Large exposures of residual mantle harzburgite at active mid-ocean ridges are generally interpreted to be the uppermost portion of the lithospheric mantle, whereas gabbroic rocks emplaced into the harzburgite likely crystallized from magmas that intruded this mantle section. Each set of rocks potentially records a complicated history that includes high-temperature processes and lower-temperature alteration and/or deformation. (Here we restrict our attention to high-temperature processes. Although most of the ultramafic rocks have been converted to serpentine and/or talc, in this section we refer to them simply by their protolith name.)
The rocks recovered from Hole 1268A fall into two distinct categories, ultramafic rocks and gabbroic rocks. Most of the ultramafic rocks are harzburgites with large modal variations of orthopyroxene that may reflect variable amounts of partial melting, variable source compositions, and/or variable amounts of melt/rock interaction such as dissolution or precipitation. Dunites are the second most abundant lithology and may reflect reaction of harzburgite with basaltic liquids that passed through them. Very minor amounts of pyroxene-rich ultramafic rocks (orthopyroxenites and harzburgite that contain coarse orthopyroxene layers) were also recovered. Variations in the textures and mineralogies of the gabbroic rocks may provide insight into melting/crystallization processes in the underlying mantle and crystallization conditions in the recovered mantle section.
In describing and interpreting these rocks we first document the stratigraphic distribution of lithologies and divide the core into major lithologic units. Next, the rock textures and mineralogy are characterized. Finally, we use these observations to discuss the high-temperature history of the rocks recovered from Hole 1268A.
Hole 1268A consists of 29 cores recovered from 147.6 m of penetration below the seafloor. The distribution of rock types defines four major lithologic units. From the top of the core down, these are Unit I: harzburgite/dunite, Unit II: intrusion breccia, Unit III: harzburgite/dunite, and Unit IV: gabbronorite/harzburgite (Fig. F3).
Unit I is composed almost exclusively of harzburgite (86%) and dunite (13.5%) with minor gabbro (0.5%). Clasts of various lithologies including aphyric basalt were sampled in the first core (Section 209-1268A-1R-1), but these probably correspond to a mixed rubble horizon and are not part of the stratigraphic sequence. Unit I is cut by a few small gabbroic intrusions (typically ~1 cm thick). Small intervals of less altered harzburgite with a small amount of clinopyroxene (classified as clinopyroxene-bearing harzburgite) were found in Sections 209-1268A-3R-1 and 3R-2 between 22.5 and 23.5 mbsf. Unit I shows an overall increase in the amount of dunite recovered with increasing depth.
Unit II is characterized by an anastomosing network of gabbroic material that locally forms intrusion breccias within the harzburgite and dunite host rock. Unit II is bounded above and below by shear zones that contain a mixture of gabbroic and ultramafic material with fault breccias below the shear zones (Fig. F4). The intrusion breccias are distinguished from the fault breccias based on the presence of gabbroic material between the clasts (or inferred presence of gabbroic material based on mineral pseudomorphs in the case of severely altered examples). Harzburgite (65.5%) is the dominant lithology recovered in this section with intrusion breccia (27.5%), gabbro (5.5%), and dunite (1.5%) present in decreasing abundance. The percentage of intrusion breccia is somewhat arbitrary, as it is a function of the clast size defined by the network of injected material. Where the injections crosscut the core the observed clast sizes are smaller and the rocks are readily classed as an intrusion breccia. Where they are subparallel to the drilling direction the brecciated nature of the rock is less apparent.
Unit III is another interval with abundant harzburgite (68%) and the most dunite (23%) of any unit; the remainder is gabbro (9%). Unit III had the highest recovery of all units, >90% of the cored interval logged. The excellent recovery allowed us to identify gradational changes in the orthopyroxene mode from harzburgites to dunites. The base of Unit III is defined by a contact between dunite and gabbro at Section 209-1268A-20R-2, 135 cm (104.38 mbsf). The gabbro has distinctive elongate pyroxene crystals that appear to have nucleated and grown perpendicular to the margin (Fig. F5). This texture suggests a high temperature for the wallrock upon intrusion of the magma and/or some amount of water dissolved in the magma.
Unit IV is composed primarily of gabbronorite and gabbro (75.5%) with an interval of harzburgite (23.5%) and dunite (1%). For the purposes of the discussion below, we have additionally subdivided Unit IV into three subunits.
Unit IVC is modally uniform with 60%–70% plagioclase, 20%–30% clinopyroxene, and 10%–15% orthopyroxene in most samples. Despite the uniform mineralogy, there are significant, but gradational, textural variations. Based primarily on the similarities of the mineralogy, we suggest that Subunit IVC could have formed as a single cooling unit but caution that additional work is required to understand the origin of the textural variations that could also indicate multiple intrusive events.
