30 km below surface; Ringwood, 1975), near the transition between plagioclase- and spinel-peridotite stability fields. This is compatible with adiabatic decompression beneath a rift zone.
The ocean/continent boundary of the West Iberia passive continental margin is characterized along its northern part, west of the Galicia Bank, by a 100-km-long peridotite ridge. This ridge was emplaced during Mesozoic continental rifting (Boillot, Girardeau, et al., 1988; Boillot, Comas, et al., 1988). Geophysical data suggest that this ridge extends southward beneath the sedimentary cover of the Iberia Abyssal Plain (Whitmarsh et al., 1990; Beslier et al., 1993). Site 897 is located in a small basin on the top of this basement ridge and was drilled to determine the nature of the basement and to test geophysical predictions. Two holes (897C and 897D), separated by 100 m, were drilled through a 678- to 694-m-thick sedimentary cover of Pleistocene to Hauterivian age. Drilling recovered basement cores composed of serpentinized peridotite (Fig. 29). In Hole 897C, 97.4 m of basement was drilled with 33.8% recovery, and in Hole 897D, 143.4 m with 54% recovery.
Three sedimentary intervals are bounded by altered peridotite in the lowest sedimentary unit (Unit IV) encountered in Hole 897C (Intervals 149-897C-63R-1, 55 cm, through -63R-2, 40 cm; -897C-64R-1 through -64R-2; and -897C-65R-2, 88 cm, through -897C-65R-3, 20 cm), and one interval in the lowest part of the Unit IV at Hole 897D (Interval 149-897D-10R-3, 40 cm, through -10R-4, 115 cm; Fig. 29). Interval 149-897C-65R-3, 0-12 cm, is a breccia of uncertain origin, either sedimentary or volcanic, that contains diverse lithic fragments, including highly altered diabase. The other intervening hard-rock sections are highly altered peridotite, except the lowest one in Hole 897C, which contains harzburgite, pyroxenite, and serpentinized peridotite breccias. The serpentinite intervals have been tentatively interpreted as large blocks within the base of the sedimentary section (Unit IV).
The basement in both Holes 897C and 897D is composed of lithologically heterogeneous peridotite. The dominant lithology (about 85%) is serpentinized peridotite that originally contained 70%-80% olivine, 15%-20% pyroxene, and l%-2% spinel. About 10% of this serpentinized peridotite is brecciated. Although relicts of clinopyroxene were recognized in most cores, extensive serpentinization inhibits the precise determination of the proportion of clinopyroxene, and the peridotite was either harzburgite or lherzolite.
Approximately 10% of the recovered rocks is plagioclase-bearing peridotite; one-third of these rocks is brecciated. The proportion of plagioclase is highly variable (averaging 15%), but ranges up to 35%-40%. This facies is also pyroxene-rich (20%-30%), with 20%-70% olivine and 1%-5% spinel. A thick (30 cm) pyroxenitic zone was encountered in Hole 897C (Interval 149-897C-70R-1, 110-135 cm).
The remaining 5% of the serpentinized peridotite is a friable serpentinite breccia found at the top of the basement in Hole 897C (Sections 149-897C-66R-1 to -66R-3), in the largest block immediately above the top of basement in Hole 897D (Intervals 149-897D-10R-2, 0-150 cm, and -10R-3, 0-39 cm), and deeper in the basement (Intervals 149-897D-18R-2, 30-70 cm, -18R-4, 0-18 cm, and -25R-6, 52-104 cm).
Two facies may be distinguished in the serpentinized harzburgitic/lherzolitic rocks, depending on the abundance and distribution of pyroxene (Fig. 29): (1) a variety in which the pyroxene is homogeneously distributed (Fig. 30), and (2) a banded variety where pyroxene-rich areas and pyroxene-poor, or even dunitic, areas alternate (Fig. 31). The transition between these two facies generally is sharp. No plagioclase was recognized in these rocks, but the extensive serpentinization makes identification of this mineral difficult. Locally, a pyroxene-rich or a pyroxenite facies occurs as a thin (1-5 cm) diffuse band or as parallel bands or dykelets. Primary zonation within the dykelets is preserved. Despite complete serpentinization, the borders are clearly visible as zones of bastite derived from pyroxenes (Fig. 32).
