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doi:10.2204/iodp.proc.345.110.2014

Metamorphic petrology

Background alteration

Lithologic Unit I is defined as a gabbroic rubble unit that contains a variety of discontinuous rock samples (Intervals 1–14) as well as a section of drill cuttings (Intervals G2–G4 in Core 345-U1415J-2G). Gabbroic rubble in Unit I is characterized by pervasive background alteration and is typically slightly altered (10%–30%) but locally can be 60%–90% altered (Fig. F34). Gabbroic rocks in Unit II exhibit pervasive background alteration and are typically slightly altered (10%–30%) but locally can be 60%–90% altered (Fig. F35). Gabbroic rocks in Unit III exhibit pervasive background alteration and are typically moderately altered (30%–60%) but locally can be completely altered (>90%) (Fig. F36). More intense alteration is associated with cataclastic zones, olivine-rich zones, intervals with a higher density of microcracks, contacts between igneous layers, and hydrothermal veins. Mineral assemblages defining distinct metamorphic zones are absent, and alteration does not appear to correlate with igneous grain size. Most of the secondary minerals are visible to the naked eye. However, for the identification of some minerals, particularly fine-grained minerals, optical petrography was required. A summary of alteration intensity and secondary modes observed in thin section is presented in Table T5. Thin sections with two or more metamorphic domains are listed in Table T6. A summary of X-ray diffraction (XRD) results for vein-filling materials and cataclasites is presented in Table T7.

Alteration in the three lithologic units in Hole U1415J varies somewhat by unit and is characterized by “normal” pervasive background alteration as well as intense alteration in cataclastic zones. In the Unit I surficial rubble, alteration is characterized by pervasive alteration of olivine to serpentine with poorly developed corona textures. Veins in Unit I are commonly prehnite, with fewer amphibole, chlorite, and clay mineral–bearing veins. Olivine is more pervasively altered with more complete corona development in the Troctolite Series of Unit III than in the Oikocryst-Bearing Layered Gabbro Series of Unit II. Prehnite is the dominant vein-filling mineral throughout Hole U1415J, and chlorite veins occur almost exclusively in the Troctolite Series. Cataclastic zones occur in Units II and III and are characterized by variable alteration of primary plagioclase, pyroxene, and cataclastic cements of prehnite and locally chlorite.

Olivine

Olivine is highly altered (70%–100%) in most samples in Hole U1415J and alteration is slightly more intense in Unit III than in Unit II (Table T5). Olivine shows a striking contrast to the freshness of plagioclase, which is commonly <20% altered. Alteration intensity, mineral assemblage, and modal proportion of secondary minerals replacing olivine are variable among pieces and even within individual pieces.

The most common alteration mineral assemblage after olivine is serpentine + magnetite ± sulfides. In samples exhibiting magmatic foliation with shape-preferred orientation (SPO) of plagioclase and olivine, serpentine veins are typically subparallel to foliation (Fig. F37). Radial cracks are observed in plagioclase surrounding serpentinized olivine, as in Hole U1415I. Clay mineral replacement occurs in olivine in contact with the dense fracturing related to serpentinization and is especially common adjacent to clinopyroxene oikocrysts in some troctolites (e.g., Samples 345-U1415J-7G-1, 26–34.5 cm [Piece 5], and 8R-3, 22.5–30 cm [Piece 5]). This alteration also seems to be related to fracturing subparallel to the magmatic fabric, as suggested by the propagation of curved fractures subparallel to plagioclase laths surrounding the oikocrysts. A notable observation is serpentine with unusually strong pleochroism from colorless to bluish gray in one sample (Thin Section 72; Sample 345-U1415J-18R-1, 141–143 cm), possibly a submicroscopic mixture of serpentine and clay minerals or tiny opaque minerals.

