doi:10.2204/iodp.proc.324.106.2010
Alteration and metamorphic petrology
Alteration
Basaltic rocks recovered from Hole U1349A have undergone extensive water-rock interactions under variable temperature conditions, resulting in complete replacement of glassy mesostasis and olivine phenocrysts. In the upper 218 mbsf (stratigraphic Unit IV; Sections 324-U1349A-7R-2 through 13R-4), both plagioclase and pyroxene are generally well preserved, whereas below this depth (bottom of Unit IV and Unit V; Sections 13R-5 through 16R-7), alteration is more pronounced with near complete replacement of pyroxene and almost complete replacement of plagioclase microliths. The overall alteration of the basaltic rocks ranges from high to complete (from 50% to ~100%) (Fig. F37), estimated visually using a binocular microscope on the archive half and an optical microscope on discrete thin section samples, without taking into account veins and vein halos.
Clay minerals are the predominant secondary minerals in Hole U1349A and were identified by optical microscopy and XRD patterns of whole rocks, vesicles, and veins. Saponite, nontronite, and montmorillonite are the main clay minerals replacing glassy mesostasis and primary phases (olivine, plagioclase, and pyroxene) and filling vesicles and veins. Calcite is the second most abundant secondary mineral in these basaltic rocks, both pervasively altering the rocks (glassy mesostasis and primary minerals) and filling vesicles, voids, and veins. Other alteration minerals present in Hole U1349A include hematite, Fe oxyhydroxides, and zeolite in the upper and middle parts of Unit IV (Sections 324-U1349A-7R-2 through 13R-4) and chlorite and rare serpentine at the bottom of Unit IV and in Unit V (Sections 13R-5 through 16R-7) (Fig. F37).
Alteration degree and mineralogy at Site U1349 will be compared with that of basaltic rocks recovered at previous sites at Shatsky Rise, Site U1346 on Shirshov Massif, and Site U1347 on Tamu Massif, with previously well studied portions of ocean crust (Alt, 1995, 2004) and 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 alteration color and mineralogy, we identified two main types of pervasive alteration separated by a gradual transition zone. Alteration colors range from reddish brown at the top of the basaltic sequence in Hole U1349A to green toward the bottom. The distribution of alteration colors, alteration degree, and mineralogy are given in Figure F37 and 324ALT.XLS in LOGS in "Supplementary materials." Two main types of veins and three main types of vesicles were identified throughout the hole, with variations in mineral infilling vesicles depending on the alteration type.
Red-brown alteration
Red-brown alteration of basaltic rocks was only encountered in the upper portion of the basaltic sequence of Hole U1349A in Unit IV (from Section 324-U1349A-7R-2 to 12R-3) (Fig. F37). Overall alteration of the basalts ranges from 50% to ~100% based on petrographic observations of thin sections. Clay minerals are the predominant secondary minerals, extensively replacing groundmass and phenocrysts. One characteristic of this alteration is the replacement of olivine phenocrysts by iddingsite (e.g., Fig. F38) and the formation of hematite (Fig. F39). Iddingsite (MgFe2Si3O10·4[H2O]) is an olivine pseudomorph consisting of a mixture of clay minerals, Fe oxides (i.e., hematite), and ferrihydrides. Based on XRD spectra from whole-rock samples, the clay minerals forming iddingsite are mainly saponite (Mg smectite) and montmorillonite (Fig. F40). Plagioclase microliths are also significantly altered to brown clays and/or a K-rich phase. In fact, the high-K content of the basaltic rocks (see "Geochemistry") and the XRD spectra (Fig. F40) may suggest that plagioclase microliths have been replaced by sanidine; however, it was difficult to identify sanidine petrographically because of the fine-grained size of the plagioclase crystals. Titanomagnetites demonstrate exsolutions of magnetite and ilmenite observed in thin sections (Fig. F41). Glassy mesostasis is altered to a mixture of likely saponite, Fe oxyhydroxides, and hematite, giving a red-brown color to the rocks (Fig. F39). The only phase that remains relatively unaffected by alteration and fluid-rock interaction is clinopyroxene (Fig. F42).
In addition, nine intervals interpreted to be subaerially weathered flow tops have been identified within Unit IV (see "Igneous petrology") (Fig. F43A). These weathered horizons demonstrate near-complete alteration of primary magmatic minerals to dark reddish brown fine-grained clay minerals, Fe hydroxides, and Fe oxides. An XRD spectrum from one weathered horizon (Sample 324-U1349A-10R-1, 10–13 cm) (Fig. F43B) indicates that this rock is likely composed predominantly of hematite (Fe2O3), goethite (FeO[OH]), sanidine, montmorillonite, and halloysite (Al2Si2O5[OH]4·2H2O), consistent with subaerial weathering.