The plagioclase-rich peridotite is a banded facies of pyroxene- and plagioclase-rich zones (Fig. 33), with a sharp transition between the two types. The Interval 149-897C-64R-5, 20-65 cm, displays a primary banding marked by four parallel pyroxenitic bands (2 cm thick). Plagioclase is particularly abundant (35%-40%) in the Interval 149-897C-66R-4 at 53-70 cm.
The entire ultramafic section in both holes has been extensively serpentinized. In addition, the upper part of basement in Hole 897D has been pervasively calcitized and altered, and is yellow to brown (Sections 149-897D-10R-3 to -15R-2 and -16R-4 to -16R-7). Elsewhere, the harzburgitic or lherzolitic rocks generally are pale green or black; the plagioclase-bearing peridotite is usually gray or black, depending on the abundance of plagioclase.
Tectonic breccias occur throughout the cores from both holes. Two main types of breccia are distinguished: (1) angular blocks of serpentinized peridotite embedded in a network of fractures filled with calcite and/or serpentine; the proportion of calcite and/or serpentine depends on the intensity of fracturing (e.g., Interval 149-897C-64R-5, 0-65 cm; Sections 149-897C-65R-1 and -19R-4); (2) more rounded, smaller blocks of serpentine embedded in a matrix of calcite and/or serpentine; these breccias are locally sheared or fractured (Sections 149-897D-17R-4 to -6). When the latter variety is altered, the matrix is friable with thin blue-gray or white layers (Sections 149-897D-10R-2 and -3; 149-897D-18R-3 to -18R-4; and Interval 149-897D-25R-6, 52-104 cm). In Sections 149-897C-66R-1 to -3, the unconsolidated matrix of green serpentine containing a few fragments of serpentinite may be such an altered breccia, but it is also comparable to serpentine sediments described from the Izu-Bonin-Mariana forearc (Fryer, Pearce, Stokking, et al, 1990).
The serpentinized peridotite has an equant texture and displays almost no high-temperature ductile deformation. However, this primary texture has been overprinted in many places by later low-temperature deformation. Features associated with this deformation include (1) the development of fractures and brecciation of the serpentinized peridotites; (2) a shear deformation marked by foliation planes and shear bands, and (3) a penetrative deformation of the serpentine mesh, either in the serpentinized rocks or in the breccias (see "Structural Geology" section, this chapter).
The late deformation is heterogeneous, and the relative timing of the development of these different features is not obvious; they appear to be more or less coeval. However, structural relationships show that the calcite-filled fractures cross-cut serpentinite breccias (Section 149-897D-17R-5).
A preliminary study of 83 thin sections confirms the extensive serpentinization of the peridotite. In a very few samples, up to 60% of the primary phases (olivine, clinopyroxene, orthopyroxene, spinel, and plagioclase) have been preserved.
Olivine has been completely altered to serpentine in most thin sections. A few samples of plagioclase-bearing peridotite, however, contain fresh olivine. In these samples (e.g., Sample 149-897D-19R-5, 120 cm), the olivine crystals are anhedral, 2 to 4 mm in diameter, and exhibit narrow kink-bands. The initial modal content of olivine ranges from 70% to 95%; however, some pyroxene- and plagioclase-rich varieties contain only 20% to 40% olivine.
Although orthopyroxenes usually have been completely replaced by secondary minerals (mainly serpentine, magnetite, and calcite), the initial crystal shape has often been preserved. The pyroxene crystals range from 0.5 to 10 mm in diameter and from 0% to 20% in abundance. The crystals (probably enstatite) sometimes display thin clinopyroxene exsolution lamellae. Some complete orthopyroxene crystals have been bent and/or kinked (Fig. 34).
Clinopyroxene (probably chromediopside) occurs as sparse, fresh remnants within serpentine, although the original crystal shape is often difficult to distinguish. Typically, clinopyroxene appears to occur as equant crystals (from 0.5 to 10 mm) that were clustered at the periphery of orthopyroxene grains in connection with exsolution. In the pyroxenerich facies, primary banding is sometimes outlined by clinopyroxene crystals with their associated curved enstatite exsolution lamellae. The modal abundance of the clinopyroxene is highly variable: from 0% in dunitic facies to 50% in some pyroxenite dikes.
