IGNEOUS AND MANTLE PETROLOGY

Drilling penetrated to a depth of 155.80 mbsf in Hole 1274A providing 28 cores; total cumulative recovery of 34.65 m of core is 22.2%. The coherent recovery of near meter-long sections in a large number of cores allows characterization of igneous rocks and residues of partial melting at a centimeter scale in the vertical dimension.

The core consists of serpentinized peridotite and dunite with minor amounts of gabbro and oxide gabbro. A curated thickness of 6.61 m of unconsolidated material with harzburgites, dunites, and gabbros was recovered at 97.9–146.1 mbsf. The total cumulative core recovery comprised 93% hard rocks and 7% unconsolidated material. The rock material recovered comprised 77% serpentinized harzburgite, 20% dunite, and the remaining 3% gabbro (Fig. F3).

Based on the lithologic recurrence of very similar peridotites (harzburgite and dunite) throughout Hole 1274A, we could have defined only one lithologic unit. However, a thick fault gouge interval consisting mostly of a fine- to medium-grained matrix of serpentinite mud/clay with clasts of harzburgite, dunite, and gabbro cuts the peridotite sequence. Therefore, we define three stratigraphic units: Unit I is a peridotite sequence that consists mainly of harzburgite, Unit II contains the fault gouge interval and the bulk of the gabbro in the hole, and Unit III is a 2.26-m-long peridotite sequence that is very similar to Unit I.

Unit I

Interval: Sections 209-1274A-1R-1 through 18R-1

Depth: 0–94.66 mbsf

Lithology: harzburgite

Unit I contains 83% harzburgite (Fig. F4), 16.5% dunite (Fig. F5), and 0.5% gabbro. The top of the sequence is 13 m of recovered (40.3 m drilled) harzburgite with very rare dunite. The modal orthopyroxene contents in harzburgite vary asystematically between 10% and 28% (average = 18% in 80 core intervals as defined in "Site 1274 Visual Core Descriptions"). Clinopyroxene is present as an interstitial phase in all samples but averages <2%, except for a 50-cm-long interval at 0.67 and 1.17 mbsf where its mode is as high as 3% and the orthopyroxene mode is ~25%. Modal spinel averages <1.5% throughout the sequence. At the base of the homogeneous harzburgite section, a sequence of alternating harzburgites and dunites characterizes the bottom of Unit I (40.3–94.7 mbsf). The dunites make their first appearance in the sequence as a few centimeter-thick intervals that alternate with orthopyroxene-poor harzburgite. These dunite bands increase toward the bottom of Unit I to as thick as 70 cm. The harzburgites have, on average, 13.5% modal orthopyroxene (47 core intervals defined in "Site 1274 Visual Core Descriptions") (54.3 m drilled), 5% less than the peridotite above this interval. Harzburgites near dunites have modal orthopyroxene content as low as 8%–10% (Fig. F5). Dunites contain some orthopyroxene (average = 2.5% in 27 samples) and a modest amount of spinel (average = 1%). Plastic deformation and recrystallization below the solidus is minor, and the textures are protogranular. One pebble of harzburgite with a mylonitic texture is present at the top of Core 209-1274A-2R (Fig. F6). A 15-cm-long interval of oxide gabbro, consisting of three pieces, is the only mafic rock found in Unit I.

Unit II

Interval: Sections 209-1274A-18R-1 through 26R-1

Depth: 94.66–142.12 mbsf

Lithology: mud/peridotite/gabbro

Unit II includes nine cores with a cumulative recovery of 8.98 m from 47.5 m drilled (recovery = 18.9%). Unit II contains significant thicknesses of a gray-green unconsolidated mud (Fig. F7). Rounded clasts of dunite and harzburgite (1–2 cm long) are dispersed in the mud matrix. A few intervals (30–60 cm) of dunite and harzburgite were cored between the unconsolidated intervals. The dunite and harzburgite are highly altered and brecciated and are, respectively, 29% and 30% of the total recovery of Unit II. Gabbro is found throughout Unit II (8%). A 20-cm-thick interval of completely rodingitized gabbro found in Core 209-1274A-21R (Fig. F8) contains clinozoisite, garnet, and vesuvianite (see "Alteration of Mafic Rocks" in "Hydrothermal Alteration" in "Metamorphic Petrology").

