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doi:10.2204/iodp.proc.345.110.2014

Igneous petrology

Ninety-one lithologic intervals were identified in the cored sections in Hole U1415J. In addition, 72 lithologic intervals were identified in the ghost cores. Table T2 lists these lithologic intervals and their division into three units and two clusters of ghost cores. The distribution of the principal lithologies within each unit and ghost cluster recovered from Hole U1415J as well as their relative position within the hole are shown in Figure F2. Analyses of cored gabbroic rock of Hole U1415J (excluding cataclasites and chromitites) revealed primitive compositions with Mg# (100 × cationic Mg/[Mg + Fe] with all Fe recalculated as Fe²⁺) varying between 78.6 and 87.4 (see “Inorganic geochemistry”). The upper three cores (345-U1415J-2G through 4R) are interpreted to represent a surficial zone of rubble, defined as lithologic Unit I.

Two further units were defined based on lithologic and structural constraints. Unit II (Cores 345-U1415J-5R through 9R) is, in general, a layered series dominated by the presence of clinopyroxene oikocrysts in gabbroic rock (Oikocryst-Bearing Layered Gabbro Series in Fig. F2). Unit II consists of oikocrystic-bearing troctolite (36%), gabbro (29%), oikocryst gabbro (12%), olivine gabbro (11%), and gabbronorite (9%), plus minor troctolite (nonoikocryst bearing) and basalt.

Unit III (Cores 345-U1415J-10R through 26R) is characterized by the presence of equigranular troctolites and olivine gabbro and the virtual absence of clinopyroxene oikocrysts (Troctolite Series in Fig. F2). The majority of this unit is troctolite (57%), but it also includes olivine gabbro (39%) plus minor heavily altered chromitite and basalt. Because of the overall low recovery in Hole U1415J (16%), a coherent lithostratigraphic column for this hole is not possible. However, we identified “packages” of probably coherent gabbroic lithologies, which are shown, including short summaries on their petrographic properties in Figure F3.

Based on lithologic and structural constraints and on the position within the hole, ghost cores were grouped into two clusters. Core 345-U1415J-2G was included in the rubble zone of Unit I at the top of the hole. Cores 6G, 7G, and 15G define ghost Cluster 1, which contains mainly rock from the cored interval of Unit II. Cores 14G, 17G, 22G, 24G, and 25G define ghost Cluster 2, which contains mainly rock from the cored interval of Unit III. The distributions of principal lithologies within the two ghost core clusters are also shown in Figure F2. Moreover, based on lithologic constraints, the first four pieces within Section 345-U1415J-23R-1 were interpreted as rubble because the RCB drill bit was used for cleaning the upper part of the hole before the hole advanced to recover Core 345-U1415J-23R.

Below are macroscopic and, where available, microscopic lithologic descriptions for the principal lithologies recovered in Hole U1415J and presented in the distribution graphs of Figure F2. The interval numbers from which these rocks were recovered are given in Table T2. For some thin sections from Hole U1415J, two or three different lithologic domains were defined. Table T3 lists the corresponding thin sections, the number and nature of the individual domains, and the characteristics of the contact between the domains, as well as a link for the corresponding image of the thin section with the domain boundaries marked.

Gabbro

Several intervals containing gabbro as the principal lithology were recovered from all three units of Hole U1415J (Table T2; Fig. F2). Gabbro in Hole U1415J often displays magmatic foliation defined by tabular plagioclase crystals and to a lesser extent by the orientation of prismatic olivine and clinopyroxene (see “Structural geology”). Gabbro in Hole U1415J is generally layered, showing centimeter- to decimeter-scale layering defined by differences in modal mineralogy and sometimes in grain size (for details see “Structural geology,” e.g., Fig. F60).

Gabbro

Gabbro (sensu stricto) occurs in Units I and II. The gabbro is fine to medium grained and has equigranular granular texture (Fig. F4). The primary mineralogy of the gabbro is dominated by plagioclase (55%–75%) and clinopyroxene (25%–45%), with trace amounts of olivine, orthopyroxene, and oxide. Plagioclase is euhedral to subhedral with a tabular habit, whereas clinopyroxene is anhedral with a subequant habit.

