METAMORPHIC PETROLOGY

A 131.0-m-long section of very highly to completely altered harzburgite and dunite and intercalated fresh to highly altered mafic units (basalt, diabase, and gabbro) was drilled in Hole 1272A (average recovery = 28.6%). In comparison to Sites 1268, 1270, and 1271, alteration of ultramafic rocks in Hole 1272A is relatively uniform. In the uppermost 100 m, harzburgite and dunite are completely altered to serpentine and magnetite, with minor brucite in some intervals. Variable amounts of the mineral iowaite are present in most samples of serpentinized harzburgite and dunite below Core 209-1272A-12R (56 mbsf). Some sections of core are characterized by unusually soft clayey serpentinites and serpentine mud. Mafic rocks in Hole 1272A are variably altered. Oxidative low-temperature alteration is usually slight and mainly affects olivine phenocrysts and microphenocrysts. Lower greenschist facies alteration is indicated by the presence of green amphibole, chlorite/smectite, talc, and minor albite and quartz (up to 70% total alteration). A quartz-biotite-amphibole gabbroic/dioritic intrusive unit in Sections 209-1272A-1R-1 and 3R-1 also shows greenschist facies alteration and a minor low-temperature oxidative alteration overprint. Low-temperature oxidative alteration of ultramafic rocks, manifest in red to brown clay and Fe oxyhydroxide and aragonite veining, is more abundant at Site 1272 than at Sites 1268, 1270, and 1271.

Alteration of Ultramafic Rocks

During serpentinization, harzburgite and dunite are very highly to completely altered to dark green to green serpentinite consisting of serpentine and magnetite. Locally, brucite and/or iowaite are common (Fig. F20; Table T2). The microtextures of serpentinized harzburgites and dunites range from pseudomorphic mesh-rim textures to transitional ribbon textures (Fig. F21) to rare nonpseudomorphic interlocking textures. Microscopic, apparently fibrous serpentine veins are common and include length-slow -serpentine (chrysotile) as well as length-fast -serpentine veins (lizardite). The latter type of microveins was not observed in cores from any of the previous sites. The uppermost 100 m is completely altered, except for relics of spinel. In the lowermost 31 m of Hole 1272A, serpentinization of olivine and orthopyroxene is incomplete and >1%–3% of these phases are preserved. The first occurrence of relict orthopyroxene in Unit II was observed in hand specimen in Section 209-1272A-22R-1 at a depth of 103.9 mbsf. The first occurrence of orthopyroxene relics (1%) in thin section is from Sample 209-1272A-21R-1 (Piece 4, 50–53 cm) (99.4 mbsf), and the first sample with a trace of fresh olivine is Sample 23R-1 (Piece 2, 7–11 cm; 108.6 mbsf). The proportions of relict primary phases can be as high as 8% total (e.g., 3% olivine and 5% orthopyroxene in Sample 209-1272A-25R-1 [Piece 15, 123–126 cm]). Sample 209-1272A-27R-2 (Piece 10, 97–99 cm) has 5% relict olivine. The overall decrease in alteration intensity in the lowermost part of Hole 1272A is displayed in Figure F22. Consistent with this observed decrease in alteration intensity are increases in density and sonic velocity in the bottom part of the hole (see "Porosity, Density, and Seismic Velocity" in "Physical Properties"). In the uppermost 30 m, oxidative alteration of serpentinized harzburgite and dunite to reddish brown clay, carbonate, and Fe oxyhydroxide is abundant (Fig. F23), particularly in proximity to carbonate-clay–cemented fault breccias (e.g., Sections 209-1272A-1R-1 and 4R-1). A fault gouge deeper in the core (Section 209-1272A-25R-2) lacks oxidative alteration. Some intervals within the oxidatively altered harzburgite and dunite of Unit I have noticeable amounts of relict olivine and orthopyroxene (e.g., 5% orthopyroxene and 3% olivine in Sample 209-1272A-1R-1, 7–11 cm).

There are two aspects of the alteration of ultramafic rocks in Hole 1272A that are unusual. The first peculiarity is that fine-grained magnetite-serpentine stringers, typically associated with replacement of olivine, are basically absent in samples from Hole 1272A. Instead, coarse magnetite is developed in the cores rather than the mesh rims of the serpentine mesh texture (Fig. F24).

