The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydro... more The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydrous tonalitic melts has been studied experimentally at 1.8 GPa and 1000–1040 °C. The knowledge of trace element partitioning between minerals and coexisting tonalitic melts is essential for geochemical models in order to explain the genesis of Archean tonalite-trondhjemite-granodiorite suites (TTGs). Our experiments were performed on a natural tonalite doped with two different trace element mixtures and with TiO2 to stabilize titanates. Trace element concentrations of crystals and coexisting melts were measured by laser ablation microprobe. The partition coefficients (D's) are independent of the trace element concentration over the concentration ranges used. The partition coefficients reported in this paper are a considerable extension of the few available data sets in intermediate to silicic systems. The D-values mostly agree well with previously published data and with predictive models. The garnet partition coefficients show steeper patterns with higher D's for heavy REE, lower D's for highly incompatible elements and lower La/Yb-ratios than previously published data for basaltic systems, whereas clinopyroxene partition coefficients show similar patterns but higher values than for basaltic systems. A set of calculated partition coefficients based on best-fit parabolas was used to model trace element fractionation in melts in equilibrium with rutile-bearing eclogitic residues. The calculated melts reproduce the overall trace element patterns of Archean TTGs, i.e. the melts show strongly fractionated REE patterns, depletions in Nb and heavy REE, and high Zr and Hf concentrations relative to Sm on normalized plots. The calculated residues have depleted trace element patterns with positive Nb anomalies and flat heavy rare earth element patterns similar to rutile-bearing xenolithic eclogites reported from Siberia and West Africa. Restites from eclogite melting are complementary to Archean TTGs and may be an important reservoir of Nb and Ta.
Volcanic domes may be emplaced rapidly and with few hazardous consequences, even at the summit of... more Volcanic domes may be emplaced rapidly and with few hazardous consequences, even at the summit of large stratovolcanoes. In this study the most recent activity of Mt. Taranaki in New Zealand is shown to have been a passive effusion of a c. 5.9 million m 3 lava dome with minor associated explosions and little syn-eruptive hazard. This event, the Sisters eruption, appears to have been unrecorded by local indigenous populations but likely occurred between A.D. 1785 and 1820. The magma erupted is chemically distinct from the preceding A.D. 1755 Tahurangi eruption. Based on breakdown of hornblende crystal rims, the Sisters magma was probably only four days outside the hornblende stability field before cooling, and the magma ascended its last four km along a conduit at rates of 0.012 ms − 1 . Based on dome surface morphology, a relatively low-viscosity magma is inferred. The dome remained in a metastable state for up to 70 years following the eruption; eventually generating a large, cool (b 350°C) collapse of at least 2 million m 3 of rock, forming a highly hazardous mass flow that travelled over 5 km. Factors contributing to the post-eruption dome instability include: its emplacement onto a steep flank of unconsolidated breccia or talus, possible oversteepening, fracturing due to spreading and subsidence, rapid cooling during heavy rain, and a hydrothermally altered inner core. The likely trigger for the collapse was a heavy rainstorm or an earthquake. Volcanic domes in the summit regions of volcanoes must be considered metastable and a potential source for hazardous, noneruption related mass flows for many decades following their emplacement.
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley ki... more Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89-0.93); pyroxene (0.88-0.93); garnet (0.72-0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53-0.84) and mostly low whole-rock SiO 2 , CaO and Al 2 O 3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO 2 ) derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene.
