Showing papers on "Peridotite published in 1976"

Carbonation and melting reactions in the system CaO–MgO–SiO2–CO2 at mantle pressures with geophysical and petrological applications

Abstract: Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO2-CO2 with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO2 on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO2, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO2 also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO2 melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO2-CO2 permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO2 subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO2-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO2 cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO2 bubbles in peridotite nodules do not represent free CO2 in mantle peridotite along normal geotherms. (4) CO2 is as effective as H2O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO2 at depths shallower than 80 km produces basic magmas, becoming more SiO2-undersaturated with depth. (6) The solubility of CO2 in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO2 at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO2-bearing SiO2-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.

Viscosities of basalt and andesite melts at high pressures

Abstract: The viscosities of melts of Kilauea 1921 olivine tholeiite and Crater Lake calc-alkaline andesite have been determined with a falling sphere method at superliquidus temperatures from 7.5- to 30-kbar pressure. Spheres of chromian diopside and of platinum were used for the tholeiite and andesite melts, respectively, and graphite capsules 10 mm long in which the temperature gradient was less than 15° were used as containers. The viscosity of the Kilauea, Hawaii, olivine tholeiite melt decreases significantly with increasing temperature at constant pressure (e.g., by a factor of 1.5 per 25°C at 15 and 20 kbar) but decreases only slightly with increasing pressure at constant temperature. Consequently, the viscosity of the tholeiite melt decreases with increasing pressure along its anhydrous liquidus, 40, 25, and 8 P at 15, 20, and 30 kbar, respectively. The viscosity of the Crater Lake, Oregon, andesite melt also decreases with increasing pressure along its anhydrous liquidus, about 3200 and 900 P at 7.5 and 20 kbar, respectively. The viscosity of the andesite melt with 4 wt % H2O, measured in a sealed Pt capsule, is lower by a factor of about 20 than that of the anhydrous melt at the same temperature and pressure. The results of the present experiments are applicable to the problems of crystal settling at high pressures, ascent of magma in the upper mantle, and separation of magma from its source region. Settling rates of garnet and clinopyroxene crystals 2 mm in diameter in basaltic (or eclogitic) magma at 30 kbar should be approximately 3 and 2 m/h, respectively. It is estimated that magmas carrying peridotite xenoliths 10 cm in diameter would have to rise from a depth of 50 km to the surface in less than 60 hours. Because of the decrease in viscosity with depth, magmas are probably more easily separated from deeper source regions in the upper mantle. It is suggested that alkali basalt magmas are probably less viscous than tholeiitic magmas under upper mantle conditions.

Sections of the Earth's crust in the equatorial Atlantic

Abstract: Crustal sections, in some cases exceeding 5 km in thickness, are exposed where the Mid-Atlantic Ridge is offset by major equatorial fracture zones Systematic sampling of some of these sections has resulted in the recovery of rocks from over 100 sites, especially concentrated on the Vema and Romanche fracture zones The vertical and lateral distribution of the various rock types (ie, basalts, metabasalts, a variety of gabbros and metagabbros including rodingites and amphibolites, serpentinized peridotites, metaserpentinites, serpentinite and basaltic breccias, sedimentary serpentinites, and various mylonites and sedimentary rocks) has been used to construct models of the structure of fracture zones It is suggested that sections of ‘normal’ oceanic crust can be exposed in fracture zones, as well as sections of ‘anomalous’ crust produced by processes occurring solely within fracture zones The northern wall of the Vema fracture valley is probably an exposed section of normal oceanic crust, as suggested by several lines of evidence, including the petrology of basalts recovered from it One of the inferences drawn from the distribution of rocks in this section is that serpentinites are probably a significant component of the oceanic crust, being emplaced as mantle-derived vertical intrusions in deep fault zones parallel to ridge axis The southern margin of the Vema fracture valley, as well as both margins of the Romanche fracture, is the locus of prominent positive topographic anomalies (transverse ridges) caused by processes of crustal generation restricted to fracture zones These processes involve (1) crustal uplift related to diapiric intrusions into the fracture zones of mantle-derived serpentinized peridotite, (2) intense tectonization of the rock units, (3) minor alkali basalt volcanism, and (4) hydrothermal activity and related metallogenesis

The physical properties of oceanic basement rocks from deep drilling on the Mid-Atlantic Ridge

