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doi:10.2204/iodp.proc.345.109.2014 Metamorphic petrologyBackground alterationUnit I is a surficial rubble unit that contains a variety of discontinuous gabbroic rock (Intervals 1–3) as well as several sections of drilling-induced disaggregated gabbro cuttings (Intervals 4–7). Despite the lithologic variability, the alteration intensity and style within Unit I are similar to alteration observed in the underlying coherent Layered Gabbro Series of Unit II, and for this reason the units are described together (Table T5). All of the lithologies in Hole U1415I show some degree of hydration, with highly variable extents of alteration. Gabbroic rock exhibits pervasive background alteration and is typically moderately altered (10%–60%) but ranges to completely altered (>90%) locally. Enhanced alteration is associated with cataclastic zones, olivine-rich zones, and regions with a higher density of microcracks, igneous contacts, and hydrothermal veins. Most of the secondary minerals are identifiable in hand specimen; optical petrography was required for the identification of some particularly fine grained minerals. Although the alteration style, in terms of replacement phases, is consistent across Units I and II, the Layered Gabbro Series of Unit II (Section 345-U1415I-4R-1) is somewhat less altered (10%–30%) relative to other sections, although alteration intensity is elevated (40%–60%) in the relatively olivine-rich zones. In these olivine-rich zones, serpentinization of olivine results in cracking around the relict olivine grains and surrounding minerals, possibly allowing enhanced fluid flow and alteration along grain margins. PyroxeneClinopyroxene is moderately to strongly altered (30%–90%) to pale green amphibole and possibly chlorite. Slightly altered clinopyroxene has a clear core that grades outward to a corroded rim with amphibole replacement along cleavage surfaces. Brown amphibole was not observed in Hole U1415I, nor was it observed in Holes U1415E, U1415G, and U1415H. Orthopyroxene, where present, is typically strongly altered (>90%) to colorless or pale green amphibole + chlorite ± talc. OlivineOlivine is partially to totally altered (30% to >90%, typically >90%) most commonly to serpentine, showing mesh textures with magnetite ± clay minerals ± talc ± pyrite. Radial cracks commonly extend through chlorite rims into otherwise relatively fresh plagioclase crystals in troctolitic gabbroic rock (Fig. F11A, F11B). These features may result from an increase in volume associated with serpentinization. In some cases, olivine is completely altered to tremolite ± talc ± chlorite; pyrite is associated with this assemblage. Corona textures of tremolite and chlorite are commonly present, even in the presence of relatively unaltered pyroxene and plagioclase (Fig. F11C, F11D). Replacement of olivine by talc occurs in some of the most altered rocks recovered from this hole. PlagioclaseIn most of the rock of Hole U1415I, plagioclase is slightly to moderately altered (<10%–60%) to assemblages of secondary plagioclase, prehnite, and chlorite. Plagioclase laths included in pyroxene oikocrysts are typically fresh with little evidence of alteration. Plagioclase is commonly altered to chlorite where it is in contact with relict olivine. The apparent volume increase associated with the serpentinization of olivine causes surrounding plagioclase grains to crack, commonly radiating away from olivine and olivine coronas, resulting in enhanced alteration (commonly to prehnite ± chlorite) along these cracks (Fig. F12). Plagioclase crystals are highly fractured with minor to moderate (<10%–30%) replacement by secondary plagioclase, prehnite, and in some cases zeolite along fracture surfaces in areas adjacent to zones of cataclasis (see “Structural geology”). VeinsVeins are mostly thin (1–2 mm), isolated, and dominated by subgreenschist facies minerals including prehnite, zeolite, and clay minerals (possibly smectite). Networks of thin prehnite veins with branched connectivity and irregular shape are common and occur in all lithologies, most often associated with cataclasis and brittle fractures (Figs. F13, F14; see also “Alteration veins”). Very thin (<1 mm) carbonate veins also occur within Core 345-U1415I-3R. Microscopic prehnite and zeolite veins often crosscut plagioclase crystals and are commonly adjacent to altered olivine (e.g., Fig. F12). Thin amphibole and epidote veins with an average thickness of <1 mm also occur in this core, although such veins are less common than the prehnite, zeolite, and clay mineral veins. Veins are irregular in shape and characterized by sharp contacts with the host rock. Networks of thin veins are present in many rocks within this core. Some of the veins are cataclastic and show prehnite replacing pulverized plagioclase. Microfractures, predominantly associated with plagioclase, are filled with fine-grained prehnite. The chronology of veining is difficult to establish because crosscutting relationships are not always apparent; however, a brief approximate chronology is, from oldest to youngest,
