The serpentinite mud recovered during Legs 125 and 195 consists mainly of clay- and silt-sized particles of lizardite, chrysotile, brucite, metabasic particles, and carbonate crystals (Shipboard Scientific Party, 1990; Savov et al., this volume), but suspended within the serpentinite mud matrix and supported by it are clasts of various rock types, including dominantly serpentinized peridotites with minor altered or metabasic rocks (Fryer et al., 1990, 2000; Fryer, 1992; Parkinson et al., 1992; Johnson, 1992; Saboda et al., 1992; Maekawa, et al., 1993, 1995; Shipboard Scientific Party, 2002). The same diversity of rocks has been recovered by dredging, which has also recovered rare fragments of chert (Hussong and Fryer, 1982; Bloomer and Hawkins, 1983; Bloomer et al., 1995; Fryer et al., 1985; Fryer and Fryer, 1987; Johnson et al., 1991), by submersible dives (Fryer et al., 1990, 1995), and by gravity and piston coring (Fryer et al., 1999). Similar rocks have been recovered on the Izu-Bonin forearc in dredges from several locations (Ishii, 1985).
The harzburgite recovered during Leg 125 from a Mariana serpentinite mud volcano (Conical Seamount) is variably serpentinized with 40%–100% replacement of original minerals (Fryer et al., 1990; Saboda et al., 1992). The clasts consist of ~76% harzburgite, ~16% dunite, and 8% metamorphosed mafic rocks (Ishii et al., 1992; Johnson, 1992). Generally, the peridotites preserve enough of the original textures to permit estimates of original mineralogy. The harzburgite was 70%–90% olivine, 5%–30% orthopyroxene, and <1% clinopyroxene (mostly as exsolution lamellae in orthopyroxene) and trace spinel (Fryer et al., 1990; Saboda et al., 1992). The serpentine forms mesh texture (after olivine) and occurs as bastitic replacement of pyroxene. Microgranulation of olivine, kink banding of pyroxene, elongation of spinel, and deformation of clinopyroxene exsolution lamellae in orthopyroxene indicate deformation of the clasts (Saboda et al., 1992). Girardeau and Lagabrielle (1992) examined petrofabrics of the serpentinized peridotites from Conical Seamount to study the formation and deformation history. After the initial formation of the peridotites under high-temperature asthenospheric conditions, they were remelted and invaded by fluids during a period of high-temperature/low-stress deformation that occurred either before subduction began or during the initial stages of subduction and proto-arc formation (Fryer, 1992; Girardeau and Lagabrielle, 1992). This sequence of preserpentinization events is similar to that observed in serpentinized peridotites exposed at the margins of rifted continents, in exposures along amagmatic slow-spreading mid-ocean ridges and at transform faults or deep abyssal troughs (see summary in Fryer, 2002). A phase of protracted retrograde metamorphism, including serpentinization, accompanied by the formation of ductile shear zones was then followed by a period of brittle deformation that overprints all earlier structures (Girardeau and Lagabrielle, 1992). The significant difference between the serpentinite found in these various locations is that the trace element compositions and isotope signatures of the serpentinites of the Izu-Bonin and Mariana forearcs and the pore fluids from the active Mariana mud volcanoes show a clear signature of the suprasubduction-zone environment.
Hard rock clasts from Hole 1200A consist predominantly of heavily serpentinized and tectonized harzburgite (64%) and dunites (28%), lherzolite (4%), and metabasic schists (4%) (D'Antonio and Kristensen, 2004). The metabasic fragments are generally less than a few millimeters in diameter and include glaucophane schist, chlorite schist, white mica schist, crossite/white mica/chlorite schist, and amphibolite schists (Shipboard Scientific Party, 2002; Gharib et al., 2002). The observed primary mineral phases are olivine: 0%–35%, orthopyroxene: 0%–35%, clinopyroxene: 0%–5%, and Cr spinel: 1%–3% (Savov et al., this volume). Savov et al. (this volume) report the bulk chemical analyses of harzburgite clasts and serpentinite mud from Sites 778, 779, and 780 on Conical Seamount and Site 1200 on South Chamorro Seamount summit. On both seamounts the depleted harzburgite protolith comprises ~80% magnesian olivine (Fo92) and 20% orthopyroxene, with very small contents (0%–2% each) of clinopyroxene and chromian spinel (see also Ishii et al., 1992; Parkinson et al., 1992; Shipboard Scientific Party, 2002). No large clasts of metabasic schist were recovered. The percent of the matrix material that was estimated aboard ship to be metabasic was ~10%, which is similar to shipboard estimates of the >1-mm fraction of the sieved muds. Clasts of serpentinized peridotite material are less abundant in Leg 195 Holes 1200D, 1200E, and 1200F, where the dominant lithology is serpentinite mud. The degree of serpentinization of the dunites varies between ~67% and 100%, whereas that of the harzburgites varies between ~40% and 100%.
