Site 1271 consists of two holes, Holes 1271A and 1271B, that recovered peridotites (dunite and harzburgite), small chromitite pods, and gabbros. In describing and interpreting these rocks we first document the stratigraphic distribution and lithology from Holes 1271A and 1271B then combine these observations to discuss igneous processes inferred from the rocks.
This short hole is composed mainly of dunite, with minor amounts of orthopyroxene, so we discuss the features of the hole without subdivision (Fig. F3). The proportion of orthopyroxene pseudomorphs observed in hand sample and in thin section (Figs. F4, F5) varies from 0% to 11%, so most of these peridotites are dunites. An exception is a small piece of altered harzburgite in Section 209-1271A-2R-1. Because of the extreme alteration (Fig. F4), little information can be extracted about the grain size or other textural aspects of the dunite, especially in the uppermost cores (Cores 209-1271A-1R and 2R; 2.56 mbsf). Locally, the dunite is less altered and the original texture and primary mineralogy are still recognizable (Figs. F5, F6). This dunite contains up to 6% orthopyroxene and up to 5% spinel.
Throughout Hole 1271A the dunite is cut by gabbroic intrusions of various sizes in intervals 209-1271A-1R-1 (Piece 7, 23–32 cm) and (Piece 9, 37–51 cm) (Fig. F49A) and 2R-1, 24–51 cm. A large pyroxene crystal (as large as 7 cm) replaced by amphibole is contained in interval 209-1271A-1R-1, 46–50 cm (Fig. F49A). Dunite xenoliths and/or olivine as xenocrysts are included in this gabbroic material (Figs. F7, F8). Gabbroic material is also found along grain boundaries and triple junctions. This material is altered to secondary minerals but is interpreted to have been predominantly plagioclase (Fig. F9). These features can be found throughout the core but are best preserved in Core 209-1271A-4R between 28.5 and 31.23 mbsf. At the bottom of the hole in Core 209-1271A-6R pieces of microgabbro were recovered but they lack lithologic context.
In Core 209-1271A-4R there is a significant amount of chromian spinel, locally as much as 45% of the rock. Examples include euhedral spinels, as large as 3 mm, that occur as small aggregates ~1 cm in diameter or as thin bands (e.g., interval 209-1271A-4R-2 [Piece 3, 61–63 cm]). A chromitite pod ~3 cm in diameter with irregular but sharp contacts in interval 209-1271A-4R-2 (Piece 2, 38–48 cm) represents the maximum enrichment in spinel (Fig. F10). The chromian spinel is associated with chlorite-altered plagioclase(?), olivine, and clinopyroxene. Chromian spinel in dunite and chromitite is reddish brown in thin section. Some small rutile grains are also present in the chromitite pod (Fig. F11). About two-thirds of the grains of chromian spinel in the chromitite are altered to magnetite and/or amorphous (hydr)oxides. The spinels locally include plagioclase and other minerals. Where the spinels are broken and completely altered, the inclusions in them are also altered (Fig. F12).
Hole 1271B consists of 20 cores recovered from 103.8 m of penetration below seafloor. Total recovery was 15.9 m, corresponding to ~15% of the drilled depth. The distribution of rock types defines four lithologic units. From the top of the core down, these are Unit I: dunite/gabbro, Unit II: harzburgite/dunite, Unit III: gabbro/troctolite/dunite, and Unit IV: dunite (Fig. F13).
Unit I is composed of dunite (63.1%) and gabbro (29.2%) with lesser amounts of harzburgite (6.8%) and breccia (0.9%). Most dunite in Unit I includes gabbroic material (i.e., plagioclase and clinopyroxene/amphibole) interstitial to olivine (see "Lithologic Characterization" below). The gabbroic material, now altered to low-temperature minerals, forms rims along grain boundaries of olivine and spinel and millimeter-wide patches at triple junctions (Sections 209-1271B-5R-1 and 6R-1), composing up to 15% of the dunites. Gabbroic material also fills microfractures crosscutting the ultramafic mineral assemblage. The proportion of gabbroic material increases downhole in Unit I. For example, gabbroic material is abundant in the lower part of Section 209-1271B-8R-1 (bottom of Piece 14) where it forms parallel stringers a few millimeters wide. However, it should be noted that the amount of gabbroic material in the upper half of Unit I (Sections 209-1271B-1R-1 through 3R-1) might have been underestimated because of the significant alteration.