Corona textures are composed of a concentric zonal aggregate of tremolite and chlorite and characteristically occur as a reaction product between olivine and plagioclase in oceanic gabbroic rocks (e.g., Nozaka and Fryer, 2011) (Fig. F38A, F38B). In many samples from Hole U1415J, coronitic tremolite-chlorite is common and locally abundant, particularly near chlorite veins that cut the magmatic foliation at a high intersection angle (Fig. F38C, F38D). The abundance of coronitic tremolite + chlorite and the completeness of corona formation is highly variable and shows no systematic downhole variation. Completion of the corona-forming reaction (i.e., absence of relict olivine inside a corona) appears more frequently in Unit III than in Unit II (Fig. F39; Table T8). A remarkable observation in Hole U1415J is the occurrence of green spinel associated with coronitic chlorite and amphibole, which is perhaps identified in situ for the first time in oceanic gabbroic rock. The spinel’s pale green color suggests enrichment of Al and Mg and deficiency of Cr compared with brown spinel, chromite, and magnetite, which are common spinel-group minerals in mafic or ultramafic rock. The green spinel is sparsely distributed in two samples of troctolite (Thin Section 67; Sample 345-U1415J-13R-1, 53–56 cm, and Thin Section 17; Sample 18R-1, 53–56 cm), showing a close association with coronitic chlorite and amphibole (Fig. F40). The green color and occurrence with chlorite strongly suggest a metamorphic origin for this spinel.

Talc occurs in the assemblage talc ± chlorite ± amphibole ± sulfides. It is not exclusive to the corona-forming reaction, suggesting that talc alteration occurred over a range of temperature conditions. In Thin Section 63 (Sample 345-U1415J-12R-1, 94–96 cm), for example, olivine is completely altered to talc + pyrite with a mesh-like texture similar to that of serpentine + magnetite.

Pyroxene

Clinopyroxene is variably altered to green or colorless amphibole and lesser amounts of chlorite along grain boundaries or cleavage surfaces (Fig. F41). Clinopyroxene is slightly to moderately altered (<10%–60%) in Unit I, slightly to highly altered (10%–90%) in Unit II, and slightly to completely altered (<10% to >90%; mean = 60%–90%) in Unit III. Slightly altered clinopyroxenes have clear cores that grade outward to irregular rims, similar to those in Hole U1415I. Hydrothermal clinopyroxene was not confidently identified in Hole U1415J samples.

Brown amphibole replacing clinopyroxene was only observed in Unit III (Thin Sections 79 and 80; Section 345-U1415J-21R-1 [Pieces 14 and 16, respectively] and Thin Sections 82 and 83; Section 23R-1 [Pieces 7 and 8, respectively]). Brown amphibole is indicative of high-temperature alteration conditions and is commonly intergrown with green amphibole (Fig. F42). Brown amphibole after clinopyroxene represents 5%–30% of the total alteration products in these rock pieces. In Section 345-U1415J-21R-1, replacement of clinopyroxene by brown amphibole occurs as a background alteration product and does not appear to be related to vein halos. In Section 23R-1, brown amphibole is also observed in the cataclastic zone of Piece 7. In Piece 8 of the same section, brown amphibole replacing clinopyroxene occurs only in weakly deformed parts but not in cataclastic zones. The development of brown amphibole does not appear to be related to cataclasis but rather appears to have formed prior to cataclasis.

Orthopyroxene is variably altered to colorless or pale green amphibole, talc, chlorite, serpentine, and clay minerals along fractures or cleavage surfaces and is highly to completely altered (60% to >90%) in Unit I, slightly to highly altered (<10% to >90%) with a mean value of <10% in Unit II, and fresh to slightly altered (<10%–30%) in Unit III. Orthopyroxene is relatively fresh (<10% altered) even in samples containing completely altered olivine (Fig. F43). Most grains of orthopyroxene show no reaction at contacts with plagioclase, in contrast to olivine, which is commonly fringed by coronitic tremolite and chlorite adjacent to plagioclase.

