Ultramafic Liquids Research Articles

Peridotites and other ultramafic rocks

Peridotites are a group within the category of ultramafic rocks. These are usually dark‐coloured rocks rich in magnesium, poor in silica and lacking feldspars. Earth's mantle, which makes up 83 percent by volume and 67 percent by weight of the planet, consists largely of peridotite, which, although relatively sparse at the surface, therefore, is Earth's most abundant rock type. Unfortunately, the mantle, which lies on average at >7 km under the oceans and >35 km under the continents, is not directly accessible for observation and our information comes largely from seismic studies, inclusions in volcanic rocks and sections of the ocean floor which have been tectonically emplaced into the crust: the ophiolite complexes and abyssal peridotites of the oceans. Such rocks also occur when mafic minerals, such as olivine, pyroxene and spinel, accumulate in magma chambers. An unusual occurrence of ultramafic rocks is the lava called komatiite, representing ultramafic liquids, which are largely restricted to Precambrian environments.

Incompatibility of argon during magma ocean crystallization

We report results from multi-anvil (MA) and laser-heated diamond anvil cell (LH-DAC) experiments that synthesize high-pressure phases, including bridgmanite, ferropericlase, stishovite, and ultramafic liquid, in the presence of an argon-rich fluid. The goal of the experiments is to constrain the equilibrium distribution of argon in magma ocean environments. Argon concentrations in LH-DAC experiments were quantified by electron microprobe analysis, while argon concentrations in MA experiments were quantified by laser-ablation mass spectrometry and electron microprobe analysis. Our LH-DAC experiments demonstrate that argon solubility in ultramafic liquid is near or above 1.5 wt.% at conditions between 13–101 GPa and 2300–6300 K. Argon concentrations in bridgmanite and ferropericlase synthesized in LH-DAC experiments range from below detection to 0.58 wt.%. Argon concentrations in bridgmanite and ferropericlase synthesized in MA experiments range from below detection to 2.16 wt.% for electron microprobe measurements and laser-ablation measurements. We interpret this wide range of argon concentrations in minerals to reflect the variable presence of argon-rich fluid inclusions in analytical volumes. Our analyses therefore provide upper limit constraints for argon solubility in high-pressure minerals (<0.015 wt.%) across all mantle pressures and temperatures. The combination of relatively high argon solubility in ultramafic liquid (∼1.5 wt.%) and low argon solubility in minerals implies argon incompatibility (Dbridgmanite−meltAr < 0.01, Dferropericlase−meltAr < 0.01) during magma ocean crystallization and that the initial distribution of argon, and likely other neutral species, may be controlled by liquids trapped in a crystallizing magma ocean. We thus predict a basal magma ocean would be enriched in noble gases relative to other regions of the mantle. Moreover, we predict that the noble gas parent-daughter ratio of magma ocean cumulates pile will increase with crystallization, assuming refractory and incompatible behavior for parent elements.

Energy Balance From a Mantle Pseudotachylyte, Balmuccia, Italy

AbstractIn the Balmuccia massif (NW Italy), a pseudotachylyte vein network (N068 trending) in a spinel lherzolite is interpreted as the product of frictional melting during a single Mw &gt; 6 earthquake. The subvertical fault underwent a metric dextral coseismic displacement, raking 60°SW. The average width of the main slip surface is ~ 5 mm. A dense network of thin (20–200 μm) injection and ultramylonite‐like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing ≈ 1% of structurally bound H2O. In the ultramylonite‐like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2–2 μm) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250°C during deformation and suggesting fast recrystallization from the frictional melt. The veins also exhibit pyroxene and recrystallized spinel, which proves that the earthquake occurred at a minimum depth of 40 km. The energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding (i.e., dynamic friction coefficient ≪ 0.1). Because of the low viscosity of slightly hydrated ultramafic liquids (≈ 1 Pa·s), we argue that lubrication was only transient, as the melt could rapidly flow into tensile fractures, which led to rapid cooling and promoted strength recovery and sliding arrest. Combined together, our observations suggest that this pseudotachylyte is the frozen record of a deep (&gt;40 km) earthquake of 6 &lt; Mw &lt; 7. Its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite‐like veins and deciphered by electron backscattered diffraction.