Redox melting refers to any process by which melt is generated by the contact of a rock with a fl... more Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH 4 -and H 2 -bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks.This oxidation mechanism causes an increase in the activity of H 2 O by the reaction of CH 4 with O 2 , and the increased aH 2 O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-date the first mechanism in the same geographical area, explaining the sequence of igneous rock types from lamproites to ultramafic lamprophyres that occurs during the development of rifts through cratons. The CRM mechanism relies on the oxidation of solid carbon as graphite or diamond that has accumulated in the lithosphere over time. The solidus temperature for rocks with both CO 2 and H 2 O is lower than in conditions with H 2 O alone; it does not occur at depths less than 65 km, but has recently been confirmed experimentally to depths of at least 200 km. Melts produced by HRM are not SiO 2 -undersaturated, even at depths of 200 km, and may often resemble lamproites or SiO 2 -rich picrites, whereas melts produced by CRM are always SiO 2 -undersaturated and range from carbonatitic to ultramafic lamprophyric or melilititic with increasing degree of melting. The operation of redox melting may be more common than has been recognized because the oxidation state of the upper mantle is not uniform as a function of depth, geodynamic setting or geological time. The general decrease of oxygen fugacity (fO 2 ) of c. 0·7 log units per 1 GPa pressure increase dictates that rapidly subducted oceanic lithosphere will be considerably more oxidized than ambient mantle peridotite at depths of 200^300 km. Hydrothermal alteration (serpentinization), addition of continental or carbonate sediments, and dehydration reactions during subduction all contribute to the heterogeneity of oxidation states in the subducted slab, which may vary over 6 log units; this raises the potential for redox reactions on local and regional scales. The oceanic lithosphere has a lower average fO 2 than either continental or cratonic mantle lithosphere at a given depth, so that the HRM mechanism dominates in recycled blocks and at the base of the continental lithosphere. The higher thermal gradients dictate that HRM is more common in the modern Earth beneath ocean islands and in upwelling mantle currents than in subduction zones. The oxidation state of the mantle is often described as having been constant since 3·5 Ga, but this overlooks the bias towards continental samples. Redox melting of oxidized recycled blocks (at approximately the fayalite^magnetite^quartz buffer) in the mantle was not important in the Hadean and Archaean, as it had to await the gradual oxidation of the mantle and the establishment of the subduction process, as well as the stabilization of the continents. The lack of CRM explains the lack of carbonatites before 2·7 Ga. However, the lower fO 2 of the Archaean asthenosphere and higher volatile contents caused more prevalent HRM in the Hadean and Archaean mantle. Degassing is controlled by solubility of volatile species in melts, which are H 2 O-rich but C-poor in reducing conditions. Silicate melts under reduced conditions contain much less carbon but more nitrogen than melts in the modern mantle, arguing for a nitrogen-rich, CO 2 -poor early atmosphere.
The structural parameters of six Nigerian coals were determined by X-ray diffraction, Raman and F... more The structural parameters of six Nigerian coals were determined by X-ray diffraction, Raman and FTIR spectroscopy. This study reveals that the coals contain crystalline carbon of turbostratic structure with some amount of highly disordered amorphous carbon. The average lateral sizes (L a ), stacking heights (L c ) and interlayer spacing (d 002 ) of the crystallite structures calculated from the X-ray intensities range from 16.47 to 25.70, 8.12 to 13.25 and 3.48 to 3.58 A, respectively. The L a values derived from the Raman spectra analyses using the classical linear relationship between 1/L a and the D/G band ratio are higher (27.53e34.67 A) than values obtained from X-ray diffraction. The FTIR spectra of the coals show the presence of stretching vibrations of eOH bonds, aliphatic eCH, eCH 2 and eCH 3 absorptions, C]C and eCH of aromatic structure, and C]O stretching vibrations of carbonyl groups. A strong linear relationship exists between the coal structural parameters (f a , d 002 , L c ) and the elemental carbon and volatile contents of the coal which reflects the dependency of the coals structure on their ranks.
Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor e... more Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor elongated, parallel-oriented ilmenite, and rarely olivine porphyroclasts and spinel. Compared with typical mantle peridotites, olivines in the Fe-rich dunites have lower forsterite (Fo 87^89 ) and NiO contents (1300^2800 ppm), which precludes a restitic origin for the dunites. Chrome-rich spinels are remnants of a metasomatic reaction that produced ilmenite and phlogopite. Trace element compositions differ between porphyroclastic and neoblastic olivine, the latter having higherTi, V, Cr and Ni and lower Zn, Zr and Nb contents, documenting their different origins. The dunites have high 187 Os/ 188 Os ratios (0Á11^0Á15) that result in young model ages for most samples, whereas three samples show isotopic mixtures between Phanerozoic neoblasts and ancient porphyroclastic material. Most Fe-rich dunite xenoliths are interpreted to be recrystallized cumulates related to fractional crystallization of Jurassic Karoo flood basalt magmatism, whereas the porphyroclasts are interpreted to be remnants from a much earlier (probably Archaean Ventersdorp) magmatic episode.The calculated parental magma for the most primitive olivine neoblasts in the Fe-rich dunites is similar to low-Ti Karoo basalts. Modelling the crystal fractionation of the inferred parental magma with pMELTS yields element fractionation trends that mirror the element variation of primitive low-Ti Karoo basalts.
