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doi:10.2204/iodp.proc.309312.203.2009

Methods and materials

Sample description

This report presents the whole-rock major and trace element compositions of 32 samples (19 basalts and 13 gabbros) and mineral compositions of 42 samples (18 basalts and 24 gabbros) obtained during Expedition 312. On the basis of microscopic observations, we selected samples with the minimum amount of secondary minerals such as chlorite, epidote, and albite so that primary igneous mineral compositions are well preserved for electron probe microanalysis. Analyzed samples are listed in Table T1.

The lithologic features of the samples are as follows. The dikes, which were recovered from Hole 1256D during Expedition 312, are metamorphosed to various grades from greenschist to pyroxene hornfels facies (see “Igneous petrology” in the “Site 1256” chapter). The development of the secondary minerals of Hole 1256D basalts and dolerites is classified into eight types in the “Site 1256” chapter and by Koepke et al. (2008). This report follows this classification, based on the primary igneous features of glass, clinopyroxene, plagioclase, and Fe-Ti oxides and secondary mineral assemblages of dusty brown material (mostly actinolite), amphibole, clinopyroxene, orthopyroxene, and Fe-Ti oxides. Type 1 is defined as completely fresh basalt with pristine glass and phenocrysts. Type 2 is characterized by the appearance of secondary minerals after glass. Type 3 is defined as slightly altered basalt where glass is altered to chlorite and oxides and <50% of primary clinopyroxene is altered to the dusty brown fibrous masses, while plagioclase is mostly primary. Type 4 is defined as the much-altered basalt in which >50% clinopyroxene is altered to dusty brown fibrous masses and Fe-Ti oxides and plagioclase is altered to dusty brown material. Type 5 is characterized by the first appearance of granular clinopyroxene, orthopyroxene, and green flaky hornblende as secondary minerals. This type is defined as metamorphosed basalt in which >90% clinopyroxene is altered to actinolite and oxide and plagioclase is replaced by submicrometer-size discrete Fe-Ti oxides and actinolite. Type 6 is defined as metamorphosed basalt where primary clinopyroxene is completely altered to prismatic actinolite, green to brown flaky hornblende, and Fe-Ti oxides grains. Types 7 and 8 are mature metabasalt. Type 7 is defined by microgranular mosaic-like texture with typical granoblastic domains, flaky and poikiloblastic green to brown hornblende, and prismatic actinolite. The typical granoblastic domain consists of microgranular clinopyroxene, orthopyroxene, and Fe-Ti oxides. Type 8 metabasalt is characterized by more or less continuous microgranular granoblastic mosaics of secondary clinopyroxene, orthopyroxene, plagioclase, hornblende, and Fe-Ti oxides (for details, see table 3 of Koepke et al., 2008). These types correspond to metamorphic grade from greenschist facies (Type 3) up to pyroxene hornfels facies (Type 7 and 8). The analyzed samples include Type 3, 4, 6, 7, and 8. The majority of the sheeted dikes are Type 3 and 4 basalts, except that the granoblastic dikes are Type 7. The upper dike screen consists of Type 8 metabasalt. Under a microscope, it is not clear whether three samples (312-1256D-233R-1, 4–7 cm, and 234R-1, 1–2 and 7–9 cm) from the lower dike screen are of igneous or metamorphic origin (see “Igneous petrology” in the “Site 1256” chapter). These rocks show a microgranular texture with granular clinopyroxene, orthopyroxene, plagioclase, and Fe-Ti oxide and include 0.5 to 1.5 mm long phenocryst-like prismatic plagioclase relics that include submicrometer-scale spherical Fe-Ti oxides and transparent inclusions (Fig. F1G, F1H). Because this mineral assemblage, texture, and appearance of phenocryst relics resemble the characteristics of the Type 8 metabasalts of the upper dike screen, these samples are classified as Type 8 metabasalt.

Gabbro 1 mainly consists of disseminated oxide orthopyroxene-bearing olivine gabbro. Gabbro 1 gabbros commonly carry Fe-Ti oxide, which decreases with depth. Olivine is present in significant amounts in the lower portions of Gabbro 1, whereas orthopyroxene is not abundant and is largely associated with olivine. Some of the large orthopyroxenes include numerous bleb-like clinopyroxene intergrowths a few micrometers in size. The clinopyroxenes of Gabbro 1 are anhedral or poikilitic between subhedral plagioclase frameworks. Multiple zoning is observed in large subhedral plagioclases. Gabbro 1 clinopyroxenes are largely altered to green or greenish brown amphibole with Fe-Ti oxide. The upper part of Gabbro 1 (Units 83 and 85) has the development of a patchy texture with clinopyroxene-rich subophitic domain and plagioclase-rich coarse-grained patch or network domain (Fig. F32 in the “Site 1256” chapter). The subophitic domain is characterized by clinopyroxene oikocrysts roughly 5 to 10 mm across with up to a few millimeter long plagioclase chadocrysts. The coarse-grained domain is characterized by coarser grained zoned euhedral prismatic plagioclase and abundant Fe-Ti oxide (Fig. F2). The subophitic domain rarely contains orthopyroxene. Remarkable exsolution lamellae occur in some of the clinopyroxenes in the coarse-grained domain. The clinopyroxene with the lamellae occurs in the rim of the subophitic clinopyroxene at the contact between the two domains (Fig. F1A, F1B). The alteration minerals in Gabbro 1 are dominated by albite, chlorite, epidote, calcite, green amphibole, and Fe-Ti oxides.