Recently discovered 3.42-3.23 Ga impact layers, Barberton Belt, South Africa: 3.8 Ga detrital zircons, Archean impact history, and tectonic implications

The Barberton greenstone belt (BGB) includes eight known layers containing spherical particles (spherules) that condensed from rock vapor clouds formed by the impact of large meteorites or asteroids 3.47–3.23 Ga. Previous studies have inferred that the spherules represent bolides at least 20–70 km across. Spherule beds S1–S4 have been previously characterized in detail: we provide here the first detailed analysis of more recently discovered beds S5–S8. All eight beds are composed of the same basic compositional and textural spherule types, including nearly pure silica spherules representing nonaluminous melt precursors, nearly pure phyllosilicate spherules representing mostly mafic and ultramafic liquids, and compositionally mixed spherules. Evidence of spherule amalgamation and surface corrosion within the rock vapor clouds is developed in some beds. Bed S6, which occurs within a thick sequence of ultramafic volcanic rocks, is overlain by a tsunami layer containing zircons as old as 3811 ± 7 Ma, suggesting deep, possibly impact-related crustal uplift and erosion in distant areas. The formation of at least 8 major impact layers representing bolides 20–70 km in diameter over an interval of ∼240 m.y. suggests impact rates greatly exceeding those of later geologic time, and provides direct evidence that terrestrial bombardment by large bolides did not end abruptly at 3.8 Ga, but waned gradually until 3.0 Ga or even later. The coincidence of at least four large impact layers and the initiation of BGB deformation at 3.26–3.23 Ga suggests that an impact cluster at this time may have disrupted a long-lived earlier geodynamic system and triggered the development of a contrasting, more modern plate tectonic regime.

A geochemical evaluation of potential magma ocean dynamics using a parameterized model for perovskite crystallization

Magnesium perovskite (MgPv) is likely the first phase to crystallize from a deep magma ocean. Consequently, MgPv crystallization has a strong control on the dynamics and chemical evolution associated with the earliest stages of silicate Earth differentiation. In order to better understand the chemical evolution associated with MgPv crystallization during a magma ocean, a parameterized model for major and trace element partitioning by MgPv has been developed. The parameterization is based on a compilation of published experimental data and is applied to batch and near-fractional crystallization scenarios of ultramafic liquids, allowing for a more complete analysis of the geochemical implications for magma ocean crystallization. The chemical signatures associated with modeled MgPv fractionation are evaluated in the context of possible dynamical outcomes to a magma ocean (e.g. basal magma ocean (BMO) or crystal settling). It is shown that fractionating MgPv from ultramafic liquids imparts diagnostic signatures (e.g. Ca/Al, HFSE anomalies, ε176Hf–ε143Nd) in both the liquid and solid phases. These signatures are not currently observed in the accessible Earth, suggesting that either early-fractionating MgPv was subsequently homogenized or crystal suspension was dominant during the earliest stages of magma ocean crystallization. A BMO that fractionates CaPv and MgPv is also considered and shown to mute many of unobserved geochemical effects associated with a MgPv-only fractionation, offering an alternative possibility for the evolution of a BMO depleted in heat producing elements.

Water and the compressibility of silicate glasses: A Brillouin spectroscopic study

