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doi:10.2204/iodp.proc.324.107.2010

Alteration and metamorphic petrology

The entire section of basaltic rocks (massive lava flows, pillow lavas, and hyaloclastites) recovered from Hole U1350A has been affected by slight to high degrees of low-temperature water-rock interaction, resulting in complete replacement of glassy mesostasis and olivine phenocrysts and slight to almost complete replacement of groundmass minerals (plagioclase and clinopyroxene). In contrast, plagioclase phenocrysts are generally well preserved throughout the hole, except in portions of stratigraphic Unit IV. Fresh glass is commonly preserved on the margins of flows and pillows in Subunits IIa and IIb, rarely preserved in Subunit IIc, and not preserved in Units III and IV. The overall degree of alteration of the basalts ranges from slight to moderate in the upper flow succession and hyaloclastites (Units II and III) to moderate to high in the plagioclase-phyric pillow succession (Unit IV). The degree of alteration was estimated visually using a binocular microscope on the archive half and using an optical microscope on discrete thin section samples, without taking into account the veins and vein halos.

Clay minerals, together with calcite, are the predominant secondary minerals in Hole U1350A, replacing primary phases and glassy mesostasis and filling vesicles and veins. Clay minerals were identified by optical microscopy and X-ray diffraction (XRD) patterns of whole rocks and veins. Nontronite and montmorillonite are the two main minerals replacing the groundmass, whereas an assemblage of saponite ± nontronite ± montmorillonite was identified in the veins. Other alteration minerals observed in the basaltic cores are pyrite, zeolites, and likely sanidine. Pyrite is the only sulfide mineral observed throughout the entire basaltic section in Hole U1350A, either disseminated in the groundmass or as constituent of veins and vesicles (Fig. F24). Zeolite was identified at various depths filling vesicles and veins, and sanidine was identified as a replacement of plagioclase phenocrysts in the plagioclase-phyric pillow succession of Unit IV.

Below, alteration degree and mineralogy at Site U1350 are compared with those of basaltic rocks recovered at previous sites at Shatsky Rise, Site U1346 (Shirshov Massif), Site U1347 (Tamu Massif), and Site U1349 (summit of Ori Massif), and with basalts recovered from the Ontong Java Plateau (OJP) (ODP Leg 192) (Mahoney, Fitton, Wallace, et al., 2001; Banerjee et al., 2004).

Alteration description

Based on core descriptions and thin section observations, alteration style of the basaltic rocks recovered from Hole U1350A is similar throughout the entire hole. Overall alteration degree varies from 10% to ~90% (Fig. F24; see 324ALT.XLS in LOGS in "Supplementary material"). One type of gray alteration was identified with significant variation in alteration degree that may be related to lithology, in particular to the intercalation of relatively thin massive lava flows and pillow lavas (Fig. F24). In Unit II, alteration is highly variable, ranging from 10% for the freshest samples to 50% for the most altered samples (Fig. F25). In the hyaloclastites of Unit III, the glass is completely altered to palagonite and brown clays, whereas plagioclase and clinopyroxene phenocrysts remain relatively unaltered. In Unit IV, alteration ranges from 40% to 90% (Fig. F25) and is likely related to the presence of interbedded limestones between the pillow lavas, which may have provided pathways for fluids to circulate and interact with the basalts.

Replacement of primary phases

Basaltic rocks recovered from Hole U1350A are divided into five petrographic units (see "Igneous petrology") and are mainly massive flows (Subunit IIa), pillow lavas (Subunit IIc and Unit IV), and hyaloclastites (Unit III). These basaltic rocks are dominated by plagioclase as phenocrysts and, in the groundmass, clinopyroxene and titanomagnetite in a cryptocrystalline to glassy groundmass (see "Igneous petrology" for detailed description). Pseudomorphs after olivine phenocrysts are observed from the bottom of Subunit IIa to the top of Unit IV (from Section 324-U1350A-13R-2 to 25R-1). Plagioclase is slightly to highly altered to brown and white clays (from ~5% to ~90%), but phenocrysts are commonly less affected by alteration than minerals forming the groundmass. However, in Unit IV, plagioclase phenocrysts are extensively altered (from 50% to 90%), being replaced by brown and green clays and sanidine (Figs. F24, F26). Green clays (possibly associated with minor chlorite) are observed along the edges of plagioclase phenocrysts, whereas brown clays and sanidine are observed as a replacement of the inner central part of the mineral. XRD spectra of whole-rock powders also suggest the presence of sanidine in the rocks. As with plagioclase in the groundmass, clinopyroxene microcrysts are slightly to highly altered to brown clays (from ~5% to ~90%; average = 40%). Olivine phenocrysts, when present, are completely pseudomorphed by calcite and brown clays (Mg saponite) in various proportions (Fig. F27). Titanomagnetites are also slightly to extensively altered (from ~10% to 80%) and the glassy mesostasis is completely replaced by brown clays and minor amounts of calcite (Fig. F28). Pyrite is rarely observed in the groundmass of the upper massive flows of Subunit IIa.