Spinel crystals usually have embayed, irregular shapes (like a holly leaf), that range in size from 0.5 to 3 mm. Frequently fractured, sometimes they are broken into pieces as small as 0.1 mm without being altered. A brown chrome-rich spinel is the most abundant variety. It becomes opaque as it is altered and invaded by pervasive exsolved-magnetite. The pyroxene- and plagioclase-rich facies contain green elongate spinel grains, perhaps with a lower iron and chromium content.
Plagioclase makes up to 40% of the whole rock in the pyroxene and green spinel-bearing facies. Most of this plagioclase has been completely altered. It normally rims corroded spinel crystals or forms irregular veinlets that pervasively invade the lherzolite and constitute from 0% to 15% of the whole rock (Fig. 35).
The primary texture of the peridotites has often been overprinted by serpentinization, calcitization, and late deformation. Primary textures, however, have been identified tentatively in some of the less altered samples. Two main types were recognized: coarse-grained equant and porphyroclastic. The majority of the rocks are coarse-grained equant, with neither macroscopic nor microscopic evidence of high-temperature deformation. Porphyroclastic textures (70% porphyroclasts on average) and locally high-temperature mylonitic textures were observed only in well-preserved pyroxenite or plagioclase-bearing lherzolitic rocks.
Secondary minerals usually constitute 80% to 100% of the rock. The freshest material occurs in Hole 897C, whereas samples from Hole 897D are both serpentinized and altered, and few primary minerals are preserved. The secondary minerals include lizardite and chrysotile, opaque minerals (magnetite, hematite, amorphous oxide phases, and sulfides), scarce amphibole and chlorite, iowaite, calcite, brucite, and, locally, quartz.
Several generations of serpentine formation can be inferred from the textural relationships in these rocks. The peridotite is mostly a mesh of thin serpentine veinlets derived from the alteration of olivine. Orthopyroxene crystals have been partly or completely serpentinized. The original crystal shapes of orthopyroxene grains usually have been preserved, making it possible to assess the original abundance of orthopyroxene. Several types of cross-cutting serpentine veins can be distinguished, including thin parallel lenses (1 with massive serpentine and thicker (5-20 mm) veins filled with a very fine-grained cream to black near-translucent variety, and greenish gray fibers (1-10 mm serpentine mesh texture of lizardite and/or chrysotile. In addition, several zones, resembling contorted pervasive veins having a faint bluish to purple color, display the XRD pattern of iowaite.
Secondary opaque minerals are mainly magnetite and pyrite. Magnetite, a by-product of the serpentinization, is observed in the serpentine mesh and darkens the deeply altered pyroxene crystals. Magnetite also was produced by the alteration of primary spinels, pseudomorphing the original crystal. Pyrite, the most common sulfide encountered, occurs as scattered grains or forms small veinlets in the fractures filled by massive serpentine (e.g., Interval 149-897C-67R-1, 38-43 cm).
Few occurrences of amphibole were observed. Some orthopyroxenes have been replaced by fibrous amphibole and are rimmed by bastite (Interval 149-897D-16R-2, 68-71 cm). The highly deformed lherzolite displays acicular amphibole (probably tremolite) (0.2-1 mm long), associated with chlorite (1.5 mm in size) in veinlets parallel to the foliation (Interval 149-897C-67R-3, 62-65 cm).
The color of the serpentinized peridotite is related to the extent of calcite alteration. The peridotite having the least amount of calcite is black or dark green. Yellow to orange-brown colors are associated with the presence of large amounts of calcite and ferric hydroxides. Calcite replacement and vein formation apparently was the last alteration stage of these rocks; calcite veins crosscut, brecciate, or replace preexisting structures related to serpentinization (Fig. 36). Extensive replacement by serpentine has sometimes induced crystallization of small pockets of chalcedony or quartz (Interval 149-897D-12R-5, 83-85 cm). Calcite alteration becomes less pervasive downhole and is essentially absent beneath 677 mbsf in Hole 897C and beneath 753 mbsf in Hole 897D. Extensive calcitization did not occur in the basement of Hole 897C, but only in the serpentinite blocks in the lowest part of the sedimentary section (Cores 149-897C-63R to -65R). However, calcitization is extensively developed in Hole 897D in the upper 60 m of the basement section.