Olivine-bearing gabbro

Olivine-bearing gabbro occurs in Units I and II and ghost core Clusters 1 and 2. This gabbro ranges from fine to coarse grained and is dominantly equigranular granular in texture (Fig. F5). The primary mineralogy is olivine (1%–5%), plagioclase (48%–73%), and clinopyroxene (25%–50%), with trace amounts of orthopyroxene and oxide. Olivine is euhedral to anhedral with a range of habits (subequant, amoeboid, and skeletal). Plagioclase is euhedral to subhedral with tabular habit. Clinopyroxene is anhedral with subequant to poikilitic habit.

Orthopyroxene-bearing gabbro

Orthopyroxene-bearing gabbro occurs as a single interval in Unit II (Interval 17). The gabbro is medium grained and equigranular granular and consists of subhedral to anhedral amoeboid olivine (3%), euhedral to subhedral tabular plagioclase (66%), anhedral subequant to poikilitic clinopyroxene (30%), and subhedral prismatic orthopyroxene (1%), with trace amounts of oxide. Photomicrographs showing details of orthopyroxene-bearing assemblages are presented in “Descriptions of igneous boundaries.”

Olivine gabbro

Olivine gabbro

Olivine gabbro was recovered from all three units and from the ghost clusters (Table T2; Fig. F2). Olivine gabbro is dominantly medium grained, displays equigranular granular to granular-poikilitic textures, and consists of olivine (5%–30%), plagioclase (45%–70%), and clinopyroxene (5%–45%), with trace amounts of orthopyroxene (Units I and II) and oxides. Olivine gabbro typically exhibits magmatic foliation defined by the tabular plagioclase crystals and to a lesser extent by aligned olivines and clinopyroxenes (see “Structural geology”). Olivine is subhedral to euhedral with amoeboid habit (Fig. F6), tabular plagioclase is euhedral to subhedral, and clinopyroxene is anhedral and dominantly subequant, and often poikilitic (Figs. F6, F7). One remarkable texture is present in Unit III (Interval 59), where a single, several centimeter–sized clinopyroxene (optically continuous across the entire thin section) is poikilitic in the olivine gabbro but interstitial to the adjacent troctolite of Interval 58 (Figs. F8, F64A, F64C, F68).

Orthopyroxene-bearing olivine gabbro

Orthopyroxene-bearing olivine gabbro occurs as two intervals (Intervals 51 and 57) in Unit II. These gabbros are medium grained and equigranular granular and consist of subhedral to anhedral amoeboid olivine (3%), euhedral to subhedral tabular plagioclase (60%–65%), anhedral subequant clinopyroxene (22%–26%), and anhedral subequant to interstitial orthopyroxene (3%–4%).

Olivine-bearing gabbronorite

Olivine-bearing gabbronorite was found only in Unit II in Sections 345-U1415J-5R-1 and 5R-2 (Table T2; Fig. F2). The olivine-bearing gabbronorite is medium- to coarse-grained rock with equigranular granular texture and displays foliation defined by tabular plagioclase crystals and to a lesser extent by aligned olivine and clinopyroxene (Fig. F9) (see “Structural geology”). Olivine-bearing gabbronorite typically consists of olivine (2%–5%), plagioclase (60%–65%), clinopyroxene (25%–28%), and orthopyroxene (5%–10%); no oxide is present. Olivine is subhedral to anhedral with amoeboid habit, plagioclase is euhedral to subhedral with tabular habit, clinopyroxene is anhedral with poikilitic habit, and orthopyroxene is subequant with prismatic habit.

