During Leg 176 a set of 188 whole rock samples from Hole 735B was analyzed using XRF for major element composition and for the abundances of the trace elements vanadium, chromium, nickel, copper, zinc, rubidium, strontium, yttrium, zirconium, and niobium. Sample preparation techniques and analytical procedures are outlined in "Geochemistry" in the "Explanatory Notes" chapter. Core samples for analysis generally weighed 20 to 30 g. Larger sample slabs were cut from the very coarse-grained intervals. As a rule, a thin section was prepared from a billet from the same or an adjacent core piece. The weight loss on ignition (LOI) was determined on the fraction of powder ignited to prepare the Li-borate beads for major element analysis. The trace element concentrations were determined in pressed powder pellets prepared from dried, not-ignited sample powder.
The main objective of the shipboard analysis was to document the downhole chemical variation of the major lithologies of the cored gabbro section. The majority of the samples are hence olivine gabbro, gabbro, and disseminated oxide gabbro. The sampling density was governed by the lithologic variability within a given part of the hole and by the analytical capacity of the shipboard laboratory. In principle, at least one sample representative of the main lithology was taken from each core (typically 8 to 9 m long), even when an apparently homogeneous unit spanned several cores. Seams of Fe-Ti oxide-rich gabbros and larger felsic veins were occasionally sampled to study the complete range of petrologic differentiation in the gabbro pile. However, in most cases the substantial amount of material required for the routine shipboard procedure precluded the sampling of the felsic veins. A limited number of samples were taken from core intervals that are strongly affected by high- or low-temperature alteration to gain insight into the first-order chemical effects of fluid flow and recrystallization attending alteration.
Of the 458 lithologic intervals identified by the igneous petrology group, about 140 are represented in the set of analyzed samples. Some of the thicker intervals have been sampled two to three times. However, many intervals exhibit significant lithologic variation, and a particular analyzed sample may not necessarily correspond to the dominant lithology. The reader is referred to the thin section descriptions (see the "Core Descriptions" contents list) for a complete sample characterization.
During the ship's transit to Hole 735B, the lower 50-m interval of core drilled during Leg 118 was restudied and partially resampled for chemical analysis. Five of the eight new samples from Cores 118-735B-74R through 88N are oxide gabbros. Due to time constraints, these oxide-rich intervals from Unit VI were not sampled and analyzed by the Leg 118 shipboard party. The remaining three samples are two olivine gabbros and one troctolite, the main lithologies from Unit VI. The chemical analyses of these eight samples are included in this volume.
Shipboard data for major elements were supplemented with postcruise determination of ferrous/ferric iron ratios by a redox-titration method, (see "Geochemistry" in the "Explanatory Notes" chapter). The chemical data for major and trace elements are listed in Table T10. The major element data have not been renormalized to sum 100% to correct for systematically high "Totals" of the XRF analyses (see "Metamorphic Petrology" in the "Explanatory Notes" chapter). It should be noted that the major element analyses are reported differently from the Leg 118 shipboard data, which consist of reconstituted, renormalized rock compositions.
Concentrations of the trace elements V, Cr, Ni, Cu, Zn, Sr, Y, and Zr are well above the respective determination limits of the technique in virtually all samples. By contrast, the elements Rb and Nb could be detected only in a small number of the analyzed samples. The XRF technique rather serves as a "screening method" to identify samples that show significant enrichment of Rb or Nb with respect to normal concentration levels (less than 1 ppm for both elements) in oceanic gabbros.
LOI values were generally low (less than 1 wt%), as expected given the overall low degree of alteration of the recovered gabbro section. The LOI values provide only a first-order indication of the amount of volatile components (H2O and/or CO2) present in the samples, because the loss of mass from expelled volatile components is counterbalanced by uptake of oxygen due to oxidation of ferrous to ferric iron. As a result of this effect, many of the iron-rich rocks actually gained weight upon ignition. To obtain more useful information for all samples exhibiting LOI in excess of 1 wt%, carbon and hydrogen were analyzed using the shipboard Carlo Erba gas analyzer (see "Metamorphic Petrology" in the "Explanatory Notes" chapter). The results are shown in Table T11. The combined contents of CO2 and H2O are indeed consistently higher than the LOI values. The main volatile component is bound water or hydroxyl in secondary alteration phases. Two samples showing substantial amounts of CO2 contain secondary carbonates in alteration veins and/or as replacement of olivine (see "Metamorphic Petrology," for a systematic discussion of alteration effects).
