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doi:10.2204/iodp.proc.345.109.2014

Igneous petrology

Coring in Hole U1415I recovered 25 lithologic intervals and 4 intervals of ghost cores. Table T2 lists these lithologic intervals and their division into two lithologic units. Principal lithologies recovered from Hole U1415I are shown in Figure F1. The upper three sections (345-U1415I-1R-1 through 1R-3) are interpreted as surficial rubble (Unit I). This includes eight lithologic intervals comprising 10% olivine gabbro and 7% gabbro (using principal rock names only). The remaining 83% of Unit I is drilling-induced disaggregated gabbro (gabbroic sand). Unit I overlies an interval of massive gabbro including a 1.1 m thick layered sequence, defined as Unit II, the Layered Gabbro Series (Sections 345-U1415I-4R-1 through 4R-2). Unit II is composed of olivine gabbro (39%), gabbronorite (27%), troctolite (26%), and gabbro (8%). The lowermost core from Hole U1415I (Core 345-U1415I-5G) was a “ghost” core recovered during hole cleaning operations from an interval that was previously cored. Analyses of cored rocks of Hole U1415I (two orthopyroxene-bearing olivine gabbro pieces, one olivine-bearing gabbro piece) revealed primitive compositions with Mg# (cationic Mg/[Mg + Fe] – all Fe as Fe²⁺) varying between 81.0 and 82.1 (see “Inorganic geochemistry”).

Olivine gabbro

Olivine gabbro

Olivine gabbro comprises four lithologic intervals, three as recovered intervals (Intervals 10, 13, and 17) within the layered gabbro sequence of Unit II and one piece from the ghost core (Interval G1; Table T2). The olivine gabbro is medium- to coarse-grained seriate to equigranular granular rock with well-developed magmatic foliation (Figs. F2, F18, F20A). The strong foliation is defined by the alignment of tabular plagioclase (see “Structural geology”). As illustrated by Thin Section 21 (Sample 345-U1415I-4R-1, 35–38 cm [Piece 6]), olivine gabbro consists of olivine (5%–25%), plagioclase (50%–70%), and clinopyroxene (10%–40%), with trace amounts of oxide and occasionally orthopyroxene (Fig. F18B–F18E). Olivine is fine grained, subhedral to anhedral, and often elongated with an irregular amoeboid habit. Despite their irregular habit, however, the long axes of the olivines are generally parallel to the foliation defined by the plagioclase in the gabbroic matrix. Plagioclase is fine grained and subhedral to euhedral with a tabular habit. Zoning in the plagioclase is variable. The presence of 120° triple junctions between plagioclase grains indicates that the plagioclase framework is close to textural equilibrium. Clinopyroxene is medium to coarse grained and anhedral with an irregular interstitial to oikocrystic habit.

Orthopyroxene-bearing olivine gabbro

Orthopyroxene-bearing olivine gabbro is found in four intervals (Interval 3 in Unit I and Intervals 12, 14, and 20 in Unit II) (Table T2). In general, the orthopyroxene-bearing olivine gabbro is a medium-grained equigranular granular rock. Two thin sections were cut from Interval 3: Thin Section 13 (Sample 345-U1415I-2R-1, 25–31 cm [Piece 7]) and Thin Section 14 (Sample 2R-1, 51–53 cm [Piece 11]). Modally, orthopyroxene-bearing olivine gabbro consists of olivine (5%–12%), plagioclase (50%–65%), clinopyroxene (20%–40%), and orthopyroxene (1%–4%), with trace amounts of oxide. Olivine is fine grained and subhedral to anhedral with an equant to subequant habit. Plagioclase is medium grained and euhedral to anhedral with a tabular habit. Intervals in the layered gabbro sequence display well-developed magmatic foliation, defined by the alignment of tabular plagioclase (see “Structural geology”). Clinopyroxene is medium to coarse grained and anhedral with an irregular interstitial and sometimes oikocrystic habit. Orthopyroxene is euhedral to subhedral with a subequant to equant, prismatic or interstitial habit.

Anorthositic olivine gabbro

Anorthositic olivine gabbro in Interval 9 (Table T2) is a medium-grained poikilitic granular rock with olivine (5%), plagioclase (80%), clinopyroxene (15%), and trace amounts of oxide.