Brucite occurs in one 1-cm-thick vein (Sample 149-897D-17R-6, 1-2 cm) and as scattered crystals (Interval 149-897D-23R-1, 59-62 cm) associated with fractures. This mineral usually is considered to be hydrothermal in origin and is characteristic of low-temperature/low-pressure conditions. The rock has been stained by purple crystals of iowaite near this vein of brucite.
Geochemical results of major elements from X-ray fluorescence (XRF) analysis of 15 serpentinized peridotite samples, taken from both Holes 897C and 897D, are presented in Table 9. These results are for ignited samples and thus are water- and carbonate-free. The loss-on-ignition values also are included; however, the ratio of ferric to ferrous iron was not determined. The quoted iron concentrations are "total iron" expressed as Fe2O3.
The results confirm the ultramafic, magnesium-rich nature of the rocks. Concentrations of Na2O, K2O, and P2O5 are low, at or below the detection limit for the XRF analytical procedures used. As expected, concentrations of Al2O3, TiO2, and MnO also are low, and concentrations of CaO are less than 1.3%, except in samples that have suffered extensive calcite alteration.
The most obvious primary variation in the majorelement geochemical data is in the concentration of MgO. The most alumina-rich samples (Samples 149-897C-64R-5, 66-70 cm and -67R-3, 51-55 cm) originally contained relatively high concentrations of plagioclase and spinel, and these results confirm the relatively undepleted nature of some of the peridotites, as suggested by thin-section studies. Abundances of all major oxides, except Fe2O3, decrease during calcitization; the reason why Fe2O3 is not affected is not clear. Alteration effects dominate the analyses.
To help characterize both the primary chemical variation within the serpent mite and the chemical effects of varying amounts of carbonate alteration, a relatively large number of samples was analyzed for the elements Nb, Zr, Y, Ba, Rb, Sr, Cu, Zn, Cr, Ni, and V. Calcium carbonate concentrations and grain densities also were measured. The results for 66 samples, taken from Holes 897C and 897D, are presented in Table 10 and Table 11. Considerable care was taken to sample the core in a representative fashion to ensure that all the different primary and secondary facies were represented. Although the calcium carbonate alteration is the youngest of the processes which have modified the chemistry of the rocks, it is discussed first because it dominates, and is superimposed on, an initial relatively homogeneous ultramafic parental material.
The pervasive nature of the carbonate alteration is apparent from the results presented in Table 10 and Table 11, which shows that some "serpentinite" samples now contain more than 80% CaCO3. Results clearly show that this alteration is most pronounced in the upper part of Hole 897D and that the bottom 60 m is largely unmodified by carbonate alteration (Fig. 37).
Abundances of Sr and Ba, but not Rb, increase directly in proportion to the intensity of carbonate alteration. In contrast, the low abundances of refractory elements such as Nb, Y, and Zr are unmodified. The concentrations of the relatively abundant transition metals Cr and Ni were reduced with the introduction of carbonate. All these results are compatible with carbonate being introduced as a result of interaction with seawater at relatively low temperatures, a model supported by the preferential alteration of the upper parts of the serpentinite in both Holes 897C and 897D.
Abundances of trace elements in the sections of serpentinite not modified by carbonate alteration are typical of dunite to lherzolitic rocks from the seafloor and from ophiolite complexes. Abundances of the elements Ba, Nb, and Y generally are less than 5 ppm; abundances of Sr and Zr are about 10 ppm or less. Concentrations of the transition metals Cr and Ni correlate negatively; these reflect varying relative modal abundances of olivine and pyroxene in the original peridotite (Fig. 38).
To make quantitative statements about, and model, the relative degree of depletion and the extent of mantle melting involved in the formation of these rocks requires microprobe analyses and an extensive trace elements data set, which were not available on board the ship.
Only a few mantle outcrops at the ocean/continent boundary of passive margins are known at present. These are important windows into the deep lithosphere beneath a continental rift. Investigating these rocks may help to constrain the mechanisms of uplift, emplacement and exposure on the seafloor during crustal thinning.