Figure F64, shows the downhole variation of Mg#, defined as the molar ratio Mg2+/(Mg2++Fe2+). Fe2+ was calculated as 85% of total Fe. To avoid needless complication of figures, a straightforward geochemical classification of the samples has been adopted in Figure F64 and all following figures in "Geochemistry". Filled diamonds represent the troctolites, troctolitic gabbros, and olivine gabbros containing 0.4 wt% or less TiO2. Samples having between 0.4 and 1.0 wt% TiO2 are represented by the half-filled diamonds. This group largely comprises noritic gabbros and disseminated oxide gabbros. Samples represented by open diamonds are oxide gabbros having more than 1.0 wt% TiO2. The half-filled squares are felsic samples or hybrid samples with a substantial felsic component; their TiO2 content, and composition in general, is variable. Although this subdivision is one of convenience, it also has some geochemical significance (see discussion of vanadium below).
The dominant rock type from Leg 176 is a moderately fractionated gabbro having Mg#s in the range of 0.70 to 0.80. The least evolved rocks are the troctolites that occur between 500 and 520 mbsf. Ferrous iron makes up 85% to almost 100% of the total iron. Throughout the entire core are numerous thin intervals of oxide gabbros and felsic rocks that are significantly to strongly differentiated and have a lower ferrous to ferric iron ratio. Although the sampling did not aim to be systematic, there are indications, nonetheless, that the abundance of oxide gabbros indeed decreases with depth as suggested by Figure F64 (see "Igneous Petrology").
The Mg# should be used with some caution as a "differentiation index" in the case of the oxide gabbros. Mg#s of these cumulate rocks decrease as a result of the accumulation of Fe-rich minerals, such that the low numbers overestimate the extent of crystallization of ferro-magnesian phases from the silicate liquid. As shown in Figure F65, the oxide gabbros have intermediate An/(An+Ab) ratios, which are a measure of the chemical/mineralogical evolution of the system (An and Ab are the normative anorthite and albite abundance, respectively). Whereas several of the "felsic" samples indeed have the characteristically low Mg#s and An/(An+Ab) ratios of strongly evolved residual liquids, the oxide gabbros appear to have crystallized from significantly less fractionated liquids. Or, they represent a mixture of evolved Fe-Ti-rich liquids and the normal variety of gabbro. Figures F64 and F65 further show that gabbros with TiO2 between 0.4 and 1.0 wt% tend to have lower Mg#s than gabbros having <0.4 wt% TiO2, but that both groups largely cover the same range of An/(An+Ab) ratios. This indicates that these gabbros do not make up a simple series of rocks that crystallized from a common, progressively evolving TiO2-poor parental magma.
The element Ni is strongly enriched in olivine in which it substitutes for Mg. Because the Ni/Mg solid/liquid distribution coefficient is substantially greater than 1, the Ni content of the residual liquid and of gabbros crystallized from it drops exponentially (i.e., almost linearly in a log-log diagram) with decreasing Mg content (Fig. F66). This figure also shows that the two groups of gabbros with less than 1% TiO2 have greatly overlapping MgO and Ni contents. This is another indication that the differences in TiO2 concentrations cannot be attributed only to simple fractional crystallization processes. Chromium exhibits a more complex variation with Mg content (Fig. F67) because the behavior of this element is governed by substitution into clinopyroxene and oxide phases and, possibly, by Cr-spinels included in olivine grains.