Altered cataclasitesComplete replacement of cataclastic matrix by secondary minerals, most commonly by prehnite but also by zeolite, occurs in several cores (e.g. Samples 345-U1415I-2R-1, 24.5–31.5 cm; 4R-2, 13–17 cm; and 5G-1, 18–24.5 cm). Comminuted plagioclase is commonly replaced by prehnite and sometimes carbonate in the cataclasite zones. Figures F14C and F15D show a cataclastic texture in Sample 345-U1415I-2R-1, 24.5–31.5 cm (Piece 7), in which clinopyroxene is broken along cleavages, forming clasts in a fine-grained dark groundmass presumed to be mainly feldspar. A replacement front can be seen where the cataclastic matrix has been completely replaced by medium-grained clear prehnite aggregates with randomly oriented clasts of clinopyroxene or amphibole left unaltered from the earlier cataclastic texture. Veins of prehnite and zeolite ± carbonate cut both the cataclasite and prehnite replacement, and in a few places in this interval (Interval 3), the medium-grained prehnite is deformed in turn. The fine-grained cataclastic plagioclase is likely extremely reactive because of the damaged grains and high surface area. Prehnite textures in the cataclastic samples are different from the fine-grained, dusty prehnite normally seen as pervasive partial replacement or oriented seams along presumed fractures in plagioclase (i.e., Fig. F15). Sample 345-U1414I-5G-1, 8.5–12.5 cm (Piece 2), composed primarily of prehnite, is inferred to be vein filling from a cataclastic zone (Fig. F16). The identification of prehnite in this vein was confirmed by X-ray diffraction (XRD) analysis. This piece has no visible contacts with gabbroic material and was recovered in a ghost core. This material is interesting because it shows clear evidence of cataclasis and intense alteration over a region at least 5 cm in width. Drill cuttingsIn lithologic Unit I, Core 345-U1415I-3R (Intervals 4–7) consists mainly of coarse sand–sized drill cuttings. Three grain mounts from these intervals were point-counted to establish primary and secondary mineralogy (Table T4). Clasts in the grain mounts are highly altered compared to the average recovered core, containing significant amphibole, prehnite, secondary plagioclase, zeolite, chlorite, and clays, including broken corona textures after olivine (Fig. F15B). The clasts also contain cataclasite (12%–20%), with prehnite sometimes replacing the matrix (Fig. F15C). Representative samples analyzed by XRD confirm the presence of calcic plagioclase, clinopyroxene, chlorite, and amphibole (Samples 345-U1415I-3R-1, 38–43 cm, and 3R-2, 70–76 cm, in Table T6). Although the reconstructed primary mode of the grain mounts is similar to the typical olivine gabbro in the core (see “Igneous petrology”), the alteration and deformation are as intense as the most altered intervals collected in lithologic Unit II. We conclude that either the cuttings were collected mainly from a fault zone or that unrecovered parts of each section consisted of such rocks. In either case, it is clear that the recovered core is not fully representative of the extent of alteration and cataclastic deformation in Hole U1415I. Metamorphic conditions and historyThe dominant alteration of the rock recovered in Hole U1415I ranges from lower amphibolite to subgreenschist facies conditions. Compelling evidence for higher temperature alteration, (i.e., dark green or brown amphibole) is absent from these samples, although it is possible that some of the amphibole replacing clinopyroxene and filling microfractures may have formed at middle to upper amphibolite facies. Coronitic textures (olivine + plagioclase = tremolite + chlorite ± talc) are variably developed, indicating middle to lower amphibolite facies hydration (Blackman et al., 2011; Nozaka and Fryer, 2011). Primary plagioclase is largely unaltered, except where in contact with relict olivine or where fractured in cataclastic zones. The presence of zeolite veins indicates that hydration occurred at or below those of the zeolite facies. The alteration asemblages recovered in Hole U1415I record a sequence of cooling during alteration. The corona textures after olivine indicate lower amphibolite facies hydration (Blackman et al., 2011; Nozaka and Fryer, 2011). Serpentininzation of olivine and prehnitization of plagioclase likely occurred at lower temperatures. This was followed by low-temperature cataclasis and prehnite mineralization of cataclasite cements. The presence of brittle deformation of epidote veins implies that cataclasis occurred below the temperatures at which epidote forms (~350°C; Bird and Spieler, 2004). Finally, crosscutting zeolite veins imply hydration occurred at temperatures of ~100–150°C. The temporal evolution of metamorphism in Hole U1415I is, from oldest to youngest,
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