Oikocryst-bearing troctolite

Abundant clinopyroxene oikocryst-bearing troctolite and minor troctolite without oikocrysts are distinctive rock types making up lithologic intervals in Units I and II (Table T2; Fig. F2). Troctolite is medium-grained, seriate poikilitic granular rock with well-developed magmatic modal layering and foliation (see also “Structural geology”). In some samples, a strong grain size contrast exists between minerals in the troctolitic matrix and chadacrysts in large clinopyroxene oikocrysts. A spectacular example is shown in Figure F10 (Unit II, Interval 49). The strong magmatic foliation is formed by the alignment of tabular plagioclase and elongated olivine. Troctolite consists of olivine (10%–42%), plagioclase (45%–70%), and clinopyroxene oikocrysts (3%–35%; 0% in minor troctolite), with trace amounts of oxide (possibly Cr-spinel) and orthopyroxene. Olivine is fine grained and subhedral to anhedral with an elongated, irregular amoeboid habit. Plagioclase is fine grained and subhedral to euhedral with tabular habit. Large clinopyroxene oikocrysts (as large as 15 mm in diameter) are anhedral and poikilitic with a distinctive population of irregularly shaped plagioclase chadacrysts (Fig. F11). The plagioclase chadacrysts are randomly oriented within the oikocrysts, in contrast to the surrounding foliated plagioclase fabric. Many of the plagioclase chadacrysts are deformed. Olivine is absent as a chadacryst phase.

Oikocryst gabbro

Oikocryst gabbro (including minor intervals of clinopyroxene oikocryst-bearing olivine gabbro) occurs in Units I and II (Table T2; Fig. F2). Note that Unit III contains very minor to no oikocrysts. Oikocryst gabbro, together with clinopyroxene oikocryst-bearing troctolite, is a very distinctive lithology recovered in Unit II (Fig. F12). Oikocryst gabbro is medium-grained, seriate poikilitic granular rock and consists of olivine (<1%–15%), plagioclase (30%–60%), and clinopyroxene (40%–55%). Olivine is subhedral to anhedral with amoeboid habit. Plagioclase is euhedral to subhedral with tabular habit. Plagioclase chadacrysts within the clinopyroxene oikocrysts are subhedral to anhedral, tabular, and sometimes show continuous or patchy zoning. Clinopyroxene is anhedral and oikocrystic. For the definition of oikocryst gabbro, as well as an explanation of the differences between oikocrysts and poikilitic clinopyroxene, see “Igneous petrology” in the “Methods” chapter (Gillis et al., 2014e).

Troctolite

“Normal” troctolite (without clinopyroxene oikocrysts) occurs in the cores of Unit III as well as in ghost Cluster 2 (Table T2; Fig. F2). Troctolite in Unit III is easily distinguishable from that in Unit II because of its coarser grain size, weak foliation, lack of clinopyroxene oikocrysts, and distinctive alteration (Fig. F13). Troctolite consists of olivine (20%–80%), plagioclase (20%–75%), clinopyroxene (<1%–10%), and trace amounts of oxide (possibly Cr-spinel). Olivine is euhedral to subhedral with subequant to amoeboid habit. Plagioclase is euhedral to anhedral with tabular habit. Clinopyroxene is anhedral with interstitial habit. Troctolite shows only sparse magmatic layering, and most of it exhibits foliation defined by plagioclase and olivine shapes (for details see “Structural geology”).

Dolerite

Several doleritic lithologic intervals were recovered from Unit I of Hole U1415J (Table T2; Fig. F2). Some of the recovered pieces are shown Figure F14.

Olivine-bearing dolerite

Olivine-bearing dolerite occurs only in Unit I (Interval 2) and is very fine grained and equigranular with granular texture. Modally, plagioclase and clinopyroxene each comprise ~50% of the rock. Plagioclase forms euhedral to subhedral laths, with interstitial clinopyroxene. Olivine-bearing dolerite in this interval contains ~5% euhedral to subhedral subequant olivine.

Dolerite

Dolerite only occurs in ghost cores in Unit I (Intervals G2 and G3) and as a dike rock in a cataclasite in Unit III (Interval 74) as very fine grained equigranular rock with textures ranging from intergranular to subophitic. Modally, plagioclase and clinopyroxene each comprise ~50% of the rock. Plagioclase forms euhedral to subhedral laths, whereas clinopyroxene is anhedral and interstitial to subophitic (Fig. F15).

Doleritic gabbro

Doleritic gabbro only occurs in Unit I (Interval 4) as fine-grained rock with equigranular, doleritic texture. Modally, plagioclase and clinopyroxene each comprise ~50% of the rock. Plagioclase forms euhedral to subhedral laths, whereas interstitial clinopyroxene is anhedral.

Basalt

Several basaltic lithologic intervals were recovered from all three units in Hole U1415J (Table T2; Fig. F2).