Detailed descriptions of the serpentinized peridotites are given in D'Antonio and Kristensen (2004). Microtextures of the South Chamorro Seamount peridotites show a similar sequence of deformation and serpentinization events to those described for Conical Seamount samples by Girardeau and Lagabrielle (1992), suggesting that the southern suprasubduction-zone mantle has undergone a similar magmatic/metamorphic/tectonic history as the central part. The less serpentinized peridotites contain relict olivine and enstatite and minor Cr spinel and diopside, similar to the phases identified at Conical Seamount. Serpentine and brucite with minor magnetite are the main secondary minerals. The most abundant serpentine phases are lizardite and chrysotile, and they have large compositional variations depending on the composition of the primary minerals from which the serpentine was derived. D'Antonio and Kristensen (2004) note that both the serpentine minerals and brucite exhibit wide Mg, Fe, and Mn substitutions, which permit them to estimate an upper temperature limit of serpentinization (200°–300°C) that is in accord with thermal models for the region and with geochemical estimates based on fluid-rock interactions (Mottl et al., 2004).
Parkinson et al. (1992) studied the compositions of clinopyroxenes in the Leg 125 peridotites and showed that the serpentinized peridotites are enriched in Sr, Ce, Zr, and Nd and somewhat enriched in Sm and Eu compared with abyssal peridotites, which suggests that the enrichment took place after the initial melting event of the protolith. Based on a forward modeling technique in which Ti and the heavy REEs were treated as incompatible elements not enriched in the peridotites and assuming 25% partial melting, the authors showed large enrichments in Rb, Nb, La, Sr, Ce, and Nd and some enrichment in Zr, Sm, and Eu in comparison with abyssal peridotites after partial melting (their fig. 12). The trace element concentrations suggest to them a multistage melting and enrichment history for the original peridotites. This involved 10%–15% melting of a normal mid-ocean-ridge basalt (N-MORB) source that produces a depleted spinel lherzolite, which is then enriched by an infusion of slab-derived constituents and then further melted by 10%–15%. They make the caveat, however, that in their attempt to compare the Leg 125 serpentinized peridotites with peridotites from ophiolite sequences interpreted to have formed in suprasubduction-zone environments, they see a wide range in patterns of enrichment. This observation led them to conclude, "if suprasubduction-zone ophiolites do represent a forearc setting, it is possible that enrichment and depletion events in this setting are highly variable and may reflect the residence time of the mantle in question in the forearc" (Parkinson et al., 1992).
Savov et al. (2002, this volume) reached a similar conclusion for the Site 1200 peridotites. They note that the presence of high bulk rock Mg numbers, high normative olivine, and low normative clinopyroxene indicate that the ultramafic protolith suffered a high degree of melt extraction, with dunites experiencing >20% extraction and harzburgites between 10% and 25%. These values are consistent with the conclusions of D'Antonio and Kristensen (2004), based on the high clinopyroxene (cpx) Mg numbers (up to 95.6) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) REE measurements revealing highly depleted cpx light REE (LREE) patterns, the peridotite clasts suffered 15% to 20% melt extraction before serpentinization. They suggested the melting started beneath the arc and backarc basin under garnet-facies conditions and continued during the upwelling of the asthenosphere to shallow, spinel-facies levels. The elevated Sr isotope values relative to the depleted mantle are consistent with this view and with the depleted mantle- and chondrite-normalized elemental patterns (Savov et al., this volume). Similar degrees of melt extraction were proposed for Conical Seamount by Parkinson et al. (1992) and Parkinson and Pearce (1998), and the South Chamorro seamount samples overlap the range noted by those authors. As noted for the Conical Seamount samples, the peridotites were then fluxed with LREE-enriched fluids or melts (Savov et al., this volume), presumably from a subducting slab.