| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.345.113.2014 Metamorphic petrologyBackground alterationSurficial rubble Unit I contains a variety of discontinuous rocks (Intervals G6–G8) as well as a section of sand and gabbroic rubble (Intervals G1–G5 in Core 345-U1415P-2G). Multitextured Layered Gabbro Series Unit II exhibits pervasive background alteration with variable intensity ranging from <10% to 90%, with no apparent trend downhole (Fig. F17). Troctolite Series Unit III exhibits pervasive background alteration of variable intensity and is typically moderately altered (30%–60%) but locally ranges to completely altered (>90%). Intense alteration is associated with cataclastic zones, igneous contacts, and hydrothermal veins. Mineral assemblages defining distinct metamorphic zones are absent, and alteration does not appear to correlate with igneous grain size. Most 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 observed in thin section in presented in Tables T5 and T6. Thin sections with two or more metamorphic domains are listed in Table T7. A summary of X-ray diffraction (XRD) results for vein-filing materials and cataclasites is presented in Table T8. OlivineThe intensity of olivine alteration is highly variable (10%–100%; Fig. F17). On average, it is slightly more altered in Unit III than Unit II (Table T5). The complicated igneous structure of this unit contributes to the variation of alteration intensity because alteration minerals are abundant at contacts between different lithologies (Fig. F18). Microscopic fracturing and veining are common and likely give rise to a local increase of alteration intensity. Olivine is more highly altered in the troctolite of Unit III han in the Multitextured Layered Gabbro Series of Unit II (70%–100% replacement based on thin section observations). Serpentinization and clay mineral formation after olivine are common throughout the hole but dominant in Unit III compared to Units I and II (Fig. F19). The formation of coronitic tremolite and chlorite is common in Unit II but rare or very localized in Unit III. For example, in an anorthosite dikelet (Interval 31; Section 345-U1415P-23R-1 [Piece 2]) and also in intensely altered intervals that contain clinozoisite, chlorite with a minor amount of acicular amphibole forms pseudomorphs after olivine (possibly vein halos; e.g., Interval 29; Section 345-U1415P-22R-1 [Piece 8]). Coronitic tremolite and chlorite are found primarily in Unit II in Hole U1415P, and the coronitic zonal structure is less well developed than in Hole U1415J (Fig. F20; see also the “Hole U1415J” chapter [Gillis et al., 2014e]). Serpentinization of olivine forms a mesh texture and commonly produces the mineral assemblage of serpentine + magnetite ± sulfides. Clay mineral pseudomorphs after olivine are common in Hole U1415P and the other Site U1415 holes. Where these clay mineral pseudomorphs are clearly observed, crosscutting relationships suggest a temporal sequence of sheet-silicate formation replacing olivine of, from older to younger, talc, serpentine, and clay minerals (Fig. F21). Radial cracks in plagioclase, commonly filled with prehnite or chlorite, adjacent to serpentinized olivine are also common characteristics of other primitive oceanic gabbro (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006; see also the “Hole U1415J” chapter [Gillis et al., 2014e]). Where olivine in troctolite is intensely serpentinized, plagioclase appears to be completely altered to prehnite + chlorite ± garnet (or possibly hydrogrossular) (Fig. F22). PyroxeneClinopyroxene is variably altered (5%–70%) in all units of Hole U1415P and displays no variation in intensity downhole (Fig. F17). Clinopyroxene is primarily altered to green or colorless amphibole and lesser amounts of chlorite along fractures and cleavage surfaces (Fig. F23). In several intervals of Unit III, secondary clinopyroxene occurs adjacent to amphibole patches in primary clinopyroxene (Fig. F24). In highly altered troctolite, clinopyroxene is serpentinized and shows a bastite-like texture (Fig. F25). Orthopyroxene is a minor magmatic phase in Unit II and is variably altered (5%–90%) to colorless or pale green amphibole, talc, chlorite, serpentine, and clay minerals along fractures or cleavage surfaces (Fig. F26); it shows no systematic variation in alteration intensity downhole (Fig. F17). PlagioclasePlagioclase is variably altered to prehnite, chlorite, and minor clay minerals as well as lesser amounts of secondary plagioclase, zeolite, and carbonate. The degree of plagioclase alteration increases downhole and is, on average, most pervasive in Unit III (Fig. F27; Tables T5, T6). Plagioclase is commonly replaced by patches of chlorite ± amphibole, as in Hole U1415J, but in Hole U1415P these patches more commonly contain carbonate. Background replacement of plagioclase by prehnite is ubiquitous, and coarse-grained prehnite is more abundant in Hole U1415P thin sections than in Hole U1415J thin sections. In thin sections of the gabbroic rock from Unit II, plagioclase is commonly fresh to moderately altered (0%–50%) (Fig. F27) to fine-grained prehnite + chlorite + minor clay minerals along microfractures. In some samples, chlorite forms a thin, continuous cryptocrystalline rim in plagioclase adjacent to olivine grains as part of the olivine corona reaction. Plagioclase is extensively altered to chlorite in vein halos, as is olivine. Carbonate is a common minor alteration product of plagioclase where it is commonly associated with chlorite or prehnite. In the Unit III Troctolite Series, plagioclase generally is more altered than in the gabbroic rock of Unit II. Plagioclase is moderately to completely altered in Unit III (50%–100%). Prehnite is the most common replacement phase after plagioclase; chlorite also occurs especially along grain boundaries with olivine (Fig. F27). Plagioclase grains are locally completely replaced by both fine-grained and locally coarse grained prehnite (Fig. F28). Clinozoisite rarely forms part of the alteration assemblage. However, in the uppermost troctolitic rock of Unit III (Section 345-U1415P-16R-1), plagioclase is only slightly altered (10%–20%) to fine-grained prehnite and chlorite along microcracks, similar to the plagioclase alteration in Units I and II (Fig. F27). An example of how the igneous lithology influences plagioclase alteration is shown in Figure F28. A contact between troctolite and olivine gabbro in the Troctolite Series of Unit III (Thin Section 140; Sample 345-U1415P-22R-2, 113–115 cm [Piece 8]) illustrates how the degree and mode of plagioclase alteration varies with lithology at the same depth within the hole. Alteration of plagioclase in the troctolitic portion of the piece is far more extensive than in the olivine gabbro portion. SulfidesSulfide minerals occur as secondary phases throughout Units II and III. These minerals are commonly associated with alteration products after clinopyroxene and olivine and in serpentine and chlorite veins. Pyrite is common in all units and is widely disseminated as a trace phase in nearly all of the lithologic intervals. Fine-grained, euhedral, isolated pyrite grains commonly occur in serpentine veins after olivine (Fig. F29A). Pyrite is also commonly intergrown with clay minerals replacing olivine. Chalcopyrite forms isolated irregular grains associated with magnetite stringers within serpentine mesh and also in the chlorite replacements of plagioclase (Fig. F29B). Sulfide assemblages of pentlandite ± heazlewoodite (an alteration phase after pentlandite) are commonly mantled by magnetite and appear to be associated with serpentinization and talc alteration after olivine (Fig. F29C). Pyrrhotite, pentlandite, and pyrite or chalcopyrite form sulfide assemblages mantled by magnetite and locally show a sense of shear associated with deformation (Fig. F29D). These sulfide assemblages and their association with magnetite are typical of serpentinization and talc alteration of mafic and ultramafic rocks and likely do not reflect magmatic segregation processes (Groves et al., 1974; Eckstrand, 1975; Groves and Keays, 1979). Similar assemblages including pentlandite, chalcopyrite, pyrrhotite, and heazlewoodite were also observed in the troctolite and olivine gabbro recovered in Hole U1309D during IODP Expedition 304/305 at Atlantis Massif, 30°N on the Mid-Atlantic Ridge (Miller et al., 2009). VeinsSeven vein types were identified in Hole U1415P (Fig. F30):
Types 4, 5, and 6 are the most common in Hole U1415P (Fig. F31). Prehnite and chlorite veins are common throughout Hole U1415P and occur with similar abundance in the Multitextured Layered Gabbro Series and Troctolite Series of Units II and III, respectively. In contrast, amphibole veins are more abundant in Unit II gabbro. Vein density is generally low, and several intervals lack veins entirely (Fig. F51; see also “Structural geology”). Microveins of prehnite commonly cut plagioclase in troctolite rocks. These veins often radiate away from serpentinized olivine and leave the bulk of the primary plagioclase unaltered. Vein networks are relatively uncommon but are more abundant in Unit II than in Unit III. Parallel, branched, or anastomosing connectivity is relatively common in Unit II but is rare in Unit III (Fig. F51). Chlorite is commonly present in veins in association with zeolite, prehnite, amphibole, and/or serpentine. Chlorite shows various textures; the most common texture is cross-fiber, but elongated or radiating fibers are also common (Figs. F32, F33). Chlorite, prehnite, and chlorite + prehnite veins are equally common in Hole U1415P (Fig. F30). These veins are commonly associated with the serpentinization of olivine and the alteration of plagioclase to prehnite along microcracks in both Units II and III. Prehnite veins are commonly flanked by vein halos in which olivine and plagioclase alteration are more intense. Veins composed of amphibole ± chlorite are common in Units I and II, with several occurrences in most intervals, but are uncommon in Unit III (Fig. F30). Amphibole ± chlorite veins may be crosscut by prehnite or chlorite veins (Fig. F33). Monomineralic zeolite or clay veins are abundant in Unit