbros from Hole 1270B.

Group 1 and 2 gabbroic rocks from Site 1275 overlap in their Ba, Sr, Zr, and Y contents (Ba = 20–60 ppm and Sr = 125–200 ppm). There is a covariation of Zr with Y, but with considerable scatter, and the Site 1275 gabbroic rocks form a wedge-shaped field on a diagram of Zr vs. Y (Fig. F79). The range of Zr and Y concentrations are 30–105 ppm and 15–85 ppm, respectively. Except for the covariation of Zr with Y, the Site 1275 gabbroic rocks do not show obvious systematic trends or groupings of Ba, Sr, Zr, and Y. We note that the granophyre, Sample 209-1275D-39-R-2, 69–71 cm, has an extremely high Zr concentration of ~1400 ppm.

Four types of rocks were distinguished at Site 1275 on the basis of petrographic and textural observations (see "Igneous and Mantle Petrology") and chemical composition: troctolites, two groups of gabbroic rocks, and granophyre. All groups are found in both Holes 1275B and 1275D.

The troctolites have high Mg# (87%–89%) and their composition plots close to that of peridotites, although they are slightly MgO depleted and FeO enriched in comparison. They may be cumulates or products of melt/rock reaction in the mantle. The chemical compositions of Group 1 gabbroic rocks are similar to ocean ridge basalts. In contrast, Group 2 gabbroic rock compositions from Site 1275 do not resemble liquid compositions. Their major and trace element compositions dominantly reflect their varying proportions of plagioclase, clinopyroxene, and oxides (magnetite and ilmenite).

Compatible elements such as Ni and Cr define the three mafic groups of Site 1275 rocks quite well. The olivine-rich troctolites are characterized by very high Ni and Cr contents. The Group 1 gabbroic rocks appear to have higher Ni contents than Group 2, although Ni was just above the detection limit when it was measurable in any of the gabbros. Cr contents in Group 1 gabbroic rocks range 250–410 ppm, whereas in Group 2 they are below the detection limit.

Oxide abundance controls much of the chemical systematics in the Site 1275 gabbroic rocks, evidenced by the broad overall covariations of Fe₂O₃ and V with TiO₂ (Figs. F75, F76). The Group 1 gabbroic rocks contain only a minor oxide component, and within this group both Fe and V increase with increasing TiO₂. The Group 2 gabbroic rocks show a more complicated behavior related to the presence of Ti-bearing phases. The Group 2 rocks contain some ilmenite (FeTiO₃) based on the XRD analyses (see "Metamorphic Petrology"). A few percent titanite (CaTiSiO₅) also replaces the oxide phases in some thin sections (e.g., in Sample 209-1275D-13R2, 49–51 cm, analyzed here) [N1].

Correlated increases in Fe₂O₃ and TiO₂ in cumulate rocks result from varying proportions of ilmenite and magnetite. At Site 1275, estimates of magnetite abundance based on the magnetic susceptibility measurements underestimate the amount of oxide observed in these rocks, consistent with the presence of a significant proportion of ilmenite in the more Ti-enriched rocks. The observation that most of the Group 2 gabbros lie along a Fe/Ti trend having a slope of approximately –1 on the Fe₂O₃–TiO₂ diagram (Fig. F75) may be accounted for by an increasing proportion of ilmenite (and/or titanite) at higher TiO₂ in these rocks.