Abstract: The physical properties of compressional and shear velocity, electrical resistivity, bulk and grain density, porosity and water content, and thermal conductivity have been measured on a large collection of basalt samples and a few gabbros and serpentinized peridotites recovered by drilling on the mid-Atlantic ridge near 37°N. Samples were recovered to 582 m into basaltic basement in one hole, and rocks that may be representative of the lower crust were recovered from another hole. The mean compressional velocity of the basalts of 5.94 km s−1 agrees well with previous fresh unweathered sea floor basalts. It, however, can be reconciled with the much lower average upper crustal layer 2 seismic refraction velocities generally observed only by the presence of extensive large-scale fracturing and voids in the crust. The laboratory velocities of the gabbros are consistent with upper layer 3, or oceanic layer, refraction velocities. Lower layer 3 refraction velocities are consistent with a composition of a mix of gabbro and unserpentinized peridotite. Poisson's ratio for little-weathered laboratory samples of basalt is found to decrease systematically with decreasing velocity. The mean electrical resistivity of the basalts is 220 ohm m at 25°C. A close dependence of resistivity on porosity and a moderate increase in resistivity with pressure suggest conduction primarily through pore fluid. The basalt resistivity decreases rapidly with increasing temperature, the mean becoming about 20 ohm m at 100°C and about 5 ohm m at 150°C. The mean basalt bulk density is 2.795 g cm−3, the mean porosity is 7.8%, and the mean thermal conductivity at 21°C is 3.97 mcal cm−1 s−1 °C−1.

Densities of fertile and sterile garnet peridotites

Abstract: Densities of a fertile peridotite (PHN 1611) and a depleted sterile peridotite (PHN 1569) have been calculated to be 3.39 and 3.30, respectively. These densities were calculated from the cell dimensions of the component minerals, the mineral analyses, and the bulk rock analyses. Studies of kimberlite nodules suggest that lighter, sterile peridotites overlie heavier, more fertile peridotites uniformly in the upper mantle beneath southern Africa. A temperature difference of 500°C reduces the density of fertile peridotite PHN 1569 to approximately 3.33, which is insufficient to cause it to float in sterile peridotite. However, if fertile peridotite PHN 1569 undergoes 25% partial melting and the garnet is dissolved, its zero pressure density would be reduced to approximately 3.2, which is less than that of depleted, sterile mantle.

Granitic magmas: possible and impossible sources, water contents, and crystallization sequences

Abstract: The calc-alkalic rocks of batholiths or their precursors may be generated in deep continental crust, in subducted oceanic crust, in the mantle wedge above, or in processes involving material from all three sources. For the series gabbro–tonalite–granite, we have phase relationships with excess H_2O to 35 kbar (3500 MPa), and the H_2O-undersaturated liquidus surfaces mapped with contours for H2O contents and with fields for near-liquidus minerals. Isobaric diagrams with low H_2O contents provide grids potentially useful in defining limits for the H_2O content of magmas, based on the sequence of crystallization. Conclusions from the experimental framework include: (1) The H_2O content of large granitic bodies is less than 1.5%. (2) Primary granite magmas can not be derived from the mantle or subducted ocean crust. (3) Primary granite magmas with low H_2O content are generated in the crust, and erupted as rhyolites. (4) Primary tonalite and andesite are not generated from mantle peridotite; the H_2O contents required are unrealistically high. (5) Primary tonalite and andesite are not generated in the crust unless temperatures are significantly higher than those of regional metamorphism. (6) Subducted ocean crust yields magmas with intermediate SiO_2 content, but not primary tonalite and andesite. (7) Batholiths are produced from crustal rocks as a normal consequence of regional metamorphism, with the formation of H_2O-undersaturated granite liquid and mobilized migmatites. Some batholiths receive in addition contributions of material and heat from mantle and subducted ocean crust.

Does CO2 cause partial melting in the low-velocity layer of the mantle?