II, whereas prehnite or serpentine veins are less common. A number of chlorite veins in Unit II show partial replacement of chlorite by zeolite and clay minerals (Fig. F32). Pervasive replacement of chlorite by prehnite and carbonate is also commonly observed in Unit II. Thick (>0.5 cm) serpentine veins occur in the lower sections of Unit III and contain abundant magnetite in the vein selvage, as well as thick (1–1.5 cm) vein halos with pervasive replacement of olivine by chlorite and plagioclase by prehnite. Thin clay and chlorite veins commonly crosscut serpentine veins. Veins filled with clay minerals or zeolites are rare in Unit III. A prominent vein of secondary clinopyroxene, confirmed by XRD, occurs in one of the troctolitic rock pieces (Sample 345-U1415P-23R1, 100–104 cm [Piece 14]; Fig. F34). A summary of XRD results for vein-filling materials and cataclasites is presented in Table T8. Alteration is very intense in the halos of some veins, particularly serpentine veins in Unit III. Vein halos are more common in the Troctolite Series of Unit III than in the Multitextured Layered Gabbro Series of Unit II. In Unit II, halos range from 0.1 to 0.3 cm and are associated with chlorite veins. Halos are characterized by more intense alteration of primary phases and especially by pseudomorphic replacement of olivine and plagioclase by chlorite. Original igneous textures are generally well preserved (Fig. F35). In Unit III, vein halos range from <0.5 to 3 cm and are associated with chlorite veins as well as with prehnite and zeolite veins. These halos are characterized by pseudomorphic replacement of primary phases by chlorite and also commonly prehnite. Alteration in cataclastic zonesIn contrast to Hole U1415J, cataclastic zones are rare in Hole U1415P (see “Structural geology”). Fractures are more common but generally do not show any appreciable displacement. Where fractures show extension, veins filled with prehnite or chlorite are common. One cataclasite occurs in Core 345-U1415P-10R at ~45 mbsf (Thin Section 121; Sample 345-U1415P-10R-1, 131–134 cm [Piece 13]) (Fig. F36). The cataclasite contains fragments of altered olivine, clasts of fine-grained prehnite, and randomly oriented fragments of clinopyroxene that are cemented by fine interlocking grains of prehnite. The cataclastic zone cuts the gabbroic host rock, and the clinopyroxene oikocrysts remain undeformed and preserve the optical continuity of apparently unconnected grains (Fig. F32A). Masses of chlorite are intergrown with minor amphibole–replaced olivine. Whereas the cataclasite cuts the previously altered gabbroic rock, small prehnite veins also cut the cataclastic zone, suggesting prehnite veining postdated cataclastic deformation. Overprinting chlorite/prehnite alteration is associated with cataclasis and brittle fracturing and is relatively common in Hole U1415J but less common in the rock of Hole U1415P and occurs exclusively in Unit III troctolite (Fig. F35). Metamorphic conditions and historyThe mineral assemblages of the rock recovered in Hole U1415P suggest a range of temperature conditions from upper amphibolite to subgreenschist facies. Compared with Hole U1415J, coronitic textures (olivine + plagioclase = tremolite + chlorite ± talc) are less developed. Minerals that clearly indicate high-temperature alteration conditions, such as brown amphibole and green spinel, were not found in Hole U1415P. These observations suggest limited hydrothermal activity under amphibolite facies conditions (Gillis, Mével, Allan, et al., 1993; Blackman et al., 2011; Nozaka and Fryer, 2011). The coronitic tremolite + chlorite assemblage is widespread in Hole U1415P, although it occurs primarily in Unit II and far less commonly in Unit III. This suggests more localized and limited hydrothermal activity under amphibolite facies conditions. Widespread serpentinization and clay mineral formation after olivine suggest pervasive alteration under lower temperature subgreenschist facies conditions. Intense chlorite and prehnite alteration overprints the normal background alteration in vein halos and cataclastic rocks. This type of alteration was also observed in Hole U1415J. In contrast to the corona formation, serpentinization of olivine and prehnite formation after plagioclase are more intense in Unit III than in Unit II. Serpentine, prehnite, and clay minerals frequently show a tendency toward preferential distribution in densely fractured domains, at igneous contacts, and within troctolite intervals. One of the possible controlling factors of the low-temperature hydration in the lower crust could be the modal composition and structure of magmatic rock. It is clear that the mode of olivine controls the overall percent of alteration; intervals with higher olivine abundance are also characterized by higher alteration intensity. The occurrence of the prehnite + chlorite assemblage as fragmented clasts, matrix, and veins in the cataclastic zones suggests syntectonic alteration at subgreenschist facies conditions. The temporal evolution of metamorphism in Hole U1415I is, from oldest to youngest,
|
|||