Alteration of chilled-margin glass

A significant number of flow and pillow margins was recovered from Hole U1350A. In Unit II, fresh glass is commonly present within a matrix of altered glass (Fig. F24). Alteration of the glass by the formation of palagonite occurs along veins and vesicle margins. Most pillow margins are rimmed by a ~1 cm thick, black soft material that is composed of a mixture of clay minerals (i.e., montmorillonite, nontronite, and possibly bannisterite, based on XRD data). This black, soft rim may be a result of either alteration of the pillow margins or clay-rich sediments that have been deposited between pillows or lava lobes. No fresh glass was preserved in the hyaloclastites of Unit III or in the intercalated pillow lavas and carbonate sediments of Unit IV. Variation in degree of alteration between the pillow margins of Subunit IIa, which contain fresh glass, and the pillow margins of Unit IV, which do not contain fresh glass, is shown in Figure F29. Spherulitic material from Subunit IIa is very fresh, with 90% reflective material (in reflected light), whereas a near identical texture in Unit IV contains only ~10% reflective material.

Vesicles

Basaltic rocks recovered from Hole U1350A show variable degrees of vesicularity depending on the lithology (massive flow versus pillow lavas). Vesicles are mainly filled with calcite, pyrite, saponite, and rarely zeolite (Fig. F24). Zeolite-filled vesicles are only observed in three cores (e.g., Sections 324-U1350A-8R-3, 20R-3, and 26R-5) (Fig. F30). Vesicles commonly contain a rim of segregated melt that has been altered to dark brown clay minerals and fine-grained oxides. These vesicles commonly contain significant pyrite grains. No systematic variation with depth is apparent in the type of mineral filling vesicles (Fig. F24). In the pillow lavas of Unit IV, large vesicles are filled with light green-brown clay, likely saponite, showing floret morphology (Fig. F30) similar to that observed in the highly altered volcanic breccia at the bottom of Hole U1349A.

Veins

Four main vein types have been identified in basaltic rocks of Units II–IV in Hole U1350A (Fig. F24; see 324VEIN.XLS in LOGS in "Supplementary material"):

1. Calcite veins (± pyrite),

2. Saponite veins,

3. Calcite and saponite veins (± pyrite), and

4. Pyrite veins.

A zeolite vein was identified in only one thin section in the upper part of Subunit IIa (Thin Section 271; Sample 324-U1350A-8R-3, 81–84 cm), and Fe oxyhydroxides were only observed in veins in two thin sections of plagioclase-phyric pillows (Unit IV) (Thin Section 329; Sample 324-U1350A-26R-5, 35–40 cm, and Thin Section 331; Sample 324-U1350A-26R-5, 84–88 cm). A total of 461 veins and vein networks were recorded in the 75 m of recovered rock (average = 6.1 veins/m). The vein frequency, however, varies significantly through the hole (Fig. F24), with, for instance, 18 veins/m in Core 324-U1350A-23R. Veins are typically thin; only three veins thicker than 1 cm were recorded (Fig. F24).

Most of the veins in Hole U1350A are calcite veins, consisting of either crystalline blocky calcite or cross-fiber calcite. The calcite veins result from symmetrical infilling of open cracks with minor or no replacement of the wall rock. Both cross-fiber and crystalline calcite veins are commonly associated with pyrite. Thin pyrite veins are commonly observed throughout Unit II (Fig. F31A). Saponite (± pyrite) veins are also very common throughout the succession. Multiple generations of veining occurred in Hole U1350A, as shown by the crosscutting relationships demonstrated in Figure F31A–F31C.

Interpretations of alteration

Basaltic rocks recovered from Hole U1350A have undergone pervasive low-temperature water-rock interaction, resulting in significant variation in the degree of alteration. Variations range from 10% alteration for the Unit II basalts, where significant glass is preserved on pillow margins and flow tops, to as much as 90% for the plagioclase-phyric pillows (Unit IV) at the bottom of the hole. The predominance of clay minerals (i.e., smectites) and calcite as secondary phases suggests relatively low temperature alteration (<100°C) (Alt, 1995, 2004; Honnorez, 2003).

Alteration mineralogy in Hole U1350A samples is comparable to that encountered in Hole U1347A on Tamu Massif, with extensive replacement of olivine phenocrysts and glassy mesostastis by clay minerals and calcite. The significant differences are the variations in alteration degree and the high degree of alteration and partial replacement of plagioclase phenocrysts by clay minerals and sanidine in the plagioclase-phyric pillows of Unit IV. A possible explanation for the high degree of alteration in Unit IV is the presence of significant intercalated carbonate sediments within the pillow lavas. The sediments and their contacts with the pillows may have acted as pathways for low-temperature fluids to percolate through the basalts more easily than in the overlying, more massive units (Subunits IIa–IIc).

The dark gray alteration in Hole U1350A is also comparable to the alteration described for the basalts of the Ontong Java Plateau (OJP) (Mahoney, Fitton, Wallace, et al., 2001; Banerjee et al., 2004). Alteration of the OJP basalts 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 U1350A are shown by the relative abundance of calcite replacing olivine phenocrysts and glassy mesostasis, the lack of celadonite, and the widespread occurrence of pyrite, disseminated in the matrix and as constituents of the veins and vesicles.

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