Gabbro is present in most sections of Unit I except Sections 209-1271B-2R-1 and 7R-1. The gabbros in Sections 209-1271B-1R-1 and 3R-1 are highly altered. From Sections 209-1271B-3R-1 (Pieces 17–20) to 8R-1, the bulk of the gabbros contain a large proportion of distinctive brown amphibole. This distinctive brown amphibole gabbro (BAG) occurs throughout Hole 1271B. Overall, the amount of gabbro increases downhole in Unit I from 25.8% in Section 209-1271B-1R-1 to 47.2% in Section 8R-1.
Unit II is composed of harzburgite (40.7%), dunite (36.0%), and gabbro (23.3%). This unit has more harzburgite than the other units of Hole 1271B. The harzburgite contains as much as 15% orthopyroxene. Coarse- and fine-grained BAGs are intercalated throughout Section 209-1271B-10R-1. A contact between an altered gabbro and dunite occurs in Section 209-1271B-10R-1 (Piece 9), where dunite is cut by an anastomosing network of gabbroic material (Fig. F14A).
Unit III is composed of olivine gabbro and troctolite (41.6%), dunite (39%), and gabbro (17.8%) with a small amount of chromitite (0.8%) and breccia (0.8%). The olivine gabbro and troctolite are interpreted to have originated as a dunite sequence that was pervasively infiltrated by melt that crystallized plagioclase and clinopyroxene along olivine grain boundaries, resulting in hybrid rocks now classified as olivine gabbros and troctolites. At a mesoscale, xenoliths of dunite are enclosed in the gabbroic rocks (Fig. F14B, F14C). Amphibole is relatively abundant in the gabbroic rocks. Parts of Hole 1271B thus contain dunite–gabbro hybrid rocks, with a transition from peridotites with small proportions of gabbroic material along grain boundaries to rocks with 15%–55% gabbroic material. There is a continuous range in size of gabbroic bodies from decimeter- to millimeter-sized intergranular patches. The olivine gabbro/troctolite/dunite hybrid rocks are abundant in the upper and lower parts of Unit III but are scarce in the middle of the unit where massive gabbro predominates. Other than the gabbro in the dunites, the most abundant gabbro type in this unit is BAG. Minor amphibolite horizons are present, but the origin of these rocks is ambiguous. They might have been clinopyroxene megacrysts, clinopyroxenites, or some other lithology that was completely modified.
Unit IV is composed of dunite (99.0%) with a minor amount of altered gabbro (1.0%). The dunite in Unit IV is like that in Unit I in that it contains various proportions of gabbroic material along olivine grain boundaries. These relations are better preserved in Unit IV and allow us to document the distribution of the gabbroic material and recognize the primary features (textures and grain sizes) of the dunites (Fig. F14D). The percentage of gabbroic material varies from piece to piece, from 1% to 15%, and is irregularly distributed within each piece. Sections 209-1271B-19R-1 (Pieces 3–6) and 19R-2 (Piece 1) contain large (2 mm) parallel gabbroic stringers. In the same section, thin intervals of mud made of serpentinized dunite, retaining the original rock texture, alternate with the rock samples through the sequence. These dunites contain the largest proportions of gabbro material, predominantly present as subparallel stringers.
Dunite has protogranular textures. Original grain sizes range from 5 to 15 mm, determined where the grain boundaries are defined by the presence of intergranular gabbroic material. Olivine is the major mineral phase (90%–100%), associated with minor orthopyroxene (<10%) and accessory spinel. Spinel is relatively coarse (1–2 mm) and subhedral to rounded. Spinel is present at triple junctions of olivine grains or is enclosed in olivine. Spinel inclusions in olivine can be found in trains of tiny grains. Some spinel-rich dunites (e.g., 10% in Section 209-1271B-2R-1 [Piece 3] and 5% in Section 3R-1 [Piece 16]) have larger spinel grains (as large as 4 mm) concentrated in 1- to 3-mm-thick bands (e.g., Sections 209-1271B-5R-1 [Pieces 9 and 10] and 7R-1 [Pieces 1, 4, 5, and 7–12]). In Section 209-1271B-20R-1 (Piece 11), a 4-mm-thick spinel-rich band is present. Despite locally high proportions of spinel elsewhere in the hole, Unit IV dunite has the most spinel, with an average of 5%. Spinel in dunite from Hole 1271A is reddish black, whereas spinel in dunite from Hole 1271B is reddish brown. This reddish color of spinel in Site 1271 peridotite suggests that it has a higher Fe3+/Fe2+ ratio than spinel in typical abyssal peridotite.