Abstract: Recent analyses of fluid inclusions in peridotite minerals suggest that CO 2 is a dominant volatile species in the upper mantle. In a CO 2 -bearing oceanic mantle, the low-velocity zone (LVZ) can be explained by a large decrease in the peridotite solidus temperature at a depth of about 90 km, causing melting by intersection with a geotherm. This decrease in the solidus temperature has been found in the system CaO-MgO-SiO 2 -CO 2 and results from a change in partial melt composition along the solidus from enstatite-normative at pressures less than 26 kb to larnite-normative (melilititic) at greater pressure. Although these liquids dissolve up to 20 wt percent CO 2 , they are silicate liquids containing at least 30 percent SiO 2 . These silica levels are appropriate for kimberlitic liquids, but the liquids are more calcic than typical kimberlites. At depths of less than 90 km in suboceanic mantle, CO 2 may be present in carbonate minerals or in vapor, depending upon the geotherm, but cannot be in solution in silicate peridotite minerals. Beneath continents, CO 2 will be present in carbonate minerals, and the mantle will not melt at least to depths of 120 km.

Geology and geochemistry of the alkali basalt—andesite association of Grenada, Lesser Antilles island arc

Abstract: Basanitoids and alkalic basalts that are strongly undersaturated in silica occur on the island of Grenada in the Lesser Antilles. Several volcanic centers have erupted basic lava of these compositions together with subalkalic basalt, andesite, and dacite from Miocene to Holocene time. The volcanic rocks overlie a folded volcanic-sedimentary formation of Eocene to Miocene age. Tuff rings and maars of explosive origin are present. Andesite and dacite are less significant volumetrically on Grenada in comparison with other islands in the Lesser Antilles. The variable trace-element geochemistry of the basanitoids and alkalic basalts is related, on the basis of rare-earth-element data, to a model of variable degrees of partial melting of an upper-mantle garnet peridotite source. It is suggested that fractional crystallization of olivine, clinopyroxene, and spinel, observed as the phenocryst assemblage in the basanitoids and alkalic basalts, takes place at high temperatures; at lower temperatures, these phenocrysts are joined by amphibole and plagioclase. A trend toward increased silica saturation is the result of this fractional crystallization process. The presence of alkalic lava rocks together with variable trace-element abundances and Sr isotope ratios are unusual features of the volcanicity.

Geochemistry and origin of basalt of the Columbia River Group, Oregon and Washington

Abstract: Strontium isotope and major- and trace-element data from the voluminous middle Miocene tholeiitic basalts of the Columbia River Group differentiate the lavas stratigraphically and reflect the origin of the basalt. The Picture Gorge Basalt has relatively low Sr 87 /Sr 86 ratios (0.7035 to 0.7039) and a major- and trace-element composition consistent with an origin from the upper mantle. The Picture Gorge Basalt is clearly distinguished from the Imnaha basalt in northeast Oregon and from the widespread, mainly younger Yakima Basalt. The Yakima Basalt, including lava flows in the Vantage, Grande Ronde, and Imnaha regions, forms a relatively coherent geochemical group with moderate to high initial Sr 87 /Sr 86 ratios (0.7045 to 0.7080), which increase with decreasing age of the lava and show an inverse relation with Sr abundance. Possible explanations include progressive contamination of parental mantle—derived magmas by crustal material or derivation from an inhomogeneous upper mantle. It is suggested that the Yakima Basalt formed during a major readjustment episode related to plate motions in an extensional tectonic regime. This caused diapiric upwelling of upper mantle peridotite with formation of great volumes of olivine tholeiite magma, which underwent extensive olivine fractionation. The Columbia Plateau may be a continental analogue of the marginal seas of the western Pacific Ocean.

Petrogenesis of McKinney (Snake River) olivine tholeiite in light of rare-earth element and Cr/Ni distributions

Abstract: Samples of McKinney Basalt, including a pillow glass, are characterized by light rare-earth element–enriched abundance patterns with small positive Eu anomalies (relative to chondrites) and high Cr/Ni ratios. In particular, the positive Eu anomaly observed in the pillow glass is considered to be characteristic of the McKinney parental magma. These compositional features are apparently inconsistent with derivation of McKinney magma by partial fusion of garnet- or plagioclase-bearing Iherzolite or clinopyroxenite or by crystal fractionation of likely liquidus phases at high or low pressure. Rather, partial melting calculations show that fusion of spinel peridotite or aluminous clinopyroxene peridotite can yield liquids with the rare-earth element patterns and Cr/Ni ratios of McKinney Basalt. An essential feature of the favored models is a relatively large contribution of clinopyroxene to the melt. This result suggests an origin of McKinney magma by fusion of mantle Iherzolite at pressures between the stability fields of plagioclase peridotite (low P ) and garnet peridotite or garnet clinopyroxenite (high P ), that is, at depths within the continental lithosphere in this region.