Aphyric basalt

Aphyric basalt occurs in Unit I (Intervals 3, G1, and G4), Unit II (Interval 56), and Unit III (Interval G56) and contains <1% phenocrysts of euhedral to subhedral plagioclase laths and euhedral to subhedral subequant olivine phenocrysts (Figs. F14, F15).

Moderately olivine phyric basalt

This lithology interval only occurs in Unit III (Interval 70) and ghost core Interval G59 and contains euhedral to subhedral olivine (6%–7%) and subhedral subequant Cr-spinel as phenocrysts (<1%) (Figs. F14, F15).

Completely altered chromitite

Chromitite was recovered from intervals in Unit III (Table T2; Fig. F2) and is completely altered, making it difficult to establish its primary mineralogy and texture (see “Chromitite”).

Detailed description of coherent gabbroic lithologies in Units II and III

Two units have been defined based on lithologic and structural constraints, which are interpreted to represent two lithologically different series of coherent gabbroic rock and are interpreted as variably layered sequences (layering is prominent in Unit II and sparse in Unit III) with boundaries based on changes in mineral mode and/or grain size. The boundaries are described in the next section. Here, we characterize the lithologic features of the rock, which are summarized in Figure F3.

The characteristic feature of Unit II is magmatic layering with respect to modal composition and grain size between troctolite and minor gabbro (for details see “Structural geology”). Because of the presence of clinopyroxene oikocrysts, this unit was named the Oikocryst-Bearing Layered Gabbro Series (Fig. F2). Typical textural features of this unit are shown in Figure F16. Of all recovered rocks in Unit II, 48% are clinopyroxene oikocryst-bearing rocks. These rocks show centimeter-sized, roundish or elongated clinopyroxene oikocrysts within a troctolitic or olivine gabbroic matrix.

Unit III is characterized by the dominance of equigranular troctolite. An apparent feature of Unit III is the virtual absence of clinopyroxene oikocrysts and the dominance of equigranular, granular troctolite and olivine gabbro which comprise 90% of the recovered rocks. Because troctolite is the most common rock, this is named the Troctolite Series (Fig. F2). The dominant rock type in this unit is homogeneous troctolite, which is characterized by the virtual absence of layering.

Typical textural features of rocks of Units II and III are shown in Figure F16. In Figure F17, the depth log for the modal compositions and mineral grain sizes for both units shows the clinopyroxene-rich nature of Unit II and the general impoverishment of clinopyroxene in Unit III. Because of the overall low recovery of Hole U1415J (16%), a coherent lithostratigraphic column for Units II and III is not possible. However, we identified packages of probably coherent gabbroic lithologies.

Unit II: Oikocryst-Bearing Layered Gabbro Series

Unit II contains six distinct packages of rock that were grouped based on similar lithology or mineralogy (Fig. F3). Typical textural features of the rock recovered in this unit are the presence of magmatic foliation, modal layering, and clinopyroxene oikocrysts. Core 345-U1415J-5R contains two packages: (1) oikocryst-bearing and (2) orthopyroxene-bearing. The oikocryst-bearing package is 0.93 m thick and contains two troctolite intervals (Intervals 19 and 24), two olivine gabbro intervals (Intervals 21 and 22), and a gabbro interval (Interval 20), all of which are oikocryst bearing. The orthopyroxene-bearing package is 0.85 m thick. The majority of this interval is gabbronorite (Intervals 25, 27, 30, and 32) but it also includes troctolite (Intervals 28, 29, and 32), and olivine gabbro (Interval 26). Core 345-U1415J-8R contains three packages: (1) olivine-bearing, (2) olivine-rich, and (3) olivine-bearing gabbro. The olivine-bearing package is 2.53 m thick and consists of four troctolite intervals (Intervals 35, 37, 40, and 41) and three olivine gabbro intervals (Intervals 34, 36, and 39); one gabbro interval (Interval 38) is also included in this package. The olivine-rich package is 0.63 m thick and consists of one olivine gabbro interval (Interval 43), one gabbro interval (Interval 46), and three troctolite intervals (Intervals 44, 45, and 47). The olivine-bearing gabbro package is 1.1 m thick and consists of two intervals that have olivine-bearing gabbro lithologies (Intervals 48 and 50). Core 345-U1415J-9R contains only one package, which is olivine-rich and 0.86 m thick. This package consists of troctolite (Interval 52), olivine gabbro (Interval 54), and olivine-bearing gabbro (Intervals 53 and 55). Figure F3B displays the average modal composition for each of the packages and the size range for dominant mineral assemblage in the packages.