The second notable difference between serpentinites of Site 1272 and those from other sites is the abundance of soft clayey serpentinized harzburgites and serpentine mud (hereafter called clay alteration). Clay alteration is most obvious in Sections 209-1272A-12R-1, 16R-1, 18R-1, and 24R-1, where large parts of the core were extremely soft and plastic when retrieved from the core barrel. After being exposed to air for more than a day, the core from these intervals developed shrinkage fractures and an overall crumbly appearance. In their most extreme form, these rocks have the appearance of mud (e.g., Sections 209-1272A-C-11R-1, 11R-2, 18R-2, and 19R-2). Compositionally, clay-altered serpentinite is not markedly different from the solid serpentinite in adjacent sections of the core, except for their unusually high H₂O contents (>20 wt%) (see "Geochemistry").

However, characteristic peaks at 8.1 and 4.07 Å in X-ray diffractograms show that these samples contain the mineral iowaite (Fig. F20). Iowaite is a rare magnesium-iron hydroxychlorite, which was discovered in Precambrian serpentinite (Kohls and Rodda, 1967). It has been recognized in Ocean Drilling Program (ODP) drill core of serpentinite mud volcanoes in the Izu-Bonin forearc (Heling and Schwarz, 1992) and from altered serpentinite at the Iberian margin (Gibson et al., 1996). The structure of iowaite is similar to brucite; however, it contains significant ferric iron and chlorine. Based on analyses by Allmann and Donnay (1969) the structural formula is

[Mg(OH)₂]₄FeOCl · nH₂O (n = 1–4).

Results of differential thermoanalyses show that the water in iowaite is gradually driven off as the temperature rises to 280°C and that the structure begins to collapse at 315°C (Kohls and Rodda, 1967). This is consistent with our results showing that the two major peaks of iowaite disappear from the diffractogram after the powder has been heated to 370°C (Fig. F25). All X-ray diffraction (XRD) analyses of samples from Section 209-1272A-12R-1 and downhole show these peaks with variable intensities, indicating that iowaite is present in the soft, visibly clay-altered serpentinites as well as in the hard serpentine (Fig. F20). In thin section, we noted brown patches with a poor polish that show wavy extinction of aggregates and straight extinction of individual fibers.

It has been inferred that iowaite may be formed from iron-bearing brucite under oxidizing conditions (Heling and Schwarz, 1992). This process involves the oxidation of Fe²⁺ in brucite to Fe³⁺, generating a charge imbalance, which is rectified by incorporating Cl^– between brucite layers. In a model suggested for iowaite formation in serpentinite muds at the Izu-Bonin arc, this reaction takes place immediately below the seafloor as a result of infiltration of ambient seawater (Heling and Schwarz, 1992). However, occurrences of iowaite in serpentinites at 710–810 mbsf at the Iberian margin (ODP Site 897) have been related to circulating low-temperature Cl-rich brines (Gibson et al., 1996). Here, iowaite is restricted to a zone with elevated Cl^– concentrations in bulk-rock analyses (up to 1 wt%) and it is inferred that alteration is this area was probably independent of the earlier serpentinization and later surficial oxidative alteration.

The observation of common iowaite distinguishes Hole 1272A from all other sites drilled during Leg 209. A distinctive stage of alteration, postdating serpentinization, may have occurred in this area and may have been associated with fluid flow along the major fault zones observed in this hole (see "Structural Geology").

In this regard it is interesting to note that all serpentinized harzburgites have similar molar (Mg + Fe)/Si ratios (1.74 ± 0.05; Table T4), consistent with that of a harzburgite precursor with 25% orthopyroxene. A dunite sample (Sample 209-1272A-23R-1 [Piece 2, 12–14 cm]) has a molar (Mg + Fe)/Si ratio = 1.95, which is close to that of olivine. These data suggest that enrichment of the rocks in silica (or removal of Mg or Fe) is insignificant, at least in relation to the large changes in (Mg + Fe)/Si observed in rocks from Hole 1268A that underwent additional hydrothermal alteration after serpentinization, including replacement of serpentine by talc.

Alteration of Mafic Rocks

Small rounded pieces of fresh to slightly altered basalt are interspersed throughout the first six cores from Hole 1272A and at the top of cores from greater depths, where they probably represent rubble that fell down the hole. These basalts show only minor replacement of olivine microphenocrysts by clay and oxyhydroxide and local red staining of plagioclase. Section 209-1272A-7R-1 represents a coherent volcanic unit composed of gray to greenish brown variably altered spherulitic basalt and microgabbro. In hand specimen, the gray spherulitic basalt does not show visible alteration. However, thin sections of Samples 209-1272A-7R-1, 87–89 cm, and 7R-1, 113–115 cm, have 5%–9% alteration of glass to very fine grained brown to gray clay. A thin section of Sample 209-1272A-7R-1 (Piece 15, 94–96 cm) reveals 60% alteration of plagioclase, pyroxene, and mesostasis to green amphibole and chlorite/smectite.