Troctolite and clinopyroxene oikocryst-bearing troctolite

Troctolite occurs in Interval 18 as part of the layered gabbro sequence in Unit II (Section 345-U1415I-4R-1) (Fig. F3). Two thin sections were cut for microscopic observation: Thin Section 19 (Sample 345-U1415I-4R-2, 9–13 cm [Piece 2]; Table T3) and Thin Section 24a/24b (Sample 5G-1, 12–14 cm [Piece 3]). Clinopyroxene oikocryst-bearing troctolite occurs in Interval 24 as part of the layered gabbro sequence in Section 4R-2 (Fig. F4) as well as a single piece recovered from Section 5G-1 (Interval G3). The troctolite and clinopyroxene oikocryst-bearing troctolite is medium-grained equigranular rock with well-developed magmatic modal layering and foliation. The strong foliation is formed by the alignment of tabular plagioclase (Fig. F4; see also “Structural geology”). Troctolite consists of olivine (20%–40%) and plagioclase (55%–70%), with trace amounts of oxide (possibly Cr-spinel) and orthopyroxene. Olivine is fine grained and subhedral to anhedral with an elongated, irregular amoeboid habit (Fig. F21B, F21C). Despite their irregular habit, the long axes of the olivines are generally parallel to the foliation defined by the plagioclase in the troctolitic matrix. Plagioclase is fine grained and subhedral to euhedral with a tabular habit. Overall, plagioclase shows no obvious zoning. Clinopyroxene is sometimes present in the troctolite either as a minor interstitial component (4%–5%), ophitic to plagioclase and olivine, or sometimes as thin films between plagioclase and olivine. Although the clinopyroxene is interstitial, alignment of the longer axes of clinopyroxene is generally parallel to the foliation of the troctolitic matrix.

Some troctolite contains large anhedral clinopyroxene oikocrysts (as large as 15 mm in diameter) that have a distinct population of subhedral to euhedral lath-shaped resorbed plagioclase chadacrysts (Figs. F4, F21). Where these are present, troctolite is referred to as clinopyroxene oikocryst-bearing troctolite (see “Igneous petrology” in the “Methods” chapter [Gillis et al., 2014e]). Olivine is conspicuous by its absence as a chadacryst. The plagioclase chadacrysts are oriented in a random manner within the oikocryst, in sharp contrast to the surrounding foliated plagioclase fabric.

Gabbro

Gabbro

Gabbro was described in Intervals 2 and 8 from Unit I as a medium-grained equigranular granular rock (Fig. F5). Thin Section 18 (Sample 345-U1415I-3R-4, 16–19 cm [Piece 2]) shows that the gabbro consists of plagioclase (50%–60%) and clinopyroxene (40%–50%), with trace amounts of orthopyroxene. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium to coarse grained and anhedral with an interstitial to oikocrystic habit.

Olivine-bearing gabbro

Olivine-bearing gabbro comprises Interval 23 as part of the layered gabbro sequence in Unit II. The olivine-bearing gabbro is a medium-grained equigranular granular rock. Modally, olivine-bearing gabbro consists of olivine (2%), plagioclase (68%), and clinopyroxene (30%), with trace amounts of oxide. Olivine is fine grained and subhedral to euhedral with a subequant habit. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is fine to medium grained and anhedral with an irregular interstitial habit.

Anorthositic gabbro

Anorthositic gabbro defines two intervals in Hole U1415I: Interval 25 from the layered gabbro sequence in Unit II and one piece from ghost Core 5G (Section 345-U1415I-5G-1; Interval G4). Anorthositic gabbro is medium-grained equigranular to seriate granular rock. This interval displays strong foliation defined by tabular plagioclase crystals similar to the layered gabbros in Unit II. Modally, anorthositic gabbro consists of plagioclase (80%) and clinopyroxene (20%). Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium to coarse grained and anhedral with an interstitial to subequant habit.

Orthopyroxene-bearing gabbro

Orthopyroxene-bearing gabbro defines Interval 1 at the top of Unit I. This orthopyroxene-bearing gabbro is a medium-grained equigranular granular rock. Modally, orthopyroxene-bearing gabbro consists of plagioclase (70%), clinopyroxene (26%), and orthopyroxene (4%), with trace amounts of olivine (Thin Section 11; Sample 345-U1415I-1R-1, 1–4 cm [Piece 1]). Plagioclase is medium grained and subhedral to anhedral with a tabular habit. Clinopyroxene is medium to coarse grained and subhedral to anhedral with an irregular to subophitic habit. Orthopyroxene is anhedral and has an irregular habit.