Representative mineral compositions determined by electron microprobe are given in Appendix B. Ga... more Representative mineral compositions determined by electron microprobe are given in Appendix B. Garnet in granulites has
Western Uganda is a key region for understanding the development of the western branch of the Eas... more Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of [5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thickskinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U-Pb age determinations (2637-2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from [140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.
Contributions to Mineralogy and Petrology, Jan 1, 1997
The effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces... more The effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces of spinel, diopside, enstatite and olivine has been determined experimentally by the sessile melt drop technique. The anisotropy,à Ac SL , of solid-liquid interfacial energies (c SL (max)-c SL (min)) can be related to the wetting angles, w, byà Ac SL / cos wðmaxÞ À cos wðminÞ j j ¼ Pwà Ac SL ð Þ . Normalising to the smallest wetting angle gives values of Pw for diopside=0.0728, olivine=0.0574, orthopyrox-ene=0.0152, and spinel=0.0075. Crystal anisotropy influences grain-scale morphology of small-degree partial melts, permeability and the melt connectivity threshold, / C . Results show that, at sufficient melt fractions, diopside should increase permeability in a peridotitic matrix, whereas enstatite should lower it. Despite its low anisotropy, spinel contributes positively to permeability and / C because of its high surface energies. These results suggest that harzburgitic mineral matrices typical of the subcratonic mantle should impede the movement of low-degree partial melts, whereas melts should flow more easily through spinel lherzolites.
Contributions to Mineralogy and Petrology, Jan 1, 1994
Experiments investigating simplified members of the alkali-titanate mineral series priderite, lin... more Experiments investigating simplified members of the alkali-titanate mineral series priderite, lindsleyite-mathiasite (LIMA) and hawthorneite-yimengite (HAWYIM) confirm their stability at pressures between 3.5 and 5 GPa for priderite and LIMA, 4.3 and 5 GPa for HAWYIM at likely mantle solidus temperatures. Starting materials with K/Ba = 1 resulted in solid-solutions for LIMA, whereas HAWYIM produced separate hawthorneite (K/Ba approx. 0.5) and nearly pure yimengite (K), indicating the existence of an asymmetric solvus for HAWYIM at high pressures. Priderite was examined with iron present as either Fe 3+ or Fe 2+ ; both are stable at 5 GPa, confirming the possible coexistence of priderite with diamond. X-ray powder diffraction data on end-members synthesized at high pressures and calculated reflection patterns improve the scope of diffraction data on these rare mantle minerals, with only small differences existing between the patterns for the synthetic minerals and those observed or expected for natural minerals.
Solid/liquid partition coefficients for large ion lithophile elements (Ba, Rb, Sr), high field st... more Solid/liquid partition coefficients for large ion lithophile elements (Ba, Rb, Sr), high field strength elements (Zr, Hf, Nb, Ta, Ti), rare earth elements (La-Yb), Pb, Th, U and selected transition elements (Sc, V) were determined by means of Secondary Ion Mass Spectrometry on potassic-richterites synthesised at upper mantle conditions (P = 1.4 GPa and T = 850-1020°C) from silica-rich lamproites. Most trace elements display an incompatible behaviour in potassic-richterites; only Sr, Ti, Sc and V show strong positive anomalies in the partitioning pattern. When S/L D for potassic-richterites are compared with those for calcic amphiboles (pargasites and kaersutites) several differences become evident. In general, S/L D are lower in potassic-richterites; also, different partitioning patterns are apparent for RE and LIL elements. These differences are discussed in terms of the distinct crystal-chemical behaviour of the involved amphibole end-members, with particular emphasis to the available charge-balance mechanisms and to the site dimensional constraints ruling incorporation of trace elements in the various sites.
