Serpentinization Processes Research Articles

Min�raux serpentineux extraits de p�ridotites serpentinis�es des Alpes Occidentales

Complete physical and chemical data presented concern serpentine minerals choosen from epizonal to non metamorphic domains. It is tried to correlate well-studied serpentine minerals, generally coming from particular outcrops with the rock-forming ones known only through the microscope. It appears that ribbons filling serpentine veins in a serpentinized dunite, the whole mesh-structure of a non metamorphic serpentine, and bastite lamellae are made of lizardite. Antigorite, only serpentine mineral present in metamorphic domains, shows no significative difference between high-pressure facies (glaucophane schist) and low-pressure facies (green-schist). The data allow a better understanding of the serpentinization process. The geochemically antinomic behaviour of iron and magnesium is emphasized.

MAGNETIC PROPERTIES OF ULTRAMAFIC ROCKS FROM RED MOUNTAIN, CALIFORNIA

A representative ultramafic intrusion at Red Mountain southeast of San Francisco, California was sampled for the purpose of studying the remanent magnetism and susceptibility of ultramafic rocks as functions of serpentinization. Due to the dependence of grain density upon the degree of serpentinization, the former was taken as a measure of the latter. Grain densities were accurately determined from bulk densities by taking into account the porosity and amount of adsorbed water contained in these rocks. In general, the porosity was found to increase with serpentinization. The magnetic data suggest that the remanent magnetization is mainly a chemical remanent magnetization acquired during the process of serpentinization whereby iron atoms, released from the silicate structure of the paramagnetic olivine and pyroxene, are oxidized to form ferrimagnetic magnetite. The intensity of both remanent and induced magnetization increases exponentially with serpentinization. However, the remanent magnetization becomes highly unstable as the rock becomes intensely serpentinized. This is due to the growth of magnetite grains to multi‐domain size and their oxidation, in some cases,to maghemite. The intensity of magnetization also depends on mode of occurrence and state of oxidation of the magnetite, and on original rock composition. Serpentinized dunites were found tobe less magnetic than equally serpentinized peridotites, because the olivine in the dunites has a lower iron content than the olivine and pyroxene in the peridotites. Therefore, it is expected that some serpentinized ultramafic bodies will have highly variable magnetic anomalies associated with them, while others of dunitic composition will have low magnetic anomalies.

Magnetic characteristics of basalts and peridotites of rift zones

The magnetic properties of peridotites are caused by the magnetite which was generated in the process of serpentinization. In principle, the peridotites of the rift zone do not differ from continental ones (according to magnetic characteristics). This seems natural as magnetite has a secondary origin; i.e., it reflects the processes in the crust and not in the magma chamber. Thermomagnetic researches, conducted according to the methods worked out by the Paleomagnetic Research Group at the Institute of Physics of the Earth, have shown that the process of serpentinization within the rift zone developed at the temperature of 400°–500°C. The study of magnetic stability allows to assume that, since the time of serpentinization, the bottom relief in the region where samples were collected (i.e., relative depth of sample deposition) did not change. The basalts containing primary titanomagnetites i.e., those originated in a magma chamber, showed an extreme magnetic stability. The continental basalts with a titanomagnetite component do not reveal such stability. (For instance, parameter Q = In xH in the titanomagnetite from the continental basalts is 10–30; in hematite-containing basalts, it reaches 70, and in extraordinary cases it is 100, while in basalts of the rift zone it is 150). Magnetic stability can be caused to a considerable extent by sudden cooling of lava during underwater eruption. The small amount of samples did not allow to make conclusions whether the observed anomalously high parameters of the rift zone basalts, characterizing magnetic stability, can be wholly allotted to the conditions of lava eruptions. The second principal difference of the rift zone basalts is the low Curie points. The assumption of the results of research conducted by the Japanese geophysicists, would mean that the formation of magma has occurred under higher pressure than the continental basalts.

Abundances of Na, Mn, Cr, Sc and Co in ultramafic rocks

A total of 113 ultramafic rocks, including dunites, peridotites, pyroxenites, kimberlites, serpentinites and peridotite inclusions, has been analyzed for sodium, manganese, chromium, scandium and cobalt by a neutron activation technique. The overall average abundances determined are: 1040 ppm Na; 1040 ppm Mn; 2980 ppm Cr; 15.7 ppm Sc; and 110 ppm Co. Na, Mn and Co values are markedly lower than the average abundances of these elements in ultramafic rocks as reported in the geochemical literature. When the fresh ultramafic rocks are compared with the serpentinites, by means of frequency histograms, the abundance ranges and the approximate variances about the mean are found to be the same, within experimental error, for each element studied. Thus, the process of serpentinization has no marked effect on the abundances of Na, Mn, Cr, Sc and Co in ultramafic rocks. For those elements whose abundances are highly dependent on mineral composition, such as Na and Sc, it appears that the pre-serpentinization mineral composition rather than the process of serpentinization determines the elemental abundances in the serpentinite.

