PETROGRAPHY AND MINERALOGY

Felsic and Plagioclase Veins

By number, felsic and plagioclase veins comprise only a few percent of the total but they make up nearly 40% by area (Table T1; Fig. F2). Thin section examination reveals no significant differences between felsic and plagioclase veins, but because they were logged separately during core description, we retain the terminology.

Felsic material may form irregular patches 1-3 cm across, anastomosing veinlets 2-3 mm wide, or single veins normally 5-10 mm wide (Pl. P1). In all cases, the veins or patches have sharp, well-fined contacts with the host rock. Many of the felsic veins are associated with oxide gabbros in lithologic Unit IV, and they are most abundant in the upper 500 m (Fig. F3). They decrease rapidly downhole and are virtually absent below ~1250 meters below seafloor (mbsf).

The felsic and plagioclase veins consist predominantly of plagioclase, with lesser amounts of amphibole, biotite, and quartz (Pls. P1, P2). Many also contain trace amounts of ilmenite, titanite, zircon, albite, apatite, and epidote as well as variable quantities of late-stage minerals such as chlorite, clay minerals, zeolites, and carbonate. A few have narrow, 2- to 3-mm-wide bands of amphibole + diopside in the center (Pl. P1, fig. 3) but most lack clinopyroxene. Narrow bands of greenish brown amphibole along the vein walls are common, particularly where the vein cuts clinopyroxene crystals in the host rock (Pl. P3, fig. 3, fig. 4).

Plagioclase is present in two modes in the felsic veins: as myrmekitic intergrowths with quartz and minor K-feldspar and as large, strongly zoned crystals. Myrmekite is relatively rare, present in <5% of the veins examined in thin section. A typical example of a myrmekite vein (Sample 176-735B-124R-1, 111-115 cm) has narrow bands of greenish brown amphibole and minor oxide along the margins, grading inward into a zone containing large, zoned plagioclase crystals and quartz, which in turn grades into myrmekite. The myrmekite varies significantly in grain size from very fine to relatively coarse (quartz blebs up to 0.5 mm in diameter).

Large, concentrically zoned plagioclase crystals are present in nearly every felsic vein examined (Pl. P2). They are commonly subhedral, blocky crystals ranging from 2 to 10 mm across. Most have pitted cores of andesine containing small inclusions of amphibole, chlorite, or dark aphanitic material and clear white rims of albite and oligoclase. In some cases, the plagioclase is extensively replaced by green or brown chlorite, locally accompanied by minor epidote or carbonate. Narrow, irregular veinlets of colorless albite and oligoclase are commonly present, and albite may replace, as well as rim, the more calcic cores.

Most of these plagioclase-rich veins have relatively high porosities, suggesting corrosion of the grains by hydrothermal fluids. The resulting pore space may be open or, more commonly, partly filled with Na-plagioclase and quartz (Pl. P2).

The quartz in the felsic veins is typically present as small, irregular interstitial grains, ranging from 0.1 to 1 mm across. Commonly, the quartz grains, although physically separated, are in optical continuity, forming large, almost poikilitic masses (Pl. P2, fig. 3). The quartz is typically concentrated in the centers of the veins where the porosity is highest. In some samples, the quartz itself is rimmed and partly replaced by light brown smectite. Smectite, chlorite, prehnite, and zeolites may also fill some of the spaces between the plagioclase grains.

Almost all of the veins that contain significant amounts of quartz (>5 modal%) also contain a few crystals of mica, up to 5 mm across. The mica is typically yellowish brown biotite or phlogopite, and it is present either as single crystals or as rims on amphibole. The mica may also be associated with small amounts of iron oxides and titanite.

Trace amounts of zircon are also present in the felsic and plagioclase veins. It forms small, euhedral crystals usually ~0.1 mm across, typically enclosed in plagioclase.

Amphibole Veins

Amphibole veins are virtually confined to the upper 600 m of the section and most are present in the upper 250 m (Fig. F3). They are most abundant in highly sheared gabbros, particularly in the amphibole gneisses near the top of the hole. Most are 0.2-3 mm wide, have sharp but irregular contacts, and are oriented at high angles to the foliation (Pl. P4, fig. 1). They both cut and are cut by the foliation in the host rocks. Many are small, discontinuous features that extend for only a few centimeters, although some may be several meters in length. Small veins commonly are not continuous across clots of colorless amphibole after olivine and orthopyroxene but, rather, merge into them.

These veins consist almost entirely of small, subhedral crystals of brown to greenish brown amphibole, 0.2 to 1 mm across, in some cases accompanied by small amounts of plagioclase. The amphibole crystals may have a random orientation or they may be aligned along the length of the vein. Optical zoning is not visible in thin section, and most amphibole grains are compositionally homogeneous. Some of the amphibole is rimmed by green chlorite.