High CO2 solubilities in mantle magmas

Abstract: Phase diagrams for the assemblage forsterite+enstatite+CO_2 in MgO-SiO_2-CO_2 show that a subsolidus carbonation reaction intersects the solidus near 44 kb–1530°C and stabilizes the carbonate molecule in the liquid, causing CO_2 solubility to increase from about 5 to 10 wt percent to about 40 wt percent. A similar increase occurs in the system CaO-MgO-SiO_2-CO_2 near 25 kb–1200°C for the liquid coexisting with forsterite+orthopyroxene+ clinopyroxene. The CO_2 solubility in diopside and enstatite liquids remains relatively low, because these are not involved in carbonation reactions at this pressure. This accounts for contrasted CO_2 solubilities in silicate melts reported in recent experimental studies. Magmas generated in a CO_2-bearing mantle below a depth of 80 km may contain up to 40 wt percent dissolved CO_2. CO_2-rich, SiO_2-undersaturated magmas can coexist with mantle peridotite through a wide temperature range. These could represent either primary carbonatite and kimberlite magmas or the carbonated alkali ultrabasic magmas cited by many petrologists as parents for the derivation of continental volcanic and plutonic associations of highly alkalic rocks.

Diamonds in an upper mantle peridotite nodule from kimberlite in southern wyoming.

Abstract: Diamonds in a serpentinized garnet peridotite nodule from a diatreme in southern Wyoming are the first known occurrence in an upper mantle peridotite xenolith from a kimberlite intrusion in North America as well as the second authenticated occurrence of diamonds from kimberlite pipes in North America. The nodule is believed to have come from a section of depleted (partially melted) lherzolite at a depth of 130 to 180 kilometers.

Major, minor, and trace element compositions of peridotitic and basaltic komatiites from the precambrian crust of Southern Africa

Abstract: Major and trace element compositional data are reported for nine mafic and ultramafic rock samples from the Barberton greenstone belt. Rocks from this province are among the oldest fragments of the Earth's crust (∼3.5 b.y.). The data are consistent with an oceanic crust related origin for these rocks. The high abundances of Ni in these samples make their origin by fractional crystallization of a primitive magma unlikely but are consistent with their generation by partial melting of an upper mantle source. The basaltic samples from the Komati formation can be related by small degrees of partial melting of a primitive upper mantle source to the peridotitic komatiite which probably derived from much more extensive partial melting of a similar source. REE and especially Ni abundances limit the proportion of olivine that is permitted in the residue.

Ultrabasic rocks of Rhum and Skye: the nature of the parent magma

Abstract: Rocks from the peridotite dykes of southern Skye (Gibb 1968) are virtually identical to those found in the large layered intrusions of Rhum and Skye, suggesting that they may have been formed in a similar manner. The rocks of the layered intrusions are generally regarded as having formed by the gravitative accumulation of the early precipitating phases from a basaltic magma, usually with subsequent extensive ad-cumulus growth. Such a mechanism is totally inapplicable to the dykes and it can be established that they were formed by flow differentiation of a suspension of olivine in an eucritic liquid which subsequently underwent near-equilibrium crystallization. The possibility therefore arises that the layered intrusions originated from an ultrabasic magma similar to that which formed the dykes. Consideration of this possibility suggests that the petrogenetic models proposed for the layered ultrabasic intrusions of Rhum and Sgurr Dubh (Skye) (Brown 1956, Wadsworth 1961, Weedon 1965, Wager & Brown 1968) be slightly modified: in particular, that the parental magma was not a basaltic liquid but rather a suspension of olivine in a eucritic liquid. Such a modification is consistent with the features of the layered intrusions and appears to overcome several problems posed by the basaltic liquid hypothesis.

