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doi:10.2204/iodp.proc.345.113.2014

Igneous petrology

Drilling in Hole U1415P recovered 31 lithologic intervals in RCB cores and 39 intervals in ghost cores. Table T2 lists these lithologic intervals and their division into three units. Principal lithologies recovered from Hole U1415P are shown in Figure F1. Analyses of cored gabbroic rock in Hole U1415P (olivine gabbro, orthopyroxene-bearing olivine gabbro, and troctolite) revealed primitive compositions with Mg# (100 × cationic Mg/[Mg + Fe] with all Fe recalculated as Fe²⁺) varying between 78.3 and 88.6 (see “Inorganic geochemistry”). The uppermost core recovered during hole cleaning (Core 345-U1415P-2G) consists of very fine grained to coarse-grained sand with basaltic and gabbroic rubble and is defined as lithologic Unit I.

Unit II, the Multitextured Layered Gabbro Series, is composed of olivine gabbro and orthopyroxene-bearing olivine gabbro that show a remarkably wide spectrum of textural variety, including a spectacular banded series, layering features, noritic bands, diffuse interfingering of modal and texturally different lithologies, skeletal olivines, and pegmatitic growth. A characteristic feature of this series is the omnipresence of late-crystallized clinopyroxene that may occur interstitially as films or ribbons on the grain boundaries of the rock matrix, forming centimeter-sized coherent anhedral crystals growing across lithologic boundaries. In pegmatitic patches, clinopyroxene sometimes forms discrete crystals with grain sizes as large as ~8 cm. Detailed descriptions of these rocks are presented below.

Below Piece 6 in Core 345-U1415P-15R-1, the lithologic character changes abruptly from heterogeneous, multitextured gabbro to homogeneous, medium- to coarse-grained granular troctolite (with small amounts of olivine gabbro), which is defined as Unit III, the Troctolite Series. Below, we present macroscopic and, where available, microscopic descriptions for the rocks recovered in Hole U1415P, including the interval numbers from which these rocks were recovered. For some thin sections in Hole U1415P, two different lithologic domains were defined. Table T3 lists the corresponding thin sections, the number and nature of the individual domains, the characteristics of the boundaries between the domains, and a link for the corresponding image of the thin section with the domain boundaries marked.

Unit I: basaltic/gabbroic rubble

Examples of recovered igneous rock from this unit are a medium-grained, granular to poikilitic olivine gabbro (Interval G6), a coarse-grained granular to poikilitic orthopyroxene-bearing olivine gabbro (Interval G7), and a sparsely phyric basalt (Interval G8). The basalt piece is moderately altered and contains <1% phenocrysts of euhedral to subhedral olivine, subhedral to anhedral plagioclase, and euhedral clinopyroxene.

Unit II: Multitextured Layered Gabbro Series

Olivine gabbro

Multitextured olivine gabbro forms the dominant lithologic interval recovered from Unit II in Hole U1415P and is the defining lithology of the series. However, thinner intervals of relatively more homogeneous equigranular olivine gabbro also occur throughout Cores 345-U1415P-6R through 13R (Fig. F1). Magmatic foliation is generally weak; layering or a characteristic banding is common (for details see “Structural geology”).

The multitextured olivine gabbro displays complex variation in both modal proportions of minerals grain size and mineral habit (Fig. F2) and consists of olivine (5%–30%), plagioclase (50%–70%), and clinopyroxene (5%–40%), with trace amounts of orthopyroxene and oxide (Cr-spinel, often altered to magnetite). Olivine is subhedral to anhedral with an amoeboid habit. Tabular plagioclase is euhedral to subhedral. Clinopyroxene is anhedral and dominantly subequant, forming large (40 mm) poikilitic grains.

