PETROGRAPHIC, MODAL, AND MINERAL COMPOSITIONAL DATA

Textural and mineralogical descriptions of Hole 735B core are based on shipboard data and descriptions of 298 thin sections by the authors. Modal mineralogy was determined by point counting (500-1500 points per thin section, depending on grain size) (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999) and by X-ray diffraction (XRD) analysis using the technique of Emmermann and Lauterjung (1990). For the XRD analyses, samples were prepared by mixing 3 g of rock powder with a few drops of polyvinyl alcohol and pressing the powder into a briquette. The analyses were carried out using a quantitative phase analysis procedure on a Siemens D 501 X-ray diffractometer using CuK radiation generated at 40 kV and 20 nA and mineral phases were identified and quantified using a "multi-iterative" algorithm developed by Emmermann and Lauterjung (1990). Samples with high water contents were analyzed several times, and in all but three samples the replicate analyses produced virtually identical results. All analyses were normalized to 100%. Comparison of the results with point-counted data indicate a 2- error on major mineral phases between 5% and 10%. Minor phases, such as calcite and chlorite, were commonly identified in the spectra but could not be determined quantitatively.

Microprobe analyses were carried out on representative samples of all rock types from Hole 735B. Analyses for Leg 118 samples were carried out at Justus-Liebig Universität, Giessen, Germany, using a Cameca SX-50 wavelength-dispersive microprobe. Operating conditions were 15 kV gun voltage and 20 nA sample current with a counting time of 120 s. A variety of natural standards was used for calibration, and counting times for different elements varied, depending on the element concentration. Duplicate analyses indicated a precision of >2% of the amount present for abundant oxides and >5% for sparse oxides such as MnO, K₂O, and P₂O₅. Instrument drift was negligible, as determined by periodic analysis of standard minerals.

Leg 176 samples were analyzed at Dalhousie University, Halifax, Canada, using a JEOL 733 energy-dispersive microprobe. Operating conditions were 15 kV gun voltage and 15 nA sample current, and each spectrum was accumulated for 40 s. Geologic standards were used for calibration, and the results were calculated using the Link's ZAF (atomic number, absorption, and fluorescence) matrix correction program. Analytical precision was >2% of the amount present for abundant oxides and >5% for sparse oxides. Mineral formulas were calculated for all analyses, and any deviating significantly from stoichiometry were rejected.

The major igneous rock types in Hole 735B are defined on variations in grain size and modal proportions of primary minerals. Average point-counted modes for the freshest samples of each rock type are given in Table T1. The metagabbros of lithologic Unit I are not included in this table because they are partly to completely recrystallized metamorphic rocks formed at high temperatures and are defined on the basis of their textures and secondary mineralogies. Their protoliths are deduced from relict porphyroclasts, metamorphic mineral assemblages, and the compositions of adjacent igneous units.

Modal data determined by XRD techniques on both fresh and altered rocks from Hole 735B are given in Table T2. In Table T3, these data are used to calculate an average modal composition for the lithologic units described above.

Average mineral compositions of the primary phases in each rock type are given in Table T4. Representative analyses of olivine, orthopyroxene, clinopyroxene, plagioclase, and ilmenite are given in Tables T5, T6, T7, T8, and T9, respectively. Downhole plots of compositional data for olivine, orthopyroxene, clinopyroxene, and plagioclase given in Figures F3, F4, F5, and F6, respectively. The range of composition for each mineral within each sample is given by the length of the bars in the figures.

Metagabbro

Metagabbros occur in shear zones throughout Hole 735B but are concentrated in lithologic Unit I at the top of the section. These are metamorphic rocks in which primary textures and minerals have been almost completely replaced. Most have well-developed mylonitic or porphyroclastic textures with good foliations. Completely metamorphosed rocks are amphibole gneisses with alternating bands of greenish brown magnesio-hornblende and recrystallized plagioclase (Table T3). A few samples still contain neoblasts of clinopyroxene, indicating metamorphism under relatively anhydrous conditions. Relict grains of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide occur as porphyroclasts in the recrystallized matrix. Most of these grains are <5 mm across and are rounded or elliptical in shape.