Gabbro 2 mainly consists of orthopyroxene-bearing gabbro, characterized by variable orthopyroxene content up to 20%. Fe-Ti oxides are present up to 5% and generally decrease with depth. Plagioclase is euhedral to subhedral with a simply or oscillatory zoned clouded core that includes numerous submicrometer opaque inclusions (Fig. F1C). In the metabasalts and Gabbros 1 and 2, clinopyroxene is classified into three types: igneous-type, amphibole-intergrowth (amphibole-type), and pale green colored (secondary-type). Igneous-type clinopyroxene is characterized by the existence of exsolution lamellae partly filled with opaque minerals (Fig. F1E). Amphibole-type clinopyroxene is variably replaced by greenish brown to brown amphiboles and Fe-Ti oxides a few to a few tens of micrometers in diameter (Fig. F1D). Secondary-type clinopyroxene is more transparent than the other types. This type is rarely observed in Type 7 metabasalt in Gabbro 1 and the upper and lower dike screen and rarely occurs as a cluster or vein (Fig. F1F). These secondary-type clinopyroxenes are observed as the pale green colored poikiloblasts which form veins or clusters and are sharply distinguished from the other types by their extremely low TiO₂ content (see “Clinopyroxene”). This pale green clinopyroxene with extremely low TiO₂ has been reported as secondary mineral in the granoblastic dikes because of retrograde metamorphism (Koepke et al., 2008). Gabbro 2 orthopyroxenes are anhedral and enclose or partially enclose euhedral plagioclases. Some large orthopyroxenes include clinopyroxene bleb-like intergrowths. Olivine rarely presents in Gabbro 2 and most olivine is pseudomorphed with iddingsite and iron oxide or talc, green to brown amphibole, and opaque minerals. Olivine commonly occurs with orthopyroxene and opaque minerals. Some olivines are poikilitically enclosed by large anhedral orthopyroxenes. Gabbro 2 includes some metabasalt or rounded metabasalt enclaves up to 1 cm in diameter (e.g., Sample 312-1256D-230R-1, 81–84 cm; 1483.81 mbsf). The texture of the enclaves corresponds to Type 7 or 8 metabasalt, which is similar to the lower dike screen. Gabbro 2 alteration minerals are dominated by greenish brown to brown amphibole and Fe-Ti oxide.

Methods

Sample preparation

Samples were prepared for X-ray fluorescence (XRF) and inductively coupled plasma–mass spectrometry (ICP-MS) as follows. To minimize the effect of alteration, visibly weathered parts and hydration veins were removed. Processed chips were washed for 15 min in an ultrasonic bath filled with ion-exchanged water and then dried for >24 h in an oven at 120°C. Dried samples were crushed using a tungsten carbide mortar followed by milling in a tungsten carbide ball mill. W, Co, Ta, and Nb were excluded from trace element analysis because of possible contamination from the tungsten carbide.

Major elements

Major elements (SiO₂, Al₂O₃, TiO₂, FeO [total Fe as FeO], MnO, CaO, MgO, Na₂O, K₂O, and P₂O₅) were analyzed with a Rigaku RIX-3000 X-ray fluorescence spectrometer at the Faculty of Science, Niigata University (Japan). Powdered samples were dried at 120°C for 1 h to remove H₂O and heated for a further 12 h at 900°C in a muffle furnace to obtain loss-on-ignition (LOI) values. A heated powder sample (0.5 g) was mixed with 5 g Li₂B₄O₇ and fused in a platinum crucible at 1200°C to form a glass bead. Calibration curves were made using the Geological Survey of Japan (GSJ) igneous rock series standard samples. To include a wide range of compositions, some standard samples were synthesized by adding pure chemical agents to GSJ standard samples (for example, pure TiO₂ was added to the JB-1b standard). The precision of analysis was verified by multiple measurements of the JB-2 standard (Table T2). These measured values are in good agreement with the reference values (Imai et al., 1995; Eggins et al., 1997).

Trace elements

Trace elements were analyzed using the ICP-MS Agilent 7500a at the Graduate school of Science and Technology, Niigata University. The elements measured were Sc, V, Ga, Rb, Sr, Y, Zr, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Hf, Th, and U. An alkali fusion method was used, following Roser et al. (2000), to avoid residue of acid-resistant minerals such as zircon. However, nitric acid was used instead of the perchloric acid used in the original method. The US Geological Survey (USGS) reference material BHVO-2 was used as a standard for the correction. The precision of the analysis was verified by multiple measurements of the USGS reference material W-2. Results are shown in Table T2.

Mineral composition

Main mineral phases were analyzed by the JXA-8600SX EPMA at the Graduate School of Science and Technology, Niigata University. Plagioclase, clinopyroxene, and orthopyroxene were analyzed for SiO₂, TiO₂, Al₂O₃, Cr₂O₃, FeO, MnO, MgO, CaO, Na₂O, K₂O, BaO, and NiO under the following analytical conditions: acceleration voltage = 15 kV, probe current = 13 nA, and beam diameter = 1 µm. Olivine was analyzed for SiO₂, TiO₂, Al₂O₃, Cr₂O₃, FeO, MnO, MgO, CaO, and NiO under 25 kV and 20 nA with a beam diameter of ~3 µm. Both natural and synthetic oxides and silicates were used as standards. Correction was made according to the ZAF (Z = atomic number, A = absorption, and F = fluorescence) method. Core and rim compositions were obtained from each mineral phase. Analyses of small grains and anhedral minerals were done irrespective of the core and rim. Detection limits are listed in Table T3.

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