Various aspects of the harzburgites are more readily observed depending upon the type and amount of alteration (low-temperature metamorphism and/or serpentinization), but all have fundamentally similar primary textures and mineral assemblages. Based on the nature of the alteration mineral phases and pseudomorphs, the major primary minerals were reconstituted as olivine (60–90 vol%) and orthopyroxene (10–40 vol%). Spinel (1–3 vol%) is mostly fresh and always present. Minor clinopyroxene (1–3 vol%), partially fresh, is observed with certainty in a very few thin sections and is thought to have been present in some other thin sections. Olivine and orthopyroxene are ubiquitously altered to secondary low-temperature minerals. In most samples, however, the grain boundaries between these two minerals are still preserved, providing information on the microstructure and texture and allowing us to roughly estimate their modal amounts. In some examples the alteration preserves small-sized (0.2–0.5 mm) polygonal neoblasts of serpentinized olivine that result from the recrystallization of coarser crystals. The original size of the primary olivine grains cannot be determined with confidence but appears to have been on the order of a centimeter, based on the olivine/orthopyroxene grain boundaries. Orthopyroxene can be as large as 1.5 cm but is generally 0.7–1.2 cm. Primary grains of orthopyroxene are anhedral with ragged boundaries (Fig. F6) and amoeboidal extensions between olivine grains (Fig. F6F), and ragged, thin grains commonly form interstitial patches in the serpentinized matrix (Fig. F7). The general outline of the orthopyroxene crystals is elongate, defining high-temperature lineations and layering. The primary texture can thus be defined as coarse-grained protogranular, although the orthopyroxene has an anhedral interstitial shape, unusual in abyssal peridotites. In many examples, orthopyroxene grains have recrystallized into 1- to 5-mm subgrains (Fig. F8), leading to clear porphyroclastic textures. Study of a single relatively fresh sample (65% serpentinization instead of the usual >99%) confirms that the texture is protogranular-porphyroclastic (Fig. F9). Where observed, clinopyroxene is present as small (<1 mm) crystals, commonly found at boundaries and junctions of orthopyroxene subgrains or along olivine/orthopyroxene and orthopyroxene/orthopyroxene grain boundaries or as interstitial grains in the serpentinized olivine matrix (Fig. F10). Small interstitial or rounded altered pyroxenes could have been former clinopyroxene based on analogy with similar textured clinopyroxene in other harzburgites (Fig. F11). In one thin section (Sample 209-1268A-13R-2, 3–6 cm), a cluster of altered pyroxenes could have been former clinopyroxene crystals. The outline of the cluster differs from that of former orthopyroxene porphyroclasts, supporting this interpretation. Spinel grains define two major textural types. One is associated with orthopyroxene and preferentially occurs at its margins, commonly forming wormy intergrowths (Fig. F12) that can be up to several millimeters in size (average generally = 0.5 mm) and interstitial to orthopyroxene neoblasts (Fig. F12). The second type of spinel grain is smaller (<0.1–0.3 mm), commonly euhedral to subhedral, and oval or rounded, although anhedral to interstitial grains are also present. This type of spinel is disseminated in the serpentinized olivine matrix where it may enclose olivine neoblasts; it is also associated with interstitial clinopyroxenes (Fig. F10). Both types are frequently flattened and may be aligned in trains or layers parallel to orthopyroxene-rich bands.
The amount of orthopyroxene varies between 13% and 22% in 64% of the ultramafic samples. About 27% of the ultramafic rocks are poorer in orthopyroxene and are classified as dunites (Fig. F13). True dunites with <8%–10% orthopyroxene form 12% of the ultramafic rocks. At the other end of the compositional range, harzburgites with >22% orthopyroxene constitute 9% of the ultramafic rocks, including 3% having 30%–40% orthopyroxene (28 pieces).
Two different types of dunite were sampled. The first type cuts the high-temperature fabrics of the harzburgites at a high angle. Traces of spinel and orthopyroxene can be observed as relics in the dunite (e.g., Section 209-1268A-20R-2 [Piece 5]). However, most dunites do not have clear-cut contacts with the harzburgites. They form a second type that occurs as bands or patches poorer in orthopyroxene (e.g., Section 209-1268A-17R-3 [Piece 7, 69 cm]). The orthopyroxene is smaller (maximum = 5 mm) but has the same anhedral to interstitial shape as in the harzburgites. Spinel in dunites also falls into the two textural groups defined above for spinel in harzburgites. However, because of the smaller amount of orthopyroxene, the spinel mainly occurs as small euhedral to subhedral crystals.
