The principal vein minerals in the Hole 735B core are, in order of abundance, plagioclase, amphibole, smectite, diopside, and quartz. All of the other minerals such as biotite, chlorite, zeolite, prehnite, titanite, ilmenite, oxyhydroxides, zircon, apatite, K-feldspar, carbonate, and epidote are typically present in only trace amounts.
Plagioclase is the most common and abundant vein mineral in Hole 735B. It is primarily present in felsic, plagioclase + diopside, and plagioclase + amphibole veins but can also be present in narrow monomineralic veinlets of Na-rich plagioclase (Robinson et al., 1991). In felsic veins, it commonly forms myrmekitic intergrowths with quartz and is accompanied by small amounts of titanite, zircon, and apatite. In plagioclase-rich veins with diopside or amphibole, it forms relatively large (1-3 mm), subhedral, strongly zoned grains, commonly with pitted and corroded cores. The monomineralic plagioclase veins consist of narrow, 0.2- to 0.3-mm-wide bands of albite or sodic oligoclase that cut more calcic plagioclase grains. Individual crystals are not visible in these veins.
Based on 439 microprobe analyses, the vein plagioclase ranges in composition from An49.4 to An0.2 and averages An16.8 (Table T2; Fig. F4). There are no systematic compositional variations in the plagioclase with depth in the hole, with vein type, or with position within the vein. The variations in composition primarily reflect zoning of individual crystals. Most grains have cores of oligoclase-andesine (An25-35) and rims of albite-oligoclase (An5-15). Some of the cores are as calcic as An49 but this is rare. In such cases, the rims are typically sodic andesine (~An30-35).
Plagioclase compositions are similar in felsic, plagioclase + diopside, and plagioclase + amphibole veins (Table T2) and show no clear patterns within individual veins. Where a vein cuts a preexisting plagioclase crystal in the host rock, the host mineral is replaced by more sodic plagioclase along a very sharp boundary. Plagioclase in monomineralic veins is typically internally uniform but varies in composition from vein to vein (Table T2).
The K2O content of the plagioclase is very low but increases gradually as the An content decreases, reaching a maximum at ~An15 (Fig. F5). It then decreases to trace amounts in albite. Pure K-feldspar is very rare in Hole 735B veins, having been identified only in a few felsic veins and one smectite + prehnite vein (Sample 176-735B-183R-3, 69-78 cm). In the felsic veins it is largely present as fine-grained interstitial material between plagioclase grains; in the smectite + prehnite vein it is present as a narrow band of aphanitic material along the vein margin.
Amphibole is the second most abundant vein mineral in Hole 735B, forming both monomineralic and plagioclase + amphibole veins. Small quantities of amphibole are also present in most diopside and diopside + plagioclase veins and in irregular felsic patches in the gabbros. Representative microprobe analyses for brown and greenish brown vein amphiboles are given in Table T3, and those for green vein amphiboles are given in Table T4. The amphibole nomenclature is based on the International Mineralogical Association (IMA) classification of Leake et al. (1997). The cation proportions and ferric/ferrous iron ratios were calculated using the program of Currie (1997) that also assigns Mg and Fe to the appropriate sites by assuming constant distribution coefficients for these elements among the possible sites.
Optically, the amphibole forms three major groups based on color: colorless to green, greenish brown to brownish green, and dark brown. These three groups are distinguished chemically primarily on their TiO2 contents; in colorless and green amphiboles TiO2 is typically <0.50 wt%, in greenish brown to brownish green varieties it ranges from 0.5 to 1.5 wt%, and in dark brown varieties it is >2 wt% (Tables T3, T4). A few green amphiboles have unusually high TiO2 contents, up to 2.37 wt% (Table T4), but these are rare. In the IMA classification scheme the colorless to green amphiboles are primarily actinolite, whereas the greenish brown, brownish green, and dark brown varieties are magnesio-hornblende or edenite, depending on the Na and K contents (Fig. F5).
The monomineralic amphibole veins at the top of the section consist chiefly of greenish brownish to brown magnesio-hornblende and edenite, the two differing only in having slightly different amounts of Na in the X position (Table T3; Fig. F5A). Individual grains show no optical zoning and are essentially homogeneous. Likewise, there is little variation among grains within a single vein, although there is some variation from sample to sample. For example, vein amphibole in Sample 176-735B-12R-1, 50-55 cm, is a dark brown, high-TiO2 variety, whereas most of the others are green to greenish brown in color, with low TiO2. The main compositional variation in the brown amphiboles is in the iron/magnesium ratio, with Mg numbers ranging from 45 to 72 (Table T3).
The monomineralic amphibole veins are most commonly present in foliated metagabbros composed of alternating bands of plagioclase and amphibole. These amphibole bands formed by replacement of clinopyroxene during brittle-ductile deformation. In these rocks there is typically no significant chemical difference between the vein amphibole and that in the foliated host rock (cf. Vanko and Stakes, 1991).