Unit III: Troctolite Series

The Troctolite Series of Unit III consists of three packages: (1) troctolite with olivine gabbro, (2) troctolite, and (3) olivine gabbro (Fig. F3). The majority of the troctolite with olivine gabbro package consists of five troctolite intervals (Intervals 58, 60, 61, 62, and 66), with olivine gabbro also included in this package (Intervals 59, 63, 64, and 65). This is the thickest (20.1 m) package of rock in Hole U1415J. The troctolite package consists only of troctolite and is 9.2 m thick (Intervals 71, 72, 73, and 75). The olivine gabbro package is only olivine gabbro (Interval 78) and is 1.5 m thick. Figure F3B displays the average modal composition for each of the packages and the size range for each mineral in the packages.

Descriptions of igneous boundaries

Within Units II and III, eight different types of igneous boundaries were recovered in Hole U1415J, for which thin sections are available. The thin sections enable microscopic characterization of the boundary, which mostly separates two different igneous lithologies as a consequence of changes in mode and/or grain size, and mostly corresponds to different magmatic layers, as defined macroscopically. Details on each of these boundaries and the corresponding microphotographs are presented as follows (Table T4).

Interval 27/28 (Thin Section 38)

The boundary in Sample 345-U1415J-5R-1, 129–143 cm (Piece 18), is defined as a grain size and modal boundary that is sutured and subparallel to the foliation (Fig. F18). The modal composition is clearly different between the olivine- and orthopyroxene-bearing gabbro domain (Interval 27) and the troctolite (Interval 28) domain. The grain size of the olivine- and orthopyroxene-bearing gabbro is coarser than the troctolite. Minerals are continuous through the boundary.

Interval G20/G21 (Thin Section 40)

The boundary in Sample 345-U1415J-7G-1, 26–34 cm (Piece 5), is defined as a grain size and modal boundary that is sutured and subparallel to the foliation (Fig. F19). The modal composition is clearly different between the clinopyroxene oikocryst-bearing troctolite domain and the gabbronorite domain. The grain size of the gabbronorite (Interval G21) is coarser than the clinopyroxene oikocryst-bearing troctolite (Interval G20), except for clinopyroxene oikocrysts. Minerals are often continuous through the boundary.

Interval G30 (Thin Section 42)

The boundary in Sample 345-U1415J-7G-1, 91–94 cm (Piece 14), is defined as a grain size and modal boundary that is sutured and subparallel to the foliation (Fig. F20). The modal composition is clearly different between the troctolite domain and the clinopyroxene oikocryst-bearing troctolite domain. The grain size of the clinopyroxene oikocryst-bearing troctolite is coarser than the troctolite. Minerals are continuous across the boundary. Both domains are assigned to Interval G30 (clinopyroxene oikocryst-bearing troctolite).

Interval 42/43 (Thin section 52)

The boundary in Sample 345-U1415J-8R-2, 105–122 cm (Piece 9), is defined as a grain size and modal boundary that is sutured and subparallel to the foliation (Fig. F21). The modal composition is clearly different between the olivine gabbro domain (Interval 43) and the olivine-bearing gabbro domain (Interval 42). The grain size of the olivine-bearing gabbro is coarser than the olivine gabbro. Minerals are continuous across the boundary.

Interval 48 (Thin Section 54)

The boundary in Sample 345-U1415J-8R-3, 31–41 cm (Piece 6), is defined as a grain size and modal boundary that is sutured and subparallel to the foliation (Fig. F22). The modal composition is clearly different between the gabbro domain and the olivine gabbro domain. The grain size of the olivine gabbro is coarser than the gabbro. Minerals are continuous across the boundary. Both domains are assigned to Interval 48 (olivine-bearing gabbro).