Chromite-magnetite

Although relict chromite is rare in Hole U1415J rock, thin Intervals 71 (Sample 345-U1415J-18R-1, 67–72 cm [Piece 9]), 77 (Sample 21R-1, 5–16 cm [Pieces 2 and 3]), and G57 (Sample 22G-1, 0–3 cm [Piece 1]) are composed primarily of magnetite and chlorite, with minor amphibole. The unusual mineral assemblage of these intervals opens the possibility that they were originally chromite-rich rock (see “Igneous petrology”). In this chromitite or chromite-rich troctolite, amphibole and chlorite show unusual bluish green colors in plane-polarized light, suggesting unusual chemical compositions (perhaps high-temperature ferric-calcic amphibole) (Fig. F44). The geochemistry of this rock shows that it is low in Cr and high in Fe (see “Inorganic geochemistry”).

Plagioclase

Plagioclase is variably altered to prehnite, chlorite, and secondary plagioclase and is slightly to moderately altered (<10%–30%) in Unit I, slightly to moderately altered (<10%–60%) in Unit II, and slightly to highly altered (<10%–90%) with a mean value of 60%–90%) in Unit III.

In thin section, magmatic plagioclase is most commonly replaced by chlorite and prehnite in all units. Secondary plagioclase is a common replacement of plagioclase in Units I and II but is less common in Unit III (Table T5). In all units in Hole U1415J, chlorite commonly forms a continuous cryptocrystalline rim in plagioclase adjacent to olivine grains (Fig. F45A). Prehnitization of plagioclase occurs as background alteration in serpentinized olivine-rich rocks and filling radiating fractures adjacent to serpentinized olivine in all units. Garnet (or hydrogarnet) is rarely developed in association with prehnite and chlorite after plagioclase (Fig. F46). Near cataclastic zones, replacement of plagioclase by prehnite becomes more intense, often with vuggy textures and cores of chlorite or clinozoisite (Fig. F47). In cataclasite samples from Unit III, clinozoisite and epidote are intergrown with and may replace prehnite after plagioclase and may also rarely replace plagioclase directly. Irregular replacement of plagioclase by zeolite occurs uncommonly in vein margins, sometimes accompanied by clay minerals.

Sulfide minerals

Sulfide minerals occur as secondary phases in all of the gabbroic lithologies recovered in Hole U1415J. These minerals occur in mineral replacement pseudomorphs and are commonly associated with alteration products after clinopyroxene and olivine. Pyrite is common in all units and is widely disseminated as a trace phase in nearly all of the lithologic intervals. Fine-grained, isolated pyrite grains commonly occur in serpentine microveins after olivine. Clusters of pyrite and pentlandite ± chalcopyrite rarely form sulfide mineral assemblages near magnetite stringers associated with olivine alteration. Chalcopyrite forms irregular grains associated with magnetite stringers within serpentine mesh and also in the chlorite replacements of plagioclase. Pyrite and perhaps pentlandite are also observed in some thin sections, intergrown with or entirely surrounded by magnetite (e.g., Thin Section 36; Sample 345-U1415J-5R-1, 62–64 cm [Piece 12]). Detailed observations of sulfide mineralization are recorded in the visual core descriptions (see “Core descriptions”).

Veins

Thin (1–2 mm), massive prehnite veins are the most common vein type in Hole U1415J (Fig. F48). The dominant vein types in lithologic Unit I are prehnite ± chlorite ± zeolite and are typically 1–2 mm thick and 2–4 cm long with massive texture. A smaller number of dominantly chlorite, dominantly amphibole, and dominantly clay mineral veins also occur in thin vein networks in Unit I. The dominant vein types in Unit II are (1) prehnite ± chlorite ~1 mm thick and 2–3 cm long with a massive or radiating texture (Fig. F49D) and (2) thin and relatively short veins of chlorite <1 mm thick and a few millimeters to a few centimeters long (Fig. F49B). Prehnite-bearing veins (Type 1) are observed throughout Unit II (Fig. F86). Halos of enhanced alteration adjacent to the veins are 0.5–2 cm thick and commonly surround prehnite veins, especially in Sections 345-U1415J-8R-1 through 8R-3 (Fig. F49A). Chlorite veins (Type 2) start to appear at the top of Section 8R-1 in Unit II. Chlorite veins typically follow grain boundaries and open out into irregular chlorite patches, often containing amphibole needles and late calcite (Fig. F45B). In some cases, tiny needles of amphibole were observed in corona rims at the contact with unaltered plagioclase adjacent to olivine and orthopyroxene. Networks of massive clay mineral veins are present in Sections 345-U1415J-8R-1 and 8R-2 but become rare in Section 8R-3 and deeper cores.