In plagioclase-amphibole and felsic veins, most of the amphibole is present as selvedges along vein walls or as replacements of hydrothermal diopside. These amphiboles are typically green to brownish green magnesio-hornblende to actinolite, although some are edenite. Brownish green and dark brown amphiboles are present in a few veins and, where present, are commonly zoned, with a brown core and a narrow green rim.
A few individual subhedral amphibole crystals are also scattered through the matrix of most felsic and plagioclase veins, although they are not abundant. These are characteristically either bright bluish green or dark brown in color and are commonly accompanied by small crystals of ilmenite and titanite. Some of these are partially altered to, or accompanied by, yellowish brown mica. The bright green amphibole is typically rich in Al2O3 and TiO2, whereas the dark brown varieties are characterized by relatively high TiO2 and intermediate Al2O3 contents (Tables T3, T4). The green amphibole is mostly actinolite with smaller amounts of ferro-actinolite, ferro-hornblende, and magnesio-hornblende (Fig. F5B). As in the veins higher in the section, the brown amphibole is mostly edenite with small variations in Fe/Mg ratios (Table T3; Fig. F5A).
A few very high iron amphiboles (FeO = 30-31.5 wt%) are also present in several veins (e.g., Samples 176-735B-144R-6, 35-47 cm, and 119R-5, 62-68 cm), a feature also noted by Vanko and Stakes (1991). These are bright green ferro-actinolite and ferro-hornblende intergrown with plagioclase in felsic veins.
Diopside typically forms anhedral, blocky crystals along vein margins, where it is almost always associated with intermediate plagioclase and greenish brown amphibole. In a few veins it forms euhedral, zoned crystals up to 2 mm across (Pl. P4, fig. 2). Most crystals are light green but a few are colorless, and most contain solid and fluid inclusions that give the crystals a pitted appearance.
Overall, the diopside is very uniform in composition, forming a tight cluster in the pyroxene quadrilateral. Most grains cluster closely around 50% Wo and vary only in their Fe/Mg ratios (Table T5; Fig. F6). Most of the diopside is compositionally homogeneous, although a few large euhedral grains show weak zoning in Fe and Mg (Table T5). Al2O3 contents range from 0.04 to 1.19 wt%, CaO is between 22.5 and 24.5 wt%, and Na2O is typically <0.5 wt%. Alumina and silica show a weak inverse relationship. TiO2, Cr2O3, and MnO contents are negligible and do not vary systematically with other oxides. The largest variations are in FeO contents, with Mg numbers ranging from 63 to nearly 87 (Table T5). The euhedral crystals have very narrow oscillatory zones that span the range of composition among crystals and become slightly more iron rich toward the rims. Many of these crystals have a narrow band of clay between the outermost band and the main part of the grain, but it is not clear when the clay formed, that is, before or after deposition of the outermost band. Nearly all the analyzed grains have totals <100%, even though clinopyroxene in the wall rock, analyzed during the same run, is close to an ideal composition. This suggests that these vein diopsides may contain small amounts of H2O or other volatiles as submicroscopic inclusions.
The absence of nonquadrilateral components in the diopside compared to igneous clinopyroxene indicates formation in a lower-temperature hydrothermal environment, that is, >300°C (Bird et al., 1984, 1986; Manning and Bird, 1986).
Mica occurs only in felsic or plagioclase-rich veins, where it forms subhedral, somewhat ragged crystals 1-3 mm long. It is typically strongly pleochroic, from yellow to dark brown, but a few grains are reddish brown in color, presumably reflecting oxidation. Although there is no clear compositional demarcation between phlogopite and biotite (Deer et al., 1992), phlogopite generally has high MgO, low TiO2, and little Al substituting for Fe and Mg in the Y position. Based on their strong pleochroism and the high FeO and TiO2 contents, most of the micas in Hole 735B veins are classified as biotite, although a few are relatively Mg rich (Table T6). However, very few of these micas have AlVI in their formulas.
Most of the analyzed grains are uniform in composition without observable zoning. However, in a few cases, rims of crystals are slightly more Fe rich than the cores (e.g., Sample 176-735B-149R-4, 82-87 cm) (Table T6). SiO2, Al2O3, and K2O contents are relatively constant, whereas TiO2, FeO, and MgO show significant variations from grain to grain. TiO2 contents are generally between 3 and 4 wt% but range from 0.28 to 4.25 wt% (Table T6). Mg numbers range from 37.2 to 59.1. Compositional variations do not correlate with color; the red grains have essentially the same composition as the other crystals.
