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doi:10.2204/iodp.proc.345.111.2014

Igneous petrology

Coring in Holes U1415K, U1415M, and U1415N recovered mostly volcanic or hypabyssal rocks as well as a few gabbroic rocks, including orthopyroxene-bearing olivine gabbro and evolved quartz-bearing lithologies such as an oxide-quartz diorite.

Hole U1415K

Sections 345-U1415K-2G-1 through 2G-4 consist of a mix of mainly fine to coarse grained sand and minor pebbles, drill cuttings, and brown mud/clay. The first interval of Section 2G-5 (Interval G5) is a mix of pebbles. The larger fragments include basalt, dolerite, and gabbro. Intervals G6–G8 contain single pieces of oxide-quartz diorite (microscopic examination reveals the presence of granophyric intergrowths of quartz and plagioclase), orthopyroxene-bearing olivine gabbro, and aphyric basalt. The distribution of principal lithologies recovered in Hole U1415K is shown in Figures F1A and F1B for all recovered lithologies, including sand (Intervals G1–G8), and for the recovered intact magmatic rocks (Intervals G6–G8).

Hole U1415M

Sections 345-U1415M-2G-1 through 2G-3 consist of very fine to medium-grained sand (Intervals G1–G3). Section 2G-4 contains a coarser grain size that also includes pebbles mostly corresponding to basalt (Interval G4). Interval G5 contains several basaltic pebbles. Cut pieces correspond to aphyric basalt. The distribution of principal lithologies recovered in Hole U1415M is shown in Figure F1C.

Hole U1415N

Coring in Hole U1415N recovered olivine-bearing volcanic and hypabyssal rock. Cores 345-U1415N-1R through 4R contain basalt (mostly olivine, moderately to highly phyric) with coarser grained hypabyssal rock in the deepest core (4R; olivine-phyric basalt, dolerite, and olivine dolerite). One ghost core (5G) recovered fine-grained olivine dolerite and olivine-highly phyric basalt. The distribution of principal lithologies recovered in Hole U1415N is shown in Figures F1D–F1F.

Rock types

Below, we give a brief lithologic description for each rock type recovered from the different intervals of Holes U1415K, U1415M, and U1415N based on macroscopic and, where available, microscopic observations. For one thin section from Hole U1415N, two different lithologic domains were defined. Table T2 lists the corresponding thin section, number and nature of the individual domains, characteristics of the contact between the domain, and a link to the corresponding thin section image with the domain boundaries marked.

Diorite

Two intervals of diorite were recovered from Core 345-U1415K-2G (Intervals G5 and G6). The diorite is medium grained and equigranular with a seriate to granular texture (Fig. F2). Based on thin section observations, diorite is composed of plagioclase (60%), clinopyroxene (25%–30%), orthopyroxene (<1%–4%), quartz (3%–10%), oxide (3%–5%), and trace amounts of amphibole that is assumed to be of primary origin (Fig. F3B). Plagioclase is euhedral to subhedral and tabular. Clinopyroxene and orthopyroxene are subhedral to anhedral with a subequant habit. Quartz is anhedral and irregular interstitial. In Sample 345-U1415K-2G-5, 13–15 cm (Piece 2), quartz forms granophyric texture together with plagioclase (Fig. F3A). The evolved composition of this rock is also expressed by the presence of apatite and zircon (Fig. F3B–F3D).

Olivine gabbro

Two intervals of olivine gabbroic lithologies were recovered (Sample 345-U1415K-2G-5, 18–25 cm [Piece 3], in Interval G7 and Sample 345-U1415N-4R-1, 48.5–57 cm [Piece 12], in Interval 16). Gabbro is fine- to medium-grained equigranular granular rocks. The primary mineralogy is olivine (2.5%–10%), plagioclase (40%–60%), clinopyroxene (35%–45%), orthopyroxene (2%–3%), and oxide (0.5%–1%). Olivine is subhedral with an equant habit. Plagioclase is subhedral to anhedral with a tabular habit. Clinopyroxene is anhedral with a subequant to poikilitic habit. Orthopyroxene is subhedral with a subequant to prismatic habit. Oxide is subhedral to anhedral with an irregular habit.

Basalt

Several basaltic lithologic intervals were recovered in Holes U1415K, U1415M, and U1415N. A series of core close-up images showing the variety of the recovered basalt rocks are presented in Figure F4. Two analyses of cored basalt in Hole U1415N (olivine-phyric basalt) revealed relatively evolved compositions, compared to the gabbroic rocks drilled at Site U1415, with Mg# (100 × cationic Mg/[Mg + Fe] with all Fe recalculated as Fe²⁺) of 63.8 and 76.1, respectively. One sample is interpreted as a former chilled margin (Fig. F4E). The intervals include aphyric basalt as well as a range of olivine-phyric basalt (sparsely to highly phyric). Olivine phenocryst contents range from 1% to 20% with minor plagioclase (<1%–5%). Plagioclase phenocrysts are subhedral to anhedral, may show abundant oscillatory zoning, and range in mode from <1 to 10 vol%.

A characteristic feature is the presence of Cr-spinel (<1%) as phenocrysts and microphenocrysts in many basalt pieces (Fig. F5). Some phenocrysts and microphenocrysts show corroded rims where the Cr-spinel reacted to magnetite. Cr-spinel also occurs as inclusions in olivine phenocrysts. In Sample 345-U1415N-1R-1, 52–54 cm (Piece 10), millimeter-sized glomerocrysts of olivine and plagioclase are visible within an intergranular matrix consisting of olivine, plagioclase, and clinopyroxene (Fig. F6A). These are regarded as xenocrysts that occasionally show subgrains (for details see “Structural geology”). Spherical inclusions of sulfide in olivine within this sample are interpreted as early immiscible sulfide droplets, indicating early sulfide saturation (Fig. F6B, F6C). Plagioclases exhibit a strong patchy zoning (Fig. F6D).

Dolerite

Several doleritic lithologic intervals were recovered in Hole U1415N. Dolerite consists of olivine (<1%–10%), plagioclase (50%–60%), clinopyroxene (40%), and oxide (<1%). A thin section was cut from Sample 345-U1415N-5G-1, 8–16 cm (Piece 2; Interval G2). Olivine, now completely altered, is euhedral to subhedral with an equant habit. Larger crystals of plagioclase are euhedral to subhedral and tabular and show strong growth zonation effects (Fig. F7A, F7B). Interstitial plagioclase in this sample is lath-shaped to acicular. Interstitial clinopyroxenes are anhedral, form subophitic clusters, and show signs of fast crystal growth (Fig. F7C, F7D). Oxide is anhedral and subequant.

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