Chlorite and prehnite veins are also the dominant vein types in Unit III (including ghost Cluster 2). Chlorite veins are commonly monominerallic but also include mixtures of chlorite-amphibole (Samples 345-U1415J-18R-1 [Pieces 1, 7, and 8], 21R-1 [Piece 2], 23R-1 [Piece 7], and 26R-1 [Piece 4]). Less commonly, chlorite veins may also include magnetite stringers (Sample 345-U1415J-18R-1 [Piece 7]). Many chlorite veins show cross-fiber texture with chlorite growing from vein rims to centers and typically lack alteration halos (Fig. F50A). Chlorite veins typically form relatively dense vein networks in the cataclastic intervals in Sections 345-U1415J-21R-1 through 26R-1 and increase in abundance with depth (Fig. F86).

Throughout Hole U1415J, prehnite veins are commonly crosscut by chlorite veins but also crosscut chlorite veins. Prehnite veins are typically thin, polycrystalline, and monominerallic. In five thin sections (Thin Sections 25, 47, 54, 69, and 73; Samples 345-U1415J-2G-1, 10–13 cm; 8R-1, 73–76 cm; 8R-3, 35–38 cm; 14G-1 [Piece 1]; and 19R-1, 18–19 cm; respectively), composite veins filled with chlorite and lesser amounts of prehnite are partially replaced by carbonates (Fig. F50). In these veins, prehnite occurs between the crossing fibers of chlorite and radiating prehnite clusters are distributed along the veins (Fig. F50E, F50F).

Rare epidote veins appear in Unit III (Figs. F49D, F51G, F51H), along with rare massive clinozoisite veins (mineralogy confirmed by XRD). Epidote veins occur in the dolerite dikes cutting cataclastic gabbro in Sections 345-U1415J-24G-1 and 25G-1 and are brecciated in cataclastic zones. In intervals containing clinozoisite veins, plagioclase is almost entirely replaced by prehnite or clinozoisite (i.e., Sample 345-U1415J-4R-1 [Piece 4]). Clay mineral veins are rare in Unit III and appear in only four pieces (Samples 345-U1415J-10R-1 [Piece 3], 21R-1 [Pieces 3 and 7], and 23R-1 [Piece 1]). Zeolite veins are also rare and observed in only five pieces (Samples 345-U1415J-11R-1 [Piece 2], 12R-1 [Pieces 1 and 6], 18R-1 [Piece 16], and 23R-1 [Piece 6]) (Figs. F50C, F50D, F51C, F51D).

Within cataclastic intervals, polycrystalline prehnite veins crosscut cataclastic zones in which prehnite ± chlorite replace the comminuted matrix minerals (Sections 345-U1415J-12R-1 and 12R-2; intervals 20R-1, 9–19 cm, and 21R-1, 16–131 cm; Section 21R-2; and interval 23R-1, 19–54 cm) (Fig. F49). These crosscutting polycrystalline prehnite veins are commonly brecciated by subsequent cataclasis.

The density of veins increases with the deformation degree of the rock. Undeformed cores contain only a few veins per 10 cm of core, whereas strongly deformed cores contain >20 veins per 10 cm. For downhole variations of vein density and vein mineralogy, see “Structural geology” and Figure F86.

Alteration in and associated with cataclasite

A wide range of cataclastic textures exist in Hole U1415J, from narrowly spaced networks of fractures a few micrometers wide to intensely comminuted zones millimeters to centimeters wide to variably brecciated core measured in decimeters and finally to foliated mylonitic cataclasites (Fig. F71). Alteration associated with the cataclasite zones is commonly intense and fracture controlled and generally appears to be superimposed on the background alteration documented in previous sections.