Transition zone
A gradual brown to green alteration zone (205–224 mbsf) from the base of Unit IV (Section 324-U1349A-12R-4) to the upper portion of Unit V (Section 14R-3), forms a transition between the two main alteration types (red-brown alteration of the upper part of the hole and green alteration at the bottom) (Fig. F37). Overall alteration of the basalts in this transitional zone ranges from 50% to ~100% based on petrographic observations of thin sections. Secondary minerals are still predominantly clay minerals replacing the glassy mesostasis and the primary phases. Olivine phenocrysts are pseudomorphed and predominantly replaced by iddingsite, saponite, and calcite (e.g., Thin Section 234; Sample 324-U1349A-12R-2, 111–113 cm) (Fig. F44). Clinopyroxene shows a higher degree of alteration in this transition zone and is replaced to various extents by brown clays. Identification of clinopyroxenes in thin sections becomes difficult, suggesting partial replacement between 205 and 216 mbsf and almost complete replacement below 216 mbsf (from Thin Section 242 to the bottom) (Fig. F37). Plagioclase microliths are also extensively altered and replaced by brown clays. Within this entire alteration zone, significant hematite is present in thin sections from Unit IV but is not present in thin sections of the breccia clasts of upper portion of Unit V. Similar to the red-brown alteration, titanomagnetite crystals show primary igneous morphology with exsolutions of magnetite and ilmenite.
In this alteration transition zone, higher temperature minerals replace groundmass glass or fill veins. Zeolite (identified as probable stilbite) was observed replacing glassy mesostasis and filling vesicles and voids (e.g., Thin Section 237; Sample 324-U1349A-13R-1, 29–32 cm). Chlorite veins were also observed as a late stage of fluid circulation in Thin Section 235 (Sample 324-U1349A-12R-3, 32–34 cm) (Fig. F45). In this sample, the crosscutting relationships allow us to roughly reconstruct the alteration history of this sample: (1) alteration of olivine to iddingsite and formation of clay minerals ± Fe oxyhydroxides on the walls of the vesicle, (2) infilling of vesicles with calcite, (3) chlorite vein, and (4) calcite veins.
Green alteration
Green alteration is only found from 224 mbsf to the bottom of the hole, affecting the volcanic breccia of Unit V (Fig. F37). In this zone, the basaltic clasts are highly to completely altered to green clays, with overall alteration degree from 70% to ~100% based on petrographic observations of thin sections. Primary minerals and glassy mesostasis are predominantly replaced by secondary clays (saponite, montmorillonite, and nontronite based on XRD patterns). Olivine phenocrysts are mainly altered to saponite and calcite, with minor replacement by serpentine along the fractures of the mineral (Fig. F46). Plagioclase microliths and pyroxene are extensively altered and replaced by brown clays. Hematite is no longer present in thin section and titanomagnetite is extensively altered to completely absent in some thin sections. This is consistent with a change in magnetic susceptibility (see "Paleomagnetism" and "Physical properties"). The highly altered breccia clasts are cemented by variable proportions of calcite and light green clays. The light green clays show floret morphology (Fig. F47) and are likely saponite and nontronite, as indicated by XRD spectra. Commonly, Fe oxyhydroxides are observed associated with the light green clays and calcite cement. In addition, XRD spectra from the breccia cement show peaks for calcite, nontronite, montmorillonite, and bannisterite. Bannisterite (or ganophyllite; [K,Ca]0.5[Mn,Fe,Zn]4[Si,Al]7O14[OH]8) is a mineral that is likely formed from metamorphism of Mn deposits.
Veins
Veins are found throughout the entire basaltic section in Hole U1349A. In the basalt flow breccia of Unit V, distinction between veins and cement infilling clasts was difficult and veins were only recorded when cutting the basaltic clasts (Cores 324-U1349A-14R to 15R). For this reason, no veins were recorded in Core 324-U1349A-16R. A total of 324 veins and vein networks were recorded in the ~49 m of basement rock recovered from Hole U1349A (Fig. F37), giving an average of 6.6 veins/m.
Two main types of veins were identified in Hole U1349A, irrespective of the alteration type: (1) calcite veins and (2) calcite, green/brown clay, and Fe oxyhydroxide veins. In addition, in the upper 186 mbsf zeolite is rarely associated with calcite in veins (Fig. F37).
Calcite veins show two main morphologies, crystalline blocky calcite and cross-fiber calcite. The cross-fiber calcite veins commonly cut the crystalline calcite veins (Fig. F48A), suggesting multiple stages of fluid circulation. The calcite veins result from symmetrical infilling of open cracks with minor or no replacement of the wall rock. Zeolites are a minor component in crystalline calcite veins in Cores 324-U1349A-8R through 10R. The calcite, green/brown clay, and Fe oxyhydroxide veins show highly variable proportions in calcite and clay minerals. Most veins have <10% clay, but occasionally they contain >70% clay minerals (e.g., Thin Section 229; Sample 324-U1349A-11R-4, 88–93 cm). The green-brown clays and Fe oxyhydroxides are typically intergrown, suggesting simultaneous formation and crystallization, with a late stage calcite cement. The calcite, green/brown clay, and Fe oxyhydroxide veins commonly cut calcite veins (Fig. F48B) and commonly show halos as wide as 1 cm of near complete alteration (Fig. F48D). In the red-brown alteration of the upper part of the hole, saponite is the dominant halo mineral. In the alteration transition zone and the green alteration zone, very pale fine-grained green chlorite is the dominant halo mineral (Fig. F45). In the red-brown alteration and transition alteration zone, the clay minerals in the veins include saponite, nontronite, and celadonite (Fig. F48C). In the green alteration, the clays are pale green with floret morphology identical to the smectite clays (likely saponite) in the breccia cement (Fig. F47). In the breccia unit, some oxidation variations have occurred that lead to juxtaposition of vein material with green clays and vein material with oxidized orange clays (Fig. F48D).
