A brief summary of the petrographic features follows (see also Table 1) but more details are available in Cornen et al. (chapter 21, this volume), Beslier at al. (this volume), and in the Leg 149 Initial Reports (Shipboard Scientific Party, 1994a, 1994b).
The peridotite samples are intensively serpentinized: more than 90% of the primary minerals are altered essentially to serpentine (lizardite, chrysotile); opaque minerals (sulfides and iron oxides), scarce chlorite, iowaite, and brucite. No talc nor antigorite were detected optically or with X-ray diffraction (XRD). Several types of serpentine were identified from the textural relationships: (1) dark or white serpentine resulting from the alteration of olivine and orthopyroxene crystals, and (2) serpentine in crosscutting veins of variable thickness (less than 1 mm to up to 20 mm), extent (1 mm to more than 10 cm), and appearance (pale-green fibrous, green, dark, white massive). The preservation of most details of the original rock texture implies that serpentinization occurred under static conditions. Pyroxenes are commonly replaced by bastite which keep their original cleavage trace. Calcite is associated with some serpentine in veins. The calcite precipitation followed the serpentine formation. The dominant vein-filling mineral changes progressively downhole from calcite to serpentine.
Amphiboles are generally uncommon in oceanic serpentinites with a few notable exceptions, including Hole 637A (Leg 103) (see also Aumento and Loubat, 1971; Roden et al., 1984; Kimball et al., 1986; Agrinier et al., 1993). Their rarity in Leg 149 ultramafics indicates that either amphiboles did not form or amphiboles that did form have been replaced by secondary phase. We are more inclined to believe in the first possibility since an exhaustive optical scan of numerous thin sections did not reveal remains of amphibole. In contrast, amphiboles are abundant in the Hole 637A serpentinites. They display a large variety of texture corresponding to a wide range of chemical composition (tremolites to pargasites) which allowed the various fluid-rock interaction stages to be described (Agrinier et al., 1988).
In the samples recovered during Leg 149 at Sites 897 and 899, different kinds of amphiboles were recorded inside mafic and ultramafic rocks. Kaersutite and Ti-richterites in unmetamorphosed alkaline lavas and microgabbros at Site 899 have a magmatic origin. Sheared gabbros recovered from the same site display pyroxene entirely replaced by chlorine-rich (up to 1.24 wt%) Ti-pargasite, by hornblende, and, at a late stage, by actinolitic hornblende and actinolite with chlorite (Fig. 2). This last association is common in the flaser gabbros of Site 900.
In the ultramafic rocks recovered during Leg 149, magmatic amphiboles are very rare. They were found only in the calcitized upper part of the Hole 897D inside some remnants of ultramafic material with unusual mineralogy. Indeed one thin section (149-897D-14R-4, 95—100 cm) displayed scarce dismembered veinlet-like ribbons with a sheared texture, set in entirely calcitized serpentinite. These ribbons are composed of clinopyroxene, Mg-ilmenite, rutile, Cr-bearing kaersutite, Ti-phlogopite, and probably former orthopyroxene, transformed to bastite. Similar mineral assemblages are known from veins inside xenoliths or peridotites with subcontinental mantle affinities. This would suggest a local metasomatism of the peridotites by alkaline melts under less severe conditions of formation than in the so-called MARID kimberlitic xenoliths (Bergman et al., 1981; Dautria et al., 1987; Lorand et al, 1990; Dawson and Smith, 1982). At Site 897, these products occur as dynamically recrystallized neoblasts, which indicates that melts infiltrated the peridotite before or during a shearing event and before the serpentinization of the orthopyroxenes.
Metamorphic amphiboles are much more abundant at Site 899 than at Site 897, although the same mineral relationships are observed at both sites. Representative amphibole compositions are reported in Table 2.
At Site 897, amphiboles are restricted to sheared plagioclase-bearing websterite where they occur around ortho- and clinopyroxene and in late brittle veins. Amphiboles are dominantly calcic, with compositions corresponding to tremolite. Amphiboles from the ultramafic rocks of Site 899 are also tremolitic and occur as (1) the main component of some clasts in the upper ultramafic breccia where they form very long, thin, and contorted white needles commonly associated to spinels (the only remnants of the primitive mineralogy), (2) syncrystallized needles with and inside large undeformed flakes of chlorite in place of primary crystals, presumably pyroxenes (Fig. 3), and (3) coarser crystals (300-500 µm) that have grown around clinopyroxenes and that constitute pockets or ribbons associated to deformation zones. In ribbons, amphiboles occur frequently as bent needles, which evidences pre- and syndeformation growths.