The sulfur isotopic compositions of barite (BaSO 4 ), anhydrite (CaSO 4 ), gypsum (CaSO 4 ·2H 2 O... more The sulfur isotopic compositions of barite (BaSO 4 ), anhydrite (CaSO 4 ), gypsum (CaSO 4 ·2H 2 O), mascagnite ((NH 4 ) 2 SO 4 ), thenardite (Na 2 SO 4 ), boetite (K 2 SO 4 ), epsomite (MgSO 4 ·7H 2 O), magnesium sulfate (MgSO 4 ·xH 2 O) and cysteine (an amino acid) were determined with a Cameca NanoSIMS 50 ion microprobe employing a Cs + primary ion beam and measuring negative secondary ions. This ion microprobe permits the analysis of sulfur isotope ratios in sulfates on 0.001-0.5 ng of sample material, enabling the analysis of individual S-bearing particles with diameters as small as 500 nm. The grain-to-grain reproducibility of measurements is typically 5‰ (1σ) for micron-sized grains, <5‰ for submicron-sized grains down to roughly 500 nm, and <2‰ for polished thin sections and ultra microtome sections which were studied for comparison. The role of chemical composition (matrix effect) and sample preparation technique on the instrumental mass fractionation (IMF) of the 34 S/ 32 S ratio in the NanoSIMS has been investigated for different sulfates and one amino acid. The IMF varies by ∼15‰ between the standards studied here, underlining the importance of a good understanding of the matrix-specific IMF correction in order to get precise S isotope data for very small samples such as aerosol particles. A good correlation between IMF and ionic radius of the cations in sulfates was found, permitting inference of IMF corrections for sulfates for which no isotope standards are available.
Contributions to Mineralogy and Petrology, Jan 1, 2000
Liquidus phase relations in the system diop-side±kalsilite±forsterite±quartz with 3 wt% F were ex... more Liquidus phase relations in the system diop-side±kalsilite±forsterite±quartz with 3 wt% F were examined at 1 bar and the locations of important invariant points were determined at 18 kbar. At all pressures within this range a large liquidus ®eld for¯uorphlogopite (Phl) exists, and has a large in¯uence on both melting and fractionation processes. One eutectic point was found to the silica-rich side of the plane Lc±Fo±Di at Di 1 Ks 30 Fo 2 Qz 67 , where a melt coexists with San, Qz, Phl and Di at 840°C and 1 bar. Another eutectic point must exist in the silica-poor part of the system because the phase topology determines that thermal barriers must exist. At this point a feldspathoid, either Lc or Ks, must coexist with Fo, Phl and a Ca-bearing phase such as Di. The exact location and phase assemblage were not determined, but the equilibrium melt must have a composition rich in Di (>29 wt%) and extremely poor in Qz (<8 wt%). The composition of the ®rst eutectic moves towards lower SiO 2 contents with increasing pressure (Di 3 Ks 40 Fo 1 Qz 56 at 18 kbar), whereas the second does not exist at 18 kbar due to the disappearance of Lc as a stable liquidus phase. Liquids which coexist with ma®c minerals such as En, Fo, Phl and Di are important for the genesis of potassium-rich ma®c rocks by partial melting in the mantle and for the early stages of fractional crystallisation. The equilibrium melt at the invariant point Fo + En + Phl + Di + L at 1125°C is very poor in Fo and Di components at atmospheric pressure (Di 5 Ks 37 Fo 5 Qz 53 ), whereas at 18 kbar the melt contains large amounts of Fo and Di (Di 19 Ks 31 -Fo 28 Qz 21 ), and has a composition close to that of natural lamproites. Kamafugites do not correspond to melts in this system under any of the studied conditions, and appear to require CO 2 in the source. Fractionation processes from primitive potassic basanite melts are controlled principally by the size (and not the mere presence) of the liquidus phase ®eld for phlogopite: at high pressures where the Phl ®eld is large, olivine is eliminated early from the fractionating assemblage and Cpx + Phl fractionation may lead to relatively silicarich rock dierentiates such as trachytes. At low pressures, extensive olivine and restricted Phl crystallisation prevents silica enrichment in the melt, resulting in phonolitic dierentiates. Later crystallisation of alkali feldspar accentuates the trends laid down in the early stages of fractionation.
When Did Plate Tectonics Begin on Planet Earth?, Jan 1, 2008
The Geological Society of America Special Paper 440 2008 A trace element perspective onArchean cr... more The Geological Society of America Special Paper 440 2008 A trace element perspective onArchean crust formation and on the presence or absence ofArchean subduction Stephen Foley* Institute of Geosciences, University of Mainz, Becherweg 21, 55099 Mainz, Germany ABSTRACT ...