Interval 58/59 (Thin Section 58)

The boundary in Sample 345-U1415J-10R-1 39–55 cm (Piece 6B), is defined as a grain size and modal boundary that is gradational and subparallel to the foliation (Fig. F23). The clinopyroxene in the olivine gabbro domain (Interval 59) is oikocrystic and continuous through the troctolite domain (Interval 58), but the modal composition is clearly different between the troctolite domain and the olivine gabbro domain. The grain size of the olivine gabbro is coarser than the troctolite, except for the clinopyroxene oikocryst.

Interval 73 (Thin Section 72)

The boundary in Sample 345-U1415J-18R-1, 141–146 cm (Piece 17B), is defined as a grain size and modal boundary that is sutured and subparallel to the foliation (Fig. F24). The modal composition is clearly different between the olivine-rich troctolite domain and the troctolite domain. The grain size of the troctolite is coarser than the olivine-rich troctolite. Minerals are continuous across the boundary. Both domains are assigned to Interval 73 (troctolite).

Interval G69 (Thin Section 85)

The boundary in Sample 345-U1415J-25G-1, 30–38 cm (Piece 5), is defined as a modal boundary that is sutured and subparallel to the foliation (Fig. F25). The modal composition is different between the olivine-bearing gabbro domain and the gabbro domain. Interfingering texture of clinopyroxene grains is observed along the boundary (Fig. F25D). The clinopyroxene of the gabbro intrudes into the clinopyroxene of the olivine-bearing gabbro and contains orthopyroxene blebs. Minerals are continuous across the boundary. Both domains are assigned to Interval G69 (olivine gabbro).

Boundary interpretation

The boundaries recovered in Hole U1415J are mainly sutured or gradational; no sharp boundaries were observed. This description implies that the boundaries were formed at the interface between magmas that were both hypersolidus. Mineral continuity across the boundaries was induced by further crystallization after the magmas were juxtaposed. The lack of chilled margins at the boundaries in Hole U1415J implies that the magmas were near thermal equilibrium.

Significance of orthopyroxene

The presence and abundance of orthopyroxene as a crystallizing phase is similar in Unit II of Holes U1415I and U1415J. Its significance (see the “Hole U1415I” chapter [Gillis et al., 2014d]) is related to the widespread perception that orthopyroxene in the ocean crust is limited to the late stages of crystallization. Mohorovicic discontinuity–crossing melts are generally considered to be undersaturated in orthopyroxene because orthopyroxene is almost never observed as a crystallizing phase in mid-ocean-ridge basalt (MORB), nor is it observed in experiments on dry MORB composition (e.g., Stolper and Walker, 1980; Grove and Bryan 1983). At the same time, the defining literature on the lower crustal sections of the Oman ophiolite, which is regarded as the best example for an ancient fast-spreading oceanic rift system, in those areas not affected by the so-called late-stage magmatism (e.g., Lippard et al., 1986) declare definitively that prismatic orthopyroxene is generally not observed (e.g., Pallister and Hopson, 1981) except in the uppermost evolved gabbro. The discovery of orthopyroxene in mafic lithologies in high quantities in the lower crustal section at Hess Deep is a novel finding, although the presence of orthopyroxene in one primitive gabbro sample from Hess Deep was reported by Coogan et al. (2002).

Three factors are critical to understanding the role of orthopyroxene in this rock. First, orthopyroxene is stabilized relative to clinopyroxene by pressure and oxidizing conditions (Feig et al 2006, 2010). Second, water in the magma destabilizes orthopyroxene in favor of clinopyroxene (Feig et al., 2006, 2010). Third, contrary to the conclusions of Pallister and Hopson (1981), orthopyroxene is found in the Oman lower crustal section (Abily, 2011) in similar abundance and occurrence to that in Holes U1415H–U1415J at Hess Deep. Although this may be related to initiation of subduction, the parallel to the occurrence observed here is striking.

