In Leg 149 ultramafic rocks, metasomatic hydrous fluids have left mineralogical imprints that imply a polyphase hydrothermal history:
Apart from the kaersutite of Sample 149-897D-14R-4, 95-100 cm, whose growth is related to an interaction high temperature (>800°C) with a magma (see Cornen et al., this volume, chapter 21), all the amphiboles from serpentinites plot in the tremolite field, along the typical line connecting tremolite and pargasite end members. These amphiboles are different from those of the flaser gabbros and the amphibolites, which are either in pargasitic field or defined a steeper trend between the hornblende and the tremolite fields. As in Hole 637A peridotites, some of these tremolite formed by the hydration of clinopyroxene (Fig. 3), but they differ by their chemical composition (low AlIV content systematically, (Fig. 2) which suggests they formed under greenschist facies conditions (<500°C, Evans, 1982; Jenkins, 1983). In contrast to Hole 637A peridotites, there is no evidence of amphiboles formed at higher temperatures by interactions with hydrothermal fluids.
Although several deformation episodes affected these peridotites (Beslier et al., this volume), only tremolites in ribbons are clearly associated with a deformation event. These tremolites were formed by the penetration of hydrous fluids within high permeability zones. The other tremolites, which occur in pockets as pseudomorphs of clinopyroxene, have identical compositions and are contemporaries. The crystallization of the tremolites also predates the extensive cold complex fracturing that occurred during the serpentinization. We interpret these tremolitic amphiboles to result from the penetration of hydrothermal fluids during the uplift of the peridotites. The question of whether the hydrothermal fluid was seawater or seawater-derived fluids or neither remains equivocal.
Chlorites formed in two different situations: (1) those that syncrystallized with amphiboles as pseudomorphs of clinopyroxene and orthopyroxene (which we concluded were formed contemporaneously with amphiboles at temperatures below 500°C); and (2) those that grew after spinel. Their relative high degree of Tschermak substitution indicates conditions of metamorphism corresponding at least to greenschist facies and temperatures close to 500°C.
During this high-temperature hydrous episode, there is potential for the serpentinization of olivine since olivine is not stable below 620°C in the presence of water. Meanwhile, the case of Zabargad Island, where peridotites are completely devoid of serpentine but do contain greenschist hornblendes and actinolites (Agrinier et al., 1993), demonstrates that hydrous episodes do not necessarily imply serpentinization. However, in the case of serpentinization, according to the serpentine phase diagram (Evans et al., 1976; Caruso and Chernovsky, 1979; Chernovsky et al., 1988), antigorite should form from olivine and talc from orthopyroxene in these conditions, while formation of chrysotile and lizardite is only possible at extremely low pressures (<2 kbar) and temperatures (<250°C).
The serpentines are predominantly lizardite, and, as shown by their 18O/16O compositions and the oxygen isotope fractionation between lizardite and magnetite, they were formed by low-temperature hydration. This hydration stage was accompanied by a complex cold fractionation associated with the emplacement of peridotites on the seafloor (Beslier et al., this volume). Apart from a single occurrence of chrysotile in a vein, which probably reflects higher temperatures of serpentinization (possibly around 300°C), we have no evidence for high-temperature (300°-400°C) serpentinization, since neither talc nor antigorite have been detected in Site 897 and 899 peridotites. Coexisting hydroxides, iowaite, and brucite confirm that the seawater-serpentinized peridotite interaction has occurred at very low temperatures and is probably still occurring. These serpentinizing conditions of the peridotites are similar to those described for the Hole 637A peridotites.
The calcites, which precipitated locally in veins and impregnate the upper parts of the serpentinized peridotites, formed at low temperature and result from circulating cold seawater within open cracks of the serpentinized peridotites. The same aspect is observed in Hole 637A peridotites.
The lack of high-temperature serpentine minerals that would form by the hydration of the Leg 149 peridotites at 5 km depth below the top of the sediment-free basement is puzzling, considering that the extent of high-temperature serpentinization of the peridotites at depth must be large—around 25% according to Christensen (1972)—to produce the decrease in Vp from 8.1 to 7 km/s.
Tremolites and chlorites are too few (less than 5%) to produce such a large decrease in Vp. Nor can this Vp decrease be due to formation of lizardite from olivine because lizardite is not stable at pressure higher than 2 kbar, according to the serpentine phase diagram (Evans et al., 1976). In the present state of the rift this lithostatic pressure is reached at 6 km below sea level. This constraint is not compatible with the serpentinization by deep penetration of cold seawater to the deep low-velocity zone (now at a depth of 10 km below sea level; Boillot et al., 1980; Whitmarsh et al, 1993) where lithostatic pressure 2.4 kbar) tops the upper stability limit of chrysotile and lizardite.
As mentioned above, Al-rich lizardite is stable to pressure and temperature conditions (greenschist facies) that are expected at 10 km depth. Accordingly, we may suggest that the S horizon is made of Al-rich lizardites formed by bastitization of orthopyroxene. Although the abundance of (bastitized) orthopyroxene is large enough, up to 20% in the Leg 149 peridotites, to account for the Vp decrease, this possibility does not explain the absence of antigorite, which should form correlatively according to the serpentine phase diagram.
Finally, we think that three explanations can be made for this absence in the Leg 149 peridotites:
1. The high-temperature phases (talc, antigorite) formed at 10 km depth but were retrogressed to low-temperature phases (lizardite) when the peridotites reached their present seafloor position. If so, since these phases are not observed, this back-reaction process must have recrystallized the original high-temperature phases totally and readjusted the oxygen isotope compositions. This complete recrystallization is not supported by two facts. First, the Hole 899B peridotites, in which the low-temperature serpentinization overprinted the peridotites much less intensively, do not show either these of high-temperature phases. One would expect the Hole 899B peridotites to preserve at least some evidence of this presumed high-temperature serpentinization. Second, unless complete dissolution-precipitation processes affect the entire high-temperature serpentines during the back reaction, it is very difficult at low temperatures (<200°C) to reset the 18O of the serpentines from the low values compatible with the high-temperature serpentinization (like that of singular vein chrysotile) to the high values compatible with the low-temperature serpentinization. Experimental studies (O'Neil and Kharaka, 1976) and geological evidence (Yeh and Savin, 1976) show that the rate of oxygen exchange between clay minerals and water is extremely slow at low temperatures (<200°C) and that the antigorite to lizardite phase transformation alone is unlikely to readjust significantly the 18O of the serpentines.
2. The Leg 149 peridotites did not record a high-temperature serpentinization episode because the high-temperature phases (talc, antigorite) did not form. Most likely, the serpentinization conditions were not met at that time but were later reached when the peridotites were set near seafloor position. As said above, the Zabargad Island peridotites exemplify such a possibility. And as far as the Leg 149 peridotites are concerned in determining the nature of the deep low-velocity zone, this possibility suggests that it would not be made of serpentines.