The Ti/V ratio in Group 2 gabbros at Site 1275 shows a large range (10–80), in comparison, the Group 1 gabbroic rocks (25–35), somewhat higher than but close to values typically measured in mid-ocean-ridge basalt (MORB) (Shervais, 1982). As briefly discussed previously (see "Geochemistry" in the "Site 1270" chapter), the igneous behavior of V depends upon its oxidation state (+3, +4, or +5), which affects its partitioning during fractional crystallization. The crystal/liquid distribution coefficient (bulk D) for V in ocean-ridge-basaltic systems is usually greater than the bulk D for Ti, so extensive silicate fractionation can drive magma to higher Ti/V ratios (Shervais, 1982). Once magnetite becomes a crystallizing phase, Ti and V concentrations will begin to decrease rapidly and there may be a dramatic increase in Ti/V ratio because V is highly compatible in magnetite (K_D for magnetite in MORB systems ranges between ~5 and 70, depending on oxygen fugacity) (Shervais, 1982). The high Ti/V ratios at higher TiO₂ in the Site 1275 gabbroic rocks (Fig. F76) are consistent with a history of crystal fractionation that includes magnetite. The trend of decreasing V with increasing TiO₂ in Group 2 gabbroic rocks from Site 1275 may also be accounted for by an increasing proportion of ilmenite at higher TiO₂ in these rocks.

The Sc budget in the Site 1275 gabbroic rocks is primarily controlled by the abundance of clinopyroxene. Sc is negatively correlated with Al₂O₃ in both the Group 1 and Group 2 gabbroic rocks from Site 1275 (Fig. F77). The Al₂O₃ budget of the Site 1275 gabbroic rocks is primarily related to the modal abundance of plagioclase. The negative correlation between Sc and Al₂O₃ is therefore an indicator of changes in the clinopyroxene/plagioclase ratio of these rocks. When the plagioclase/pyroxene ratio is low, the bulk rock Sc content is higher and the Al₂O₃ content is lower. Together with the positive Sc–Ti covariation (Fig. F78), these relations are consistent with a simple mass balance involving corresponding increases in the proportion of clinopyroxene plus oxides as plagioclase proportion decreases. The apparent maximum Sc content of ~70 ppm in the Site 1275 gabbroic rocks corresponds to a ~45% modal abundance of clinopyroxene.

The elements Ba, Sr, Zr, and Y do not appear to correlate well with other geochemical indicators in the Site 1275 gabbroic rocks. There is a Zr–Y correlation, but it shows significant scatter (Fig. F79). The scattered diverging trends indicate that there are (at least) two controls on the Zr + Y concentrations. One control may simply be a ~1:1 enrichment in each of these incompatible elements due to increasing concentration of both elements in melts undergoing fractional crystallization. Samples lying along the lower edge of the Zr–Y trend have ~1:1 variation in Zr/Y. The gabbroic rocks lying along the upper edge of the scattered trend are enriched in Zr relative to Y. This suggests that either Zr behaves more incompatibly than Y during late-stage evolution of these rocks or that some trace amount of zircon may control the Zr concentration in some samples. Zircon is present as a trace constituent in some of the Group 2 gabbroic rocks (see "Site 1275 Thin Sections"). It is also notable that the granophyre from Hole 1275D has an extremely high Zr content of 1400 ppm, and high Zr contents were observed in many plutonic rocks during Leg 153 (Agar et al., 1997), indicating that rocks with such enriched compositions are indeed present in the oceanic crust.

The incompatible trace element concentrations measured in the Site 1275 gabbros indicate that small amounts of evolved magma, highly enriched in some trace elements, could be present within the crust along the Mid-Atlantic Ridge near 15°20´N (see similar discussion based on the Site 1270 oxide gabbros in "Geochemistry" in the "Site 1270" chapter). For example, taking bulk rock Y and Zr contents of 20 and 50 ppm, respectively (averages for the Site 1275 gabbroic rocks), assuming all the Y and Zr in the rock resides in clinopyroxene with a modal abundance of 40% and assuming appropriate K_D values for clinopyroxene (Y = 0.47 and Zr = 0.12) (Hart and Dunn, 1993), the equilibrium liquid contains Y = 106 ppm and Zr = 1040 ppm. These concentrations are similar to those measured in the granophyre from Hole 1275D and are much higher than Y and Zr concentrations observed for MORB magmas that have average values of 24 and 78 ppm, respectively (Sun and McDonough, 1989). For additional discussion of highly evolved plutonic rocks sampled along the mid-ocean ridges, see Figure F58 and accompanying text in the "Leg 209 Summary" chapter.