Unit III

Interval: Sections 209-1274A-26R-1 through 28R-1

Depth: 142.12–151.38 mbsf

Lithology: harzburgite

Unit III includes two cores (three sections) with a cumulative recovery of 2.09 m from 4.98 m drilled (recovery = 42%). Unit III is similar to Unit I and consists primarily of harzburgite (90% of Unit III recovery). Two horizons of gabbro (6%) and dunite (4%) account for the rest of Unit III. The harzburgite averages 22% orthopyroxene, 1% clinopyroxene, and <1% spinel. The dunite contains as much as 5% orthopyroxene as an anhedral to interstitial phase. Plastic deformation and recrystallization below the solidus are essentially absent, and the textures are protogranular.

Harzburgite

The harzburgites recovered from Hole 1274A are the freshest (40%–50% of the primary minerals are preserved) drilled so far during Leg 209. They have a coarse protogranular texture and contain a mineral assemblage consisting largely of olivine (70%–90%) and lesser orthopyroxene (10%–28%) with minor amounts of clinopyroxene (<1%–3%) and spinel (1%–2%). There is a gradational decrease in the modal proportion of pyroxene toward dunite at the centimeter scale, but in general the modal proportions of minerals in harzburgite do not show a systematic gradient downhole (Fig. F9). Below, we characterize the textural aspects of the constituent minerals of the harzburgite.

Olivine

Olivine composes >70% of the harzburgite from Hole 1274A. The olivine is moderately serpentinized but is well preserved locally. Determination of the original grain size of olivine in hand sample was not possible because of the pervasive alteration. In thin section, fresh olivine occurs as small (generally <0.1–1 mm) grains consisting of subangular to subrounded kernels separated by anastomosing serpentine veinlets and secondary magnetite. Optically continuous olivine kernels indicate that the original crystals were considerably larger than 3–4 mm. Small grains of olivine are partially enclosed in the orthopyroxene margins.

Pyroxenes

Orthopyroxene composes as much as 30% of the volume of the peridotites. Most of the orthopyroxene grains are anhedral with smoothly curved grain boundaries at the contact with olivine, typical of protogranular texture in abyssal peridotites (Fig. F10). Orthopyroxene margins partially enclose smaller-sized olivine grains, many of which are euhedral. Some orthopyroxene grains are more equant, and these tend to form clusters of several orthopyroxene grains with granular to polygonal grain boundaries. Clinopyroxene is a common interstitial phase along the triple junctions in the orthopyroxene clusters. Subhedral to euhedral spinel is commonly included in orthopyroxene grains, especially in those that are more rounded. Spinel, orthopyroxene, and olivine symplectite are common at or within the margins of anhedral orthopyroxene (Fig. F11). These characteristics are also observed in the peridotites from Hole 1272A, and like Hole 1272A harzburgite, Hole 1274A harzburgite has textures and mineralogies characteristic of partial melting residues. Clinopyroxene abundance in harzburgite from Hole 1274A, although low (2%), is greater than that in peridotites from Hole 1272A and locally may amount to 3% of the volume of harzburgite. Clinopyroxene occurs as rims or small patches at the tips of anhedral orthopyroxene or all along the margins of large orthopyroxene grains. Generally, clinopyroxene is optically continuous when it rims orthopyroxene (Fig. F12). Clinopyroxene also tends to enclose small grains of olivine and orthopyroxene, and we refer to this as poikilitic clinopyroxene (Fig. F13). In one thin section (Sample 209-1274A-1R-1, 15–17 cm), a poikilitic clinopyroxene totally encloses an oval orthopyroxene. Clinopyroxene also occurs in association with spinel in symplectitic or granular intergrowths. Symplectites commonly occur at the margins of orthopyroxene and seem to grow toward the olivine. The boundary between orthopyroxene and spinel symplectites is sharp and straight. Rare, relatively large, single crystals of clinopyroxene with intergranular extensions are also present (Fig. F14).