The compressional and shear wave velocities of seven series of hydrous aluminosilicate glasses were measured by Brillouin scattering at room temperature and pressure as a function of water content. The glasses were quenched from high temperature and 0.2 or 0.3 GPa pressure. The dry end-members range from highly polymerized albitic and granitic compositions, to depolymerized synthetic analogues of mantle-derived melts. For each set of glasses, the adiabatic shear and bulk moduli have been calculated from the measured sound velocities and densities. These moduli are linear functions of water content up to 5 wt% H 2 O, the highest concentration investigated, indicating that both are independent of water speciation in all series. For anhydrous glasses, (∂V/∂P) S changes from about -0.3 to -0.7 × 10 −15 m 3 /(Pa·mol) with increasing degree of polymerization. For the same compositions, the partial molar (∂V/∂P) S of the water component changes from near zero to -0.8 × 10 −15 m 3 /(Pa⋅mol) with increasing degree of polymerization, such that dissolved water amplifies the differences in rigidity between the anhydrous glasses. This strong variation indicates that the solubility mechanisms of water depend strongly on silicate composition. A simple linear model reproduces (∂V/∂P) S to within measurement uncertainty, with glasses becoming more compressible as NBO/T decreases and as alkalinity increases. The dissolved water component also becomes more compressible as NBO/T decreases, but less compressible as alkalinity increases. In strongly depolymerized structures, water causes an increase in sound velocities even though it induces a decrease in density. The general rule according to which sound velocities correlate with density is, therefore, violated, for in these cases water dissolution results in a decrease of compressibility. Barring any strongly anomalous effect of water on configurational compressibility, the trends observed for glasses should be still more divergent for melts. Consequently, hydrous mafic and ultramafic liquids are predicted to have a very low compressibility. The shear modulus of hydrous glasses varies little with bulk composition or water content, supporting the use of a single composition-independent pre-exponential factor in models of melt viscosity.

Geochemistry and mineralogy of the Platreef and “Critical Zone” of the northern lobe of the Bushveld Complex, South Africa: implications for Bushveld stratigraphy and the development of PGE mineralisation

The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration. This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone.

Intrusive calcite-carbonatite occurrence from Mt. Vulture volcano, southern Italy

AbstractIntrusive calcite-carbonatite ejecta (sovite) in the lowermost tephra layers of the Mt. Vulture alkaline mafic-ultramafic volcano (Upper Pleistocene), is the first intrusive carbonatite sample from southern Europe. The sovite is of coarse granularity and shows some textural and mineralogical layering. It is mainly formed of calcite (up to 3.5 wt.% MgO, and 0.53 wt.% SrO), less dolomite (average 18.2 wt.% MgO, and up to 2.1 wt.% SrO), spinel (60 wt.% Al2O3, 26.5 wt.% MgO, 10.7 wt.% FeO) and olivine (Fo99). Perovskite and apatite have been found only as microlites. Intergranular vugs are scattered throughout the rocks and small composite inclusions occur in calcite.The mineral chemistry, high temperature melt inclusions, overall isotopic compositions, and the REE distribution are consistent with a primary igneous origin. Compared with world average sovite compositions, the Vulture sovite has lower LILE and HFSE but Rb, Sr, Th and U are high. The REE abundance is typical of carbonatites, having an LREE/HREE value of ∼100. The δ13C (−4.8% SMOW) is in the range for mantle-derived carbonatites. The 143Nd/144Nd (0.512648±15) and 87Sr/86Sr (0.705978±10) ratios show close similarity between the sovite and the Vulture alkaline mafic rocks. The sovite ejecta are interpreted as a crystallization product of carbonate ultramafic liquid. In common with many shallow-level carbonatites from other localities, the recrystallization of rather pure Mg-calcite, the presence of dissolution vugs and the depletion of some HFSE and the relatively high δ18O values, suggest that a secondary process, such as hydrothermal leaching, affected the rock.

Geochemical Study of Ultramafic Volcanic and Plutonic Rocks from Gorgona Island, Colombia: the Plumbing System of an Oceanic Plateau