Clinopyroxene textures and the origin of oikocrysts in troctolites in Hole U1415J

The textures of clinopyroxene observed in Hole U1415J gabbro are remarkable for their complexity and diversity. We observed a range of textural habits from interstitial, intergranular, and subophitic to ophitic, sometimes all within the same thin section. Despite this complexity, in a broad sense the observed textures in gabbro vary between two end-members. The first end-member is a relatively medium grained equigranular gabbro containing discrete, relatively equant nonpoikilitic granular clinopyroxene. The second end-member is a relatively coarse grained, strongly seriate gabbro with very large pyroxene oikocrysts, charged with chadacrysts forming what appears to be a continuous interconnecting network of grains (see a definition for oikocryst and poikilitic clinopyroxene in “Igneous petrology” in the “Methods” chapter [Gillis et al., 2014e]). For any particular gabbro in Hole U1415J, it is possible to place the observed textures within a continuum between these two end-members. This continuum may reflect an interplay between (1) clinopyroxene resulting from cumulus crystallization at one end and (2) clinopyroxene crystallizing from the rapid cooling of a crystal mush or in melt channels. Imposed on this continuum is magmatic foliation ranging from weak to strong, so it is possible to have both isotropic to foliated equigranular gabbro, for example. Below, we present the features of the end-member textural types and give some intermediate examples.

Clinopyroxene textures in Hole U1415J troctolite

Clinopyroxene oikocrysts in Unit II troctolite are considered separately because of their distinctive nature, reflected in the very large grain size contrast between the relatively fine grained foliated troctolitic matrix and the large clinopyroxene oikocrysts (Figs. F26, F27). The oikocrysts appear as isolated large subhedral crystals in a finer grained troctolitic fabric that appears to “swirl”’ around the oikocrysts (Fig. F26A, F26B). Visually, the comparison to snowball porphyroblasts in some metamorphic rock is inescapable, though their origin is obviously different.

Hole U1415H–U1415J troctolite clinopyroxene oikocrysts show grain boundaries that range from interstitial and optically continuous with the oikocryst (Fig. F26C) to sharper boundaries with no interstitial domain. In these oikocrysts, the troctolite fabric wraps around the oikocryst, with olivine and plagioclase in direct contact with clinopyroxene and some of the plagioclase projecting from the troctolite matrix into the interior of the oikocryst (Fig. F27).

Another feature of the oikocrysts is the distinctive chadacryst population. Olivine is never a chadacryst despite its presence at oikocryst margins and in the interstitial poikilitic domains of the oikocrysts. The plagioclase chadacryst population is complex, displaying euhedral to irregular shapes with some deformation features (see “Structural geology”) (Fig. F28). Hence, clinopyroxene crystallized after deformation for it to have captured deformed plagioclase. The chadacrysts are randomly oriented, in marked contrast to the tabular euhedral to subhedral plagioclase in the surrounding strongly foliated troctolitic matrix (Figs. F26C, F27C, F28). If the clinopyroxene crystallized late, we would expect it to have captured the surrounding troctolite foliation, and indeed this has occurred in the interstitial margins, yet it is not present in the oikocryst cores. If the oikocryst was the result of late-stage melt-rock reaction, we would also expect to see abundant partly reacted chadacrysts reflecting the original fabric of the troctolite.

In summary, the oikocryst textures suggest that the clinopyroxene

• Crystallized after deformation of the plagioclase chadacrysts (see “Structural geology”),

• Appears to not be in a reaction relationship with olivine and plagioclase of the troctolite matrix, and

• Contains a plagioclase chadacryst population that is difficult to explain as being derived from the surrounding plagioclase population.

Olivine morphology

Skeletal and dendritic olivine textures are preserved in Hole U1415J. Figure F29 shows some examples of skeletal olivine observed in recovered core. Many of these skeletal olivines have a “C” or hook shape associated with them, with plagioclase commonly enclosed in the hook (Fig. F29A, F29C, F29D, F29F). These plagioclases display evidence of resorption in the form of “elbow” joints or 120° grain boundaries. The majority of these skeletal olivines occur in Cores 345-U1415J-3R through 10R.

Other olivine morphologies lack this hook shape but are still classified as skeletal because of their “branching” morphology (Fig. F29B, F29E). This type of morphology has been labeled dendritic. Piece 4 from Section 345-U1415J-11R-1 displays clear macro- and microscopic evidence of this morphology. The red outline in Figure F30A enhances the branches of the dendritic olivine. Clinopyroxene fills the gaps where the branches have broken. The branching nature of the olivine in this sample is further demonstrated in thin section. In Figure F30B, dendritic olivine with pink birefringence is observed next to poikilitic clinopyroxene (tan birefringence). This morphology has been termed “starburst.” Another troctolitic sample (345-U1415J-25G-1, 28–30 cm [Piece 4]) also displays signs of dendritic olivine; however, the olivine in this sample is more altered, and the branches are difficult to identify.