Harzburgite abundance increases toward the bottom of Unit I. The rocks contain olivine (75–88 vol%) and orthopyroxene (10%–25%), with accessory spinel (<1%). No clinopyroxene was found. However, most harzburgite samples are highly altered, making it difficult to estimate modal composition and texture. Where less altered, harzburgite shows a protogranular texture, in which orthopyroxene has smooth, curved boundaries with surrounding olivine. Grain sizes vary from 1 to 7 mm, with some as large as 15 mm (Section 209-1271B-10R-1 [Piece 24]). Spinel occurs as tiny disseminated grains (0.2 mm) or as large as 4-mm grains in close proximity to orthopyroxene. Harzburgite, like dunite, may contain a small amount of intergranular gabbroic material originally present as plagioclase and clinopyroxene but now altered.
Almost all dunite in Holes 1271A and 1271B contains some amount of gabbroic material (1%–15%) that reacted with spinel and orthopyroxene, and possibly with olivine. This gabbroic material, probably mostly plagioclase (now altered), is distributed in an intergranular network along olivine and spinel grain boundaries (Fig. F14D). Millimeter-wide patches of plagioclase and clinopyroxene, now largely altered to amphibole, crystallized at olivine triple junctions. Parallel gabbro stringers (<1 mm thick) are replaced by amphibole or low-temperature alteration minerals. Where alteration is limited, spinel near gabbroic material shows a reaction rim characterized by symplectites of spinel and an altered mineral, possibly originally plagioclase (Fig. F15A, F15B). Intergrowths of spinel, olivine, and an alteration phase also occur along the margins of olivine (Fig. F15C–F15F). Similar intergrowths are also observed at olivine triple junctions and enclosed within olivine (Fig. F16A, F16B). The alteration phase in these intergrowths might have replaced pyroxene, plagioclase, or even amphibole. Where the amount of gabbroic material reaches ~20%–30%, disaggregation of the peridotite minerals can be observed. Coarse olivine aggregates are dispersed into isolated grains separated by amphibole ± plagioclase assemblages, so that very few of the olivine grains show grain/grain contact (Fig. F17A–F17C). The peridotite in Unit III has been most extensively modified. The olivine gabbros and troctolites in this unit contain dunite xenoliths and xenocrysts (olivine and spinel) that may have partially reequilibrated with the gabbroic material.
Oikocrystic olivine gabbro has subhedral olivine grains as large as 12 mm enclosed in a plagioclase matrix (Fig. F18A) and/or coarse poikilitic clinopyroxene (Fig. F18B, F18C). Euhedral laths of plagioclase are in direct contact with olivine, with no reaction relationships, and this assemblage is itself enclosed in clinopyroxene oikocrysts (Fig. F18D). In troctolites, a rim of tiny amphibole crystals is developed around olivine grains in contact with plagioclase. This texture is interpreted as a reaction rim. In other places, olivine grains are more extensively mantled and/or partially replaced by amphiboles and clusters of plagioclase form the remainder of the rock (Fig. F18E, F18F). In the more extensive mantles, the amphibole assemblages may not be reaction rims between plagioclase and olivine but may also include some former igneous pyroxene that crystallized in association with plagioclase. Other samples contain olivine set in a finer-grained matrix of amphibole and 5%–25% plagioclase (Fig. F16C). Euhedral magnetite is a common accessory mineral in all these rocks. It is enclosed in olivine or occurs at its border and in the amphibole/plagioclase assemblages.