Plagioclase + Amphibole Veins

Plagioclase + amphibole veins are the third most abundant variety in Hole 735B (Table T1) and make up nearly 8% by area (Fig. F2). They are primarily present between 550 and 950 mbsf, that is, below the monomineralic amphibole veins and below most of the plagioclase + diopside veins (Fig. F3). Most of these veins consist dominantly of plagioclase, with only small amounts of amphibole, and hence are difficult to distinguish macroscopically from plagioclase or felsic varieties. A few contain small relict grains of diopside, completely surrounded by amphibole, suggesting that these two types of veins are gradational.

The plagioclase + amphibole veins typically range from ~2 to 10 mm in width and have sharp boundaries with the host rock (Pl. P1, fig. 3, fig. 4). Typically, where these veins cut clinopyroxene in the wall rock, a narrow band of amphibole is developed along the contact (Pl. P3, fig. 3, fig. 4); where the veins cut plagioclase, the contact is marked by secondary plagioclase. Many of the veins are compositionally zoned with alternating bands of amphibole and plagioclase (Pl. P3, fig. 3, fig. 4). In some cases, the amphibole may be present as a discrete band along the center of the vein (Pl. P1, fig. 3, fig. 4).

The plagioclase in these veins typically forms subhedral, blocky, strongly zoned crystals, often with a pitted appearance. The plagioclase-rich portions are typically quite porous, with many open spaces between the grains. Nearly all of these veins also contain small amounts of titanite, which forms subhedral to euhedral grains, typically close to the vein wall (Pl. P3, fig. 2). Chlorite is present in about a third of the veins, rimming amphibole or filling spaces between the plagioclase grains.

The amphibole along the vein walls is greenish brown in color and commonly forms ragged grains oriented perpendicular to the wall. Most veins also contain scattered subhedral to euhedral crystals of either brownish green or green amphibole intergrown with the plagioclase. Many of these grains, particularly the larger ones, exhibit some optical zoning, from brown or greenish brown in the cores to brighter green on the margins. The most spectacular example of zoning is from a small felsic patch in Sample 176-735B-202R-7, 96-101 cm. In this patch, amphibole forms clots up to 5 mm across that replace plagioclase. The amphibole adjacent to the plagioclase is dark brown, but it grades rapidly into light brown material and then into green amphibole (Pl. P5, fig. 1). The green amphibole also exhibits strong zoning.

A few of the plagioclase + amphibole veins contain small patches of dark, very poorly crystallized material with tiny skeletal needles of bright green actinolite.

Diopside and Plagioclase + Diopside Veins

These veins are particularly abundant in a 235-m-thick interval between ~165 and 400 mbsf, but several smaller clusters are also present between 460 and 750 mbsf (Fig. F3). Most are 2-10 mm wide and typically have sharp boundaries with the host rocks, which are chiefly gabbro and olivine-bearing gabbro (<5% olivine) or, more rarely, olivine gabbro.

The diopside is typically present as irregular to tabular, light green to colorless grains along or near the vein walls, but in a few samples it forms a 1-mm-wide band down the center of the vein (Pl. P3, fig. 1). Large (up to 4 mm), euhedral, zoned crystals are present in a few veins, for example, Sample 176-735B-69R-5, 31-41 cm (Pl. P4, fig. 2). Many of the grains are pitted and irregular, and most are rimmed and partly replaced by greenish brown amphibole. Typically, diopside makes up no more than 5 to 10 modal% of a given vein but can be up to 90%. It is invariably associated with intermediate plagioclase, which makes up the bulk of most veins, and with variable amounts of amphibole. Other common minerals in these veins include chlorite, epidote, titanite, Na-plagioclase, clay minerals, and zeolites. Quartz and biotite have not been found in the diopside-bearing veins. The chlorite is nearly always in the groundmass of the vein, filling spaces between plagioclase crystals. Na-plagioclase rims and fills spaces between more calcic plagioclase grains, producing highly zoned crystals like those in the felsic and plagioclase-rich veins. In a few cases, it also fills narrow veinlets or cracks cutting through the other grains. Epidote is present in only trace amounts, usually as an alteration product of plagioclase. At least a few grains of titanite are present in most diopside-bearing veins (Pl. P3, fig. 1), and in some cases they are associated with ilmenite.

