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doi:10.2204/iodp.proc.335.203.2016

Methods and material

Samples

Selection

We selected sand and gravels from 12 fishing runs over the 14 conducted during hole cleaning operations (Table T1). Runs 2 and 6 recovered cuttings from an unstable section of the hole located in the lava sequence (~922 mbsf), whereas the other runs (11–15 and 17–21) recovered cuttings from the bottom of the hole (1518–1520.2 mbsf) (see Table T1 in the “Site 1256” chapter [Expedition 335 Scientists, 2012c]). For each fishing run, the fine-grained fraction of the cuttings (grain size from ~0.5 to 5 mm) was sampled to rule out single mineral grains or mixed grains and to have sufficient grains per thin section for good statistics. For Runs 12, 15, and 19, we selected two different fine-grained fractions (coarser and finer), and for Run 20 we selected cuttings from various fishing assemblies (bit sub junk basket [BSJB] and external junk basket 1 [EXJB1]) (Table T1).

Grains were randomly selected and mounted in thin sections (Fig. F1A). A total of 25 polished thin sections were prepared with at least 2 thin sections for each run (except for Runs 2, 11, and 13) in order to evaluate the heterogeneity of the cuttings (Table T1). The number of grains per thin section varies from 24 to 400 according to the grain size, but most thin sections include around 35 grains (Table T1; Fig. F1A).

Description

The lithology of each grain was identified on the basis of its mineral paragenesis and texture, accurate modal proportions being difficult to estimate in these fine-grained cuttings. We identified 11 major lithologies: glass, aphyric basalt, phyric basalt, dolerite, gabbro, diorite, gabbronorite, albitite, and three kinds of granoblastic basalt (Fig. F1B). These lithologies are similar to those described on board the ship from Hole 1256D cores and reflect the different levels of the hole (see the “Site 1256” chapter [Expedition 335 Scientists, 2012c] and Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).

Grains of fresh and devitrified glass are present in most thin sections. They sporadically include microcrysts of plagioclase (pl) and microphenocrysts of plagioclase and clinopyroxene (cpx). Grains of basalt have microphyric to fine-grained textures. Groundmass phases consist of plagioclase, clinopyroxene, glass, and Fe-Ti oxides (magnetite and minor ilmenite). The microphenocryst phases (i.e., visible under magnification) are mainly plagioclase and clinopyroxene. Sulfides and secondary quartz are commonly observed. Basalts were classified as aphyric or phyric according to the relative abundance of microphenocrysts (see example on Fig. F1). Glass and basalt grains likely come from the extrusive sequence of the hole, although some grains (phyric basalts and glass) may come from the upper part of the sheeted-dike complex (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).

Dolerite grains have medium- to fine-grained holocrystalline textures and are mainly composed of plagioclase, clinopyroxene, amphibole, magnetite, and ilmenite. Glass is rarely observed. Dolerite grains present variable degrees of alteration; sulfides and secondary quartz are occasionally observed. These grains most likely come from the sheeted-dike complex of the hole.

Gabbro, gabbronorite, and diorite grains have medium- to coarse-grained macrocrystalline textures and show various degrees of alteration. The main paragenesis of the gabbros is cpx + pl + Fe-Ti oxides ± amphibole ± olivine (ol), and the main paragenesis of the gabbronorites is cpx + orthopyroxene (opx) + pl + Fe-Ti oxides ± ol. Olivine-bearing grains are, however, very rare. Accessory phases like quartz, apatite, zircon, titanite, and sulfides are commonly observed. Diorite grains consist of clinopyroxene, amphibole, plagioclase, quartz, and Fe-Ti oxides with accessory apatite, zircon, and epidote. In all gabbroic grains, pyroxenes are variably altered to secondary amphiboles and Fe-Ti oxides correspond to ilmenite and magnetite. Most gabbroic grains probably come from the two gabbro units located in the plutonic section of the hole, but some grains may represent gabbroic dikes cutting the base of the sheeted-dike complex or the plutonic section (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).

Grains of granoblastic basalt consist of two pyroxenes, plagioclase, ilmenite, and magnetite with minor quartz, and sulfides and are characterized by recrystallized medium-grained doleritic to fine-grained granular textures. Three kinds of granoblastic basalt grains were identified according to their recrystallization degree using the 0–6 textural scale defined on board the ship (see Fig. F11 in the “Methods” chapter [Expedition 335 Scientists, 2012b]). Low recrystallization degrees (1 and 2) are characterized by the growth of small, isolated micrometer-sized opx and/or cpx granules replacing clinopyroxenes. As the recrystallization proceeds, the pyroxene granules develop and the igneous texture is progressively replaced. In this way, granoblastic basalts were classified as strongly recrystallized (Degrees 3 and 4) if the igneous texture is only partially erased (i.e., laths of plagioclase and relics of primary cpx and Fe-Ti oxides) and as completely recrystallized (Degrees 5 and 6) if the igneous texture is obliterated and overprinted by an equigranular granoblastic assemblage of secondary pyroxenes, plagioclase, and Fe-Ti oxides exhibiting flat grain boundaries and 120° triple junctions. Granoblastic basalt grains are usually rather fresh, although grains altered into hydrous assemblages are occasionally observed and often display amphibole and orthopyroxene veins. These granoblastic basalts were cored at the base of the sheeted-dike complex and in the plutonic section (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006) and were interpreted as produced by high-temperature metamorphism of hydrothermally altered dikes (Koepke et al., 2008).