In many cases, the origin of the amphibole in gabbros and troctolites is ambiguous. It may have replaced clinopyroxene, orthopyroxene, olivine, or earlier amphibole. There are several generations of amphibole, characterized by various grain sizes and textural relationships with the other minerals. Rarely, some coarser, more equant amphibole crystals seem to be clinopyroxene pseudomorphs. Early amphibole might have been in equilibrium with olivine, as suggested by sharp contact between the two phases and by the presence of discrete, euhedral, fresh amphibole enclosed within olivine (Fig. F16D).
The origin of the olivine in these hybrid rocks is not always clear. Their interpretation as olivine xenocrysts is based on the presence of coarse-grained olivine in dunites that are partly disaggregated into smaller grains separated by amphibole ± plagioclase as described above. Interval 209-1271B-17R-1, 43–48 cm, contains a 1-cm dunitic xenolith (Fig. F19A). Although very altered, a reaction rim is still recognizable between the olivine and the amphibolite matrix (Fig. F19B, F19C) with pseudomorphs of symplectites, much like those observed in symplectite-bearing dunites (Fig. F15C, F15D) and troctolites. Many of the olivine grains included in gabbroic material are large with rounded shapes, suggesting that they were originally large olivine crystals in the dunite but were disaggregated and perhaps partially resorbed. However, igneous olivine may also be present in some samples that have plagioclase closely associated with the olivine. The rounded morphology of olivine in these rocks may therefore reflect overgrowths on xenocrysts. The olivine gabbros and troctolites are thus interpreted as hybrid rocks resulting from the inclusion of olivine from dunite into a melt crystallizing a gabbroic phase assemblage. Although the gabbroic portions of these rocks are generally altered to amphiboles, the olivine is mostly fresh and either in equilibrium with amphibole or being replaced by amphibole along grain boundaries. The amount of gabbroic/amphibolitic material in olivine gabbros and troctolites varies from 15% to 80%, producing various textures in these rocks. When gabbroic material composes as much as 80% of the rock, the olivine crystals occur either as isolated single crystals or in small clusters in the gabbroic/amphibolite matrix.
The gabbroic rocks in Hole 1271B include gabbro (sensu stricto), microgabbronorite, and BAG. The gabbro contains only primary plagioclase and clinopyroxene, is coarse grained with ophitic clinopyroxene, and has plagioclase grains >35 mm. A gabbronorite with a magmatic foliation from Section 209-1271B-8R-1 (Piece 9) contains 55% plagioclase, 35% clinopyroxene, 8% orthopyroxene, and 2% oxide.
The BAG contains 35%–55% plagioclase (1–13 mm), 40%–60% brown amphibole (2–20 mm), and accessory minerals, most notably rutile and in some samples ilmenite (the proportion of ilmenite is as high as 20% in a few samples). The amphibole is generally completely fresh, whereas the plagioclase is almost completely altered. The amphibole in the BAG is of uncertain paragenesis. We consider three possibilities for its origin: (1) it is a primary magmatic phase, (2) it is a metamorphic replacement of clinopyroxene, and/or (3) it is a late-stage magmatic reaction product. The presence of euhedral amphibole with sharp grain boundaries supports a magmatic origin (F20A). The euhedral rutile grains included in the amphibole would then require that the rutile is also magmatic, as it is too concentrated to be exsolved from the amphibole (Fig. F20B). Gabbro that has textural characteristics similar to the BAG is found in the same sections as the BAG and is a potential example of the igneous progenitor, subsequently modified by late-magmatic or metamorphic replacement to produce a BAG (Fig. F20C). The presence of latticework extensions of amphibole into plagioclase-rich areas suggests that the amphibole is replacing both clinopyroxene and plagioclase (Fig. F20D). However, close inspection of the gabbro reveals fine tendrils of the ophitic clinopyroxene between plagioclase crystals. If the brown amphibole crystallized from a magma with a similar morphology, then after alteration of the plagioclase the relict amphibole tendrils might be misinterpreted as replacing the plagioclase. Likewise, if the amphibole is metamorphic and replaced igneous clinopyroxene with this habit, it would explain the observed characteristics of the amphibole without requiring that plagioclase be replaced by amphibole.
A small number of amphibole-rich rocks (composed of amphibole [>95%] with minor plagioclase [<5%]) are also present. They are similar to the altered interstitial material in hybridized dunites, olivine gabbros, and troctolites. Their origin is ambiguous, but some might have been clinopyroxene megacrysts, gabbros, or igneous rocks rich in primary amphibole.