Petrological and geochemical evidence for convection in the Earth’s mantle

Various lines of evidence suggest that layer 3 of the oceanic material is partially serpentinized peridotite. Hess (1962) suggested that this layer originated by serpentinization of the upper part of the mantle, the necessary water being derived from the degassing of rising columns of mantle convection cells beneath the mid-ocean ridges. In this paper it is shown that support for this hypothesis can be derived from two entirely different lines of evidence. The first concerns the characters of volcanic lavas extruded in oceanic areas. Although these are, in general terms, basaltic, significant minor differences exist between basalts dredged from different parts of the ocean floor and between many dredged basalts and the lavas of oceanic islands. In particular high-alumina tholeiitic basalt of Warner type has been obtained from the more deeply submerged parts of the Mid-Atlantic Ridge. Experimental work has shown that, although alkali basalts and normal tholeiitic basalt could be derived by partial fusion of a dry mantle, the presence of water in the upper mantle is probably necessary for the genesis of Warner type basalts. The apparent restriction of Warner type basalts to the mid-ocean ridge areas suggests a similar restriction of the presence of water in the upper mantle and thus the restriction of the serpentinization process to these localized areas as suggested by Hess. The wide distribution of layer 3 in the oceanic crust must then imply some mechanism such as mantle convection currents for its dispersal from the areas of origin. The second concerns the contrast in composition between the total volatiles that have emerged from the Earth’s interior throughout geological time and the gases emerging from oceanic volcanoes at the present time. The state of oxidation of modern oceanic volcanic gases is very close to that predicted for gases in equilibrium with a Mg-Fe-silicate upper mantle, which suggests they have suffered no appreciable contamination on their way to the surface. They are, however, on a percentage basis, much richer in compounds of G, S and N and poorer in those of Cl and H20 than the total emergent volatiles. The amount of water which must have emerged from some other source than oceanic volcanoes in order to explain the total volatiles composition can be calculated to be 8500 to 12600 x 1020 g. On the basis of the Hess hypothesis the amount of water liberated by dehydration of serpentinized peridotite in convective down currents can also be calculated. It is 13 000 x 1020 g for a convective system moving at a rate of 1 cm/y. Two-channel escape of volatiles from the Earth’s interior should result in minor changes in the partial pressure of carbon dioxide in the atmosphere. These should be reflected in the geochemistry of sedimentary rocks. The agreement between deduced and observed effects is surprisingly good in view of the uncertainties in the data and supports the hypothesis of transfer of the serpentinized perioditite layer from areas of convective up-current in the ocean basins to those of down-currents under the continents or continental margins.

NATIVE METALS IN THE MUSKOX INTRUSION

The following native metals have been identified in the Muskox intrusion: native iron, native nickel–iron (awaruite), native cobalt–iron (wairauite), and native copper. Mineral distributions and textures indicate that the native metals formed more or less contemporaneously, during the period of serpentinization of the host dunites and related rocks.Conditions during serpentinization must have been more reducing in the central and lower parts of the layered series than in the margins and upper parts of the intrusion. This is indicated by the fact that most native metals are abundant in the central regions and are essentially lacking elsewhere, even in strongly serpentinized zones. This zoning suggests that reducing conditions may have been generated internally, possibly as a result of the serpentinization process itself. The composition of the primary olivine of forsterite80–88together with the presence of abundant secondary magnetite in equivalent serpentinites indicates that a redox reaction, olivine + water = serpentine + magnetite + hydrogen, contributed to the development of a progressively more reducing, or hydrogen-rich, fluid phase.Natural phase relations indicate that each native metal formed primarily in situ as a result of the decomposition of specific earlier formed minerals that had become unstable in the reducing environment. Native iron appears to have been formed by the reduction of magnetite; awaruite by the reduction of pentlandite; wairauite by the reduction of an unknown phase, possibly cobalt pentlandite or cobaltian pyrite; and native copper by the reduction of chalcopyrite. The feasibility of most of these reactions was confirmed by experimental studies carried out in systems open to moist hydrogen.