Platinum metals in the Stillwater Complex, Montana

Abstract: Platinum-group metals show a wide variation in content and relative proportions in the rocks of the Basal, Ultramafic, Banded, and Upper zones of the stratiform mafic and ultramafic Stillwater Complex, southwestern Montana. Reported values range from the limits of determination to approximately 8 ppm Pt, 11 ppm Pd, 1.7 ppm Rh, 0.5 ppm Ir, 1.0 ppm Ru, and 0.1 ppm Au as analyzed by fire assay-spectrochemical and neutron activation techniques. The highest concentrations occur within particular horizons of the Basal zone; in the chromitites of the Peridotite member of the Ultramafic zone, in pegmatitic bronzitites in the Bronzitite member of the Ultramafic zone, in a specific horizon in the Banded zone, and in disseminated sulfide horizons in the Upper zone. These occurrences represent manifold enrichment over the estimated levels of about 12 ppb Pt, 55 ppb Pd, and <5 ppb Rh in the parent basaltic magma(s) of the complex. The presence of platinum-iron alloy, isoferroplatinum, platinian rhodium, rhodian platinum, palladian gold, cooperite, laurite, braggite, vysotskite, moncheite, kotulskite, merenskyite, sperrylite, and stibiopalladinite has been confirmed by various investigators. The variations of Pt, Pd, and Rh content with stratigraphic position in the Basal zone and in individual cumulate units of the Peridotite member show repetitive, cyclic patterns similar to the patterns exhibited by the grain size, modal proportions, whole-rock compositions, and compositions of individual silicate minerals. Correlation between chemical and physical properties of the cumulates and the platinum-group metal distributions suggests that the distribution patterns may be ascribed to oscillation of physical conditions or composition of the parent basaltic magma, or to oscillation of both.

Na, P, Ti and coordination of Si in garnet from peridotite and eclogite xenoliths

Abstract: THEORY and experiment show that some cations are surrounded by extra oxygen anions at elevated pressure: for example, Si changes from 4-coordination in quartz and coesite to 6-coordination in stishovite. Sobolev and Lavrent'lev1 suggested that substitution of Na in garnet derived from the mantle is coupled with 6-coordinated Si according to the equation CaVIII+AlVI→NaVIII+SiVI. We show here from sensitive electron-microprobe analyses that Na in garnets from ultramafic and eclogite xenoliths can be explained by coupled substitutions with Ti and P without invoking 6-coordinated Si.

Structure of the Vulcan Peak alpine-type peridotite, southwestern Oregon

Abstract: The Vulcan Peak alpine-type peridotite forms part of the Josephine ultramafic complex in the Klamath Mountains geologic province. The peridotite is a partly serpentinized highly deformed harzburgite-dunite complex, in which three episodes of high-temperature plastic deformation are recognized. The first deformation was the most intense and produced the dominant metamorphic foliation and scattered folds in crosscutting layers and in the foliation itself. The first deformation seems also to have produced an olivine fabric in which X is normal to the foliation. The second deformation superposed a similar and pervasive fabric on the first, in which X olivine is normal to a spotty weak subvertical north-striking foliation that crosscuts the first foliation. These olivine fabrics are analogous to fabrics produced experimentally by either gliding or syntectonic recrystallization at temperatures in the range 1000° to 1200°C. This temperature range agrees with the temperatures of formation calculated from the distribution of Mg and Fe in mineral pairs. The third deformation was characterized by a limited plasticity, in which deformation was restricted to scattered narrow northeast-striking subvertical plastic shear zones. The sense of movement on the shear zones is consistently down on the northwest. A homotactic olivine fabric is present in the shear zones, consisting of a strong Z-point maximum approximately parallel to the zone. This fabric suggests glide on the system {Ok1} [100], which has been produced experimentally in the temperature range 800° to 1000°C. After the high-temperature and presumably deep-seated plastic deformation, the relatively cold peridotite was thrust, probably in post-Middle Jurassic time, northward against a complex of igneous and high-grade metamorphic rocks. Later, probably in Late Cretaceous or Tertiary time, the peridotite and the complex were thrust together westward against the low-grade Dothan Formation.