Orthopyroxene-bearing olivine gabbro

Orthopyroxene-bearing olivine gabbro occurs as a major lithologic interval in the Unit II Multitextured Layered Gabbro Series. Orthopyroxene-bearing olivine gabbro appears either as a more homogeneous equigranular gabbro, which is the dominant lithology in the upper part of Unit II (Cores 345-U1415P-5R through 7R), or as domains within the dominantly multitextured gabbro in the lower part of Unit II (e.g., Intervals 4 and 8 in Fig. F3 and Interval 21 in Fig. F4). Multitextured orthopyroxene-bearing olivine gabbro is also a major lithology, along with the dominant multitextured olivine gabbro in the lower part of Unit II (Cores 345-U1415P-8R through 12R). In general, orthopyroxene-bearing gabbro consists of olivine (15%–20%), plagioclase (50%–70%), clinopyroxene (15%–30%), orthopyroxene (1%–4%), and trace amounts of oxide (Cr-spinel, often altered to magnetite). Olivine is subhedral to anhedral with an amoeboid habit. Plagioclase is euhedral to subhedral with a tabular habit. Clinopyroxene is anhedral, forming large interstitial grains. Orthopyroxene is anhedral, forming smaller interstitial grains. Coexisting clinopyroxenes and orthopyroxenes are common without any visible reaction between these two minerals.

Olivine-bearing gabbro

Olivine-bearing gabbro occurs in two ghost core intervals (Intervals G9 and G10) of Unit II. The gabbro is coarse-grained seriate poikilitic granular rock. The primary mineralogy is olivine (1%), plagioclase (65%), and clinopyroxene (34%). Olivine is subhedral to anhedral with an amoeboid habit. Plagioclase is subhedral to anhedral with a tabular habit. Clinopyroxene is anhedral and poikilitic with a subequant habit and contains plagioclase chadacrysts.

Unit III: Troctolite Series

Troctolite is the major lithology in both cores (Intervals 29 and 31) and ghost cores (Intervals G29 - G32, G33 - G37, G39) of the Unit III Troctolite Series (Fig. F1). Magmatic foliation defined by tabular plagioclase and prismatic olivine is common (for details see “Structural geology”). In contrast to the extreme variability in textures displayed by the Unit II Multitextured Layered Gabbro Series, troctolite shows a remarkable degree of homogeneity in both modal composition and grain size (Figs. F2, F5, F6). Troctolite generally consists of olivine (30–54 vol%) and plagioclase (45–70 vol%), with trace amounts of clinopyroxene (<1%) and oxide (<1% Cr-spinel). Olivine is euhedral to subhedral with a subequant habit. Although olivine in Unit II often shows skeletal textures (see section below), it is generally prismatic in Unit III. Plagioclase is subhedral to anhedral with a tabular habit. Clinopyroxene is anhedral with an interstitial habit. Interstitial clinopyroxene in many cases is optically continuous, forming what appears to be large coherent grains with grain sizes often >20 mm. For example, interstitial clinopyroxene in Figure F6 is formed by two large, centimeter-sized clinopyroxene grains.

Macroscopic characterization of multitextured olivine gabbro

In the multitextured orthopyroxene-bearing olivine gabbro of Unit II, large variations in modal composition and grain size of the constituent minerals and in textural appearance are observed. Based on their macroscopic features, the rocks of this unit have been classified into five lithologic groups:

1. Homogeneous medium-grained orthopyroxene-bearing olivine gabbro: the mineral assemblage of this rock type is plagioclase + olivine + clinopyroxene + orthopyroxene, which is the dominant lithology in Unit II. The rocks are homogeneous, granular, and medium grained.

2. Orthopyroxene-bearing pegmatitic olivine gabbro: this mineral assemblage is the same as for the first group (plagioclase + olivine + clinopyroxene + orthopyroxene), but these rocks show much more variation in texture, which is the reason for assigning them to the second lithologic group. The textures are inequigranular granular and in general have a much coarser grain size, as coarse as pegmatitic. These rocks include enclave-like patches of homogeneous, medium-grained orthopyroxene-bearing olivine gabbro (Fig. F7). One characteristic feature of the rocks within this group is the presence of skeletal olivine (see “Skeletal olivine in Unit II”).