Partition coefficients for compatible and incompatible trace elements for use in modelling the or... more Partition coefficients for compatible and incompatible trace elements for use in modelling the origin and evolution of lamproitic and other potassic rocks are reported. These are based on values obtained by in situ determination using laser ablation microprobe-inductively ...
II materials (AGV-2, GSP-2, JG-1a) usually agree within 10 % with published values. Similar accor... more II materials (AGV-2, GSP-2, JG-1a) usually agree within 10 % with published values. Similar accordance was observed between data from solution ICP-MS and laser ablation-ICP-MS of natural rock samples. Advantages of the fusion method are lower detection limits compared to XRF analyses and avoidance of wet-chemical processing and use of strong acids as in solution ICP-MS as well as smaller sample volumes compared to the other methods. Thus, the method is an user-friendly analytical tool that can be easily adopted wherever laser ablation-ICP-MS is used for data acquisition.
The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydro... more The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydrous tonalitic melts has been studied experimentally at 1.8 GPa and 1000–1040 °C. The knowledge of trace element partitioning between minerals and coexisting tonalitic melts is essential for geochemical models in order to explain the genesis of Archean tonalite-trondhjemite-granodiorite suites (TTGs). Our experiments were performed on a natural tonalite doped with two different trace element mixtures and with TiO2 to stabilize titanates. Trace element concentrations of crystals and coexisting melts were measured by laser ablation microprobe. The partition coefficients (D's) are independent of the trace element concentration over the concentration ranges used. The partition coefficients reported in this paper are a considerable extension of the few available data sets in intermediate to silicic systems. The D-values mostly agree well with previously published data and with predictive models. The garnet partition coefficients show steeper patterns with higher D's for heavy REE, lower D's for highly incompatible elements and lower La/Yb-ratios than previously published data for basaltic systems, whereas clinopyroxene partition coefficients show similar patterns but higher values than for basaltic systems. A set of calculated partition coefficients based on best-fit parabolas was used to model trace element fractionation in melts in equilibrium with rutile-bearing eclogitic residues. The calculated melts reproduce the overall trace element patterns of Archean TTGs, i.e. the melts show strongly fractionated REE patterns, depletions in Nb and heavy REE, and high Zr and Hf concentrations relative to Sm on normalized plots. The calculated residues have depleted trace element patterns with positive Nb anomalies and flat heavy rare earth element patterns similar to rutile-bearing xenolithic eclogites reported from Siberia and West Africa. Restites from eclogite melting are complementary to Archean TTGs and may be an important reservoir of Nb and Ta.
Volcanic domes may be emplaced rapidly and with few hazardous consequences, even at the summit of... more Volcanic domes may be emplaced rapidly and with few hazardous consequences, even at the summit of large stratovolcanoes. In this study the most recent activity of Mt. Taranaki in New Zealand is shown to have been a passive effusion of a c. 5.9 million m 3 lava dome with minor associated explosions and little syn-eruptive hazard. This event, the Sisters eruption, appears to have been unrecorded by local indigenous populations but likely occurred between A.D. 1785 and 1820. The magma erupted is chemically distinct from the preceding A.D. 1755 Tahurangi eruption. Based on breakdown of hornblende crystal rims, the Sisters magma was probably only four days outside the hornblende stability field before cooling, and the magma ascended its last four km along a conduit at rates of 0.012 ms − 1 . Based on dome surface morphology, a relatively low-viscosity magma is inferred. The dome remained in a metastable state for up to 70 years following the eruption; eventually generating a large, cool (b 350°C) collapse of at least 2 million m 3 of rock, forming a highly hazardous mass flow that travelled over 5 km. Factors contributing to the post-eruption dome instability include: its emplacement onto a steep flank of unconsolidated breccia or talus, possible oversteepening, fracturing due to spreading and subsidence, rapid cooling during heavy rain, and a hydrothermally altered inner core. The likely trigger for the collapse was a heavy rainstorm or an earthquake. Volcanic domes in the summit regions of volcanoes must be considered metastable and a potential source for hazardous, noneruption related mass flows for many decades following their emplacement.
Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley ki... more Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re-Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89-0.93); pyroxene (0.88-0.93); garnet (0.72-0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53-0.84) and mostly low whole-rock SiO 2 , CaO and Al 2 O 3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO 2 ) derived by partial melting of eclogite before or during cratonization. The melt-peridotite interaction converted olivine into orthopyroxene.