Experimental studies suggest that skeletal and branching crystals grow relatively rapidly under conditions of strong supersaturation or magmatic undercooling (e.g., Lofgren and Donaldson, 1975; Donaldson, 1976; Lofgren, 1980). Donaldson (1974, 1977) attributed dendritic development to in situ crystallization under conditions of supersaturation of the magma with respect to olivine. He speculated this might arise from two possible alternative mechanisms: a sudden decrease in the water content of a water-undersaturated feldspathic peridotitic liquid and the transition of a water-saturated peridotitic magma to an unsaturated state by exsolution of volatiles. Hort (1998) also concluded that exsolution of volatiles from magma could induce such an increase in undercooling, giving rise to dendritic and skeletal crystal morphologies. This suggests that replenishment could involve both continual small influxes of magma as well as intermittent large influxes of magma (Butcher et al., 1985; Faithfull, 1985; Renner and Palacz, 1987) and that this periodic influx could be the fundamental cause of much of the layering observed in this section.

Chromitite

Heavily altered chromitite was recovered from two intervals in Unit III (Intervals 70 [Sample 345-U1415J-18R-1, 67–69 cm] and 77 [Sample 21R-1, 5–16 cm]) (Figs. F31, F32, F33) and from one ghost core (Sample 22G-1, 0–3.5 cm [Piece 1]). Chromite grains are also present in the troctolite adjacent to the chromitite, including one small aggregate of chromite grains in one piece of troctolite (Sample 345-U1415J-19R-1, 39–46 cm [Piece 6]). Chromitite is embedded in a series of primitive, olivine-rich, chromite-bearing troctolite and olivine gabbro (between the sequences of troctolite in Sections 345-U1415J-10R-1 through 20R-1 and olivine gabbro in Sections 21R-1 through 26R-1). Two contacts between domains with aggregates of chromitite and heavily altered troctolite within individual pieces are shown in Figures F31 and F33. Although obscured by strong alteration, the primary contacts appear to have a sutured character. One chromite aggregate contains roundish inclusions of now heavily altered material, which is interpreted as former inclusions of olivine (Fig. F31). All these observations imply that these chromitite aggregates represent accumulation of Cr-spinel in an early stage of MORB crystallization.

Because of the severe alteration, estimating the primary lithologies of chromitite is difficult. Several grains in two samples represent relics of the magmatic stage now forming a central core surrounded by “orbicular” aggregates of fine-grained magnetite (Figs. F31, F33; Thin Section 71; Sample 345-U1415J-18R-1 [Piece 9] and Thin Section 76; Sample 21R-1 [Piece 3]). The host rock of the chromitite is troctolite, although it is composed of minerals that are completely altered to chlorite and amphibole. In one sample (21R-1 [Piece 2]), the primary phases are completely replaced by magnetite and/or iron-rich chromite, chlorite, and amphibole with bluish green pleochromism (Fig. F32; see also “Metamorphic petrology”). The texture and mineral assemblage of Sample 21R-1 (Piece 2) is different from other chromitite samples in Hole U1415J (Figs. F31, F33). This rock is interpreted to have formed during chloritization of the surrounding olivine/chromite associations. The chromite grains are surrounded by chlorite, broken apart into elongated magnetite grains, and cut through by chlorite veins (Kelemen, Kikawa, Miller, et al., 2004; Abe, 2011). These observations imply that the classification of this rock (Sample 345-U1415J-21R-1, 4.5–12 cm [Piece 2]) as “chromitite” is justified, although it is now completely metamorphosed. In order to express the generally high grade of alteration within the chromitites, the term “completely altered chromitite” for the principal rock name was used. Only two reported examples of chromitite recoveries from previous ocean drilling cruises exist. One example is several aggregates of chromite and a chromitite minipod from peridotite cores recovered at Hess Deep from Ocean Drilling Program (ODP) Leg 147 Holes 895C and 895E (Arai and Matsukage, 1996). The other example is from ODP Leg 209 Site 1271 peridotite cores (Shipboard Scientific Party, 2004; Abe, 2011).

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