During petrographic observations, we found a type of microscopic vein in 31 (~14%) of 217 lithologies in 199 samples (Tables T1, T2). Samples containing the veins are olivine gabbros and gabbros, troctolites, and oxide-rich gabbros, and we did not find the veins in any microgabbros or felsic rocks. However, it is noted that we examined very few microgabbros and felsic rocks.
Because the veins are composed of high-temperature minerals (i.e., clinopyroxene, orthopyroxene, brown amphibole, and plagioclase), for convenience, these veins are called the "high-temperature microscopic veins" in this paper. Figure F1 shows the downhole distribution of the high-temperature microscopic veins. Within the interval of 500-1500 mbsf drilled during Leg 176, the veins are relatively rich at 620-680 and 950-1070 mbsf. We selected four thin sections of olivine gabbro for detailed examination; that is, Samples 176-735B-101R-1 (Piece 7, 106-108 cm [586.16 mbsf]), 176-735B-112R-2 (Piece 1, 64-66 cm [644.62 mbsf]), 176-735B-130R-3 (Piece 11, 93-95 cm [797.57 mbsf]), and 176-735B-156R-3 (Piece 5, 51-54 cm [1027.59 mbsf]). In the next section, we describe the petrography of the examined samples. Petrographic summaries and representative microprobe analyses of the constituent minerals are given in Table T3 and Tables T4, T5, T6, T7, and T8, respectively.
The olivine gabbros we examined are similar to each other in petrography. These olivine gabbros show coarse- to medium-grained, equigranular to seriate textures and contain olivine, plagioclase, and clinopyroxene as the predominant phases (Fig. F2). There is almost no evidence of ductile and/or brittle deformation, except for the weak undulatory extinction and curvature of twinning of plagioclase. Olivine and plagioclase grains have subhedral to euhedral granular shapes. Olivine is very homogeneous in composition within a single grain and also within a single thin section. The average composition of olivine within the examined thin section ranges from Fo70.3 to Fo63.6 (Table T3). Thus, the examined olivine gabbros are not primitive in chemistry. The composition of plagioclase is very diverse even in a single thin section due to magmatic zoning in cumulus grains and the presence of less calcic plagioclase in the veins, which are targets of this paper. The compositional range of magmatic plagioclase within a single thin section is from An59.7-An47.9 to An53.3-An45.6 (Table T3). Clinopyroxene grains are subhedral to anhedral with granular to ophitic-poikilitic texture. Clinopyroxene compositions are also diverse and heterogeneous within a single thin section because of their complex magmatic and postmagmatic histories. The average Mg# of magmatic clinopyroxenes in the examined samples is from 81.1 to 74.7 (Table T3). Orthopyroxene occurs as thin rims around olivine, and brown amphibole occurs as rims around clinopyroxene and also around Fe-Ti oxides with irregular shapes. These phases are thought to be magmatic products, based on these textures. Orthopyroxene and brown amphibole are also present as small blebs within grains of clinopyroxene. Small amounts of Fe-Ti oxides, sulfides, and apatite are present as accessory minerals. In some samples, brownish green, greenish, and colorless amphiboles occur as replacement products of clinopyroxene and brown amphibole and also are filling brittle fractures as thin veins within primary phases. These grains of amphibole are clearly metamorphic products based on these textures. The compositions of amphibole are very diverse, as is expected from the variations in texture and color. Chemical features of clinopyroxene, orthopyroxene, and amphibole will be discussed in a later section.
Figure F3A shows a high-temperature microscopic vein from olivine gabbro (Sample 176-735B-101R-1 [Piece 7, 106-108 cm]) (see Fig. F2A, F2B). In the cumulus grain of olivine (Fo70.3), the vein is composed of an aggregate of orthopyroxene (Mg# = 74.9) + brown amphibole + plagioclase. A very narrow magmatic rim of orthopyroxene (inner side) and brown amphibole (outer side) surrounds the cumulus olivine grains. The vein extends into nearby magmatic plagioclase (~An60-An48), where it is composed of less calcic plagioclase (~An42-An25). The distinction between the magmatic plagioclase and the vein plagioclase under the microscope is easy because of the difference in birefringence (Fig. F3A). A compositional map clearly shows a difference between the magmatic plagioclase and vein plagioclase (Fig. F3B, F3C, F3D). The boundaries between the two stages of plagioclase are diffuse and very irregular. In general, the widths of these veins are thicker in magmatic plagioclase than in mafic silicates and are not constant in magmatic plagioclase, that is, they irregularly expand and contract (Fig. F3A). In this case, the width of the vein is ~400-800 µm at most. Figure F4 shows the histogram of An% of the analyzed plagioclase in this area of the thin section, clearly indicating a bimodal distribution of calcic plagioclase of magmatic origin and less calcic plagioclase in the veins, although this does not indicate the true proportions of these two stages of plagioclase. It is noteworthy, however, that the vein plagioclase is not always less calcic. Some parts of the veins, especially within the olivine grains, contain plagioclase as calcic as the magmatic plagioclase (up to An57.5 in this case) (Figs. F3B, F3C, F3D, F4). Therefore, it is likely that during the formation of the veins (at least in the earlier stage), physicochemical conditions were not very different from those for the magmatic plagioclase. This may be also suggested by the same mineralogy in the magmatic rims on olivine and in the veins (i.e., orthopyroxene and brown amphibole).