Albitite grains have strongly altered medium-grained macrocrystalline textures and essentially consist of dusty plagioclase of albitic composition with minor clinopyroxene, titanite, chlorite, apatite, epidote, and Fe-Ti oxides (ilmenite and minor magnetite). The dusty appearance of plagioclases is usually related to scattered microcrystals of hydrated calcium-aluminium silicates. No quartz is observed. This kind of leucocratic rock was not reported in previous expeditions but was described in cores and pebbles/cobbles during Expedition 335. They were interpreted on board the ship as strongly altered granoblastic basalts on the basis of textural observations (see the “Site 1256” chapter [Expedition 335 Scientists, 2012c]).

Methods

Image processing

The abundance of each lithology in each thin section was estimated using the Java-based image processing program ImageJ on high-quality photos of the thin sections (JPEG file extension, 2400–4000 dpi; Fig. F1A). First, each grain of the thin section was manually outlined and labeled according to the lithologic groups previously described. Then, the area occupied by all grains of a same lithologic group was measured in number of pixels by the program and converted into relative abundance (Fig. F1B). For a better illustration of the results, a simplified process in which each grain was labeled according to the four main lithologies defined in the hole (basaltic lavas, dolerites, gabbroic lithologies, and granoblastic basalts) was also undertaken (Fig. F1C). The “others” group corresponds to isomodal and metal grains in the detailed process and to isomodal, metal, and albitite grains in the simplified process. The results are illustrated in Figure F2.

Mineral composition

Major and minor element compositions of main mineral phases (cpx, opx, feldspar, olivine, Fe-Ti oxides, and amphibole) and glass were determined using an electron microprobe (CAMECA SX50 at Toulouse University, CAMECA SX100 at Brest University, and JEOL 8800 at Hokkaido University). A standard analysis program with an accelerating voltage of 15 kV and a beam current of 10–20 nA was used at Toulouse and Brest Universities. An accelerating voltage of 20 kV, a beam current of 20 nA, and a probe diameter of 3 µm were used at Hokkaido University. Counting time was 10–20 s on the peak and 5–10 s on the background for both programs. Detection limits were similar for the three instruments with values of ~0.07% for Al₂O₃ and Na₂O and ~0.09% for TiO₂ and Cr₂O₃ for all phases analyzed. The full data set is available in RESULTS in “Supplementary material.”

About 4400 analyses were performed on 16 grain-mount thin sections. To simplify the chemical diagrams and to allow better comparison between cuttings and core data, grains were grouped into seven major lithologies: basaltic lavas (basalts and glass), dolerites, gabbroic lithologies (gabbro, gabbronorite, and diorite), albitites, and the three kinds of granoblastic basalt (low, strong, and complete recrystallization). This report aimed to make an inventory of the mineral composition of the entire hole; no distinction was made between primary (magmatic) and secondary (related to alteration or recrystallization) main phases. In the same way, the distinction between microphenocrysts and microcrysts in basaltic lava grains has not been investigated in detail; only a few obvious microphenocrysts were analyzed for comparison.

Thermometry and oxybarometry

Several geothermometers and oxybarometers were used to estimate the equilibrium temperature and redox conditions prevailing in the different levels of the hole (Table T2). The 2-pyroxene thermometers of Brey and Kohler (1990) and Andersen et al. (1993; QUILF program) were used for lithologies with coexisting cpx and opx, and the single-clinopyroxene thermometer of France et al. (2010b) was used for cpx-bearing holocrystalline rocks. For hornblende- and edenite-bearing lithologies (except basalts), we used the amphibole-plagioclase thermometer of Holland and Blundy (1994). The 2-oxide oxythermobarometers of Sauerzapf et al. (2008) and Andersen et al. (1993; QUILF program) were used to estimate the temperature and oxygen fugacity in grains with coexisting ilmenite and magnetite. The clinopyroxene-plagioclase oxybarometer of France et al. (2010a) was used to estimate the oxidation state of grains bearing this mineral pair except basalts and glasses (the model being not recommended for skeletal and dendritic crystals). To apply this oxybarometer, we used the temperatures estimated with the 2-pyroxene and single-clinopyroxene thermometers (Table T2) and the mean composition of the glass grains as an equivalent of the melt composition. For oxybarometers, the oxidation state was expressed as ΔNNO (oxygen fugacity relative to the nickel-nickel-oxide buffer) and ΔFMQ (oxygen fugacity relative to the fayalite-magnetite-quartz buffer).

For all calculations, we applied a pressure of 1 kbar (0 kbar for the amphibole-plagioclase thermometer) corresponding to the upper crust conditions, and we worked with average mineral compositions per grain. The results are available in RESULTS in “Supplementary material” and summarized in Table T2.

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