In Hole 735B, chlorite is typically present as irregular patches in the groundmass of felsic or plagioclase-rich veins. In a few cases, it rims amphibole or mica or partially replaces plagioclase. Although widely distributed, it rarely makes up more than 1 modal% of any given vein.
Most of the chlorite is light green or colorless, but brown varieties are locally present. Although it is difficult to obtain reliable microprobe analyses of such fine-grained hydrous minerals, the data reported in Table T7 are very consistent. Duplicate analyses within any patch show only minor differences, and these are primarily in the Fe/Mg ratios (Table T7). All of the analyzed grains are very close to an ideal formula with 20 cations when calculated on the basis of 36 (O, OH).
As expected, chlorite compositions vary widely from vein to vein, ranging from 24.76 to 30.52 wt% SiO2 and from 16.21 to 20.10 wt% Al2O3. FeO and MgO also vary widely and are negatively correlated. Most other oxides are present in very small quantities, although MnO is as high as 1.45 wt% in one specimen. There are no systematic variations in chlorite compositions, either with depth in the hole or with mode of occurrence within individual veins.
Quartz is a common, although not voluminous, mineral in many felsic and plagioclase-rich veins. In felsic veins with a myrmekitic texture, the quartz forms small blebs and irregular patches intergrown with sodic plagioclase and is more or less regularly distributed throughout the vein. The more common occurrence is as irregular masses in the groundmass of plagioclase veins, where it forms small, anhedral grains that are in optical continuity over distances to 1-3 mm. Typically, the quartz is colorless to white and lacks inclusions. In a few cases, it is marginally replaced by yellowish brown clay minerals.
As expected, the quartz consists of nearly pure SiO2, occasionally accompanied by trace amounts of Al2O3, TiO2, FeO, or CaO (Table T8). The nonsilica components never exceed 0.19 wt% and do not vary with the mode of occurrence of the quartz.
Epidote occurs in many plagioclase-rich veins but is typically present in very small quantities (<1 modal%). Most commonly, it replaces the cores of plagioclase crystals, but in a few cases it is present as discrete clusters of grains within the vein matrix. It is typically white to very pale brown in color and forms subhedral to anhedral crystals 0.1 to 0.5 mm across. In Sample 176-735B-120R-1, 85-91 cm, epidote forms subhedral, prismatic to bladed crystals up to 4 mm long. These are in a matrix of dark brown, aphanitic material and are associated with plagioclase, diopside, and chlorite.
The epidote has nearly constant SiO2 and CaO but varies significantly in iron and alumina (Table T9). In nearly all of the analyzed grains, silica fills the tetrahedral position; thus, the aluminum is all in sixfold coordination. Iron in epidote can be assumed to be mostly in the ferric state and thus to also occur in sixfold coordination. This assumption is supported by the strong negative correlation of Fe2O3 and Al2O3 in the analyzed grains (Fig. F7). Fe2O3 contents range from 3.14 to 10.56 wt% and Fe2O3/Al2O3 ratios range from 0.10 to 0.41 (Table T9). Although there are no systematic downhole variations in composition, the high-iron epidotes are present in the deepest levels in the hole (>700 mbsf). However, they are associated with relatively low-iron varieties in the same sample (Table T9).
None of the epidote grains show optical zoning, even though compositional zoning is apparent from the microprobe analyses. This is particularly apparent in Sample 176-735B-120R-1, 86-91 cm, where iron contents within single crystals range from just under 6 to over 10 wt% (Table T9).
Prehnite is present only in the lower part of the hole in composite veins with smectite and carbonate. These veins typically have narrow bands of greenish brown smectite along the margins and have prehnite, with or without carbonate, in the central part. The prehnite forms colorless to very light brown bladed crystals, 0.1-0.2 mm long, typically aligned subparallel to the vein axis. All of the analyzed prehnite is very uniform in composition and shows no evidence of zoning within individual minerals or within veins, nor is there any variation in composition with depth (Table T10).
Ilmenite is by far the most abundant opaque mineral in Hole 735B veins, but trace amounts of magnetite, hematite, and sulfide may also be present. Ilmenite is almost exclusively present in the felsic and plagioclase-rich veins, where it forms individual anhedral grains, usually 0.2-0.5 mm across, or anhedral aggregates along the vein margins. In the latter case, the ilmenite may extend from the vein into the host gabbro. It can occur alone or, more commonly, in small clots associated with green or brown amphibole, titanite, and rare biotite.
The ilmenite is quite uniform in composition, consisting almost entirely of FeO and TiO2 with minor amounts of MnO (up to 2.45 wt%) and MgO (up to 0.88 wt%) (Table T11). Formulas calculated on the basis of six oxygens have slightly high iron because all of the iron was calculated as Fe+2. Compared to ilmenite in the host rock, the vein ilmenite has somewhat higher MnO and lower TiO2 and MgO.