Interval 209-1268A-18R-4 (Pieces 10 and 11, 83–96 cm) represents a 13-cm fragment of orthopyroxenite (Sample 18R-4, 88–91 cm). The rock is composed of orthopyroxene (86 vol%), olivine (8 vol%), minor clinopyroxene (2 vol%), and spinel (4 vol%) that are only slightly altered (~5% serpentinization). The texture suggests that the rock was a coarse-grained granular orthopyroxenite that underwent high-temperature deformation and recrystallization and partial annealing. Orthopyroxene forms a heterogranular (0.2–12 mm) assemblage of anhedral to polygonal crystals (Fig. F14). Olivine, clinopyroxene, and most of the spinel are smaller (0.5–1, 0.05–0.2, and 0.1–5.0 mm, respectively) and are thought to have formed during subsolidus deformation, recrystallization, and equilibration. Primary spinel may have been present, as suggested by the larger crystals and inclusions in orthopyroxene (Fig. F14). Contacts with the surrounding harzburgite wallrock were not recovered.
Coarse orthopyroxene forms 0.5- to 1.5-cm-thick bands in several other places in the harzburgite. These bands are boudinaged and composed of >90 vol% orthopyroxene. The orthopyroxene was formerly present as crystals as large as 1.5 cm but recrystallized into 0.5- to 1-mm subgrains. The remaining ~10% of the bands is composed of spinel and olivine. The olivine may, however, belong to the host harzburgite (Samples 209-1268A-12R-2, 93–98 cm, and 18R-1, 61–63 cm). These orthopyroxene-rich bands are parallel to the high-temperature fabric of the harzburgite, and some of the spinel grains are also flattened along this direction. These bands could be orthopyroxene segregations in the harzburgite or may have formed as a consequence of injections of magma intruded into the harzburgite followed by flattening and transposition into the high-temperature foliation. Three observations support their origin as thin injections into the harzburgite:
The gabbros and gabbronorites are interpreted to reflect at least four separate intrusive events. There is little evidence for crosscutting relationships between different gabbroic intrusions, which suggests that the gabbroic rocks nearest the seafloor are the oldest or that the intrusions were emplaced by lateral injections relative to the orientation of the drilling. Given the near-vertical orientation of some of the smaller gabbroic bodies, we favor an age/position relation and so discuss the gabbroic rocks from the top of the core downward, possibly corresponding to oldest to youngest.
The first significant occurrence of gabbroic rocks in the core is in Unit II where gabbroic material forms an anastomosing network around pieces of harzburgite, resulting in an intrusion breccia (Fig. F15). The injected material is almost completely altered so its original mineralogy and textures are uncertain, but pseudomorphs of plagioclase and clinopyroxene indicate that they were gabbroic. The alteration has obliterated any crosscutting relationships, so we cannot say if the intrusion breccia of Unit II formed in a single event or as a progression of many small injections.
The next significant gabbroic intrusion is found in Unit III, Sections 209-1268A-16R-4 (86.61 mbsf) and 17R-1, with a maximum thickness of only 0.67 m. This gabbro has a variable texture, ranging from pegmatitic to microgabbroic over tens of centimeters. The microgabbroic texture could be related to rapid cooling at the intrusion margins, but the contacts were not recovered. Alternatively, this range of textures could indicate that this gabbro was relatively water rich. Upon emplacement it crystallized large grains, but a sudden rupture of the system decreased the partial pressure of H2O and fine-grained microgabbros formed, by analogy to pegmatites and aplites in granitic systems thought to form via a "pressure quench" process. In thin section it can be seen that the clinopyroxene in this gabbro is completely altered, whereas ~50% of the original plagioclase remains. This large extent of alteration makes it impossible to determine whether primary amphibole or minor phases were present.
Another pegmatitic textured gabbro marks the upper boundary of Unit IV and extends to 107.16 mbsf (Section 209-1268A-21R-1). Like the gabbro described above, it has large elongate clinopyroxene crystals (Fig. F16) along contacts between gabbro and harzburgite, indicating that no quenching or rapid undercooling of the basaltic magma took place. Although it is coarse grained, the gabbro beneath this contact has significant textural and modal variations that may represent more than one injection and crystallization event.