3. Pegmatoidal veins: this rock type, which is less common in Unit II, has a plagioclase + olivine + clinopyroxene + orthopyroxene mineral assemblage. A characteristic feature is veins of pegmatitic orthopyroxene-bearing olivine gabbro that interfinger with the homogeneous medium-grained orthopyroxene-bearing olivine gabbro of Group 1 (Figs. F8, F9). The boundaries between the pegmatitic veins and the Group 1 gabbros are sutured. Greenish pegmatitic clinopyroxene (several centimeters in size) occurs in a vein within Sample 345-U1415P-4G-1, 62–93.5 cm (Piece 8) (see the upper right part of the piece shown in Fig. F9A). In this vein, plagioclase grows perpendicular to the vein wall in a typical comb-structure mode. The outer part of the pegmatitic vein is composed of plagioclase, whereas both orthopyroxene and clinopyroxene occur only in the inner part. Skeletal olivine is also observed in this rock (see the lower part of pieces in Fig. F9).

4. Heterogeneous olivine gabbro: the mineral assemblage of this group is plagioclase + olivine + clinopyroxene + orthopyroxene and defines a markedly heterogeneous lithology, characterized by the presence of elongated monomineralic domains that often display layering or banding or are disrupted to elongated aggregates, often with parallel alignment of the minerals (Fig. F10). Large plagioclase-rich domains are aggregates of fine-grained plagioclase crystals (average size = 0.3 mm). Often, clusters of isometric euhedral to subhedral spinel crystals occur in the plagioclase-rich domains. The pale greenish smoky part is composed of fine-grained plagioclase and poikilitic clinopyroxene. Skeletal olivine is also present.

5. Vertical bands: these rocks (plagioclase + olivine + clinopyroxene + orthopyroxene mineral assemblage) show spectacular subvertically oriented ~5 cm thick bands (Fig. F11) that may possibly be interpreted as subvertical “intrusions” (or banded layers) into heterogeneous olivine gabbro or as originally horizontal nonplanar layers now rotated into a subvertical position. Within the layer at the boundary to the host rock (heterogeneous olivine gabbro; Group 4), subvertically elongated monomineralic domains of constituent minerals (olivine, plagioclase, and clinopyroxene) can be observed. The subvertical bands are composed of two outermost plagioclase-rich layers (~5–10 mm thick) followed by an inner band of olivine gabbro composed of plagioclase, olivine, clinopyroxene, and orthopyroxene (~40 mm thick). In the outermost plagioclase-rich layers, the longer axes of plagioclase grains are arranged perpendicular to the boundary with the host rock. The gabbroic band shows characteristic asymmetrical olivine distribution. As shown Figure F11, at one side of the subvertical band a thin layer of dominantly fine grained equant olivine is observed. From here, the grain size of olivine increases toward the center of the band, and its shape changes from an equant to a more skeletal form. The boundary between plagioclase-rich and olivine-rich layers at one side is straight, and the boundary between the plagioclase-rich layer and the surrounding host gabbro is wavy. In contrast, the boundary between plagioclase-rich and olivine-rich layers at the other side is wavy and the boundary between the plagioclase-rich layer and the host gabbro is straight.

Descriptions of igneous boundaries

Within the Multitextured Layered Gabbro Series (lithologic Unit II) and the Troctolite Series (Unit III), four samples with igneous boundaries were recovered in Hole U1415P. The available thin sections of these boundaries enable microscopic characterization of the two igneous lithologies, which are differentiated by changes in mode and/or grain size (Table T4). The first two boundaries are within the Multitextured Layered Gabbro Series (Unit II), defining the boundary between relatively coarse grained olivine gabbro host rock that bears diffuse patches interpreted as enclaves of more finely grained mafic orthopyroxene-bearing olivine gabbro (Figs. F12, F13). The two other examples illustrate boundaries in the Troctolite Series (Unit III) between medium-grained troctolite and a finer grained dark-colored olivine gabbro, one of which is orthopyroxene bearing (Figs. F14, F15).

Interval 13 (Thin Section 114)

The boundary in Sample 345-U1415P-7R-2, 9.5–56 cm (Piece 2), is defined as a grain size and modal boundary (Fig. F12). The modal composition is slightly different between the orthopyroxene-bearing troctolite domain and the olivine gabbro host rock, described as an “anorthosite domain” because the corresponding thin section contains only plagioclase because of the heterogeneous distribution of large minerals (or mineral aggregates). The grain size of the anorthosite domain is slightly coarser than the orthopyroxene-bearing troctolite domain. The boundary is sutured, and the minerals are continuous through the boundary. Macroscopic and microscopic observations are consistent with a model of diffuse intermingling of two types of coherent magma batches under hypersolidus condition. The finer grained orthopyroxene-bearing troctolite domain could alternatively be interpreted as some kind of “enclave” hosted by the coarser grained olivine gabbro (see “Enclaves and magma mixing”).