In intervals affected by cataclasis, chlorite appears to form complete pseudomorphs with relict serpentine mesh textures after olivine (Fig. F52). These pseudomorphs often appear to be deformed in the cataclasite zones, forming chlorite foliae. Epidote occurs as brecciated vein material within chlorite in the cataclastic matrix of some highly altered samples. Turbid and fluid inclusion-rich, possibly secondary, plagioclase is locally associated with chlorite and is fairly common as a clast in the cataclasite.

Comminuted plagioclase within cataclasite zones is commonly replaced by prehnite and rarely by carbonate (Fig. F47), whereas clinopyroxene is relatively fresh and occurs as fragments dispersed within the prehnite (Fig. F53A). Replacement of plagioclase by prehnite is variable, with some cataclastic zones being essentially prehnite-free. In other intervals, relatively coarse grained prehnite replaces the matrix (primarily plagioclase) in the relatively fine grained cataclastic intervals in Units II and III. Relatively coarse grained prehnite is commonly brecciated and included as clasts in successive cataclasite. Epidote may occur within prehnite, and prehnite + chlorite + epidote may be a stable assemblage in some samples (Fig. F53).

Drill cuttings

Core 345-U1415J-2G consists mainly of coarse sand–size (0.5–1 mm) drill cuttings. Two grain-mount sections from this interval were point-counted to establish primary and secondary mineralogy (Table T9). Clasts in the grain mounts were highly altered compared to the average recovered core, containing significant amphibole, prehnite, secondary plagioclase, zeolite, chlorite, and clay minerals and broken coronitic aggregates of amphibole and chlorite after olivine. The clasts also contain cataclasite (12%–20%) with local overprints of prehnite.

Although the reconstructed primary mode of the grain mounts is similar to the typical olivine gabbro in Hole U1415J (Table T9), alteration and deformation are as intense as the most deformed and altered intervals in the Unit II gabbroic lithologies. These observations suggest that the drilling sand was 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 the hole.

Metamorphic conditions and deformation processes

Background alteration

The mineral assemblages of the rocks recovered in Hole U1415J suggest a range of temperature conditions from upper amphibolite to subgreenschist facies. Plagioclase alteration is variable in mineral association and spatial distribution, suggesting a wide range of temperature conditions and localized fluid infiltration along microcracks, grain boundaries, fractures, and cataclastic zones. 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). A significant difference of Hole U1415J from the other Expedition 345 holes is the localized occurrence of brown amphibole and green spinel, suggesting upper amphibolite facies. Green spinel has been reported from metagabbro of the Oman ophiolite and is interpreted there as the product of high-temperature (700°–800°C) hydrothermal alteration (Abily et al., 2011). Low-temperature alteration of olivine to serpentinite and clay minerals appears to result from fracturing controlled by the physical properties and/or magmatic fabrics of the original rock. Veins are mostly thin and isolated and are dominated by prehnite, zeolite, and clay minerals, implying relatively low temperature alteration conditions.

The background alteration of gabbroic rock in Hole U1415J is similar to alteration in the Site 894 gabbroic rock, although far less secondary clinopyroxene and green amphibole along plagioclase grain boundaries and filling microfractures were observed in the rock of Hole U1415J (Shipboard Scientific Party, 1993; Früh-Green et al., 1996).

Cataclastic zones

The replacement of magmatic minerals by metamorphic alteration products requires introduction of aqueous fluids, and cataclastic zones are obvious potential pathways for their introduction. A wide range of cataclastic zones exist in Hole U1415J, from narrowly spaced networks of 1 mm wide fractures to foliated mylonitic cataclasite with potentially large displacements (see “Structural geology”). It is possible that the presence of poorly cemented fault rock was responsible for the many meters of core that were not recovered and for the meters of sand containing fragments of cataclasite that were recovered (see “Drill cuttings”).

Chlorite and prehnite are the dominant secondary minerals in the cataclastic zones. These minerals primarily cement broken and angular shards of plagioclase and pyroxene (e.g., Fig. F54). Not all cataclastic zones are associated with new mineral growth, however. Figure F55 shows an incipient cataclastic zone in which crystallization of hydrous minerals is limited to the narrow chlorite-filled shear zones and extensional “pull apart” structure where calcite, chlorite, and prehnite precipitated. Fibrous chlorite veins are commonly truncated by the cataclasite and demonstrate that fluids infiltrated into this rock prior to the cataclastic event.