Protolith compositions are difficult to determine because of the extensive metamorphism but can be estimated from the least altered samples. Most of Unit I down to 26.5 mbsf (upper part of Core 176-735B-7D) was probably originally gabbronorite judging from the common appearance of orthopyroxene porphyroclasts and the paucity of olivine. These rocks have highly variable proportions of plagioclase, clinopyroxene, orthopyroxene, and amphibole. Altered olivine is recognized in only three samples but could have been more abundant originally. Small grains of Fe-Ti oxide are present in some samples but rarely exceed 3 modal%. Most of the oxide is clearly secondary because it fills fractures perpendicular to the foliation, occurs preferentially in the porous parts of the rocks, and is sometimes associated with postdeformational development of green amphibole.

Below 26.5 mbsf in Unit I, relict olivine is found in most metagabbros, whereas orthopyroxene rarely exceeds 2 modal% and is commonly absent altogether. A few samples have up to 5 modal% orthopyroxene, but these are rare. The olivine metagabbros consist of ~5 modal% olivine, 43% plagioclase (including neoblasts), 38% clinopyroxene (including neoblasts), 1% orthopyroxene, 2% magnetite, and 10% amphibole (mostly replacing pyroxene).

Olivine Gabbro and Olivine Microgabbro

Olivine gabbros and olivine microgabbros (>5 modal% olivine) are by far the most abundant lithologies in Hole 735B, making up ~60% of the total core. They are the principal lithologies in Units II, III, V, VI, VIII, X, XI, and XII, and they occur locally in all the other units (Fig. F2). The olivine gabbros and microgabbros are very similar in modal composition and differ primarily in grain size. Modal mineralogies vary considerably because of phase layering and the irregular distribution of minerals in the rock, but an average mode consists of ~60% plagioclase, 30% clinopyroxene, and 10% olivine, with trace amounts of orthopyroxene, opaque minerals, and amphibole (Table T1). Grain size variations in these rocks can be either abrupt or gradational, but sharp intrusive contacts are rare.

In the olivine gabbros, the grain size is typically 1-2 mm, but crystals can range up to 15 mm across in very coarse patches. Olivine forms irregular, commonly amoeboidal grains, many of which have narrow, discontinuous rims of orthopyroxene. The average olivine composition in these rocks is Fo₈₂ and the orthopyroxene rims average about En₆₇ (Table T4), although both show a wide range in composition (Tables T5, T6). Clinopyroxene forms large tabular grains, commonly with exsolution lamellae of orthopyroxene. The clinopyroxene is all augite with an average composition of En₄₆ (Tables T4, T7). Plagioclase consists of subhedral, lath-shaped, tabular or blocky grains, typically 1-3 mm in maximum direction. It has an average composition of An₅₁ (Table T4), but it varies widely because of zoning and background alteration (Table T8).

Alteration in the olivine gabbros and microgabbros varies considerably depending on their position in the hole and proximity to veins. In the lower 500 m of the hole, background alteration is typically <5%. In these rocks, olivine is slightly replaced by smectite and secondary magnetite; clinopyroxene is partly rimmed by greenish brown amphibole; and plagioclase is fresh. Elsewhere in the section, olivine is partly to completely replaced by mixtures of Mg-amphibole and talc, or rarely by red iron oxides. Clinopyroxene commonly contains flakes of brown amphibole within the crystals and exhibits extensive marginal replacement by greenish brown amphibole. Plagioclase is partly replaced by more sodic feldspar, epidote, chlorite, clay minerals, and less commonly by amphibole.

Troctolite and Troctolitic Gabbro

Troctolites in Hole 735B are very mafic rocks with <5 modal% pyroxene and up to 30 modal% olivine, whereas the troctolitic gabbros have >10 modal% pyroxene and ~15 modal% olivine. The volume of these rocks is relatively small, and most of them occur in lithologic Units VI, VII, X, and XII (Fig. F2). They typically form intrusive veins and dikes, up to 1.5 m thick, in the host olivine gabbros.

The troctolites have an average modal composition of 66.5% plagioclase, 29.5% olivine, 3.5% augite, and 0.5% of orthopyroxene, opaque minerals, and amphibole combined (Table T1). Traces of apatite and rare spinel are also present. The troctolitic gabbros are similar in composition but have notably lower olivine (17 modal%) and higher augite (Table T1). Olivine forms euhedral to subhedral crystals up to ~3 mm in diameter in the troctolites and appears to be cumulus in origin. In the troctolitic gabbros, it is finer grained (average = ~1 mm) and equigranular. Augite forms anhedral, interstitial grains in both troctolites and troctolitic gabbros and ranges from 0.5 to 1 mm in diameter. Plagioclase occurs both as relatively large subhedral prisms and as anhedral interstitial grains intergrown with pyroxene.