Interval 27 (Thin Section 127)

The boundary in Sample 345-U1415P-14R-1, 16–25.5 cm (Piece 3), is defined as a grain size and modal boundary that is sutured (Fig. F13). The modal composition is different between the orthopyroxene-bearing olivine gabbro domain and the gabbro domain. Based on thin section observation, the grain size of the orthopyroxene-bearing olivine gabbro domain is coarser than that of the gabbro domain, although macroscopic observation suggests large plagioclase grain sizes. However, thin section observations imply that these larger grains are “anorthositic” aggregates of much finer plagioclases, now well equilibrated to a granoblastic mosaic-like texture, which implies an annealing stage resulting in recrystallization of the coarse primary plagioclase grains to a fine-grained network. As in the previous sample, this boundary may be either interpreted as a frozen boundary between two types of coherent magma batches intermingling with each other under hypersolidus conditions or as an enclave of orthopyroxene-bearing olivine gabbro in the more heterogeneous olivine gabbro.

Interval 29 (Thin Section 134)

The boundary in Sample 345-U1415P-18R-1, 78–83.5 cm (Piece 7), is defined as a grain size and modal boundary that is sutured (Fig. F14). The modal composition is different between the anorthositic troctolite domain and the orthopyroxene-bearing olivine gabbro domain. The grain size of the anorthositic troctolite domain is coarser than that of the orthopyroxene-bearing olivine gabbro domain. Minerals are continuous across the boundary. Because of the strong alteration observed in this sample, detailed microscopic characterization is hampered. Based on macroscopic observation, this boundary was interpreted as intrusive (see “Structural geology”).

Interval 29/30 boundary (Thin Section 140)

The boundary in Sample 345-U1415P-22R-2, 99–115.5 cm (Piece 8), is defined as a grain size and modal boundary that is sutured (Fig. F15). The modal composition is different between the troctolite domain and the olivine gabbro domain. The grain size of the troctolite domain is coarser than that of the olivine gabbro domain. Plagioclase in the finer grained olivine gabbro defines strong foliation parallel to the boundary. This, together with macroscopic observation, implies an intrusive boundary of the finer grained olivine gabbro into the troctolite.

Downhole variations of mode and grain size within Units II and III

Rock in Hole U1415P shows variations in mineral modal abundances, average mineral size, and maximum grain size as a function of depth (Fig. F2). These variations are also a function of rock type. A major change in lithologic features in this hole is the boundary between Units II and III, and the mineral modes correspond with this lithology contrast.

Olivine

Olivine occurs in both units. In Unit II, olivine has a modal amount of ~20% but increases in Unit III to ~35%, on average. This change in modal composition is not reflected in the maximum size or size mode of olivine in the two units. In Unit II, olivine has a heterogeneous grain size distribution, varying from as large as 14 mm in Core 345-U1415P-7R (where skeletal olivines are observed) to as small as 2 mm. The average, however, is weighted to ~10 mm because of the occurrence of skeletal/dendritic olivines throughout this unit.

In Unit III, the modal composition, size mode, and maximum size display a homogeneous distribution. Modal abundance ranges from 30% to 54% (average = ~35%), with a size mode of ~4 mm and maximum size of 10 mm. The abrupt change in the olivine mode and maximum size to low values in Core 345-U1415P-22R (Fig. F2) corresponds to the boundary between olivine gabbro and troctolite (Intervals 29 and 30, respectively). This boundary is interpreted as an intrusion of olivine gabbro into troctolite (see “Structural geology”).