The only known post-Archaean komatiites are found on Gorgona, INTRODUCTION a small island off the Colombian coast that forms part of the Surprisingly little is known about the internal structure Caribbean oceanic plateau. Mafic and ultramafic intrusions are and magmatic evolution of oceanic plateaux. This is due located in the interior of the island. To establish the relationship in part to the relative inaccessibility of type examples between intrusive and extrusive phases of ultramafic magmatism, such as Ontong Java and other plateaux in the Pacific, and to help understand how an oceanic plateau is constructed, we which are located far from continents, in deep waters, undertook the first petrological and geochemical study of the intrusive and often beneath a deep layer of sediments. Samples rocks. Rare earth element patterns in gabbros range from almost flat obtained by dredging or drilling the submerged portions, to moderately depleted; in dunites and wehrlites, the depletion is or through field work on the few sections exposed on more pronounced. These patterns fall midway in the range measured islands around the Ontong Java plateau consist prein Gorgona volcanics, whose compositions vary from slightly enriched dominantly of tholeiitic basalt with relatively uniform, to extremely depleted. Nd isotope compositions indicate two distinct magmatically evolved compositions (Mahoney et al., 1993; mantle sources, one highly depleted, the other less depleted. MgO Neal et al., 1997). These compositions are far removed contents of parental liquids are estimated from olivine compositions from those of the primary picritic magmas that formed at 20–25% in ultramafic lavas, and 12–13% in the intrusives. through partial melting of the mantle source at subPetrographic observations and similarities in trace-element contents lithosphere depths (Cox, 1980). The generation of basalt indicate that the two magma types are comagmatic, related through from picrite involves crystallization of large volumes olivine fractionation. Modelling of major and trace elements indicates of olivine, which presumably accumulated in magma that the primary ultramafic magmas formed by advanced critical chambers at deeper levels in the plateau. It is commonly melting at high pressure in a rising mantle plume. The plumbing assumed that these chambers are located at the base of system that fed the Gorgona plateau was complex, being characterized the crust where magma would pool as it passes from by a series of magma chambers at different crustal levels. Mantlederived ultramafic liquids either travelled directly to the surface to dense mantle peridotite to less dense basaltic rocks. This erupt as komatiite flows, or were trapped in magma chambers where assumption is supported by geophysical data—layers with they differentiated into basaltic liquid and mafic to ultramafic high seismic velocities in the lower parts of oceanic cumulates. Gorgona gabbros and peridotites formed in shallow-level plateaux are commonly equated with olivine-rich cuexamples of these intrusions. mulates (Coffin & Eldholm, 1994; Leroy & Mauffret, 1996; Mauffret & Leroy, 1997)—and from numerical modelling. Farnetani et al. (1996) have shown that the

Metal–silicate partitioning and the incompatibility of S and Si during core formation

We have determined the partitioning of a number of siderophile and lithophile elements between liquid metal and liquid silicate phases under the high temperature, low pressure conditions at which core segregation is considered to have originated [2]. Oxygen fugacity was varied from 2 to 6 log units below the iron–wüstite (IW) buffer by increasing the Si content of the metallic phase from 100 ppm up to the 8% proposed by Allègre et al. [8] as being cosmochemically required in the earth's core. We find that Si and S, two of the principal candidates for the light element in the core are mutually exclusive in the metal phase. As oxygen fugacity is reduced, Si becomes increasingly siderophile and S increasingly lithophile. At 8% Si in the metal, all S, up to at least 1%, enters the coexisting ultramafic liquid. Thus, Si and S are incompatible during a single-stage low pressure core formation event. Mn and Cr, which are lithophile under oxidising conditions, become slightly siderophile if there is 8% Si in the metal, corresponding to an oxygen fugacity of about 6 log units below the iron–wüstite (IW) buffer. Ti remains lithophile even under these conditions, only becoming siderophile at about 7 log units below IW. These results demonstrate that the Mn and Cr contents of the core proposed in [8], coupled with lithophile behaviour of Ti, are consistent with core segregation under strongly reducing conditions. Such conditions could not generate a core containing 2% S, however, and the problem of the high contents of oxidised Fe, Ni, Co and other siderophile elements in the mantle would remain. The only plausible way of introducing S into the core is to add it as part of a late-stage accretion of oxidised material to the earth. A second, minor phase of core segregation under oxidising conditions would supply S, but no additional Si, to the core.