The troctolites and troctolitic gabbros are the most mafic rocks encountered in Hole 735B. Average olivine compositions range from Fo_80.6 in the troctolites to Fo_83.2 in the troctolitic gabbros (Table T4), and individual grains range up to Fo_83.7 (Table T5). Orthopyroxene is rare in these rocks, occurring only as narrow rims on some olivine crystals. The sparse clinopyroxene forms subhedral, intergranular to poikilitic grains with rare exsolution lamellae of orthopyroxene. Both the orthopyroxene and clinopyroxene are highly magnesian (Tables T4, T6, T7), although the En content of the clinopyroxene is similar to that of the gabbros and olivine gabbros (Table T4). Plagioclase is highly calcic, ranging from an average of An_65.0 in the troctolites to An_74.3 in the troctolitic gabbros (Tables T4, T8).

Most of the troctolites and troctolitic gabbros occur in the lower parts of the hole where veins are sparse and alteration is slight. Olivine in these rocks is commonly fresh or exhibits only minor replacement by smectite along cracks. Pyroxene is also relatively fresh, typically having only a few flakes of brown amphibole within individual grains. In a few cases, clinopyroxene grains also show marginal replacement by brownish green amphibole. Plagioclase is fresh or very slightly replaced by clay minerals or green amphibole.

Gabbro

Gabbros are relatively sparse in Hole 735B and occur as thin intervals in the dominant olivine gabbro, especially in lithologic Units X, XI, and XII (Fig. F2). Nearly all of these rocks are olivine bearing and are mineralogically similar to the olivine gabbros. An average mode consists of 58% plagioclase, 37% clinopyroxene, 3.5% olivine, and small amounts of orthopyroxene, opaque minerals, amphibole, and apatite (Table T1). Modal layering is locally present, with the rocks alternating between gabbro and olivine gabbro.

Most of the gabbros are coarse-grained, granular rocks commonly with a weak magmatic foliation, although a few microgabbros are also present. Plagioclase typically forms relatively large, subhedral, prismatic to blocky grains, 1-3 mm across. Clinopyroxene occurs as subhedral, tabular grains ranging up to 10 mm across in very coarse grained intervals. Olivine forms small, anhedral grains, commonly with narrow, discontinuous rims of orthopyroxene where it is in contact with plagioclase.

Although most of the gabbros are olivine bearing, they are somewhat more evolved than the olivine gabbros and troctolites. Olivine is relatively uniform in composition with an average of Fo_71.6 distinctly more iron rich than that in the olivine gabbros, whereas plagioclase is nearly identical in composition in the two rock types (Tables T4, T5, T8). The sparse orthopyroxene has an average composition of En_69.6, slightly more magnesian than the average in olivine gabbros (Table T4), but this may be a function of the small sample population. Clinopyroxene has almost identical En contents in the two rock types (Table T4) but is slightly more iron rich in the gabbros with Mg#s ranging from 75-80.

Like the olivine gabbros, most of these rocks are only weakly altered, although a few samples in the upper part of the hole show extensive replacement of olivine and pyroxene. Where alteration is most intense, the olivine is partly to completely replaced by colorless amphibole and rare talc; elsewhere it is partially replaced by green smectite. Pyroxene is typically rimmed and partially replaced by brownish green amphibole, and plagioclase is locally bleached and replaced by more sodic varieties.

Gabbronorite

Gabbronorite occurs primarily in lithologic Units I, VII, and IX, although it forms thin intervals elsewhere in the sequence (Fig. F2). The gabbronorites are closely associated with Fe-Ti oxide gabbros and gabbronorites and are concentrated in zones of moderate to intense crystal-plastic deformation.

In Unit I, most of the rocks are strongly deformed metagabbros but primary orthopyroxene appears to have been present throughout the section, based on the presence of numerous porphyroclasts. The original abundance of orthopyroxene is difficult to determine but may have been as much as 10 modal%. An average modal composition for unmetamorphosed gabbronorites elsewhere in the core is 64% plagioclase, 2.5% olivine, 27% clinopyroxene, 4.5% orthopyroxene, 1% opaque minerals and 1% amphibole (Table T1). Although these rocks rarely contain more than 5 modal% orthopyroxene, they were classified as gabbronorites by the shipboard party to emphasize their petrologic importance. The few samples containing more than 5 modal% olivine are best classified as olivine gabbronorite.