Vesicles
Basaltic rocks in Hole U1349A are poorly to highly vesicular, the latter especially in the upper part of the hole. Three main types of minerals filling vesicles have been identified in Hole U1349A, calcite with minor zeolite, saponite, and zeolite. Calcite vesicles are observed in the entire section and show two main textures, blocky and/or fibrous (Fig. F49A). In some samples (e.g., Thin Section 215; Sample 324-U1349A-9R-1, 36–40 cm), tabular zeolite minerals are observed at the rim of the calcite vesicle (Fig. F49A, F49B). In contrast, zeolite- and saponite-filled vesicles are restricted to some intervals and units. Saponite-filled vesicles are only present in the basalt flow breccia of Unit V (Fig. F49C), whereas zeolite-filled vesicles are restricted to the basaltic rocks of the transition alteration zone (Fig. F49D). In addition, basalts from the upper part of the hole (i.e., Core 324-U1349A-7R) have vesicles that have been partially filled with different successive layers of fine-grained micrite, clays, and Fe oxyhydroxides (e.g., Thin Section 206; Sample 324-U1349A-7R-1, 112–115 cm). In general the vesicles show a sharp horizontal contact with crystalline calcite and are interpreted as geopetal vesicles.
Interpretations of alteration
Basaltic rocks in Hole U1349A have undergone extensive fluid-rock interaction with variations in redox and temperature conditions of the circulating fluids. The predominance of clay minerals with iddingsite, calcite, and hematite in the upper and middle part of Unit IV (165–205 mbsf) suggests alteration under relatively oxidizing and relatively low temperature conditions. Iddingsite is an olivine pseudomorph consisting of a mixture of clay minerals and Fe oxides (i.e., hematite) and is commonly observed as an alteration product of olivine in extrusive or subvolcanic igneous rocks formed by injection of magma near or at the surface. Evidence for subaerial exposure, and possibly extrusion of Unit IV basalts, comes from the presence of subaerially weathered horizons within Unit IV, geopetal vesicles, and magnetite-ilmenite exsolutions after titanomagnetite. Such exsolution is commonly observed in subaerially erupted basalt flows in which deuteric oxidation of titanomagnetite occurs (Dunlop and Özdemir, 1997). The highly oxidized nature of the upper and middle lavas of Unit IV suggests subaerial tropical weathering processes (Widdowson, 2007), which is consistent with Shatsky Rise's equatorial paleolatitude. Furthermore, the high K content of basalts from Unit IV (see "Geochemistry") and the likely presence of sanidine indicate that once the lavas were submerged, more typical oceanic alteration occurred with interaction of the lavas with low-temperature circulating seawater leading to K uptake into the basalts (Alt 1995, 2004).
Secondary mineral assemblages change in the transition alteration zone and toward the bottom of the hole, reflecting different temperature and redox conditions of alteration. The replacement of olivine by saponite and calcite, the occurrences of chlorite veins and zeolite replacing glassy mesostasis and filling vesicles, and the persistence of hematite suggest higher temperature and less oxidizing conditions than the upper part of the hole. In the basalt flow breccia (Unit V), the replacement of olivine by saponite and minor serpentine, the lack of hematite and the vesicles filled by saponite, and the possible occurrence of metamorphosed Mn deposits (e.g., bannisterite) suggest more reducing conditions and likely higher temperature conditions than in the basaltic rocks above.
The majority of the highly altered basaltic rocks recovered from Hole U1349A may be compared with typical alteration occurring in the oceanic crust (Alt, 1995, 2004). The red-brown alteration zone affecting the basalts in the upper part of Hole U1349A is similar to the low-temperature oxidative alteration observed in the upper portion of the oceanic crust, with the formation of clay minerals (e.g., nontronite and celadonite) and the uptake of alkalis from seawater (for temperature <70°C). However, hematite formation does not occur in common alteration of MORBs and is more typical of tropically weathered basalts (Widdowson, 2007). The transition alteration zone shows similar alteration conditions and mineralogy to that observed deeper in the oceanic crust, where an increase in the temperature of circulating seawater-derived fluids (>150°C) and more reducing conditions allows formation of higher temperature minerals, such as chlorite and zeolite. Therefore, the lavas in Unit IV record two stages of alteration: first, subaerial alteration immediately after extrusion, and second, seawater alteration upon submergence of the lavas.
Alteration in Hole U1349A is characterized by variable degrees and conditions of fluid-rock interaction, from oxidative at the top to reducing conditions at the bottom, and consequently differs from the suboxic and CO2-rich alteration observed at Site U1346, the low degree of alteration at Site U1347, and the dark gray alteration observed for the basalts of the OJP (Mahoney, Fitton, Wallace, et al., 2001; Banerjee et al., 2004).