The lowest intrusive gabbroic unit is composed of variably textured gabbronorite that makes up the bulk of Subunit IVA and all of Subunit IVC. The orthopyroxene in these rocks has been completely altered and is identified based on its larger grain size and subophitic texture. In contrast, the clinopyroxene is smaller, subeuhedral, and can be >90% fresh even adjacent to completely altered orthopyroxene grains. The contact between the pegmatitic gabbro described above and the underlying gabbronorite was not recovered. The significant amount of low-temperature alteration and brittle deformation of the uppermost section of the gabbronorite suggests that the two are in fault contact. Below the horizon of alteration and deformation associated with the fault zone, the textures of the gabbronorite vary from coarse grained (orthopyroxene = 15 mm) with a weak primary magmatic foliation to medium grained (orthopyroxene = 6 mm) and massive. The plagioclase in these rocks is 90% fresh and records evidence of modest high-temperature deformation (e.g., bent plagioclase twins, strain twinning, and broken crystals) from which the plagioclase grain boundaries have partly recovered. Little or no evidence of this deformation is recorded by the pyroxenes, suggesting they may have behaved rigidly while the plagioclase accommodated the strain. A high-temperature shear zone was also logged at 136.66 mbsf (Section 209-1268A-27R-1 [Piece 8C]). It shows no alteration but has significant grain-size reduction and more abundant plagioclase than the surrounding gabbronorite.
The recovered rocks provide some insight into the high-temperature processes operating at slow-spreading ridges in the uppermost mantle. A key observation is that there is a continuous decrease in the number of samples with a given proportion of modal orthopyroxene from harzburgites to dunitic rocks (Fig. F13). This distribution is not typical in oceanic mantle rocks, in which there is commonly a bimodal distribution of harzburgite with >15% orthopyroxene and dunite with <5% orthopyroxene and a discrete gap in the abundance of rocks with modal percentages of orthopyroxene in between. Samples with less than ~2.5% orthopyroxene are present in much greater abundance than expected based on the decreasing trend in orthopyroxene mode from harzburgites to dunites (Fig. F13). We suggest that two different processes are responsible for generating rocks with less than ~2.5% orthopyroxene. One of these processes corresponds to the first type of dunite described, which crosscuts the harzburgite fabric at a high angle. These dunites are found in relatively small discrete intervals with sharp boundaries. The other dunites are less easily understood and seem to have formed as an end-member in the process that produced the regular decrease in modal orthopyroxene in the harzburgites. In addition to the unusual gradation between the harzburgites and dunites, the textures of the orthopyroxenes are unusual in that they have irregular grain boundaries and locally extend between olivine grains, similar to how the pseudopods of amoebas extend beyond the main mass. The shape of the orthopyroxene suggests progressive dissolution of the orthopyroxene that may explain the continuous variation in observed mode. There is also a large proportion of harzburgite (35.7%) with ~20% orthopyroxene compared to that with ~25% orthopyroxene (~8.8% of harzburgites), suggesting that the orthopyroxene-rich harzburgites are different from the bulk of the harzburgites (Fig. F13). Orthopyroxene-rich harzburgites are spatially associated with some or all of the following: orthopyroxene-rich or orthopyroxenite bands in harzburgite, orthopyroxenites, and gabbroic intrusions (Fig. F17). A potential explanation for the high modal orthopyroxene in these rocks is that they represent tectonically disaggregated orthopyroxene-rich rocks (a kind of mechanical homogenization).
The several different generations of gabbroic rocks attest to a potentially extended history of magmatism in this section of the shallow mantle. The liquid that formed the intrusion breccia of Unit II must have had relatively low viscosity and/or intruded into relatively hot harzburgite to form such a network. The stockwork character of the breccias suggests that this liquid was relatively evolved and water rich, such as might be derived from a fractionating intrusion at depth. The fact that the gabbroic rocks below the intrusion breccia are not cut by similar intrusions likely discounts them as a source of this liquid. The pegmatitic textures of the gabbros in the lower parts of Unit III and in Subunit IVA are also consistent with the idea that hydrous basaltic liquids were present. This hypothesis may ultimately prove difficult to test because the rocks are highly altered (especially in Unit II). The gabbronorites in Subunits IVA and IVC appear very similar, especially with regard to their mineralogy. It may be that they formed during a single intrusive event and that the intervening mantle material in Subunit IVB represents either a xenolith or roof pendant near the top of the gabbronorite. However, the textural variations seen in both Subunits IVA and IVC are significant and could be interpreted to indicate that, although modally similar, this apparently massive gabbronorite is actually a composite intrusion with individual units on the scale of a meter or less, similar to the scale of the gabbros in the lower part of Unit III.
The atypical character of the mantle rocks recovered, both in terms of the progressive decrease in orthopyroxene from harzburgite to dunites and with regard to the textures of the orthopyroxene, may reflect the role of reaction with migrating hydrous melt in this mantle section. The idea that substantial H2O was available during the high-temperature modification of this mantle sections finds support in the apparent importance of H2O contents during the crystallization of the gabbroic rocks.