The plagioclase in these rocks forms subhedral laths or tabular crystals up to ~2 mm across or minute neoblasts in the groundmass of deformed samples. Clinopyroxene can also occur as neoblasts but more commonly forms tabular crystals or subrounded porphyroclasts 2-3 mm across. Olivine rarely forms >2-3 modal% in these rocks and occurs as anhedral grains 1-2 mm across. Although orthopyroxene is not abundant, it typically forms discrete grains, usually 0.5 to 1 mm across. It can also occur as narrow rims on olivine or as exsolution lamellae in clinopyroxene grains.

The gabbronorites are highly evolved rocks in which plagioclase has an average composition of An₃₉ with a standard deviation of 6.1 (Table T4). The most calcic plagioclase in these rocks is An_48.5 (Table T8); thus strictly speaking, they should be classified as diorites. Orthopyroxene has an average composition of En₆₁, significantly more iron rich than the orthopyroxene in the gabbros and olivine gabbros (Tables T4, T6). Clinopyroxene is also relatively iron rich with an average composition of En₃₉ (Table T7), some of which are low-Ca inverted pigeonite.

Many of these rocks are moderately to intensely altered, particularly those that are highly sheared. Olivine is almost always replaced by colorless amphibole or smectite and locally by red Fe oxyhydroxides. Both orthopyroxene and clinopyroxene are partially replaced by brown to brownish green amphibole and plagioclase by more sodic feldspar, epidote, or clay minerals.

Oxide Gabbro and Oxide Microgabbro

Oxide gabbros and microgabbros are best developed in Unit IV but occur as thin intervals throughout the upper 1000 m of the section. Many, but not all, are closely associated with zones of intense crystal-plastic deformation (Shipboard Scientific Party, 1999). The oxide gabbros and microgabbros typically have sharp contacts with the enclosing olivine gabbros, sometimes even truncating single crystals in the host rock (Ozawa et al., 1991).

Textures range from strongly foliated to granular. In the highly deformed rocks, plagioclase forms bands of minute neoblasts that enclose porphyroclasts of plagioclase, pyroxene, and less commonly olivine. The porphyroclasts are ovoid in form and typically 1-2 mm across. Most of the undeformed rocks are granular in texture with grain sizes ranging from 1-4 mm across. However, some of these display a pronounced magmatic foliation.

The oxide gabbros and microgabbros differ significantly from the other rocks in their modal mineralogy. The gabbros are characterized by relatively high modal percentages of Fe-Ti oxides and clinopyroxene and low modal olivine and plagioclase compared to the microgabbros (Table T1). The abundance of oxide varies widely, averaging ~12 modal% in the gabbros but ranging up to as much as 50 modal% in some samples. In undeformed rocks, the oxide occurs as relatively large (1-3 mm) anhedral grains that partially enclose the silicate minerals. In deformed samples, it occurs as narrow, anastomosing bands and stringers parallel to the foliation, wrapping around both neoblasts and porphyroclasts. In a few cases, it also fills cracks in the silicate minerals. The oxide is ilmenite (Table T9), with an average composition of 50% TiO₂, 48% FeO, 1% MnO, and 0.4% MgO. Many of the ilmenite grains contain small globules of magmatic sulfide (Natland et al., 1991).

Based on their mineral compositions, most of these rocks are highly evolved. Plagioclase is highly variable in composition, ranging from An₆₂ to An₁₂ (Table T8) but has an average composition of An₃₈ with a standard deviation of 12 (Table T4). Thus, strictly speaking, most of the rocks are diorites, even though many contain small amounts of olivine. One reason for the sodic nature of the plagioclase in these rocks is the abundance of plagioclase neoblasts in the groundmass, which are consistently more sodic (average about An₃₅) than undeformed grains or porphyroclasts. In addition, many of porphyroclasts are moderately to strongly zoned with cores of about An₅₀ and rims of An_30-36 (Table T8). Bloomer et al. (1991) noted that reverse zoning occurs in some plagioclases from Unit III and the top of Unit IV. Olivine in these rocks averages about Fo₅₄ (Table T4) but like the plagioclase varies widely, ranging from Fo₇₁ to Fo₃₃ (Table T5). Orthopyroxene averages En₆₁ and clinopyroxene En₄₁, with significantly less variation than either the olivine or plagioclase (Tables T4, T6, T7). Most of these rocks also have small amounts of apatite and titanite, both of which are closely associated with the oxide bands.

