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doi:10.2204/iodp.proc.324.104.2010

Alteration and metamorphic petrology

The entire section of basaltic rocks (massive and pillow lavas) recovered from Hole U1347A has been affected by slight to moderate low-temperature water-rock interactions, resulting in almost complete replacement of glassy mesostasis and complete replacement of olivine. In contrast, plagioclase and clinopyroxene are generally well preserved throughout the hole, either in the groundmass or as phenocrysts. The overall alteration of the basalt pieces ranges from slight to moderate (from 5% to 50%), with the majority of the rocks showing ~15% alteration, estimated visually using binocular microscope on the archive half and optical microscope on discrete thin section samples, without taking veins and vein halos into account. Fresh glass was observed in some slightly altered basaltic sections and at pillow margins (Fig. F34).

Clay minerals, together with calcite, are the most abundant secondary minerals in Hole U1347A, replacing primary phases, glassy mesostasis, and filling vesicles and veins. As a result of the clays' fine-grained size, identification by optical microscopy was difficult and clays are therefore named according to color (observed on thin sections) (i.e., brown clays and white clays). Other alteration minerals observed in the basaltic cores are pyrite and Fe oxyhydroxides. Pyrite is the only sulfide mineral observed throughout Hole U1347A, either disseminated in the groundmass or as a constituent of veins and vesicles (Fig. F34).

Alteration degree and mineralogy at Site U1347 will be compared with those of basaltic rocks recovered at Site U1346 (Shirshov Massif), previously well studied portions of ocean crust (Alt, 1995, 2004), and basalts recovered from the OJP (ODP Leg 192; Mahoney, Fitton, Wallace, et al., 2001; Banerjee et al., 2004).

Low-temperature pervasive alteration processes

Based on core descriptions and thin section observations, alteration of the basaltic rocks recovered from Hole U1347A is similar throughout the entire hole, with variations only in the degree of alteration (see Figure F34 and 324ALT.XLS in LOGS in "Supplementary material").

Primary phase replacement

Basalts recovered from Hole U1347A are dominated by plagioclase and pyroxene (as phenocrysts and in the groundmass) with minor olivine microphenocrysts and titanomagnetite in a cryptocrystalline to glassy groundmass (see "Igneous petrology" for detailed description). Plagioclases and pyroxenes are slightly to moderately altered to brown and white clays (typically, from ~5% to ~40%), phenocrysts being commonly less affected by alteration than minerals forming the groundmass (Fig. F35). Olivine phenocrysts are completely pseudomorphed by various proportions of calcite and brown clays (likely Mg saponite) (Fig. F36) throughout the basement lava flows. Only three occurrences of relics of fresh olivine were observed in Hole U1347A, all of which were present in glassy pillow margins (see "Igneous petrology"). Titanomagnetites, commonly present in the groundmass, are 5%–20% altered to fine-grained nonreflective opaque minerals, such as Fe oxyhydroxides.

Glassy mesostasis is commonly totally altered to brown clays in the basaltic rocks, whereas relics of interstitial glass have been observed in few samples (Fig. F37).

Alteration of chilled-margin glass

Glass at the margin of pillows and tops of sheet flows is commonly well preserved in Hole U1347A. Alteration of the glass consists of palagonite replacement along veins and vesicles (Fig. F38). Most pillow margins are rimmed by a black soft material, which is composed of a mixture of clays (i.e., montmorillonite and nontronite) (Fig. F39). This ~1 cm thick, dark, soft rim may either be a result of alteration of the pillow margins or clay-rich sediment that has been deposited between pillows or lava lobes.

Vesicles

Basaltic rocks recovered from Hole U1347A are all poorly vesicular. The vesicles are mainly filled with calcite and gray clay (most likely nontronite and saponite) (Fig. F40). Traces of pyrite are present in vesicles in all of the lava units described in "Igneous petrology" (Figs. F34, F41A, F41B). Vesicles commonly contain a rim of segregated melt that has been altered to dark brown clay minerals and fine-grained oxides (Fig. F41C). There is no systematic variation in mineral infilling vesicles with depth (Fig. F34), but variation occurs within lava flows and calcite vesicles are commonly observed close to lava flow contacts. Occasionally calcite shows radial growth patterns in vesicles (Fig. F41D).

Veins

Three main vein types have been identified in Hole U1347A: (1) calcite veins, (2) green clay veins, and (3) calcite and green clay veins together (see Fig. F34 and 324VEIN.XLS in LOGS in "Supplementary material"), all with or without pyrite (Figs. F34, F42). Calcite veins are the dominant vein type (N = 546) and are found throughout Hole U1347A (Fig. F34). Calcite and green clay veins (N = 152 and N = 108, respectively) are also distributed throughout the hole (see Fig. F34 and 324VEIN.XLS in LOGS in "Supplementary material"). There is an average of ~3 veins/m in the basement lavas, and average vein thickness is ~1 mm (Fig. F34).

Interpretations of alteration

Basaltic rocks recovered from Hole U1347A have all undergone similar pervasive alteration throughout the hole, with variations only in alteration degree (from 5% to 50%). The predominance of clay minerals and calcite as secondary phases suggest relatively low temperature of alteration (<100°C) (Alt, 1995; Honnorez, 2003).

Alteration in Hole U1347A is clearly different from that encountered in Hole U1346A on Shirshov Massif in terms of fluid flux and/or fluid chemistry. On Shirshov Massif, three types of extensive pervasive low-temperature alteration were identified, suggesting chemically variable fluid flow (and maybe fluid flux). The predominant alteration type in Hole U1346A resulted from interaction of basalts with CO₂-rich seawater-derived hydrothermal fluids at relatively low temperature (<100°C) (Honnorez, 2003). In contrast, in Hole U1347A on Tamu Massif calcite replacement of the primary phases is less abundant and affects only olivine (micro-) phenocrysts and, rarely, the groundmass. This suggests that fluids circulating through the succession of lava units in Hole U1347A were compositionally different (less CO₂ rich) than those in Hole U1346A. The lower degree of alteration in Hole U1347A also suggests lower fluid fluxes and lower water-rock ratios.

Similar dark gray alteration has been described for the basalts of the OJP (Mahoney, Fitton, Wallace, et al., 2001; Banerjee et al., 2004) with replacement of olivine microphenocrysts and glassy mesostasis by clay minerals. This alteration type has been interpreted as a result of interaction between basalt and seawater-derived fluids under anoxic-suboxic conditions at low temperature and low water-rock ratios (Banerjee et al., 2004). Differences in alteration mineralogy in Hole U1347A are shown by the relative abundance of calcite replacing the olivine phenocrysts, the lack of celadonite, and the widespread occurrences of pyrite disseminated in the matrix or as a constituent of the veins and vesicles.

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