Mantle composition derived from the chemistry of ultramafic lavas

Abstract: ESTIMATES of the composition of the Earth's mantle have been based on both geochemical and geophysical studies (refs 1–3). As might be expected, there are some discrepancies between the estimates particularly for Na2O and TiO2. Such discrepancies in part arise from uncertainties concerning the degrees of melting which many volcanics represent and, possibly, the restricted tectonic settings of the ultramafic nodules studied. High MgO basalts and ultramafic lavas, some of which have been termed komatiites4 occur among the Archaean volcanic rocks. They represent exceptionally high degrees of melting (up to 50% or more) of peridotite source regions. Potentially such volcanics may be used to make a precise estimate of the composition of their mantle source region. High MgO lavas are well preserved in the Archaean Belingwe greenstone belt of southern Rhodesia, where they form part of a sequence deposited unconformably on granitic crust5. We assume that the occurrence of pillow lavas in this sequence containing both skeletal olivine spinifex (Fo92.5 in a rock with 28% MgO, E. G. Nisbet, M. J. Bickle and A. Martin, unpublished) and skeletal olivine microphenocrysts indicates that these particular lavas have a composition close to that of the liquid.

Reaction trends shown by chrome-spinels of the Rhum layered intrusion

Abstract: Chrome-spinels in the stratiform complex of Rhum, Scotland, have compositions that lie along two reaction trends, with the composition of a given crystal being determined by its local environment. Crystals in chrome-spinel rich seams between allivalite and peridotite, or disseminated in adcumulates of allivalite or peridotite, have compositions defining a reaction trend involving a marked change in the Cr/Al ratio and little variation in Fe3+ (Al-trend). Chrome-spinel crystals in cumulates that contain crystallized trapped liquid, define a reaction trend involving a marked change in Fe3+ (Fe-trend). Evidence is presented to show that the Al-trend results from reaction of cumulus chromite, cumulus olivine and either plagioclase or a liquid rich in the plagioclase component, whereas the Fe-trend results from reaction of only cumulus chromite and trapped liquid. Some of the implications of these findings are discussed.

The nature of chrysotile asbestos occurrences in southern Africa; a review

Abstract: The principal chrysotile asbestos occurrences in Rhodesia, South Africa, and Swaziland are described, emphasis being placed on the regional geological settings and host rock stratigraphy of the mineralized areas. All the more important asbestos ore deposits in southern Africa are Archean in age ( approximately 2.5-3.5 b.y. old), being associated with ultramafic complexes occurring either as sill-like bodies in greenstone belts or as later cross-cutting intrusions.The ultramafic bodies can be grouped into three varieties. These, in order of decreasing age, are: (1) the layered complexes associated with basaltic and peridotitic komatiite extrusives and forming part of the Lower Ultramafic Unit of southern African greenstone belts, (2) layered ultramafic bodies associated with the intermediate to acid volcanic rocks that constitute part of the Mafic-to-Felsic Unit of greenstone belts, and (3) ultramafic intrusive bodies that postdate the greenstone belts but which are still affected by Archean tectonic disturbances that arise from the emplacement of granites.All the principal asbestos-bearing complexes show magmatic segregation into layered, often cyclically repetitive, differentiation sequences. These single or multicyclic sequences may consist of two or more of the following rock types: dunite, peridotite, harzburgite, Iherzolite, wehrlite, bronzitite-enstatolite, websterite, gabbro, norite, and gabbroic anorthosite. Where fractional crystallization of the ultramafic magma has been most efficient, many layers at, or near, the base of the complexes comprise monomineralic cumulate phases. These commonly consist of dunites and Mg-rich orthopyroxenites. With increasing distance from the base, progressive Mg depletion and Fe enrichment of the successive layers take place. Although chrysotile asbestos may commonly be encountered in all serpentinized ultramafic rock types, optimum development of economically exploitable fiber generally occurs in dunites, peridotites, or harzburgites.The Archean layered complexes were derived from magma of ultramafic composition in contrast to magmas of tholeiitic parentage that gave rise to the great stratiform intrusions like the Bushveld, Stillwater, and many others, including the Great Dyke in Rhodesia. The Great Dyke, unlike the others, however, acts as host to small chrysotile deposits developed in serpentinized dunites or harzburgites.In addition to the asbestos mineralization found in the layered complexes, subordinate deposits, occurring in serpentinized dolomitic rocks associated with the approximately 2.0-b.y.-old Transvaal Supergroup, are briefly described.Whereas faulting and fracturing is generally acknowledged as being largely responsible for the local development of asbestos fiber, examples from the southern African green-stone belts demonstrate that folding is often a dominant regional controlling factor in the localization of asbestos mineralization in ultramafic rocks.