Activities of nickel, cobalt, and manganese silicates in magmatic liquids and applications to olivine/liquid and to silicate/metal partitioning

The activities of NiSi 0.5O 2, CoSi 0.5O 2, and MnSi 0.5O 2 in magmatic liquids have been calibrated from experimentally determined distributions of Ni, Co, and Mn between olivine and silicate melt. Chemical potentials of these species in silicate liquids are extracted from an expression for the chemical potentials of minor components in olivine and are described by an extension of the regular solution model of Ghiorso and Sack (1994). The extended liquid mixing model, together with the mixing model for multicomponent olivine, retrieves olivine/liquid D M ol liq for Ni, Co, and Mn with average absolute values of deviations equal to 14%, 13%, and 14%, respectively. Partitioning of Ca between olivine and liquid, using previously calibrated chemical potentials of CaMgSiO 4 in olivine and CaSiO 3 in silicate melt, is reproduced with an average relative deviation of 18%. Extrapolation of these models successfully predicts experimentally determined partitioning of Ni and Co between olivine and ultramafic liquids to > 2000°C and 100 kbar, thereby casting doubt on the importance of pressure-induced coordination changes to the geochemistry of Ni and Co under these conditions. Calculated partitioning of Ni and Co between silicate liquid and molten metal indicates that these elements remain markedly siderophile to temperatures greater than 3000 K. Calculations suggest that the siderophile behavior of Ni and Co may be reduced by increasing pressure. Forward modeling of phase equilibria and Ni partitioning during crystallization of mafic melts from Hawaii suggests that the Ni/MgO systematics of Hawaiian basaltic glasses are consistent with a parental liquid having 17% MgO and 760 ppm Ni. Activity coefficients of NiSi 0.5O 2, CoSi 0.5O 2, and MnSi 0.5O 2 in natural magmas are generally greater than unity, owing in a large part to positive energetic interactions between transition metals and SiO 2. The primary determining compositional variable for these liquid activity coefficients and partition coefficients is molar silica content. The effects of changing temperature and liquid composition on olivine/ liquid partitioning of Ni, Co, and Mn are comparable and complementary, leading to more compatible behavior at lower temperatures and in more evolved liquids. Activity coefficients are greatest for NiSi 0.5O 2, and smaller for CoSi 0.5O 2 and MnSi 0.5O 2. Compositional variation of mineral/liquid partitioning and of activity coefficients owing to interactions with SiO 2 are likely to be important for many other trace elements.

Constraints on the origin of mantle-derived low Ca garnets

Current hypotheses for the source rock of low Ca garnets hosted in mantle-derived diamonds and xenoliths range from residues of komatiite generation, to subducted serpentinite, to subducted mid-ocean ridge (MORB) harzburgite. Experiments designed to test these hypotheses were undertaken. The stability and compositional variation of garnets at pressures above 4 GPa through the melting interval of hydrous peridotite, in the subsolidus of depleted harzburgite and peridotite compositions, and along the liquidus of aluminium-undepleted and aluminium-depleted komatiites were examined, and compared with petrological data for natural low Ca garnets. Partitioning of Cr between garnet and ultramafic liquid along the liquidus of komatiites and within the melting interval of peridotite, indicates that garnets in mantle residues after single stage Archean ultramafic liquid removal would contain 2 to 4 wt% Cr2O3. Thus, the more Cr-poor population of mantle-derived low Ca garnets, with Cr2O3 less than 4 wt%, could have originated by such a process. Experimental results for other compositions indicate that average cratonic peridotite or its hydrated equivalent is typically too Cr-poor to be the protolith from which low Ca garnets containing greater than 4 wt% Cr2O3 could have crystallized in the upper mantle. Experiments on a spinel harzburgite composition indicate that an extremely Cr-rich protolith (Cr/Cr+Al>0.3) is required to crystallize spinel and Cr-rich low Ca garnets, at pressures deduced for the ultramafic inclusion suite in diamonds (5 to 7 GPa). Natural examples of such Cr-rich protoliths are represented in some ophiolite harzburgites. All the experimental data taken together require that low Ca garnets with greater than 4 wt% Cr2O3 originated from residues that underwent multiple melt extraction. Whether such multi-stage events formed protoliths for low Ca garnets at shallow (i.e. MORB source region) or deep (i.e. komatiite source region) levels in the Precambrian mantle is not completely resolvable. The former environment can better account for the abundance of spinel in many diamonds hosting low Ca garnets, but the latter scenario best explains the presence of low Ca garnets in harzburgite xenoliths with ‘cratonic’ bulk compositions well removed from typical MORB residues.