Chromitites (35%–45% modal spinel) are found in Hole 1271A (Section 209-1271A-4R-2) and at two horizons in Hole 1271B (Sections 209-1271B-13R-1 [Piece 6] and 14R-1 [Piece 7]). The horizon in Section 209-1271B-14R-1 forms a 3-mm-wide band in amphibolite. The spinel is reddish brown, euhedral, and 0.5–2 mm in diameter. The proportion of spinel varies from 35% to 45%, and the grain size ranges from 1 to 5 mm with euhedral to subhedral grains. Spinel crystals are surrounded by a matrix of olivine with a small amount of chlorite (replacing plagioclase or formed by reactions between olivine, pyroxene, and spinel) filling cracks in spinel.
Almost all of the peridotite from Site 1271 contains some amount of gabbroic material (typically 1%–15%) crystallized from a liquid that migrated along the grain boundaries and reacted with spinel, orthopyroxene, and olivine. Where the volume of gabbroic material is large (15%–40%), it is associated with the mechanical disaggregation of the peridotite. The best examples of this process are preserved in Hole 1271B, Unit III. When the volume fraction of gabbroic material exceeds ~40%, the rocks have the appearance of gabbroic cumulates and these rocks are named olivine gabbros and troctolites. They have a matrix that was originally (and in some examples still is) composed of plagioclase and clinopyroxene that enclosed dunite xenoliths and xenocrysts (olivine and spinel).
On the basis of the lithologic variation in Hole 1271B we envision the following genesis of these rocks:
A remarkable feature of Site 1271 is the occurrence of chromitite pods (up to 4 cm in diameter) and several chromite layers included in chromian spinel–rich dunite and in amphibolite. This is the first podiform chromitite in abyssal peridotite reported from the Mid-Atlantic Ridge. A small chromitite "minipod" included in a troctolite (with a hybrid origin as described above) was reported from Hess Deep, East Pacific Rise (Leg 147 Site 895; Arai and Matsukage, 1996). Podiform chromitite, usually surrounded by dunite within harzburgite, is common in the mantle section of ophiolites (e.g., Nicolas, 1989), but lherzolite-hosted podiform chromitite is very rare (Irvine, 1977). It is therefore generally believed that the origin of podiform chromitite requires interaction between liquid and moderately depleted harzburgite (e.g., Zhou et al., 1994; Arai and Yurimoto, 1994; Arai and Abe, 1995), as seen at Site 1271.
Recent experimental study suggests that crystallization of water-rich basaltic liquid can duplicate the nodular textures typical of podiform chromitite (Matveev and Ballhaus, 2002). Surprisingly, given our findings, Matveev and Ballhaus (2002) conclude that podiform chromitites are not expected to occur in oceanic lithosphere forming at mid-ocean ridges because mid-ocean-ridge basalt does not contain sufficient H2O.
An intriguing feature of Site 1271, and especially Hole 1271B, is the abundance of the amphibole in nearly all the different lithologies. Several generations of amphibole are present, so that in many cases amphibole is clearly replacing former igneous minerals. Textural evidence, such as curved olivine/amphibole grain boundaries and euhedral amphibole inclusions in olivine, indicates that at least some of the amphibole formed in equilibrium with olivine. The chromitite pods found at Site 1271 are all associated with amphibole-rich gabbro and amphibole-bearing dunite. It is also notable that one of the three chromitite bands is enclosed within amphibolite rather than dunite. The association of chromitite and amphibole at Site 1271 is probably not a coincidence and suggests that even in a mid-ocean-ridge environment, abundant H2O may be involved in chromitite genesis.
The scale of the chromitites (a few centimeters in maximum dimension) and high concentration of chromian spinel in the dunite throughout Site 1271 suggests that these chromitites are similar to podiform chromitites found in many ophiolites. Lithologic variations similar to those in Hole 1271 have been reported from some ophiolites. The association of partially hybridized peridotite with chromitite, gabbroic rocks, and troctolites with various amounts of plagioclase is common in the Mohorovicic (Moho) transition zone of the Oman ophiolite (Nicolas, 1989; Boudier and Nicolas, 1995). However, in the Oman ophiolite amphibole is rare in the Moho transition zone.