IGNEOUS AND MANTLE PETROLOGY

Four holes were drilled at Site 1270; three of these recovered predominantly mantle peridotite and the last recovered almost exclusively oxide-rich gabbronorites. Here we discuss each of the holes separately and then combine these observations to compare the variations within Site 1270 and then between Sites 1270 and 1268.

Hole 1270A

Lithology and Stratigraphy

The predominant rock type in this hole is serpentinized harzburgite (88 vol%) with minor serpentinized dunite (7 vol%), gabbro (3 vol%), and breccia (1 vol%) (Fig. F3). There are two major gaps in the downhole stratigraphy. The first gap in recovery (9.5 m) is located at the transition between the first two sections of core. The second gap (9.5 m) corresponds to two intervals where no rock was recovered between the second and third (4.75 m) and third and fourth cores (4.16 m). All of Core 209-1270A-3R (0.34 m) is serpentinite fault gouge and small pebbles of peridotite that are too altered to identify the protolith more completely. The percentage recovery is so limited that statistical analysis of the observations and the construction of a stratigraphy may not be meaningful. Nevertheless, we observe consistency in the modal variation of the harzburgite that forms the three recovered portions of Hole 1270A. In these rocks, the modal proportions of orthopyroxene decrease from ~30% at the top of the cored interval to ~10% at the bottom. In addition, the (apparent) grain size in the harzburgite protoliths of the three units decreases downhole and they have undergone different extents of high-temperature deformation. Dunite is most abundant in the upper part of Section 209-1270A-1R-1 and in the gabbros in the lower half of Section 2R-1. Based on these observations, we define three lithologic units (Fig. F3).

Unit I

Interval: Sections 209-1270A-1R-1 through 2R-1
Depth: 0–12.12 mbsf
Lithology: orthopyroxene-rich harzburgite/dunite

Unit I is characterized by orthopyroxene-rich harzburgite with dunite bands, orthopyroxene-rich bands, and a single thin crosscutting orthopyroxenite. Orthopyroxene constitutes 23%–30% of this coarse-grained harzburgite, which contains olivine and orthopyroxene as large as 2 cm, although 1.5 cm is the most common size (Fig. F4). Relict orthopyroxene is kinked and recrystallized into relatively coarse subgrains, defining a protogranular porphyroclastic texture. The top of Section 209-1270A-1R-1 is a mixture of dunite and harzburgite pebbles, and we identified four dunites that are 2–5 cm wide in the first meter of core. This section also contains several (five were identified) thin (<1 cm thick), boudinaged pyroxene-rich bands characterized by coarse recrystallized orthopyroxene crystals intergrown with large spinel. At Section 209-1270A-1R-1 (Piece 2, 5 cm), the orthopyroxenite is in contact with dunite. Sections 209-1270A-1R-2 and 2R-1 down to 12 mbsf are the continuation of the same harzburgite, but no dunite or orthopyroxenite is present. At the top of Section 209-1270A-1R-2, the harzburgite is strongly deformed but the deformation intensity decreases with depth. The harzburgite at the top of Section 209-1270A-2R-1 is little deformed but contains two intervals of harzburgitic pebbles and sand.

Unit II

Interval: Sections 209-1270A-2R-1 through 4R-1
Depth: 12.2–22.03 mbsf
Lithology: harzburgite with gabbro

Unit II is harzburgite with gabbroic intervals. The harzburgite contains 15–20 vol% orthopyroxene. With respect to the Unit I harzburgite, the Unit II harzburgite is poorer in orthopyroxene and has smaller grains; the original size is estimated to be ~1 cm. The top of the unit (Section 209-1270A-2R-1 [Piece 17]) is strongly deformed with significant grain size reduction, but the texture changes to porphyroclastic in Pieces 18–21 then to protogranular/porphyroclastic in Pieces 21–27 to the bottom of the unit. The harzburgite is cut by two gabbro layers, 2–3 cm thick, in intervals 209-1270A-2R-1, 81–85 cm, and 113–117 cm, and by breccia in interval 2R-1, 101–104 cm. Brown amphibole-rich veins, a few millimeters wide, are common in the breccia and the harzburgite underneath it. In Section 209-1270A-4R-1, the Unit II harzburgite is cut by a 6-cm-thick gabbro. However, the bottom of Section 209-1270A-2R-1 and the top of Section 4R-1 are separated by 28 cm of serpentinite and altered ultramafic fault gouge (the protolith is not identifiable). This fault gouge constitutes all of Section 209-1270A-3R-1. The base of Unit II is more strongly deformed and has porphyroclastic textures.

Unit III

Interval: Section 209-1270A-4R-1
Depth: 22.03–26.9 mbsf
Lithology: harzburgite/dunite

Unit III is characterized by a peridotite that has ~10 vol% orthopyroxene and so is classified at the harzburgite/dunite boundary. This harzburgite, forming the lowermost 37 cm of the core in Hole 1270A, has a porphyroclastic texture. It is crosscut by millimeter-wide bands of amphibole-bearing ultramafic rock in Section 209-1270A-4R-1 (Pieces 14 and 15) (Fig. F5).

Lithologic Characterization

The harzburgites are highly altered (>99%), so reconstruction of the high-temperature mineral assemblages is based on the nature of the alteration and pseudomorphs. In many samples, the grain boundaries between the primary olivine and pyroxene grains are preserved and provide information on the microstructure and texture, allowing us to roughly estimate their modal proportions. Partially fresh clinopyroxene is present in a thin section (Sample 209-1270A-4R-1, 54–56 cm) from Unit III, suggesting that this sample had ~2% modal clinopyroxene. In the other harzburgite thin sections, altered pyroxenes with similar character could be interpreted as former clinopyroxene. There may have been relatively large clinopyroxene grains (up to 2 mm) in the orthopyroxene-rich harzburgites of Unit I. The amount of former clinopyroxene, estimated based on the geometry of the grain boundaries and the nature of the serpentinization (pseudomorphs of thin, regular exsolution lamellae), is as high as 3–4 vol% in Sample 209-1270A-1R-1, 139–141 cm. This amount of clinopyroxene is not atypical for harzburgites containing ~70 vol% olivine like those in Unit I. However, the other three thin sections from Unit I do not include anything that we interpret as having been clinopyroxene. The bulk of the Unit I harzburgite thus appears to be an unusual harzburgite characterized by high orthopyroxene contents (23–30 vol%) but low clinopyroxene and spinel (2 vol%) contents. Unit II harzburgite is poorer in orthopyroxene, with 15%–20% modal orthopyroxene, 1%–2% clinopyroxene, and 1%–3% spinel, with the remainder olivine. Unit III peridotite is even poorer in modal orthopyroxene, with 10% orthopyroxene, 2% clinopyroxene, and 1% spinel. In all three units, the texture before plastic deformation and recrystallization was protogranular. The original sizes of the primary olivine and orthopyroxene grains cannot be determined with confidence, but, based on the olivine/orthopyroxene grain boundaries, they must have been on the order of 1.5–2 cm in Unit I, 1 cm in Unit II, and 1.2 cm in Unit III.

Orthopyroxene pseudomorphs are anhedral with embayments filled with olivine (Figs. F6, F7). They tend to be equant (Fig. F6), and although they have some irregular portions interstitial to olivine grains, these irregularities are far less common than in harzburgite from Hole 1268A. Likewise, none of the amoeboidal orthopyroxene, so common in Hole 1268A, is found in the Hole 1270A harzburgites. Orthopyroxene is commonly recrystallized into relatively large subgrains of 1–5 mm size (Fig. F7), but the primary olivine is recrystallized into smaller neoblasts. Where observed, clinopyroxene is present as small (<0.5 mm) crystals at boundaries and junctions of orthopyroxene subgrains and probably results from orthopyroxene exsolution during deformation and recrystallization. Holly leaf–shaped spinel grains, as large as 1–3 mm, are commonly found as intergrowths with orthopyroxene. Smaller grains disseminated in the serpentinized olivine matrix are less common.

The dunites are totally altered, with 2%–5% relict orthopyroxene and 1% spinel. The original size of the olivine grains is difficult to determine but was probably on the order of 1 cm, based on the size of the orthopyroxenes.

Thin orthopyroxenites and orthopyroxene-rich bands in harzburgites are characterized by coarse primary orthopyroxene recrystallized in relatively large subgrains and intergrown with skeletal, large spinels (Fig. F8). Small interstitial grains of spinel and clinopyroxene (Fig. F8B) are interpreted as exsolution products generated by recrystallization.

A 3-mm-wide ultramafic dike or vein that cuts Unit III harzburgite (Section 209-1270A-4R-1 [Pieces 14 and 15]) is made up of 20% olivine, 20% orthopyroxene, 53% clinopyroxene, 5% magmatic interstitial brown amphibole (pargasite) (Fig. F9), and 2% oxides. This dike is intruded into a mylonitic band, and we cannot exclude the possibility that the olivine and orthopyroxene grains are derived from the harzburgite wallrock. Indeed, these minerals occur as 0.5-mm-size neoblasts, as shown in Figure F9, whereas clinopyroxene forms larger (1 mm) subhedral crystals, oriented parallel to the margins of the intrusion.

A single large piece of pegmatitic gabbro (with 20% clinopyroxene and 80% plagioclase) was recovered from Unit II (Section 209-1270A-2R-1 [Piece 20]).

Hole 1270B

Hole 1270B was drilled to a depth of 45.9 mbsf and its 10 cores are composed of 99% oxide gabbronorite, gabbronorite, and microgabbro, with the remainder being altered harzburgite. Because of problems with drilling, three of these cores are not well located stratigraphically. These "M" cores were placed into a stratigraphic context using a combination of their reported starting depth from the driller's estimates and observed modal, textural, and lithologic variations. Most of the rocks have undergone high-temperature deformation and now are appropriately described as gabbroic gneisses. However, the original mineralogy is preserved, so in this section we characterize these rocks in terms of their igneous protolith and focus on evidence of their magmatic history.

Grain size, variations in the fraction of oxide minerals (magnetite and ilmenite), and lithologic variations were used to define seven units in Hole 1270B (Fig. F10). Units I, III, and VII compose 85.2% of the drilled interval and are predominately coarse-grained oxide gabbronorite with lesser amounts of gabbronorite. (This gabbronorite is not free of oxides but lacks the 5% modal required for the mineralogical modifier.) Units II and VI are microgabbros (4.3% of the recovered core), and Unit V is a gabbronorite (1.1% of the recovered core). Unit IV is distinct in that it contains harzburgite interpreted to be the upper mantle host rock of the gabbroic rocks. Although Unit IV makes up 9.46% of the drilled interval, it corresponds to only 2.5% of the recovered core.

Oxide Gabbronorite Units

Unit I
Interval: Sections 209-1270B-1R-1 through 4M-2
Depth: 0–16.16 mbsf

Lithology: oxide gabbronorite

Unit III
Interval: Sections 209-1270B-4M-2 through 6R-1
Depth: 16.74–27.51 mbsf
Lithology: oxide gabbronorite
Unit V
Interval: Section 209-1270B-7R-1
Depth: 31.74–32.16 mbsf
Lithology: oxide gabbronorite
Unit VII
Interval: Sections 209-1270B-7R-1 through 10M-1
Depth: 33.63–45.9 mbsf
Lithology: oxide gabbronorite

There are three oxide gabbronorite/gabbronorite units. Unit V is similar to the gabbronorite parts of the other three units and was only designated as separate unit because of intervening lithologies. In nearly all of the units the rocks are relatively fresh with <10% alteration. This makes estimation of modal proportion of plagioclase, oxides, and pyroxene straightforward. However, both clinopyroxene and orthopyroxene are present and are very difficult to distinguish reliably. Calculation of modal proportions based on the bulk rock composition is dependent on the mineral compositions used but generally suggests that 5%–10% orthopyroxene is present. In most samples the plagioclase abundance is 40%–55% and does not vary stratigraphically in any discernable pattern.

We estimated the amount of magnetite in the samples by measuring the bulk magnetic susceptibility via a susceptibility loop on individual pieces. The susceptibility loop averages some distance along the length of the core such that pieces that are at least 7 cm in length account for 95% of the response curve. In the measurement procedure we considered a 7-cm piece to give a full response and estimated the size of any smaller pieces as a fraction of the volume of a 7-cm half round. In practice, no pieces smaller than an estimated 50% of this volume were measured and the raw data were adjusted upward to compensate for the smaller volumes. The raw data were corrected for the expected cross-sectional area for which the loop was designed (6.6 cm whole round). The corrected susceptibilities were then converted into volume percent estimates of the magnetite content by dividing the corrected susceptibility by 10,000 and multiplying by 3.3, corresponding to a susceptibility of magnetite of 3 million µSI units (Fig. F11). The amount of magnetite estimated using this susceptibility method generally agrees with visual estimates of oxide proportions in thin sections, except in Unit I where the visual estimate is much larger than the susceptibility-based estimate of magnetite proportion. These samples contain discrete ilmenite crystals and may have some TiO2 in the magnetite, lowering its bulk susceptibility and thus causing our susceptibility-based estimates to be systematically low.

There is a variable amount of deformation in these units that decreases with depth. All the rocks in Unit I are moderately to strongly deformed. Most rocks in Unit III are moderately deformed; some are strongly deformed. Rocks in Unit VII are moderately deformed at the top of the unit, but some relatively undeformed rocks are found near the base of the core (Figs. F12, F71). The least deformed rocks are medium grained and have lath-shaped plagioclase grains that preserve both optical zonation and polysynthetic twins. These rocks have subhedral to euhedral clinopyroxene and orthopyroxene and interstitial oxides. Preservation of some igneous features in the more deformed rocks indicates that the original grain size in some samples was >20 mm for both clinopyroxene and plagioclase. The largest orthopyroxene grains preserved suggest that orthopyroxene had a maximum size of 10–15 mm. The crystals have been variably rounded, partially deformed, and recrystallized into small neoblasts in these coarse-grained samples. Locally, clinopyroxene grains that include euhedral oxide grains are found (Fig. F13). One example of an unstrained euhedral plagioclase crystal was also noted (Fig. F14). Clinopyroxene is mostly equant, but a few examples of large ophitic clinopyroxene crystals are preserved (Fig. F15), most notably in Section 209-1270B-10M-1 (Piece 23), in which a 70 mm x 40 mm oikocryst of clinopyroxene is found.

It has been suggested that some amount of oxide was added to the igneous protolith during the deformation process, as was proposed for some oxide gabbros recovered from Leg 118 Hole 735B. Based on the following observations, we find no compelling reason to consider that oxide addition during deformation was an important process in the genesis of the oxide-rich gabbronorites in Hole 1270B:

  1. The least deformed rocks contain the same proportion of oxide as the deformed rocks.
  2. Samples with igneous textures have interstitial oxide that crystallized with the surrounding silicate minerals.
  3. In the most deformed rocks, euhedral oxide grains preserved in the clinopyroxene indicate that oxides were present prior to deformation.
  4. We have not found accessory phases such as zircon, apatite, or titanite associated with strongly deformed, oxide-rich parts of these rocks, as might be expected if an evolved oxide-rich liquid were added.

These observations do not eliminate the possibility that the rock compositions were modified, but this process is not required to explain the observed oxide textures and proportions. However, for an alternative viewpoint, see "Structural Geology" in the "Site 1274" chapter and Figure F24 and accompanying text in the "Leg 209 Summary" chapter.

Microgabbro Units

Unit II
Interval: Sections 209-1270B-4M-2 through 4M-3
Depth: 16.16–16.74 mbsf
Lithology: microgabbro
Unit VI
Interval: Sections 209-1270B-7R-1 through 7R-2
Depth: 32.16–33.63 mbsf
Lithology: microgabbro

Units II and VI are microgabbros crystallized from a more primitive liquid that intruded the oxide gabbronorite after it cooled. This interpretation is based on the fine grain size of the rocks, their relatively low abundance of oxides, and their distinctive chemical composition compared to the gabbronorites (see "Geochemistry;" Table T4). Both of these units have been deformed, so the observed grain size may reflect some amount of grain size reduction during recrystallization. The grain size in Unit II decreases as the lower contact is approached. The portion of Unit II near the contact is severely altered, and it is not clear if the reduction in grain size reflects more rapid cooling at the margin of the microgabbro or deformation-related recrystallization associated with the lithologic boundary.

Unit IV

Interval: Sections 209-1270B-6R-1 through 7R-1
Depth: 27.51–31.74 mbsf
Lithology: harzburgite

Unit IV contains the only ultramafic rocks in Hole 1270B. They are extremely altered but are interpreted to have originally been harzburgites. Harzburgite was recovered in three cobble-sized pieces with no lithologic contacts. The harzburgite cobbles are interspersed with coarse-grained oxide gabbronorite in the uppermost part of Section 209-1270B-6R-1. This mixture may simply be a consequence of oxide gabbronorite falling into the hole during core retrieval.

Hole 1270C

Hole 1270C consists of three cores with total drilled length of 19.8 m (recovery = 10.65%). The predominant rock type is harzburgite (79%) and dunite (21%), with minor amounts of gabbro and orthopyroxenite (Fig. F16). The core is lithologically uniform, so we have not subdivided it into units.

The harzburgite has significant variations in both texture and modal proportions. The texture of the harzburgite varies from protogranular to porphyroclastic to mylonitic (see "Structural Geology;" Fig. F17). The proportion of orthopyroxene porphyroclasts (now as pseudomorphs) ranges as high as 30% and can be heterogeneously distributed, resulting in modal variations in a single piece (e.g., Fig. F18). In the orthopyroxene-rich harzburgites, a few grains of olivine, orthopyroxene, and the spinel are enclosed by orthopyroxene grains that have escaped alteration (Fig. F19). No contact between harzburgite and dunite is preserved. Alteration makes the textures of dunites difficult to recognize, but they typically contain <3% orthopyroxene and ~1% spinel.

Anastomosing gabbroic intrusions (5–10 mm thick) cut both harzburgite and dunite in eight discrete intervals in the core. These gabbros are highly deformed with asymmetric structure and irregular-shaped boundaries, probably related to localized shear deformation (Fig. F20). Gabbro intrusions consist of clinopyroxene porphyroclasts surrounded by fine-grained clinopyroxene and plagioclase with mosaic texture. Some large clinopyroxenes poikilitically enclose plagioclase, and secondary brown amphibole partially replaces clinopyroxene. The gabbros contain trace amounts of subhedral zircon. The host rock of the gabbro is brecciated and deformed at the contact, resulting in elongated lenses of fresh olivine and orthopyroxene with porphyroclastic to mylonitic texture (Fig. F21). Orthopyroxene porphyroclasts form aggregates next to the olivine matrix.

Hole 1270D

Drilling penetrated to a depth of 57.3 mbsf in Hole 1270D with a total recovery of 7.68 m (recovery = 13.4%). The core consists mainly of deformed harzburgite (98.7%) with a minor amount of dunite (1.3%) and so is treated as a single unit (Fig. F22). These peridotites are cut by pervasive small intrusions of gabbroic material prior to deformation (Fig. F23). The mixed ultramafic and gabbroic rocks are very similar to those recovered in Hole 1270C. Centimeter-scale dunites are distributed throughout the core. There is ~20%–25% orthopyroxene in the deformed harzburgite and <5% in the dunite. The strong deformation, recrystallization, and alteration of the rocks diminishes the reliability of these modal estimates. The percentage of gabbroic material cutting the harzburgites and dunites was visually estimated and compared with the natural gamma radiation (NGR) (Fig. F22). There are two peaks in the NGR in Sections 209-1270D-3R-1 and 4R-1, in which zircon and apatite were found in thin section.

Deformation in Hole 1270D is concentrated in the gabbroic intrusions, and large domains of porphyroclastic harzburgite are preserved. Microscopic observations of these domains show that the primary grain size of the host rock was 4–8 mm and texture is granular to porphyroclastic. The harzburgite clasts contain as much as 25% orthopyroxene and up to 90% fresh recrystallized olivine. Inclusions of fresh or serpentinized olivine are found in orthopyroxene. Rare interstitial clinopyroxene runs in discontinuous parallel bands cutting through a serpentinized olivine matrix (Fig. F24).

Strongly altered dunites usually occur in 1- to 3-cm-sized domains within the harzburgite but also as discrete portions of the core (pebbles) with a few bands enriched in orthopyroxene. The only primary phase that survived alteration is spinel, up to 1%–2% in abundance, whereas orthopyroxene pseudomorphs make up 2% of the mode. It is not possible to determine the original size of the olivine crystals because they are totally altered.

Anastomosing gabbroic intrusions, originally composed of plagioclase, clinopyroxene, and amphibole, are pervasive in the harzburgite. The plagioclase is now completely altered, but there is some relict clinopyroxene. Euhedral brown amphibole crystals (up to 2 mm in size) with well-developed 60°–120° cleavages are preserved in the sheared gabbro (Fig. F25). We interpret this amphibole to be a late magmatic phase. Zircon and apatite occur as discrete accessory minerals in these sheared gabbros (Fig. F26). They are dispersed in the gabbroic matrix and are locally included in the amphibole rims (Fig. F25).

Discussion

Three of the four holes drilled at Site 1270 are fundamentally different. The obvious outlier is Hole 1270B, which recovered almost exclusively gabbroic rocks, whereas the others are predominately peridotite. Holes 1270C and 1270D are very similar but are different from Hole 1270A. There are two main distinctions between these sets of holes: (1) Hole 1270A lacks the pervasive small gabbroic bodies that characterize much of the rock from Holes 1270C and 1270D and (2) Hole 1270A shows a systematic decrease in modal orthopyroxene downhole, whereas no such systematic variations are discerned in Holes 1270C and 1270D. Both points should be viewed with caution because of the very limited recovery from Hole 1270A. The harzburgite of Unit I from Hole 1270A is also remarkably coarse grained and rich in orthopyroxene while poor in clinopyroxene. None of the peridotite from Holes 1270C and 1270D is as coarse grained, but as these two cores have been more extensively recrystallized during deformation, the original texture and grain size have been obscured.

The peridotite from Site 1270 contrasts with the peridotite from Site 1268 in showing no continuous variation in modal orthopyroxene content from harzburgites to dunites. The habit of the orthopyroxenes also differs. Orthopyroxene from Hole 1268A is distinctly "ragged" or amoeboidal in most samples, whereas this texture is scarce or absent at Site 1270. Taken together these observations might indicate that no reaction between melt/fluid and residual harzburgite occurred or that such reaction was not a dominant process, as is proposed for Hole 1268A.

Holes 1270C and 1270D contain a network of gabbroic intrusions hosting local concentrations of accessory minerals that are unusual in abyssal peridotites. These include zircon, apatite, and epidote. Could these evolved gabbroic intrusions be related to the oxide-rich gabbronorites from Hole 1270B? The oxide gabbronorites clearly crystallized from an evolved liquid, but their low incompatible element concentrations and lack of accessory minerals such as apatite and zircon indicate that they are cumulates without a significant crystallized liquid component. It is therefore likely that an even more evolved liquid was extracted during their crystallization. This type of liquid could have given rise to the gabbroic material seen in Holes 1270C and 1270D. However, unlike the gabbroic rocks at Site 1268 that we interpreted to have crystallized from relatively H2O-rich liquids, there is no evidence for high H2O contents in the oxide gabbronorites from Hole 1270B.

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