Gabbronorite

Olivine-bearing gabbronorite

Olivine-bearing gabbronorite defines four intervals (Intervals 11, 16, 19, and 21) from the layered gabbro sequence of Unit II. The olivine-bearing gabbronorite is medium-grained equigranular granular rock with a well-developed magmatic foliation (Fig. F6; see also “Structural geology”). The strong foliation is formed by the alignment of tabular plagioclase, similar to that displayed by troctolite. One thin section was cut for microscopic observations (Thin Section 22; Sample 345-U1415I-4R-1, 48–50 cm [Piece 8A]). Modally, olivine gabbronorite consists of olivine (5%–10%), plagioclase (55%–60%), clinopyroxene (20%–25%), and orthopyroxene (5%–15%). Olivine is fine grained and subhedral to anhedral with an irregular amoeboid habit. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium grained and subhedral to anhedral with an equant habit. Orthopyroxene is subhedral to euhedral with a subequant habit.

Gabbronorite

Gabbronorite was observed in Interval 22 in Section 345-U1415I-4R-2. The gabbronorite is medium grained with a seriate and poikilitic-granular texture. Thin Section 23 (Sample 345-U1415I-4R-1, 136–139 cm [Piece 12]) was cut for microscopic observations. Modally, gabbronorite consists of plagioclase (50%), clinopyroxene (40%), and orthopyroxene (10%), with trace amounts of oxide. Plagioclase is medium grained and subhedral to euhedral with a tabular habit. Clinopyroxene is medium grained and anhedral with an irregular to poikilitic habit. Orthopyroxene is anhedral with an irregular to equant habit.

Drilling-induced disaggregated gabbro

Drilling-induced disaggregated gabbro was recovered in Sections 345-U1415I-3R-1 through 3R-3 and most of 3R-4 (Intervals 4–7) and is included in Unit I. Point counting of grain mounts under a petrographic microscope revealed that this gabbro is ~75% altered. A reconstructed primary igneous mode based on the characteristic alteration assemblages of olivine, plagioclase, and pyroxenes is plagioclase (61%), clinopyroxene (31%), and olivine (8%), corresponding to olivine gabbro (Table T4; see “Metamorphic petrology” for details of reconstruction). The reconstructed modes are consistent with that obtained from olivine-bearing gabbro recovered from Hole U1415I in general.

Layered Gabbro Series of Unit II

Core 345-U1415I-4R recovered an exceptional series of coherent layered gabbroic rock with a total curated length of 1.1 m and consisting of five pieces (Pieces 6–12). Principal lithologies within this series include olivine gabbro, troctolite, and gabbronorite, all with equigranular to seriate textures. One characteristic feature of this series is a moderate to strong magmatic foliation defined by the strong alignment of tabular plagioclase and, if present, elongated anhedral olivine. All boundaries between the different lithologies are parallel to the foliation and sutured. When boundaries were recovered, they were always modal, sometimes in combination with slight changes in texture and grain size (Table T2; Fig. F3).

Details of this series are shown in Figure F7. The series starts with Interval 15, a 5 cm thick medium-grained troctolitic olivine gabbro followed by a 25 cm thick sequence of medium-grained olivine-bearing gabbronorite (Interval 16). The next interval consists of 11 cm of medium-grained olivine gabbro (Interval 17) followed by a relatively thin layer (2 cm) of troctolite (Interval 18) that shows a relatively sharp boundary relative to the neighboring gabbroic layers as well as a slightly larger grain size than other gabbroic rock of the Layered Gabbro Series. Interval 19 is an 11 cm thick medium-grained olivine-bearing gabbronorite that shows a gradational transition (see next section) to the following Interval 20, which is a 17 cm thick layer of medium-grained orthopyroxene-bearing olivine gabbro. The boundary to the next interval, which is a 6 cm thick layer of medium-grained olivine-bearing gabbronorite (Interval 21), is relatively sharp. Although all intervals within this series show relatively strong magmatic foliation (see “Structural geology”), the last interval of this series, a gabbronorite (Interval 22), shows markedly weaker magmatic foliation but still with an orientation parallel to the boundaries and subparallel to the modal layering. This last interval consists of three similar pieces without recovered boundaries, so the length of this interval, 33 cm, must be regarded as a minimum thickness.