The concentration of zinc is strongly correlated with total iron content in all rock types from Hole 735B (Fig. F68). The Zn/Fe ratio remains almost constant over 1 order of magnitude variation in Zn and Fe concentrations, which implies that the behavior of Zn is largely controlled by Fe-Ti oxide minerals. Somewhat lower Zn/Fe ratios are observed in felsic veins and some oxide gabbros. It is tempting to ascribe the Zn/Fe fractionation to minor sulfide phases which appear in these rocks. The element Cu is not correlated with any of the analyzed major or minor elements. Judging from the study of Leg 118 samples, the variation of Cu in the gabbro pile appears to be entirely controlled by sulfide phases (Alt and Anderson, 1991).
The trace element vanadium follows titanium in magmatic processes. The vanadium contents are high in the oxide gabbros (Fig. F69), but they are not as high as expected from an extrapolation of the linear V-correlation trend observed in the low-TiO2 troctolites and olivine gabbros (Fig. F70). An inflection in the V-TiO2 trend at about 0.4 wt% TiO2 marks a change in the geochemical processes controlling the abundances of Ti and V. At low Ti-content clinopyroxene is the major host phase of Ti and V. (The nominal abundance of 0.6 wt% TiO2 and 340 ppm of clinopyroxenes from oceanic gabbros is based on analyses culled from various literature sources.) The steep V-TiO2 trend of the troctolites and olivine gabbros shown in Figure F70 most likely represents a mixing line between clinopyroxene, olivine, and plagioclase, as the latter minerals have negligible Ti and V contents. The inflection at 0.4 wt% TiO2 then reflects a higher abundance of intercumulus liquid and/or cumulus Fe-Ti oxide minerals, both of which have lower V/Ti ratios than clinopyroxene. The lower V/Ti ratio of the oxide phases is most likely the result of charge balance difficulties attending substitution of titanium by vanadium. The inferred change of geochemical control of Ti-geochemistry provided a rationale for choosing 0.4 wt% TiO2 as the criterion for subdividing the gabbros into two groups.
Although the element niobium could be determined in only a limited number of samples, the data indicate that this trace element shows quite complex behavior. It is markedly enriched in some oxide gabbros that also have high vanadium concentrations (Samples 118-735B-78R-1, 114-118 cm; 118-735B-79R-7, 115-118 cm; 118-735B-82R-1, 13-16 cm; 118-735B-85R-3, 47-54 cm; 176-735B-102R-1, 17-23 cm; 176-735B-103R-1, 72-80 cm; 176-735B-114R-5, 23-29 cm; 176-735B-119R-4, 5-11 cm; 176-735B-137R-5, 33-38 cm; 176-735B-157R-4, 66-72 cm; 176-735B-157R-5, 134-140 cm; 176-735B-159R-2, 41-47 cm; 176-735B-171R-2, 56-62 cm; 176-735B-190R-3, 68-77 cm). Such observations are consistent with significant substitution of pentavalent niobium in vanadium-bearing oxide phases. However, niobium also shows strong enrichment in felsic samples having rather low vanadium contents but showing pronounced enrichment of other incompatible trace elements such as yttrium and zirconium (Samples 176-735B-99R-4, 106-108 cm; 176-735B-101R-1, 63-68 cm; 176-735B-110R-4, 0-5 cm; 176-735B-120R-4, 62-67 cm; 176-735B-126R-5, 91-97 cm; 176-735B-135R-3, 98-99 cm; 176-735B-154R-3, 67-72 cm; 176-735B-159R-4, 111-116 cm). This enrichment indicates that the small, highly charged niobium ions are enriched in residual fluids through complexing with minor ligands, such as hydroxyl, fluorine, and chlorine.
Enrichment in residual fluids followed by trapping in accessory minerals such as apatite, titanite, and zircon is the most likely explanation for the occasionally large enrichment of yttrium and zirconium in Ti-rich gabbros, felsic veins, and hybrid samples consisting of oxide-rich and felsic material. The zirconium data reported in Table T10 and Table T1 in the "Leg Summary" chapter for troctolites, troctolitic gabbros, and gabbros overestimate the real abundances because of systematic analytical error (see "Metamorphic Petrology" in the "Explanatory Notes" chapter). The Leg 118 shipboard data appear to be a more reliable source of Zr data for the rock types recovered from Hole 735B. Further discussion of the enrichment of incompatible elements in Hole 735B is given below.