Plagioclase

The multitextured nature of Unit II is reflected in the variable maximum grain size of plagioclase. Grain size is very heterogeneous, whereas the modal abundance of the plagioclase is relatively homogeneous (50%–70%). Plagioclase grain size varies from 2 to 10 mm in this unit (Fig. F2). At the Unit II/III boundary, plagioclase grain size is more homogeneous and averages ~10 mm. The modal amount of plagioclase in the Troctolite Series varies between 45% and 79%. The Troctolite Series is composed of only two troctolite intervals that span tens of meters. A marked change in plagioclase grain size is associated with the olivine gabbro/troctolite boundary recovered in Core 345-U1415P-22R.

Clinopyroxene

Modal abundance for clinopyroxene ranges from ~20% to 30% in Unit II (Multitextured Layered Gabbro Series), with grains as large as 60 mm (Core 345-U1415-5R; Fig. F2), the largest clinopyroxene grains observed in Hole U1415P. Below Core 5R, clinopyroxene reaches a maximum size of 50 mm, with an average of ~30 mm. A sharp change in clinopyroxene mode and grain size occurs at 64.2 mbsf (i.e., the boundary between the Multitextured Layered Gabbro Series [Unit II] and the equigranular Troctolite Series [Unit III]). Clinopyroxene is rare or absent in the Troctolite Series, in which it is observed macroscopically as an interstitial phase, and is generally <1 mm.

In addition to the phases described above, the occurrence of orthopyroxene and oxides (chromite/magnetite) also changes with depth and rock type. In Unit II, the modal amount of oxide varies between trace and 1%; in Unit III the oxide mode varies between trace and 0.8%. Orthopyroxene is only observed in Unit II and locally makes up as much as 5% of the modal composition; it is not observed in Unit III except for a small orthopyroxene-bearing olivine gabbro patch in Sample 345-U1415P-18R-1, 78–83.5 cm (Piece 7) (Fig. F14). The maximum size of observed orthopyroxene is 40 mm (Core 345-U1415P-4G).

Skeletal olivine in Unit II

Similar to the skeletal olivine interval at the top of Unit III (Troctolite Series) in Hole U1415J, olivine in Hole U1415P displays a skeletal morphology. The skeletal olivine in Hole U1415P is part of Unit II (Multitextured Layered Gabbro Series). However, unlike Hole U1415J where skeletal olivine occurs for ~15 m, skeletal olivine in Hole U1415P occurs within a ~50 m interval that makes up the bulk of the stratigraphy of Unit II. Skeletal olivine is generally interpreted as a disequilibrium cooling phenomenon and grows normal to the cooling surface along a thermal gradient (Donaldson, 1976).

Figure F16 shows various olivine morphologies and occurrences observed in Hole U1415P, which include

• A ~21 cm section filled solely with skeletal olivine (Sample 345-U1415P-7R-2, 9.5–30 cm [Piece 2]) (Fig. F16B). The bottom of this section hosts a rectangular patch of orthopyroxene-bearing olivine gabbro interpreted as an enclave (Fig. F7);

• Skeletal olivine that shows very elongated, subvertically orientated structures (“channel-like”) anastomosing with the host rock (Sample 345-U1415P-8R-1, 133–146 cm [Piece 9]) (Fig. F16C). This interval in particular occurs beneath the part with the marked banded feature, suggestive of magma mingling;

• A ~25 mm long poikilitic olivine grain with orthopyroxene chadacrysts (Sample 345-U1415P-9R-1, 29–42 cm [Piece 3]) (Fig. F16D). This centimeter-sized olivine oikocryst occurs at the boundary of a centimeter-sized poikilitic clinopyroxene. In the next section downhole, another centimeter-sized olivine was observed;

• A 30 mm diameter dendritic (or star burst) olivine (Sample 345-U1415P-10R-1, 55.8–68 cm [Piece 7]) (Fig. F16E); and

• An olivine “bud” that crosscuts a plagioclase-rich vein and branches out. The “stalks” on these branches are end-capped by equigranular 6 mm olivine grains. Interstitial to the olivine dendrites is centimeter-sized interstitial clinopyroxene (Sample 345-U1415P-13R-2, 40–50 cm [Piece 5]) (Fig. F16H).

The images in Figure F16 are only a small sampling of the skeletal textures within Unit II. Complete sections within Unit II of Hole U1415P are entirely skeletal.

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