Element partitioning between majorite and liquid: A secondary ion mass spectrometric study

The secondary ion mass spectrometry (SIMS) was applied to measure the trace element abundances in majorite and the ultramafic liquid equilibrating at high pressure and temperature for the first time, and was proved to be useful for the high pressure experimental petrology. The analysis by SIMS was made to determine the abundance of 15 lithophile elements (Li, Be, B, Sc, Ti, V, Cr, Mn, Y, Zr, La, Sm, Gd, Yb, and Hf), one siderophile (Co), and two chalcophile elements (Cu, Zn). The electron microprobe (EPMA) analysis was also made to obtain the major element abundance. Some lithophile elements (Al, Be, and V) are enriched in majorite, whereas the lithophile elements (Li, B, Na, K, Ca, Ti, Zr, Y, La, Sm, Gd, and Hf), and siderophile and chalcophile elements (Co, Cu, Zn) are favored in the liquid. These results give important constraint for the genesis of komatiites and chemical heterogeneity in the Archean mantle.

Evidence for liquid immiscibility in the upper mantle

Liquid immiscibility, the separation of magmas into two or more immiscible liquid phases, has been shown to occur both experimentally1–3 and in nature4,5, but this phenomenon has received only limited attention in modelling magmatic processes, as known cases of liquid immiscibility are generally restricted to magma compositions encountered during late stages of crystal fractionation, and relatively shallow crustal levels. Here I report the results of a study of trapped quenched liquids in spinel Iherzolite xenoliths, these are fragments of upper mantle material carried to the surface in volcanic rocks. These xenoliths carry quenched coexisting basaltic, carbonatitic and volatile-rich ultramafic liquids, showing the existence of a previously unknown field of three-liquid immiscibility. This type of immiscibility occurs in primitive volatile-rich magma compositions at upper mantle to crustal levels. Furthermore, metasomatic growth of amphibole ± apatite ± phlogopite in the Iherzolites is associated with introduction of carbonatitic and volatile-rich ultramafic liquids, indicating that such liquids may play an important role in mantle metasomatism.

The geochemistry of mafic and ultramafic from the Archaean greenstone belts of Sierra Leone

AbstractThe Archaean (c. 2800 Ma) ultramafic rocks in eastern Sierra Leone cut basalt lavas and are mostly olivine-rich cumulates either iron-rich (Fo85–86) and derived from a basaltic or picritic parent, or more magnesian (Fo92–93) derived from an ultramafic melt with c. 18–25 wt. % MgO. In central Sierra Leone the ultramafic rocks are lavas predating tholeiitic basalts.The basalts show a wide variation in Zr/Y, suggesting that garnet was present in the source region of some of these rocks but not others. This implies that melting took place at different depths in the mantle. The REE evidence for basaltic rocks in the upper part of the Nimini belt succession suggests that they were derived from a mantle source region which had already suffered melt extraction. Ti/Zr ratios in the basaltic rocks are also variable and individual belts define different trends on a Ti vs. Zr plot implying that the basaltic rocks evolved in geographically distinct magma chambers. It is likely that the basaltic rocks evolved from a parental liquid with Ti/Zr = 90 via shallow level crystal fractionation. The source region for these rocks therefore had a lower than chondritic Ti/Zr.There are two possible models for the basaltic and ultramafic magmas in the Sierra Leone greenstone belts. First that the ultramafic and basaltic liquids were derived from mantle diapirs of differing size, but originating in the same region of the mantle. Ultramafic liquids were produced in small diapirs, which store large melt fractions, and basaltic liquids in larger diapirs which segregate larger melt fractions. A second model is based upon the double diffusion process suggested for magma chambers at mid-ocean ridges and involves a transient magma chamber from which basalts, derived from parental ultramafic liquids, are erupted, with ultramafic liquids rising directly to the surface when the magma chamber is frozen. The available data does not discriminate between these two models.