Plagioclase is the major host phase of the trace element strontium. Sr correlates well with the Al2O3/MgO ratio, which is a chemical measure of the ratio of plagioclase to ferromagnesian phases (Fig. F71). Several oxide gabbros have lower Sr concentrations probably as a result of accumulation of Sr-poor oxide phases. Plagioclase abundance, however, does not appear to be the sole factor that governs Sr geochemistry in the gabbros. The variation of Sr/Al ratio, which is almost completely determined by plagioclase, with An/(An+Ab) ratio (Fig. F72) indicates that the partitioning of Sr into plagioclase increases with increasing albite content of plagioclase and/or strongly depends on the composition of the liquid from which the plagioclase crystallized. The lower Sr contents of felsic samples can be ascribed to depletion of residual liquids in Sr by protracted plagioclase crystallization.
A limited number of samples were analyzed to obtain a first estimate of the extent of chemical change brought about by alteration. The pervasively altered Sample 176-735B-91R-3, 92-100 cm, from the oxidized zone at ~520 mbsf is enriched in Ca-carbonate, bound water, K, and Rb (Tables T10, T11). The MgO content is low due to replacement of olivine by secondary carbonate. Two samples that contain low-temperature secondary smectite (Samples 176-735B-168R-3, 51-56 cm, and 176-735B-181R-2, 83-100 cm) mainly show appreciable enhancement of K and Rb contents and of bound water (see "Metamorphic Petrology").
Figure F64 shows that oxide gabbros and felsic material are present throughout the entire section studied during Leg 176, although with decreasing abundance toward the bottom of the hole. Because of the limited sampling and large chemical and mineralogical variability, these two rock types are of little value in establishing a chemical stratigraphy. The downhole stratigraphy will hence be entirely based on the chemistry of the main gabbro types (rocks having less than 1 wt% TiO2). The "master variable" for present purposes is the Mg# (Fig. F73), because all other chemical variables of the main gabbro types are directly or indirectly correlated with this master variable.
The first 20 m of the Leg 176 core from Hole 735B consists of fairly primitive gabbros and troctolitic rocks. This interval forms the lower part of lithologic Unit VI, as defined by the Leg 118 shipboard party.
Unit VI terminates at about 520 mbsf in a zone of sheared gabbros, colored brown due to extensive oxidative alteration (e.g., Sample 176-735B-91R-3, 92-100 cm). The altered zone is underlain by relatively fractionated gabbros and disseminated oxide gabbros. At approximately 705 mbsf, the chemical composition abruptly changes, as the underlying gabbros have a higher Mg# and lower TiO2 content. The 705 mbsf depth also marks the onset of a core interval characterized by a very low degree of high-temperature alteration (see "Metamorphic Petrology"), and by the near absence of crystal-plastic deformation (see "Structural Geology"). The interval between 520 and 705 mbsf coincides with Units VII, VIII, and IX, as defined by the Leg 176 igneous petrology group (see "Igneous Petrology"). However, this threefold lithologic subdivision is not clearly expressed in the chemical stratigraphy, possibly as a result of the limited number of analyzed samples.
At 800 mbsf, the degree of high-temperature alteration and deformation abruptly changes again. This transition also appears to coincide with a subtle change in rock chemistry because the underlying cores contain a much higher fraction of gabbros with between 0.4 and 1.0 wt% TiO2. It is highly improbable that the absence of disseminated-oxide gabbros in the 705 to 800 mbsf interval is an artifact due to limited sampling density. As this interval also has distinct alteration and structural characteristics, it is possible that the rocks also have an igneous history different from the overlying and underlying gabbros.
The chemical unit that started at 800 mbsf terminates in a sheared zone at 960 mbsf. The gabbros between 800 and 960 mbsf exhibit considerable variation of Mg# (0.83 down to 0.68) and TiO2 content. The two chemical units recognized between 705 and 960 mbsf closely coincide with lithologic Unit X (714-960 mbsf).