Most of the deformed oxide gabbros and microgabbros exhibit significant amounts of alteration. Pyroxene neoblasts and porphyroclasts are partly to completely replaced by brownish green amphibole, and olivine is partly altered to colorless amphibole. In addition, subhedral to euhedral grains of bright green amphibole are commonly associated with titanite and apatite in the oxide-rich layers and bands.

Oxide Gabbronorite

Oxide gabbronorites are very similar in texture and mineralogy to the oxide gabbros, differing only in having notably higher modal percentages of orthopyroxene (Table T1). Like the oxide gabbros, most of these rocks are deformed and have well developed foliations with bands and layers of plagioclase neoblasts enclosing porphyroclasts of plagioclase, clinopyroxene, and orthopyroxene. Undeformed specimens have granular textures with grain sizes generally between 1 and 3 mm.

An average modal composition for the oxide gabbronorites is 49% plagioclase, 1% olivine, 35% clinopyroxene, 3% orthopyroxene, 11% Fe-Ti oxide, 0.5% amphibole, 0.4% apatite, and traces of titanite (Table T1). The mineral compositions are identical to those in the oxide gabbros within one standard deviation (Table T4). In addition, the degree and nature of alteration in the oxide gabbronorites is the same as in the oxide gabbros.

Downhole plots of mineral compositions reveal a number of significant variations, both within and between lithologic units (Figs. F3, F4, F5, F6). Olivine and clinopyroxene show similar trends in Mg#, with a number of cyclic variations (Figs. F3, F5); orthopyroxene compositions vary in a similar fashion but the trends are much less well defined because of the paucity of data (Fig. F4). Plagioclase compositions show the widest variations, reflecting both variable degrees of deformation and alteration as well as crystal fractionation (Fig. F6). In most cases, major mineral compositional breaks correspond reasonably well to lithologic unit boundaries.

Both olivine and clinopyroxene compositions show clear cyclic variations in the lower half of the core with the minerals in each cycle generally becoming more iron rich upward (Figs. F3, F5). The first cycle extends from the base of the hole to a depth of ~1270 mbsf, somewhat above the boundary between Units XII and XI. The boundary between the second and third cycles corresponds closely to the boundary between Units XI and X at 960 mbsf. The patterns become more complex in Unit X, with olivine becoming slightly more magnesian upward and clinopyroxene showing a bimodal distribution. Unit X consists of interlayered gabbro and olivine gabbro, and the bimodal distribution of the clinopyroxene suggests the presence of two parental melts. There is a gap in the data between ~825 and 750 mbsf, but it appears that the third cycle extends to the top of Unit IX at 714 mbsf. Despite the cyclic variations described above, both olivine and clinopyroxene show a clear upward decrease in Mg# in the lower half of the core.

Plagioclase compositions follow a similar pattern in the lower part of the core, although the cycles are not as clearly defined (Fig. F6). The most pronounced break in the sequence is at the boundary of Units XI and X, where one of the cycles begins. There are three zones of very sodic plagioclase (>An₂₀) and these correspond closely to the more evolved olivine and pyroxene compositions. Thus, they are interpreted as primary features rather than as the result of deformation and alteration.

Mineral compositions in the upper half of the core are much more varied and complex than in the lower part. There are no data for Unit VIII and only very limited pyroxene and plagioclase data for Unit VII. Unit VI, which consists of olivine gabbro and troctolite, is the most primitive in the entire core, with olivine compositions as high as Fo₈₇ and plagioclase as high as An₈₀. The plagioclase in this unit is quite varied, and the very sodic compositions near the base of the unit are due to alteration and deformation. Unit V has slightly more evolved mineral compositions than Unit VI, although overall, the minerals are much more uniform.

Unit IV is composed of highly evolved Fe-Ti oxide gabbro and has very iron-rich olivine (as low as Fo₃₀) and pyroxene and very sodic plagioclase (mostly less than An₄₀). Unit III consists of olivine gabbro but is much more evolved than any of the olivine gabbros lower in the section (Figs. F3, F4, F5, F6).

Mineral compositions in lithologic Unit II are some of the most interesting in the section. This unit consists primarily of olivine gabbro and olivine-bearing gabbro but has a number of thin bands of Fe-Ti oxide gabbro, commonly associated with shear zones. This results in a strong bimodal distribution of plagioclase and a similar, but less pronounced, pattern for clinopyroxene. Because olivine is rare in the oxide gabbros, it is much more uniform in composition, but a few grains have compositions below Fo₆₀. Unit I consists largely of metagabbro and almost no primary minerals are preserved.