In Figure F5 (Sample 176-735B-130R-3 [Piece 11, 93-95 cm]), two high-temperature microscopic veins are cutting magmatic grains of olivine, clinopyroxene, and plagioclase. In this case, the width of the vein is ~80-200 µm at most. The petrographic features of these veins within the grains of olivine and plagioclase are very similar to those in Figure F3. In the magmatic plagioclase (An53.3-An52.0), the boundary between the magmatic plagioclase and less-calcic plagioclase (An48.7-An37.3) of the vein is diffuse and irregular. The extension of this vein into the magmatic clinopyroxene is marked by a discontinuous, irregular alignment of elongated blebs composed of brown amphibole (±plagioclase). This discontinuous and irregular occurrence is very peculiar by contrast with the microgabbro veins and lower-temperature hydrothermal veins reported from Hole 735B cores. The latter veins have straight and sharp contacts against the host minerals and a nearly constant width in microscopic or macroscopic scale, in general. In this thin section, the high-temperature microscopic vein is furthermore cut by a later, lower-temperature vein of brownish green to green amphibole, which looks like fillings within brittle fractures of the plagioclase grains. Similar greenish amphiboles are also observed within the high-temperature veins as discontinuous alignments with very irregular shapes and are also filling brittle fractures in the magmatic plagioclase. These observations suggest that this high-temperature microscopic vein survived during the rather long transition from higher- to lower-temperature conditions or was reactivated at lower-temperature conditions. Based on this occurrence, these brownish green to green amphiboles are referred to as "later amphiboles" in this paper.
Figure F6A and F6B shows a photomicrograph of olivine gabbro (Sample 176-735B-156R-3 [Piece 5, 51-54 cm]) (see Fig. F2C, F2D). A subhedral to anhedral grain of clinopyroxene (Mg# = 80.7-70.7) is penetrated by a high-temperature microscopic vein. This vein is composed of plagioclase (An35.3-An30.3) and brown amphibole within the grain of clinopyroxene and can be traced into the nearby magmatic plagioclase chadacryst (An54.7-An52.7) enclosed in the clinopyroxene grain. In this case, the width of the vein is ~100-300 µm at most. The mode of occurrence and compositional features of magmatic and vein plagioclase are the same as shown in Figure F3A. It is noteworthy in this figure that vermicular intergrowths of brown amphibole + orthopyroxene (Mg# = 71.8-68.4) occurred in clinopyroxene on both sides of the vein. This very symmetrical occurrence of the intergrowth relative to the high-temperature microscopic vein strongly suggests that the origin of the intergrowth is related to the penetration of the vein. The chemical compositions of the clinopyroxene are different in the parts with and without the intergrowth (Fig. F6C, F6D). This difference can be related to the penetration of the vein and formation of the intergrowth.
One of the most important features of these high-temperature microscopic veins is the mineral assemblage. Most veins are composed of clinopyroxene, orthopyroxene, brown amphibole, and plagioclase and virtually do not contain lower-temperature minerals, such as chlorite, smectite, epidote, prehnite, and/or talc, for example. Although some veins contain these low-temperature products (e.g., the later amphiboles mentioned above), these phases overprinted the earlier, higher-temperature phases. Another important feature is a dependence of mineralogy on the host magmatic minerals; that is, the mineral assemblages of the veins are strongly controlled by the host magmatic minerals penetrated by the veins. The "along-vein variation" in mineralogy of veins has also been described from the gabbros of Hole 735B (e.g., Mével and Cannat, 1991) and from the Mid-Atlantic Ridge at the Kane Fracture Zone (MARK) area (Gillis et al., 1993), but in veins that consist of lower-temperature minerals and are completely different from those described here. The significance of the along-vein variations will be discussed in a later section.
In the previous section, the intergrowth of brown amphibole + orthopyroxene in clinopyroxene grains along the high-temperature microscopic veins was described (Fig. F6). The very symmetrical occurrence of the intergrowth to the high-temperature microscopic vein strongly suggests that these two features are genetically linked to each other. In this section we describe another occurrence of brown amphibole + orthopyroxene intergrowth in clinopyroxene.
Figure F7A and F7B shows grains of clinopyroxene ~2 cm away from the clinopyroxene grain shown in Figure F6 (see Fig. F2C, F2D). Contacts between the two grains of clinopyroxene are interpenetrating and interlocking to each other. We tentatively call this texture an "interfingering" texture. This is a very common texture in olivine gabbros recovered from Hole 735B, and we observed this texture in ~62% of the examined samples (see Table T9; Fig. F1). In some cases, dependent on the cutting direction of thin sections, a grain of interfingering clinopyroxene looks like an inclusion within another grain of clinopyroxene (Fig. F6A).
An important feature of this texture is the occurrence of intergrowths of brown amphibole + orthopyroxene in these interfingering areas. Figure F7C and F7D show compositional maps for nearly the same area (see Fig. F2C, F2D). The interfingering area of clinopyroxene grains contains blebs of orthopyroxene and brown amphibole. Whereas, in general, grains of magmatic clinopyroxene in olivine gabbros are slightly dark under the microscope due to the presence of many thin exsolution lamellae of Fe-Ti oxide, within the interfingering area such lamellae are scarce and the development of (100) cleavage is also very weak. As a result, the interfingering parts of clinopyroxene look very clear under the microscope. These same features of clinopyroxene are also observed in clinopyroxene with intergrowths near the high-temperature microscopic veins (Fig. F5A, F5B) and are also common in secondary clinopyroxenes observed in oceanic gabbros (e.g., Ito and Anderson, 1983; Mével, 1987, 1988; Gillis et al., 1993; Gillis, 1995, 1996; Manning and MacLeod, 1996). Overall, it is noteworthy that two kinds of brown amphibole + orthopyroxene intergrowth are very similar in petrography and are not distinguished from each other.