From 960 to 1320 mbsf, the chemical composition displays a gradual change from moderately fractionated gabbros at the top (Mg# of 0.72) to less fractionated at the bottom (Mg# of 0.80). This section of the core corresponds closely to lithologic Unit XI (960-1314 mbsf).
The trend toward more primitive compositions is interrupted at 1320 mbsf by the appearance of an interval that comprises a large fraction of gabbros that have consistently higher TiO2 contents (0.40-0.55 wt% vs. 0.25-0.35 wt%). This boundary is marked by a small sheared zone but shows no change of alteration properties. This chemical unit is part of lithologic Unit XII (1314-1508 mbsf).
Figure F74, shows the downhole variation of Mg# over the whole 1500-m section drilled during Leg 118 and Leg 176. The figure is based on 357 whole-rock analyses. The data for the upper 500 m include the Leg 118 shipboard analyses of 97 samples, the eight additional samples analyzed during Leg 176 (Table T10), and 72 unpublished analyses (J. Hertogen and P. Meyer, pers. comm., 1997).
The most striking aspect of the downhole chemical variation is that the gabbro section consists of a stacking of separate units with a thickness varying from 100 to 300 m. The average degree of fractionation of the various units differs. However, almost all units show a trend of increasing degree of fractionation upward within each unit. Most likely these chemical units represent the scale at which individual magmatic events add to the construction of oceanic Layer 3 at slow-spreading centers.
A second salient feature is the ubiquity of oxide-rich gabbro throughout the core (Fig. F75). This type of gabbro predominantly occurs in centimeter- to decimeter-thick intervals, and the degree of Fe-Ti enrichment is quite variable. However, the 50-m-thick Unit IV at a depth of 223 to 274 mbsf in the upper part of Hole 735B is almost entirely made up of massive oxide gabbro, which crystallized from differentiated Fe-Ti-rich liquids that undoubtedly migrated from elsewhere. However, the geochemical data offer as yet few clues to the origin of the Fe-Ti-rich liquids from which the oxide seams in other units formed. They all might have migrated from elsewhere, or might have been produced by fractionation within the unit itself.
The downhole distribution of TiO2 (Fig. F75) reflects the downward decrease in abundance of Fe-Ti-rich gabbros with depth in Hole 735B. The reasons are not obvious. It appears, however, that the decrease cannot be ascribed to a decrease of the titanium abundance of the parental liquids. Indeed, gabbros of similar modal composition from the various units have virtually identical average Ti contents. It is important to note that in the upper part of the core where oxide gabbros are more prominent, the units nevertheless have a large fraction of gabbros with low TiO2 content. A case in point is Unit VI, where one might deduce that the development of oxide gabbros within a given unit might also depend upon emplacement and crystallization conditions that favor protracted crystallization and prevent premature expulsion of evolved residual liquids.
Felsic material present in dispersed form in the section is likely a product of extensive differentiation. The nature of the material is variable; whereas some thicker veins are made up of K-poor trondhjemite, others consist of granodiorite with appreciable potassium feldspar.
The downhole distribution of the trace element yttrium is shown in Figure F76. The concentrations are recast as chondrite-normalized values to facilitate comparison with published abundances of heavy rare earth elements (REEs), for which yttrium is a proxy. From the yttrium data one can conclude that most of the gabbros from Hole 735B have three to five times lower REE abundances than in common mantle magmas, depending upon modal composition and amount of trapped interstitial liquid. The clinopyroxene-poor troctolites and troctolitic gabbros have very low heavy REE abundances, as this mineral is the main mineral host of heavy REE in oceanic gabbros (see Barling et al., 1997, for a discussion of systematics of REE abundances of oceanic gabbros). Hence, a large fraction of the REE (and, by inference, of other incompatible trace elements) either must have been lost from the gabbro pile or is present in strongly concentrated form in minor lithologies. The rather high yttrium content of many oxide gabbros and felsic veins from Hole 735B indicates that the latter is partly the case.
The main conclusions that can be drawn from the shipboard chemical analysis of 180 gabbro samples from the 1000-m-thick section of oceanic Layer 3 drilled during Leg 176 are as follows: