Drilling in Hole 1272A provided 27 cores from a penetration depth of 131.5 m (recovery = 31.5%). The upper part is composed almost entirely of fine- to medium-grained mafic rocks with a subordinate amount of peridotite. The section below 56 mbsf is nearly the inverse, almost exclusively peridotite with only a small amount of mafic material. Thus, we divide the core into two lithologic units, Unit I diabase and Unit II harzburgite.
Unit I includes the first 11 cores of the hole and contains 75% diabase and 25% peridotite (Fig. F3). It is unclear if the rocks that compose Unit I (1) are part of a talus pile, (2) represent a coherent package of rocks, or (3) are a talus pile that has been intruded by dikes. Fine-grained cobbles with weathering rinds that appear to be concentric in Section 209-1272A-7R-1 suggest these are fragments that were weathered on or near the seafloor. Medium-grained gabbroic rocks make up much of Sections 209-1272A-3R-1 and 4R-1 and appear to be sampled from a coherent section. The paleomagnetic and structural data are ambiguous as to the nature of Unit I. For simplicity, we assume that this section is coherent and so name all the fine-grained mafic rocks accordingly.
The top of Unit I contains peridotite, almost entirely dunite, that has been subjected to extensive alteration, completely masking its original texture (both serpentinization and near-surface oxidation). A small amount of harzburgite is present in the lower portion of Unit I in Sections 209-1272A-8R-1, 9R-1, and 10R-1 (Fig. F3). The mafic rocks in Unit I have significant variations in their grain size, from completely aphanitic to fine grained and even medium grained. Unit I is composed mainly of diabase but includes some microgabbros and gabbros. These are distinguished based on grain size and texture. Diabases have grain sizes that range from aphanitic to fine grained, microgabbros are fine grained, and gabbros are medium grained or coarser. The textural distinction between diabases and microgabbros is based on the modal homogeneity of the sample. Diabases are defined as having a felty texture with randomly distributed plagioclase laths throughout and are modally homogeneous. Microgabbros can have felty textured plagioclase or be more massive, but those with a felty texture have distinctive modal segregations of plagioclase laths into a framework in the sample (Fig. F4). Using these criteria, the bulk of the gabbroic rocks in Unit I are classified as diabase. They are very fine grained, and in thin section some can be seen to contain devitrified glass. The only microgabbro in Unit I is found in Section 209-1272A-1R-1 (Pieces 6–8). There are two intervals of gabbroic rocks in Unit I in intervals 209-1272A-3R-1, 0–110 cm, and 4R-1, 68–140 cm.
Unit II consists of a thick (75.0 m drilled; 27.7 m recovered) sequence of serpentinized harzburgite. A limited number of small pieces of fine-grained mafic rocks (3.5%, representing 0.97 m recovered) were recovered at the top of several drilled sections and are interpreted as talus introduced during drilling. No peridotite/gabbro contacts are present. Five dunite horizons (3.4%) are also present (Fig. F3). The dunite is defined by areas that are orthopyroxene-free (or contain <5% orthopyroxene) within harzburgite. These areas are bands or ovoid patches in cross section (Figs. F5, F6) and might correspond to lens-shaped (interfingered?) material, a few centimeters wide. The harzburgite is texturally quite homogeneous at the scale of the hole, having coarse-grained protogranular textures. High-temperature banding defined by orthopyroxene-rich layers is present in some places (Fig. F7A). Plastic deformation and recrystallization below the solidus is absent except for some slight deformation in a few places, and the textures are truly protogranular (see "Crystal-Plastic Fabrics and Deformation Intensity" in "Harzburgite Section (56–131 mbsf)" in "Structural Geology"). The two dominant mineral phases are olivine and orthopyroxene, forming 96%–98% of the rocks. Spinel is almost exclusively associated with orthopyroxene, forming aggregates as large as 5 mm. Its abundance varies with that of orthopyroxene and is in the 1%–2% range. Clinopyroxene can be observed where the rocks are less serpentinized. It forms tiny grains associated with orthopyroxene crystals, never exceeding 2% in modal proportion.
In >83% of the harzburgites, modal orthopyroxene contents vary between 15% (28% of samples) and 25% (20% of the samples), with 35% of the samples having ~20% orthopyroxene. The ratio of olivine to orthopyroxene generally varies at the decimeter scale, but finer-scale variations also occur in some intervals (Fig. F7B). Locally, the amount of orthopyroxene increases to as much as 30%–40% (Fig. F7C) or is as low as 10% (Fig. F8). Orthopyroxene-rich harzburgite forms a mere 2.5% of the Unit II, whereas harzburgite with <15% orthopyroxene constitutes ~15% of Unit II. Although the harzburgite is generally uniform in composition, several discrete sequences can be discerned in Unit II.
Between 60.7 and 63.4 mbsf (Sections 209-1272A-13R-1 and 13R-2), a sequence of orthopyroxene-rich harzburgite (30%–35% orthopyroxene) sandwiches a meter of harzburgite (10%–20% orthopyroxene). Most of Cores 209-1272A-14R and 15R (starting at the top of Section 14R-1 [65.7 mbsf] down to the bottom of Section 15R-1 [70.6 mbsf]) consist of a meter-thick sequence of harzburgite with 10%–15% orthopyroxene interlayered with harzburgite containing 25% orthopyroxene. The orthopyroxene-poor sequence contains the uppermost dunite horizon.
Below 75.1 mbsf (in Section 209-1272A-16R-1), orthopyroxene content ranges from 10% to 30% on a centimeter to meter scale. Nevertheless, the averaged contents of orthopyroxene mode allow us to determine a sequence starting in Core 209-1272A-16R (75.1 mbsf) and finishing at the bottom of Section 24R-1 (114.9 mbsf). In this sequence, corresponding to ~13 m of recovered peridotite, nearly 80% of harzburgites contain 20%–25% orthopyroxene. Intervals of harzburgites containing <15% orthopyroxene with thicknesses varying from 5 to 15 cm form four horizons (Fig. F3). Two of these horizons are dunite. Core 209-1272A-23R contains the longest piece of dunite. In this interval, dunite alternates with harzburgite (Fig. F5). There are 1- to 3-cm-wide orthopyroxene-rich bands of harzburgite within the dunite. The harzburgite below the dunite contains as much as 40% orthopyroxene (Fig. F5). Orthopyroxene-rich harzburgite (30%) also constitutes the bottom of the Section 209-1272A-23R-2, but is only 13 cm thick, with a recovery gap beneath it.
A last sequence (Section 209-1272A-24R-2 [114.9–119.9 mbsf to the bottom of the hole]) is composed of 8.54 m of recovered peridotites in which harzburgite containing 15%–20% orthopyroxene forms ~85% of the rocks. Harzburgite containing >20% orthopyroxene is not present, whereas peridotites with <15%–10% orthopyroxene constitute 14% of the rocks. There are two dunite horizons in this sequence. One is 3 cm thick in Section 209-1272A-24R-2, and the other is 7 cm thick in Section 27R-1 (Fig. F3).
Based on their grain size, texture, and composition, all of the gabbroic rocks in Unit I are interpreted as having liquid compositions—none have a significant cumulate component. The diabases and microgabbros are relatively uniform in appearance in hand sample and thin section. Clear evidence of quench crystallization in some diabases is preserved in the form of skeletal olivine and plagioclase and spherulites composed of devitrified glass and needles of plagioclase and clinopyroxene (Fig. F9). The most distinctive rocks in Unit I are the quartz-olivine gabbros found in intervals 209-1272A-3R-1, 0–110 cm, and 4R-1, 68–137 cm. They are visually distinctive in hand sample, having a strong modal segregation with patches of mafic minerals surrounded by a network of felsic minerals (Fig. F10). They also contain ~15% pore space into which plagioclase, quartz, amphibole, and other igneous minerals have grown. The mineralogy of these rocks is complicated and includes (in decreasing abundance): plagioclase, olivine, clinopyroxene, quartz, oxides, amphibole, biotite, and apatite. The mafic-rich areas contain poikilitic crystals of olivine and subophitic clinopyroxene with a minor amount of orthopyroxene. The olivine crystals are often intergrown with single plagioclase crystals and cut by others (Fig. F11). The felsic areas are composed predominantly of plagioclase with a lesser amount of quartz and accessory oxides, amphibole, orthopyroxene, biotite, and apatite. Most of the quartz is associated with the pore space, in places as subhedral or even euhedral crystals (Fig. F12). The quartz does not display undulatory extinction, a feature common to igneous quartz induced by strain generated by volume changes on cooling. Small acicular apatite crystals are included in both plagioclase and quartz, with the highest abundance of these needles in felsic minerals near pores (Fig. F13). The shapes of the pores are defined by the negative crystal faces of plagioclase, quartz, oxides, and less commonly by pyroxene or amphibole.
Most of the mafic rocks in Unit II are aphanitic, and some even have glass rinds. They occur near the top of the cored intervals, and we interpret them as having fallen into the hole during recovery processes. However, in Section 209-1272A-19R-1 there is an interval of coarse-grained oxide gabbronorite (Fig. F14). The orthopyroxene in this rock has coarse exsolution lamellae of clinopyroxene, suggesting that it cooled slowly. The rock has been deformed at submagmatic temperatures, as indicated by the development of pyroxene and plagioclase neoblasts around the subhedral to euhedral igneous crystals. The oxide is interstitial and there is no evidence that the deformation is localized into the more oxide-rich portions of the rock.
The harzburgite in Hole 1272A has a coarse-grained protogranular texture defined by varying proportions of olivine, orthopyroxene, and minor clinopyroxene and spinel. In hand sample, orthopyroxene appears as anhedral grains that partially enclose large (1–2.5 cm wide) olivine-only domains (Fig. F15A). Some orthopyroxene grains are thinner and somewhat elongated along olivine grain boundaries and larger and more equant at the junctions of several olivine grains (Fig. F15). With increasing orthopyroxene/olivine ratios, the orthopyroxene forms clusters of a few crystals that occupy areas as large as the olivine-only domains (Fig. F15B).
In thin section, several features characterize the harzburgite textures:
Dunite is present in both Units I and II. The dunite from Unit I is completely altered except for spinel, which has the same reddish color as spinel in dunite from Site 1271. Spinel occurs as small euhedral to subhedral grains disseminated throughout the serpentinized matrix. A thin section from Unit II that includes a dunite/harzburgite contact in Section 209-1272A-23R-1 (Fig. F6) reveals several striking features. Spinel is absent from the dunite but is abundant in the harzburgite, associated with the orthopyroxene. This spinel forms large (#1 mm) euhedral to blocky anhedral grains within and near orthopyroxene (Fig. F19A, F19B). Two centimeters from the contact with the dunite, some of the spinel is still blocky but some of it is vermicular along orthopyroxene grain boundaries (Fig. F19C). The different spinel shapes suggest that the vermicular, symplectite-like spinel typical of Site 1272 harzburgite texturally reequilibrated into euhedral grains at the dunite contact (Fig. F19D). Similar gradual changes in the spinel texture are described in ophiolites from peridotite adjacent to dunites (Matsumoto and Arai, 2001).
Whether Unit I is a coherent outcrop, a mass of talus, or something in between, it is clear that the break between Unit I and Unit II is very sharp, and the presence of a crushed interval between them suggests they are juxtaposed by a fault (see "Structural Geology"). All the mafic rocks in Unit I crystallized rapidly at relatively low pressures. The fine-grained rocks have quench textures, and some contain intersertal glass. Even the medium-grained quartz-olivine gabbro has the characteristics of a quenched rock. The pore spaces are interpreted to be miarolitic cavities formed by the trapping of exsolved volatile elements from the rapidly crystallizing liquid. The presence of olivine and quartz in the same rock but segregated into discrete mafic and felsic portions indicates that the crystallizing liquid did not remain in equilibrium with the solid assemblage.
The harzburgite of Unit II has a protogranular texture and little clinopyroxene, suggesting it is residual mantle from which extensive melt extraction has taken place. However, the amount of dunite in Hole 1272A is less than that in other peridotite-bearing holes sampled during Leg 209. These observations appear to be at odds since dunite is considered to be a reaction product between melt and harzburgite (e.g., Boudier and Nicolas, 1972; Dick, 1977; Quick, 1981; Kelemen, 1990), so one might expect more dunite to be associated with more extensive melt extraction. One explanation for this apparent discrepancy is that the centimeter- to meter-scale variation of modal orthopyroxene in the harzburgite may have been produced by dissolution of orthopyroxene by interaction with basaltic liquid migrating by porous flow. However, the orthopyroxene grain size does not decrease as the orthopyroxene proportion decreases as is expected if orthopyroxene-poor harzburgite and dunite were formed by varying degrees of dissolution. Alternatively, it could be that the liquid was efficiently focused into a few narrow channels now represented by dunites, or the lack of dunite in this section might indicate that this portion of the mantle was strongly depleted prior to the current melting event.
The common occurrence of symplectic intergrowths between spinel and orthopyroxene in harzburgites may be indicative of some sort of reaction in the shallow upper mantle. Because the orthopyroxene-spinel-(olivine?) intergrowths occur preferentially along olivine/orthopyroxene boundaries, it can also be interpreted as the product of rapid crystallization of melt at the end of the partial melting. This would be analogous to the way clinopyroxene–spinel–olivine intergrowths form in residual clinopyroxene-bearing peridotites (Seyler et al., 2001, 2003).