Different varieties of fine- to medium-grained mafic rocks, including diabase, gabbro, and diorite, were cored in Hole 1272A. The first occurrence of mafic rocks is in Section 209-1272A-1R-1, where a dike of moderately altered (20%), fine-grained diabase/microgabbro crosscuts dunite. Plagioclase in the diabase is slightly altered to chlorite and minor talc, whereas pyroxene is moderately altered, dominantly to green amphibole and chlorite, and olivine is completely altered to talc and magnetite (Fig. F26). Locally, the diabase underwent oxidative alteration, indicated by brown discoloration of the rock. A second intrusive body was cored in Sections 209-1272A-3R-1 and 4R-1 (Fig. F27). This medium-grained myarolitic rock is probably dioritic, indicated by the abundance of quartz, biotite, apatite, amphibole, and its chemical composition (see "Igneous and Mantle Petrology" and "Geochemistry"). Quartz in this rock is clearly primary, although traces of secondary quartz are also present, replacing plagioclase along with chlorite/smectite. Plagioclase alteration to chlorite/smectite is locally intense (Fig. F28). Orthopyroxene and clinopyroxene are also partly replaced by green amphibole, chlorite/smectite, and minor talc. The total alteration is between 15% and 20%. Fluid inclusions in primary quartz are abundant. Multiphase liquid-vapor-solid inclusions (Fig. F29) have as many as three solid phases, including one or two optically isotropic minerals (halite and sylvite?), a birefringent mineral, and a vapor bubble and could have formed as a result of supercritical phase separation of seawater-derived fluids or trapping of magmatic fluids.

A third type of gabbroic intrusion is highly altered (60%) oxide gabbro in Section 209-1272A-19R-1. A thin section of Sample 209-1272A-19R-1, 33–35 cm, indicates that the plagioclase and pyroxene are replaced predominantly by fibrous to acicular green amphibole, subordinate chlorite, and a trace of talc.

The total volume of veins accounts for 0.7% of the core in Hole 1272A, which is substantially less than that observed at Sites 1268, 1270, and 1271. In fact, several sections of completely altered dunite or harzburgite are devoid of any veining. Furthermore, slightly to moderately altered basalt, diabase, and microgabbro in Hole 1272A were affected by veining (Fig. F30). The vein mineralogy is dominated by serpentine (84.8%) and carbonate (13%) with minor magnetite (1.9%) and clay (0.3%) (Fig. F31). Several generations of veins can be distinguished that show consistent crosscutting relationships and systematic changes in abundance with depth. Carbonate veins (chiefly aragonite) are only present in the uppermost 30 m, whereas serpentine veins are abundant below 60 mbsf (Fig. F30; Table T3).

The first vein generation is represented by locally banded, composite, black serpentine-magnetite veins (e.g., Section 209-1272A-13R-1). In proximity of the fault gouge in Section 209-1272A-25R-2, the abundance of serpentine-magnetite veins increases somewhat. This vein type is crosscut by magnetite-free green picrolite veins.

Sigmoidal, fibrous chrysotile veins usually form wispy en echelon arrays and crosscut both the black to green serpentine-magnetite veins and the green picrolite veins. An exceptional 15-cm-long and 3-mm-wide serpentine-magnetite vein in interval 209-1272A-24R-1, 85–110 cm, probably formed synchronously with sigmoidal chrysotile veins. This can be deduced from the complex crosscutting relations: some of the sigmoidal chrysotile veins can be traced across the large serpentine-magnetite vein, whereas other chrysotile veins appear to be cut and offset by the large vein. The abundance of wispy chrysotile veins decreases with depth, and virtually no veins of this generation occur below Section 209-1272A-25R-1. Locally, these chrysotile veins contain some clay, which imparts a dusty appearance to the vein (e.g., interval 209-1272A-24R-1, 17–19 cm). Throughout most of Hole 1272A these veins are paragranular. However, in Section 209-1272A-22R-1 (Pieces 3, 7, 11, and 16) transgranular chrysotile veins crosscut orthopyroxene grains and in Section 15R-1 the veins are oriented at a high angle to the main foliation of the host rock.

Gray, vuggy aragonite veins are restricted to the upper 30 m of Hole 1272A and crosscut the three different types of serpentine veins described above. Locally (e.g., Sections 209-1272A-2R-1 and 5R-1), the carbonate veins contain a dark claylike material, possibly celadonite (Sample 2R-1, 66–68 cm). These veins are similar to the carbonate veins at Site 1271, and they are interpreted as the result of late low-temperature oxidative alteration.