Orthopyroxene is affected by minor ductile-brittle deformation that is manifest as incipient breakdown of the coarse protogranular crystals into subgrains. Intergranular clinopyroxene is present at the boundaries of the subgrains (Fig. F15A), clearly healing microfractures and crystallizing as small lamellae along kink bands (Fig. F15B), and in orthopyroxene cleavages (Fig. F15C, F15D, F15E, F15F). Moreover, some microfractures are filled with small grains of olivine, clinopyroxene, and spinel and only clinopyroxene and spinel in other places. Similar crack-filling assemblages are present between larger orthopyroxenes (Fig. F16A–F16D). Locally, small clinopyroxene grains with magmatic twinning crystallized between olivine and orthopyroxene grains (Fig. F17).

Maximum and minimum grain sizes of orthopyroxene and clinopyroxene in the peridotites of Hole 1274A were estimated during visual core description. Grain size was estimated more accurately from 20 thin sections. Figure F18 shows that the grain size in Hole 1274A is bimodal.

Spinel

Chrome spinel is generally well preserved and composes 1%–2% of the harzburgite and dunite. Spinel shapes, from observation of the entire core, can be divided into three categories, equant, interstitial, and vermicular:

1. Equant: the shape is equidimensional with straight and/or curved surfaces.
2. Interstitial: a transitional category between vermicular and equant. The boundaries of these spinels are commonly curved and have thin tips extending from the corners of the grains.
3. Vermicular: the shape is intricate, forming symplectitic intergrowths with orthopyroxene, clinopyroxene, and/or olivine.

In Hole 1274A, all spinel morphologies are present throughout the hole and many core pieces contain spinel with more than one textural type (Fig. F19). However, in dunite (e.g., Cores 209-1274A-2R to 5R) equant spinel is dominant, whereas in harzburgite (e.g., Cores 8R, 20R, 21R, and 25R) vermicular spinel predominates (Figs. F19, F20, F21). In thin section, spinel in harzburgite is deep red to black in plane-polarized light (indicating variable composition).

Dunite

Dunite is highly serpentinized compared to harzburgite and is composed of olivine and spinel (1%–2% modal) with or without pyroxene (<10% modal). Relict olivine generally composes <10% of the dunite, but locally, clusters of olivine kernels are preserved. Spinel is euhedral to subhedral and is brown to deep brown. Orthopyroxene in dunite is anhedral and has curved contacts with olivine, typical of protogranular textures. Small clinopyroxene crystals associated with spinel are present along some orthopyroxene grain edges. This microstructure is identical to that in the harzburgite, except that orthopyroxene in dunite may also contain subhedral, 1-mm-size spinel inclusions. Clinopyroxene associated with orthopyroxene locally forms relatively large patches of two or more grains with spinel, as in the harzburgite. Relics of vermicular intergrowths of spinel and clinopyroxene are between olivine grains or are elongated lenses crosscutting the olivine matrix (Fig. F22A, F22B). Polygonal, poikilitic clinopyroxene enclosing small olivine grains and rimmed by symplectitic spinel-clinopyroxene intergrowth is present at olivine grain junctions. In thin section, Sample 209-1274A-8R-1, 109–111 cm, shows four 1-mm euhedral spinels at olivine junctions and four spinel-clinopyroxene symplectitic intergrowths. In thin section, Sample 209-1274A-15R-1, 23–26 cm, shows 1-mm euhedral spinel and a few pseudomorphs of spinel-clinopyroxene symplectite. These symplectitic relics may have replaced former orthopyroxene. Most symplectites are along orthopyroxene grain boundaries in the harzburgites.

Gabbros

The first occurrence of gabbro in Hole 1274A is in Unit I at a depth of ~55 mbsf in interval 209-1274A-11R-1 (Pieces 9–11, 41–56 cm) (Fig. F23). The gabbro is rather fresh and rich in oxide (~12%, mainly ilmenite) and contains 64% plagioclase, 16% clinopyroxene, 4% olivine, and 2% orthopyroxene as primary magmatic phases. Based on its primary mineralogy, this rock is an oxide gabbro. In thin section (Sample 209-1274A-11R-1 [Piece 10, 46–49 cm]), the rock appears to have recrystallized and the original igneous texture is destroyed. Triple junctions between crystals (120°) (Fig. F24) suggest high-temperature annealing. Substantial recovery has occurred, leading to coarsening of neoblasts. Plagioclase twins are bent, indicating deformation (Fig. F25). Olivine is associated with clinopyroxene, except for a small patch of olivine associated with oxide. Plagioclase contains sulfide inclusions that could be primary igneous phases.

The bulk of the gabbroic rocks in this hole are in Unit II. In hand specimen, they are similar to each other. All are extensively altered, so their original lithology and texture are obscured. The contact between gabbroic rocks and the host peridotite is preserved in many pieces of Cores 209-1274A-21R, 22R, and 23R (Fig. F26). At the contact there appears to be a reaction rim separating gabbroic rocks from harzburgite. Primary mineralogy has been inferred from two thin sections (Samples 209-1274A-21R-1 [Piece 5, 12–17 cm] and 21R-1 [Piece 23, 93–98 cm]) and from interpretation of alteration products and pseudomorphs of the primary minerals. These gabbros likely contained variable amounts of plagioclase (45%–60%), clinopyroxene (29%–45%), orthopyroxene (7%–8%), and oxides (3%). Fresh clinopyroxene (5%) is still preserved in interval 209-1274A-21R-1 (Piece 5, 12–17 cm). The presence of orthopyroxene is inferred based on pseudomorphs with a lattice of oxides. Thus, we classify the gabbroic rocks of Unit II as gabbronorites.

A small amount of gabbroic material is also present in Unit III. It appears similar to gabbroic rocks in Unit II. No contacts were recovered between gabbroic rocks and host peridotite in Unit III.

Site 1274 harzburgites display mineralogical and textural characteristics similar to those of Site 1272. In particular, peridotites from both sites have a protogranular texture. The low abundance of residual clinopyroxene and the dark red chromium spinel (that usually corresponds to high chromium numbers, i.e., high degrees of melting) suggest that the harzburgites from Hole 1274A are very depleted, in accordance with geochemical investigations on dredged peridotite samples from the same region (Bonatti et al., 1992; Sobolev et al., 1992a, 1992b; Cannat and Casey, 1995; Cannat et al., 1992; Dick and Kelemen, 1992). Clinopyroxene, locally with magmatic twinning, occurs as an interstitial phase, with or without symplectitic intergrowths of spinel, olivine, and orthopyroxene in both harzburgite and dunite from Sites 1272 and 1274. This texture was described in abyssal peridotite samples by Seyler et al. (2001) and has been interpreted to result from the crystallization of interstitial melt in the residual solid at the end of partial melting. The occurrence of intergranular clinopyroxene along subgrain boundaries in orthopyroxene indicates that the crystallization of the clinopyroxene occurred during or just after orthopyroxene deformation. However, at Site 1274 the interstitial clinopyroxene is more abundant than at Site 1272 and forms better-developed crystals along orthopyroxene grain boundaries and within cracks between orthopyroxene subgrains. Based on the logic of Seyler et al. (2001), we suggest that the amount of crystallized melt was large (>2 vol%) (see discussion in Seyler et al., 2001). Perhaps this is related to the relatively cold thermal regime in the shallow mantle adjacent to the 15°20´ Fracture Zone. An alternative is that a genetically unrelated melt could have impregnated the peridotite during upwelling. A third hypothesis calls for partial melt accumulation at the top of the melting column. Finally, it could be that all or much of the clinopyroxene, together with some associated spinel, was exsolved from high-temperature orthopyroxene during cooling of the harzburgites.

At Site 1274, dunite is not very abundant. However, it is important to note that interstitial clinopyroxene occurs in dunite as well as in harzburgite and the contacts between harzburgite and dunite are gradational. Possibly, the dunite is a simple residue of melting, formed by more extensive melt extraction compared to the harzburgite. Orthopyroxene in dunite, with or without spinel-clinopyroxene symplectites, could also be relics of dissolved or reacted orthopyroxene, suggesting the formation of dunite by dissolution of orthopyroxene from harzburgite.