Several types of chlorite occur in the recovered basement sections. According to the nomenclature of Hey (1954), ripidolite is the main component of some clasts and schistose boulders from Site 899. These ripidolites differ significantly from chlorite, with similar blue anomalous birefringence that locally rims spinels in ultramafic blocks recovered in the same hole. They differ also from those found in sheared amphibolites and flaser gabbros, which have a lower Fe. Cornen et al. (chapter 26, this volume) have deduced from their mineralogy that these chlorite- and sphene-bearing rocks were retrograded in the greenschist facies and that they were derived from leucogabbro or plagiogranite. A similar rock (Schärer et al., 1995), has already been sampled in situ at the top of the peridotite section in the Galicia Bank (Dive 10, Beslier et al., 1990). Other associated clasts represent former mafic lavas inside which epidote, prehnite, and Al-chlorite have crystallized close to equilibrium. According to Liou et al. (1985), this association and the absence of pumpellyite is indicative of temperature conditions in the range of 200° to 400°C and a maximum pressure of 0.22 GPa.
Chlorite is present in the ultramafic rocks from at both Sites 897 and 899. It displays two types of occurrence: as reaction rims around spinel and as pseudomorphs after presumably clinopyroxene. In the latter case, they appear with strong birefringence color (second or third order) due to the syncrystallization of very fine needles of amphibole. Compositions are reported in Table 3. Following Hey (1954), they are clinochlore (Fig. 4A). According to Figure 4B (Laird, 1988), these chlorites are clinochlore, which plot near the 1 to 1 line corresponding to the Tschermak (TK) substitution. In ultramafic rocks, chlorites are the Al phase known to span a large range of pressure-temperature conditions from prehnite-pumpellyite facies up to upper amphibolite facies (Abbott and Raymond, 1984). At Sites 897 and 899 their relative high degree of TK substitution would indicate conditions of metamorphism corresponding to at least greenschist facies. This agrees with the stability conditions inferred by Sanford (1982) for the assemblage of chlorite-Ca amphibole occurring in reaction zones between ultramafics and varied country rocks.
XRD patterns on bulk samples confirm that serpentine is mainly lizardite and accessory chrysotile. Neither antigorite nor talc was detected. Serpentine analyses are reported in the Table 4. Their composition is rather uniform in the two sites. Figure 5 discriminates clearly between aluminous-poor serpentine that replaces olivine and bastites after pyroxene, which is aluminum rich. Vein serpentines have intermediate compositions and it is suspected that the most aluminous types represent a mixture of chlorite and serpentine. One cluster of serpentine compositions displays low Al and Fe and are from chrysotile veins and their immediately adjacent serpentine from both Sites 897 and 899. Fluorine contents are low, but chlorine may be present in small concentration, up to 0.77 wt% Cl content (Fig. 6). Mesh serpentines and vein serpentines have similar Cl content. This distribution is very similar to that exemplified in Galicia Margin serpentinites by Agrinier et al. (1988).
Iowaite (Mg4Fe(OH)8OCl.2-4H2O) was first described by Kohls and Rodda (1967) in serpentinites. It has since been found in several places, including in the Mariana and Izu-Bonin forearcs drilled during ODP Leg 125 (Helig and Swartz, 1992). During Leg 149 iowaite was identified by XRD aboard ship, mainly characterized by the peaks at 8.04 and 7.32 Å (Gibson et al., this volume). Microprobe analyses were performed on two samples that display well-developed grains. In Sample 149-897C-70R-1, 6-9 cm, iowaite rims olivine remnants in a serpentinized dunite and looks like brucite with higher birefringence (Fig. 7). Cores 897C-70R and 71R, which are composed of dunite and harzburgite, display several occurrences of this mineral. The composition reported in the Table 5, shows that iowaite contains less than 0.5% of SiO2 and about 1% of alumina by weight. The mineral is a little more magnesium-rich (Mg# = 81 to 83 vs. 78) and the chlorine content, which is near 5%, appears lower than the composition (8.5%) reported by Kohls and Rodda (1967). In this sample, beside the iowaite that rims olivine Fo90, the mesh serpentine has a "normal" composition, with Mg# close to 94 and chlorine up to 0.7%.
In another occurrence (Sample 149-897D-17R-6, 5-10 cm), a mineral with a distinct purple color appears close to brucite and calcite veins. XRD analysis revealed iowaite. In thin section, the pink-colored mineral, supposed to be iowaite, does not appear very different from serpentine between crossed polars: it composes lamellae with low birefringence and dull aspect. This material was stable under the microbeam, and analyses reveal low alumina and low FeO contents, the presence of silica in the range of 12% to 20%, chlorine up to 2.14%, and a surprisingly high Cr content (between 11% and 14% Cr2O3). Locally small spinel crystals (less than 100 µm in size) are enclosed in this pink "mineral." Serpentine from the same sample has low chlorine content (up to 0.25%). The aspect of this purple mineral and the absence of good reliability between analyses seem to correspond to a mixture of iowaite (since its characteristic X-ray peaks were detected), serpentine (which would account for the high content of silica), and an unknown Cr-bearing hydrate (which may be responsible for the color).
Brucite from the vein close to this purple mineral has a typical composition, with low FeO content (less than 0.05%) mineral and no chlorine.
Only pure calcite (Milliken and Morgan, this volume) analyses have been recorded in veins and in the carbonated mesh serpentine.