Ultramafic flows of the Crixas greenstone belt, Goias — Brazil

Abstract The Crixas greenstone belt, together with the Guarinos and Pilar belts, has a basal, intermediate and an upper unit consisting of extrusive and intrusive ultramafic, mafic and sedimentary rocks, respectively. The Crixas belt, the main object of this study, has a basal unit formed of peridotitic and pyroxenitic lava flows intercalated with iron formation, chert, graphite schist, mafic and acid tuffs, and intrusive bodies of pyroxenite and gabbro. The ultramafic unit becomes thicker towards the south end of this belt and it is there that the flows are best exposed, particularly along the Alagadinho creek, where detailed mapping was carried out. Three types of ultramafic flows were recognized: flows with olivine spinifex texture in the central zone, flows with polyhedral jointing at the base, clinopyroxene spinifex texture towards the top and flows without spinifex, but showing polyhedral jointing throughout. The flows are composed of olivine, clinopyroxene, glass and opaques. Greenschistfacies metamorphism, together with deformation, have obliterated part of the primary volcanic textures and structures in these rocks, and partially changed olivine to serpentine and chlorite, clinopyroxene to tremolite, and glass to a very fine chlorite. Preliminary chemical work suggests that the ultramafic rocks are komatiites, characterised by MgO > 20%, CaO/Al 2 O 3 > 1 and low TiO 2 , Na 2 O and K 2 O; these compositions coupled with the presence of spinifex texture indicate an origin from an ultramafic liquid. The flows from Crixas represent the first recognition of spinifex-textured komatiites in the central-western part of Brazil.

Iridium and palladium as discriminants of volcanic-exhalative, hydrothermal, and magmatic nickel sulfide mineralization

Neutron activation of Pd, Au, and Ir in samples from (1) economically significant Ni sulfide deposits of Western Australia, (2) the host rocks of these deposits, (3) hydrothermal and/or metasomatic Ni sulfide (mainly millerite) mineralization in peridotite at Nullagine, Western Australia, (4) hydrothermal replacement (volcanic-exhalative?) Ni-Cu sulfide mineralization at Sherlock Bay, Western Australia, and (5) volcanic-exhalative sulfides in inter flow sediments at Kambalda are used to show that Ir and Pd can be used to discriminate between the various origins (magmatic, volcanic-exhalative, hydrothermal/metasomatic, and metamorphic) ascribed to Ni sulfide deposits.All the economically significant Ni sulfide deposits in Western Australia are strongly enriched in the precious metals relative to other crustal rocks and have similar Pd/Ni and Ir/Ni ratios to each other. These ratios persist regardless of whether the nickel sulfides occur at the base of volcanic ultramafic piles (e.g., Kambalda) or as centrally disposed interstitial sulfides in intrusive dunite bodies. The strong similarities of the Pd/Ir ratios of the Ni sulfide ores and of the spinilex-textured komatiites, which represent ultramafic liquids from which the Ni sulfide liquids were derived, together with other features of the precious metal geochemistry of the ores and host rocks of the Ni sulfide deposits, are used to show that the deposits had magmatic origins.In contrast, the Pd/Ir and Ir/Ni ratios of the hydrothermal and/or metasomatic Ni sulfide mineralization at Nullagine are distinctly different from those of the major Ni deposits. The former contains high Au, moderate but variable Pd, and exceptionally low Ir. Similarly, the volcanic-exhalative sulfides at Kambalda and Sherlock Bay contain negligible quantities of Pd and Ir. All the Ir in both the interflow sediments and the hydrothermal metasomatic mineralization is contributed by the host rocks and not by the sulfides. The Western Australian data, together with precious metal data for other examples of hydrothermal Ni-Cu sulfide mineralization, strongly suggest that although Ni, Au, and Pd may be transported in aqueous solution, Ir cannot. Hence, the relative abundance of Ir can be used as a discriminant of magmatic sulfides.