Minute grains of secondary sulfide are commonly associated with ilmenite in the veins, particularly along grain boundaries. Pyrite or pyrrhotite and chalcopyrite are the most common varieties but are present in only trace amounts.
A few veins also have small amounts of reddish brown ferric oxide, either rimming some of the ilmenite grains or as small patches associated with carbonate in the vein matrix. The distribution of ferric oxide in the core is directly related to the abundance of carbonate veins, but most of the oxide replaces olivine or orthopyroxene in the host rock rather than occurring in veins.
In Hole 735B, zeolites are primarily present in the groundmass of felsic or plagioclase-rich veins, where they are associated with, and appear to replace, dark, aphanitic material. In a few veins they fill late-stage cracks cutting through the centers of felsic and plagioclase veins. The zeolites are associated with amphibole, chlorite, clay minerals, prehnite, and, rarely, carbonate.
Two zeolite species are recognized in these veins: natrolite and thomsonite. Both are typically present as small, radiating clusters of white to light brown acicular crystals (Pl. P5, fig. 2), although natrolite also forms minute white prismatic crystals with square cross sections (Pl. P5, fig. 3). For zeolites, both of these minerals are relatively uniform in composition (Table T12). Natrolite averages about 50 wt% SiO2, 27 wt% Al2O3, and 10 wt% Na2O. CaO is typically <1 wt% but ranges up to 2.41 wt% (Table T12). Thomsonite has significantly higher CaO and lower SiO2 and Na2O than natrolite. CaO/Na2O ratios vary widely in the thomsonite, ranging from 0.21 to 0.45.
Apatite is a common constituent of many of the shear zones impregnated with Fe-Ti oxide melts but is relatively rare as a vein mineral. It is restricted to felsic and plagioclase-rich veins, where it forms small (usually <0.1 mm) subhedral to euhedral grains in the matrix. The apatite crystals occur either singly or in small clusters, sometimes associated with titanite or ilmenite. They are relatively uniform in composition but contain small amounts of SiO2, Al2O3, and Na2O (Table T13).
Approximately half of the felsic and plagioclase veins examined contain trace amounts of zircon. The zircon is present as minute (0.05 mm or less), colorless, euhedral crystals sometimes associated with apatite. Only rarely is there more than one grain per thin section of a vein. The zircon is relatively uniform in composition, the major variation being in P2O5, which ranges from 0 to a maximum of 2.27 wt% (Table T13).
Virtually every felsic and plagioclase-rich vein in Hole 735B core has at least small quantities of titanite. The titanite typically forms subhedral to euhedral crystals from 0.1 to 1 mm across that are almost always associated with ilmenite or green or brown amphibole. Most grains are colorless to very light brown but some have red pleochroic cores, suggesting high rare earth element concentrations (Pl. P3, fig. 2). The grains may be randomly distributed in the veins but are most abundant near the vein walls. In a few cases they are concentrated in narrow bands or fractures.
The titanite is very uniform in its bulk composition (Table T14), showing only small variations in FeO and Al2O3. Some of the red pleochroic crystals have La2O3 contents up to 0.67 wt%.
Late-stage smectite-coated cracks are common throughout Hole 735B core and some amphibole veins in the upper part contain bands of smectite, but significant clay mineral veins are present only in the lower parts of the section. In these veins, the clay minerals are commonly associated with prehnite and carbonate. In most veins, the clay minerals form platy or tabular crystals oriented perpendicular to the vein walls. Where they occur with other minerals, the clay minerals typically line the vein walls, whereas carbonate and prehnite fill later cracks. Clay minerals are also present in small quantities in felsic veins, where they partly replace plagioclase or other minerals in the groundmass. In a few of these veins, very light brown clay minerals replace quartz in the groundmass.
Most of the analyzed clay minerals are high-magnesia smectite, although a few green varieties have relatively high iron (Table T15). Neither the Fe- nor Mg-rich clay minerals are associated with ferromagnesian minerals, and generally the composition is independent of the host mineral. Exceptions to this rule are found in Samples 176-735B-203R-2, 11-16 cm, and 210R-5, 116-120 cm, where what appears optically to be a clay mineral is actually partly altered plagioclase. A few clay mineral samples with high CaO (not shown in Table T15) contain small amounts of finely divided carbonate.
Small amounts of carbonate are widely scattered in the Hole 735B veins. The carbonate either replaces groundmass minerals or forms narrow, discrete veins. In both cases, it is typically associated with clay minerals, zeolites, or prehnite. Most of the carbonate, particularly that in discrete veins, is white, but some of the material is light brown or is intimately intermixed with light brown clay minerals. It is entirely calcite in composition, having a maximum of 5.67 wt% MgO and only trace amounts of FeO, MgO, SiO2, and SrO (Table T16).