Chlorite is locally a significant alteration product after plagioclase; however, prehnite is the more common replacement mineral in the cataclastic zones (e.g., line CC-CC, Figs. F56, F57). In a typical cataclasite (Sample 345-U1415J-21R-1, 103–107 cm [Piece 14]), chlorite embays the plagioclase clasts in the cataclasite, suggesting substantial replacement (Fig. F58). The prehnite vein cuts plagioclase with smooth boundaries (yellow arrows), whereas chlorite embays plagioclase clasts in the phyllonitic matrix and widens the walls of the adjacent fractured grains; this feature is most conspicuous in reflected light that highlights the two-dimensional interface (white arrows). In this case, these sutured boundaries suggest that plagioclase was dissolved and replaced by the adjacent chlorite.

Where slip subsequent to chlorite formation occurs, fibrous chlorite veins appear to have developed (Figs. F55, F56), and the slip seems to establish a preferred orientation of the chlorite flakes. The chlorite content of some cataclasites may be high enough and the preferred orientation strong enough that the rock becomes a foliated cataclasite; in some cases multiple deformation events are evident (Fig. F59). In spite of the relatively strong cleavage developed in this rock, it was still cut by fractures that were later filled with prehnite.

Multiple cycles of deformation are apparent in some samples, and crosscutting relationships help to establish the order of brecciation. Figure F54 shows an example of a cataclasite in which five or more separate bands of breccia are visible. Each band is composed primarily of prehnite in interlocking crystals <50 µm in diameter. This matrix contains many inclusions of coarser grained prehnite with angular outlines, suggesting that they were relatively coarse grained veins broken up by cataclastic events. Band C5 also contains fragments of relatively fine grained prehnite with a texture similar to that in Band C2. Most bands contain a few small fragments of plagioclase and clinopyroxene, especially Band C1. Band C5 may be the youngest because it truncates layering (arrows in Fig. F54) in adjacent bands (C2, C3, and C4) in the cataclasite veins. In turn, Band C3 truncates layering in both Bands C2 and C1, suggesting the order of brecciation. Band C1 also contains more relict clinopyroxene and less coarse-grained prehnite, implying that this band may have been one of the earlier generations of cataclasite. Figure F54 identifies one domain (C3A) that might exist as either a separate cataclastic event or a large individual clast. The small size of the sample does not allow discrimination between these possibilities.

Rare brittle-plastic deformation occurs in two samples (Thin Section 77; Sample 345-U1415J-21R-1, 17–19 cm [Piece 4] and Thin Section 79; Sample 21R-1, 107–108.5 cm [Piece 14]). In Sample 21R-1, 103–107 cm (Piece 14), two contrasting cataclasites are juxtaposed across a chlorite vein (Fig. F57). The lower part of the figure is dominated by relatively coarse and fine-grained prehnite microbreccia, whereas the cataclasite in the upper part of the image contains more abundant plagioclase than prehnite. Some plagioclase is cloudy because of abundant tiny inclusions, suggesting that it is secondary plagioclase. Much of the chlorite between the two domains is in random orientation, but chlorite adjacent to the walls of the vein has a strong preferred orientation (white arrows in the figure) and phyllonitic bands have developed. Apparently, the randomly oriented chlorite acquired its preferred orientation by brittle-plastic deformation along the walls of the vein. The same relationships are evident in Sample 345-U1415J-21R1, 17–19 cm (Piece 4) (Fig. F59), where chlorite in the cataclastic matrix defines a strong preferred orientation, suggesting that slip along these grains postdated the cataclasis. Apparently, the fractures that allowed the precipitation of chlorite were reactivated in a ductile fashion to create phyllonite (“Ph” in Fig. F59). Nevertheless, renewed deformation involved brittle fracturing, as evident from the crosscutting prehnite-filled fractures.

From shipboard observations and interpretations, we conclude that the cataclastic zones were formed by brittle deformation in close association with repeated prehnite and chlorite formation under subgreenschist facies conditions.

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