Geochemistry of some ultramafic komatiite lava flows from the Barberton Mountain Land, South Africa

Thin ultramafic flows from the Komati Formation in the Barberton Greenstone Belt show striking textural variations similar to those found in some ultramafic flows from Munro Township, Ontario and Western Australia. The spinifex textured A2 zone has been divided into an A2r zone consisting of short randomly oriented olivine blades with vesicles in the interblade areas and an A2p zone made up of booklets of olivine blades forming a ‘thermotactic’ spinifex texture. The composition of the A2r zone is considered to be representative of the initial ultramafic liquid, while the variation in the composition of the A2p zone reflects the evolving residual liquid composition as the flow crystallized. Detailed modelling of major and trace element data obtained from a 3.1 m thick flow demonstrates that olivine fractionation and accumulation after extrusion probably generated the observed compositional range (25.7–31.7 wt.% MgO). The Ni content of olivine cores analysed from the B zone of this flow are consistent with these crystals having existed in equilibrium with a liquid composition represented by the random spinifex textured A2r zone. A DCr (Cr % in olivine/Cr % in ‘liquid’) of 0.5 found for these olivine crystals suggests that the high Cr content of olivines from Archaean ultramafic lavas is due to the very high Cr content of the liquids from which they crystallized. The textural and chemical data are combined to develop a cooling and crystallization model for this flow.

Picrite basalts and related lavas from the Deccan Traps of Western India

Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO.

Archean Ultramafic Flows in Munro Township, Ontario

Research Article| March 01, 1973 Archean Ultramafic Flows in Munro Township, Ontario D. R. PYKE; D. R. PYKE 1Ontario Division of Mines, Geological Branch, Whitney Block, Queen's Park, Toronto, Ontario, Canada Search for other works by this author on: GSW Google Scholar A. J. NALDRETT; A. J. NALDRETT 2Department of Geology, University of Toronto, Toronto, Ontario, Canada Search for other works by this author on: GSW Google Scholar O. R. ECKSTRAND O. R. ECKSTRAND 3Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (1973) 84 (3): 955–978. https://doi.org/10.1130/0016-7606(1973)84<955:AUFIMT>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. R. PYKE, A. J. NALDRETT, O. R. ECKSTRAND; Archean Ultramafic Flows in Munro Township, Ontario. GSA Bulletin 1973;; 84 (3): 955–978. doi: https://doi.org/10.1130/0016-7606(1973)84<955:AUFIMT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A unique exposure within the Abitibi orogenic belt of northeastern Ontario provides an exceptionally clear view of early Precambrian (Archean) ultramafic volcanic rocks. Approximately 60 ultramafic flow units are exposed over a stratigraphic thickness of 125 m. Individual flow units range in thickness from 0.5 to 15 m, average about 3 m, and display remarkable strike lengths and internal structure, which suggest extreme fluidity.The upper margin of the flow units (0.1 to 1.0 m) is thicker than the lower margin (about 1 cm), and the upper margin is highly fractured, thereby providing an easily recognizable top determinator. Many flow units contain a zone of spinifex, ranging in width from zero to 5 m, which underlies and is gradational with the flow top. Spinifex is a textural term to describe an array of criss-crossing sheafs or booklets characterized by numerous closely spaced and parallel blade- or plate-like crystals of olivine. The texture is generally accepted as having formed by rapid cooling of a crystal-free ultramafic liquid in situ. The variation in size of bladed olivine crystals within a spinifex zone is always from coarse (up to 1 m long) at the bottom to fine at the top, thereby providing a primary top determination. In flow units not containing a spinifex zone the chilled and fractured flow top grades downward into a zone of moderately foliated peridotite which generally constitutes the remaining part of a flow unit.Chemically, the flow units resemble similar ultramafic rocks described from northeastern Ontario, western Australia, and the Barberton area of South Africa; one unique feature is the exceptionally high alumina content of the clinopyroxene This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.