The occurrence of mantle rocks at Site 897 demonstrates that the Galicia Bank peridotite ridge, drilled during Leg 103, is not an isolated occurrence, but a major feature of this margin along the ocean/ continent boundary. Geophysical and geologic data show that this ridge forms at least two segments that are offset eastward from north to south. The first segment extends continuously for more than 100 km along the Galicia Bank margin (Boillot, Girardeau, et al., 1988; Boillot, Comas, et al., 1988; Boillot et al., 1992). The southern segment continues for another 200 km beneath the sediments of the Iberia Abyssal Plain (Whitmarsh et al., 1990; Whitmarsh et al., 1993; Beslier et al., 1993). Preliminary Site 897 petrographic and mineralogical studies indicate that the mantle cores are plagioclase- and spinel-bearing peridotites. The presence of plagioclase rimming spinel in some of the rocks attests to late equilibration at low pressures (0.9-1 GPa 30 km below surface; Ringwood, 1975), near the transition between plagioclase- and spinel-peridotite stability fields. This is compatible with adiabatic decompression beneath a rift zone.
Another striking feature of Site 897 is the variety of rocks encountered, which range from plagioclase- and pyroxene-rich varieties to lherzolite or harzburgite and dunite. Although less than 150 m of basement was drilled at each hole, the succession of rock types is similar in both holes: the depleted rocks occur below the enriched ones. The high proportion of plagioclase, and its distribution within the plagioclase facies, suggest partial melting in the mantle and upward melt migration. The wide range of compositions is unusual for abyssal peridotite occurrences (Dick, 1989).
Eventually, geochemical and petrological studies of the Site 897 peridotites will give an estimate of the degree of melting that took place in the rocks and the genetic relations of the different facies. This will help to constrain the processes of mantle emplacement.
The Site 897 serpentinized peridotites typically display an equant primary texture, with a porphyroclastic, or locally mylonitic, texture limited to some fresh plagioclase- and pyroxene-rich facies. Two late deformation fabrics have been heterogeneously developed throughout the recovered section. The chronology of formation of the different features associated with this late deformation is not obvious. The features may have developed during the last rift-related deformation events at the margin, when the peridotites were already near, or at, the surface and already partially serpentinized (see "Structural Geology" section, this chapter).
The peridotites of the Galicia Bank margin were emplaced at the end of continental rifting and/or at the very beginning of oceanic accretion (Girardeau et al., 1988; Fraud et al., 1988; Boillot et al., 1989). These rocks were sampled by drilling during ODP Leg 103 (Site 637) and with the French submersible Nautile at five sites along the ridge (Boillot, Comas, et al., 1988). These rocks are serpentinized plagioclase-bearing harzburgite and lherzolite. During their ascent beneath the continental rift, they experienced limited melting (<10%) and high-temperature/low-stress ductile deformation, followed by intense mylonitization in a rotational regime accompanied by decreasing temperature (1000°-850°C) and increasing deviatoric stress (>102 MPa), thereby suggesting lithospheric conditions (Girardeau et al., 1988; Evans and Girardeau, 1988; Beslier et al., 1990). This mylonitization formed in a normal shear zone gently dipping toward the continent. Serpentinization occurred mainly below 300°C as a consequence of the introduction of seawater as the rocks were intensely fractured during a late brittle deformation (Agrinier et al., 1988).
The preliminary studies of the Site 897 peridotites allow for only preliminary comparison with the peridotites at Site 637. It is clear, however, that these peridotites display some similar features, with the occurrence of (1) plagioclase- and spinel-bearing peridotites; (2) textures suggestive of partial melting; (3) extensive late serpentinization; (4) intense late brittle deformation that formed fractures filled with calcite and/or serpentine, and local brecciation.
In contrast, the Galicia Bank margin peridotites of Site 637 differ from those of Site 897 in that no evidence can be observed at Site 897 of extensive intense high-temperature mylonitization. Instead, the Site 897 peridotites underwent a late shear deformation event, leading to the development of foliation and shear bands in the serpentinite and local brecciation of the serpentinite. This suggests that the structural evolution of the mantle rocks during their uplift differed at the two places. Similarly, the absence of high-temperature amphibole in the serpentinite at Site 897 suggests different retrometamorphic conditions. Other striking differences observed at Site 897 are (1) the higher proportion of plagioclase in the plagioclase peridotite and (2) the occurrence of dunite indicative of the wide variety of rocks types encountered in this site.