Analysis of enhanced images from Section 345-U1415I-4R-1A highlights the modal variations along the complete Layered Gabbro Series of Intervals 16–18. The rock types vary from troctolite, olivine gabbro, and olivine-bearing gabbronorite to troctolitic olivine gabbro (from the bottom of Piece 8 to the top of Piece 6). Estimation of the mineral modes was based on 1 cm increments parallel to the layering (or foliation). The result, presented in Figure F8, shows continuous trends for the individual minerals, with smooth transitions at the boundaries between the different lithologies. The total length of the coherent layered sequence in Hole U1415I is 1.1 m, which comprises 69% of the recovered core and 15% of the hole. This is a minimum estimation, as some layered gabbro may not have been recovered.

Details on the relation between oikocrysts and background rock

Large (up to 15 mm) clinopyroxene oikocrysts are unusual in oceanic gabbroic rocks. Moreover, one investigated gabbro also contains 10 mm sized clinopyroxene oikocrysts. The relationship between the fabric of plagioclase hosted in the matrix and oikocrysts is variable. Plagioclase chadacrysts may be oriented in a random manner within the oikocryst, in accordance with the surrounding randomly oriented plagioclases of the gabbroic matrix (e.g., Thin Section 18; Sample 345-U1415I-3R-4, 16–19 cm [Piece 2]) (Fig. F9). By contrast, in a clinopyroxene oikocryst-bearing troctolite sample (Interval 19; Sample 345-U1415I-4R-2, 8.5–13 cm [Piece 2]) plagioclase chadacrysts show random orientation, whereas plagioclase of the troctolitic matrix define a strong foliation (e.g., Thin Section 19; Sample 345-U1415I-4R-2, 9–13 cm [Piece 2]) (Fig. F9). This implies that the oikocrysts do not correspond to simple poikilitic growth in a later stage of magmatic evolution, which would be expected assuming the standard model of simple fractional crystallization of mid-ocean-ridge basalt (MORB) (e.g. Grove and Bryan 1983; Berndt et al., 2005). In the same thin section, matrix plagioclases as well as the chadacrysts within the oikocrysts show a very strong patchy zonation (Fig. F10). Moreover, some of the plagioclase chadacrysts show plastic deformation (as observed in the troctolitic matrix), whereas the clinopyroxene oikocrysts are virtually undeformed (Fig. F21). All of these observations, together with the absence of olivine as chadacryst, although it is omnipresent in the troctolitic matrix, support a complex model of oikocryst formation that is significantly different from a simple late-stage growth in an evolved magma enclosing earlier crystallized minerals.

Significance of magmatic features

Coring in this hole recovered a number of remarkable textural and mineralogic features that are unusual and highly significant for the formation of the lower ocean crust. In the upper part of the hole, comprising rounded rubble clasts of Unit I, the general relationships are the same as in Holes U1415D–U1415H (i.e., lithologies mostly consistent with derivation from a wide variety of structural levels with in the lower crust). The Layered Gabbro Series of Unit II penetrated a larger slump block, perhaps more nearly in place, and displays both clinopyroxene oikocrystic gabbro types observed in Hole U1415H, well-developed magmatic layering, and orthopyroxene-bearing olivine gabbro.

Magmatic layering is considered one of the key observational features expected of ocean crust that to date has been rarely directly observed. Many authors have observed microscopic textural evidence of crystal accumulation typical of layered rocks (Engel and Fisher, 1969; Melson and Thompson, 1970; Meyer et al., 1989; Francheteau et al., 1990) that occurs because of a number of processes operating on the scale of a gabbroic magma chamber and is thought of as a ubiquitous feature of the oceanic lower crust. However, actual modal and rhythmic layering implied by the microscopic observations have not been found in any great quantity in any oceanic sampling campaign.

The occurrence of primitive orthopyroxene-bearing mafic magmatic rock from deep fast-spread oceanic crust is also new in Hole U1415I. Normally, orthopyroxene is not common as a phenocryst mineral in MORB and is thought of generally as a very late forming phase in conjunction with significant oxides in the upper part of the gabbroic section in the ocean crust. It is thus quite unusual to find orthopyroxene nearly throughout the rhythmically layered section in Hole U1415I.

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