Smectite Prehnite Carbonate Veins

Smectite veins are present throughout the Hole 735B core but are best developed in the lower 500 m of the section and in a 260-m-thick interval between 575 and 835 mbsf (Fig. F3). Even in the lower 500 m of the section, however, their distribution is uneven, with a concentration of veins between 1230 and 1330 mbsf and a barren zone between 1340 and 1380 mbsf (Fig. F3). Although abundant in number (Table T1), most of these veins are simply minute, smectite-lined cracks <1 mm wide, and they make up only ~3% of the total area of the core (Fig. F2). A few smectite + carbonate and smectite + zeolite veins are also present, but these are less abundant. The smectite veins may be present either as individual features or as narrow cracks in the center of felsic veins. In some cases, they form networks of hairline cracks, particularly in plagioclase where the host mineral is partly replaced.

The well-developed smectite veins in the lower part of the hole are generally 2-5 mm wide and have relatively sharp contacts (Pl. P5, fig. 4), except where the vein intersects olivine. Where that happens, the olivine is extensively altered to smectite + magnetite + pyrite, whereas adjacent plagioclase and clinopyroxene are little affected. Veins between ~575 and 835 mbsf are filled with dark green smectite, commonly accompanied by variable amounts of pyrite. Those deeper in the hole consist of light green or white, well-crystallized smectite, with or without prehnite. In most cases, the smectite crystals are oriented perpendicular to the vein walls, whereas prehnite grains have a random orientation. Where the two are present together, smectite lines the vein walls and prehnite fills the center. Small amounts of carbonate are commonly included in these veins (Pl. P5, fig. 4), but other minerals are absent.

Zeolite Prehnite Carbonate Veins

Separate zeolite veins are relatively rare and make up <2% of the total number (Table T1). However, zeolites are also present as minor groundmass phases in felsic veins and occasionally they fill late-stage cracks in such veins (e.g., Sample 118-735B-84R-6, 31-41 cm) (Pl. P4, fig. 4). Most zeolite veins are present in the lower part of the hole between ~1385 and 1455 mbsf, where they are associated with smectite and prehnite veins. The zeolite veins are typically 1-2 mm wide, 10-20 mm long, and have relatively sharp but irregular boundaries. They are filled primarily with sheaflike clusters of light brown natrolite crystals, sometimes accompanied by smectite or carbonate (Pl. P5, fig. 2, fig. 3). A few thomsonite veins are present in the lower 100 m of the core, but this mineral is most commonly present in the groundmass of felsic veins.

Prehnite veins are restricted to the lower 200 m of the core and are most abundant between ~1300 and 1450 mbsf. They are similar to the zeolite veins in size and appearance, and the two are typically present together. Many of the prehnite veins are zoned, with thin bands of smectite along their margins. The cores may be filled with prehnite alone or with mixtures of prehnite, zeolite, and minor carbonate. The prehnite is typically present as small, colorless, tabular crystals randomly oriented in the vein.

Carbonate Veins

Carbonate veins are widespread and abundant in the core but make up <2% of the total area (Table T1). They are concentrated in a 100-m-thick interval between 500 and 600 mbsf, which is characterized by brittle deformation and by extensive low-temperature groundmass alteration (Fig. F3). In other parts of the core, carbonate veins are typically present in small clusters and are associated with smectite, zeolite, and prehnite veins. Wherever carbonate veins are abundant, the adjacent host rock is reddish in color because of oxidation of iron in the silicate minerals.

Most of these veins are <2 mm wide, and they rarely extend for more than 10-15 cm in the core (Pl. P4, fig. 3). Many are just hairline cracks that radiate outward from an irregular mass of groundmass carbonate. The carbonate in these veins consists either of minute fibers oriented perpendicular to the vein walls or of small anhedral, interlocking crystals. In addition to carbonate many of these veins contain small amounts of zeolite, prehnite, and, especially, smectite. In such mixed veins the carbonate is usually present in the center and appears to be the latest mineral deposited (Pl. P5, fig. 3, fig. 4). Four of the logged veins consist of mixtures of carbonate and iron oxyhydroxide similar to that replacing olivine in the host rock.

Other Veins

A few veins of quartz, chlorite, and epidote are also present in the core, but these are rare. Eight quartz veins were logged, mostly between 1264 and 1505 mbsf, and three epidote veins are present, the largest of which is at 656.4 mbsf. The quartz veins are <2 mm wide, ~10 cm long, and have sharp, planar contacts. A little smectite is present along the vein walls. The only significant epidote vein is ~15 mm wide and 20 cm long, pinching out downward in the core. It is filled with a mixture of epidote and amphibole and has a chloritic halo 1-2 cm wide.

Chlorite is relatively common in the groundmass of many felsic and plagioclase veins, but only a few separate chlorite veins were recognized, most of which are present in the lower 50 m of the core (Fig. F3). A cluster of five small chlorite veins are present in Core 176-735B-121R at 721 mbsf. Those in the lower part of the hole typically contain small amounts of smectite, prehnite, or zeolite along with the chlorite.

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