Redox melting refers to any process by which melt is generated by the contact of a rock with a fl... more Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH 4 -and H 2 -bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks.This oxidation mechanism causes an increase in the activity of H 2 O by the reaction of CH 4 with O 2 , and the increased aH 2 O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-date the first mechanism in the same geographical area, explaining the sequence of igneous rock types from lamproites to ultramafic lamprophyres that occurs during the development of rifts through cratons. The CRM mechanism relies on the oxidation of solid carbon as graphite or diamond that has accumulated in the lithosphere over time. The solidus temperature for rocks with both CO 2 and H 2 O is lower than in conditions with H 2 O alone; it does not occur at depths less than 65 km, but has recently been confirmed experimentally to depths of at least 200 km. Melts produced by HRM are not SiO 2 -undersaturated, even at depths of 200 km, and may often resemble lamproites or SiO 2 -rich picrites, whereas melts produced by CRM are always SiO 2 -undersaturated and range from carbonatitic to ultramafic lamprophyric or melilititic with increasing degree of melting. The operation of redox melting may be more common than has been recognized because the oxidation state of the upper mantle is not uniform as a function of depth, geodynamic setting or geological time. The general decrease of oxygen fugacity (fO 2 ) of c. 0·7 log units per 1 GPa pressure increase dictates that rapidly subducted oceanic lithosphere will be considerably more oxidized than ambient mantle peridotite at depths of 200^300 km. Hydrothermal alteration (serpentinization), addition of continental or carbonate sediments, and dehydration reactions during subduction all contribute to the heterogeneity of oxidation states in the subducted slab, which may vary over 6 log units; this raises the potential for redox reactions on local and regional scales. The oceanic lithosphere has a lower average fO 2 than either continental or cratonic mantle lithosphere at a given depth, so that the HRM mechanism dominates in recycled blocks and at the base of the continental lithosphere. The higher thermal gradients dictate that HRM is more common in the modern Earth beneath ocean islands and in upwelling mantle currents than in subduction zones. The oxidation state of the mantle is often described as having been constant since 3·5 Ga, but this overlooks the bias towards continental samples. Redox melting of oxidized recycled blocks (at approximately the fayalite^magnetite^quartz buffer) in the mantle was not important in the Hadean and Archaean, as it had to await the gradual oxidation of the mantle and the establishment of the subduction process, as well as the stabilization of the continents. The lack of CRM explains the lack of carbonatites before 2·7 Ga. However, the lower fO 2 of the Archaean asthenosphere and higher volatile contents caused more prevalent HRM in the Hadean and Archaean mantle. Degassing is controlled by solubility of volatile species in melts, which are H 2 O-rich but C-poor in reducing conditions. Silicate melts under reduced conditions contain much less carbon but more nitrogen than melts in the modern mantle, arguing for a nitrogen-rich, CO 2 -poor early atmosphere.
The structural parameters of six Nigerian coals were determined by X-ray diffraction, Raman and F... more The structural parameters of six Nigerian coals were determined by X-ray diffraction, Raman and FTIR spectroscopy. This study reveals that the coals contain crystalline carbon of turbostratic structure with some amount of highly disordered amorphous carbon. The average lateral sizes (L a ), stacking heights (L c ) and interlayer spacing (d 002 ) of the crystallite structures calculated from the X-ray intensities range from 16.47 to 25.70, 8.12 to 13.25 and 3.48 to 3.58 A, respectively. The L a values derived from the Raman spectra analyses using the classical linear relationship between 1/L a and the D/G band ratio are higher (27.53e34.67 A) than values obtained from X-ray diffraction. The FTIR spectra of the coals show the presence of stretching vibrations of eOH bonds, aliphatic eCH, eCH 2 and eCH 3 absorptions, C]C and eCH of aromatic structure, and C]O stretching vibrations of carbonyl groups. A strong linear relationship exists between the coal structural parameters (f a , d 002 , L c ) and the elemental carbon and volatile contents of the coal which reflects the dependency of the coals structure on their ranks.
Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor e... more Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor elongated, parallel-oriented ilmenite, and rarely olivine porphyroclasts and spinel. Compared with typical mantle peridotites, olivines in the Fe-rich dunites have lower forsterite (Fo 87^89 ) and NiO contents (1300^2800 ppm), which precludes a restitic origin for the dunites. Chrome-rich spinels are remnants of a metasomatic reaction that produced ilmenite and phlogopite. Trace element compositions differ between porphyroclastic and neoblastic olivine, the latter having higherTi, V, Cr and Ni and lower Zn, Zr and Nb contents, documenting their different origins. The dunites have high 187 Os/ 188 Os ratios (0Á11^0Á15) that result in young model ages for most samples, whereas three samples show isotopic mixtures between Phanerozoic neoblasts and ancient porphyroclastic material. Most Fe-rich dunite xenoliths are interpreted to be recrystallized cumulates related to fractional crystallization of Jurassic Karoo flood basalt magmatism, whereas the porphyroclasts are interpreted to be remnants from a much earlier (probably Archaean Ventersdorp) magmatic episode.The calculated parental magma for the most primitive olivine neoblasts in the Fe-rich dunites is similar to low-Ti Karoo basalts. Modelling the crystal fractionation of the inferred parental magma with pMELTS yields element fractionation trends that mirror the element variation of primitive low-Ti Karoo basalts.
Representative mineral compositions determined by electron microprobe are given in Appendix B. Ga... more Representative mineral compositions determined by electron microprobe are given in Appendix B. Garnet in granulites has
Western Uganda is a key region for understanding the development of the western branch of the Eas... more Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of [5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thickskinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U-Pb age determinations (2637-2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from [140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.
Contributions to Mineralogy and Petrology, Jan 1, 1997
The effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces... more The effect of crystal anisotropy on wetting angles of equilibrium silicate melts on crystal faces of spinel, diopside, enstatite and olivine has been determined experimentally by the sessile melt drop technique. The anisotropy,à Ac SL , of solid-liquid interfacial energies (c SL (max)-c SL (min)) can be related to the wetting angles, w, byà Ac SL / cos wðmaxÞ À cos wðminÞ j j ¼ Pwà Ac SL ð Þ . Normalising to the smallest wetting angle gives values of Pw for diopside=0.0728, olivine=0.0574, orthopyrox-ene=0.0152, and spinel=0.0075. Crystal anisotropy influences grain-scale morphology of small-degree partial melts, permeability and the melt connectivity threshold, / C . Results show that, at sufficient melt fractions, diopside should increase permeability in a peridotitic matrix, whereas enstatite should lower it. Despite its low anisotropy, spinel contributes positively to permeability and / C because of its high surface energies. These results suggest that harzburgitic mineral matrices typical of the subcratonic mantle should impede the movement of low-degree partial melts, whereas melts should flow more easily through spinel lherzolites.
Contributions to Mineralogy and Petrology, Jan 1, 1994
Experiments investigating simplified members of the alkali-titanate mineral series priderite, lin... more Experiments investigating simplified members of the alkali-titanate mineral series priderite, lindsleyite-mathiasite (LIMA) and hawthorneite-yimengite (HAWYIM) confirm their stability at pressures between 3.5 and 5 GPa for priderite and LIMA, 4.3 and 5 GPa for HAWYIM at likely mantle solidus temperatures. Starting materials with K/Ba = 1 resulted in solid-solutions for LIMA, whereas HAWYIM produced separate hawthorneite (K/Ba approx. 0.5) and nearly pure yimengite (K), indicating the existence of an asymmetric solvus for HAWYIM at high pressures. Priderite was examined with iron present as either Fe 3+ or Fe 2+ ; both are stable at 5 GPa, confirming the possible coexistence of priderite with diamond. X-ray powder diffraction data on end-members synthesized at high pressures and calculated reflection patterns improve the scope of diffraction data on these rare mantle minerals, with only small differences existing between the patterns for the synthetic minerals and those observed or expected for natural minerals.
Solid/liquid partition coefficients for large ion lithophile elements (Ba, Rb, Sr), high field st... more Solid/liquid partition coefficients for large ion lithophile elements (Ba, Rb, Sr), high field strength elements (Zr, Hf, Nb, Ta, Ti), rare earth elements (La-Yb), Pb, Th, U and selected transition elements (Sc, V) were determined by means of Secondary Ion Mass Spectrometry on potassic-richterites synthesised at upper mantle conditions (P = 1.4 GPa and T = 850-1020°C) from silica-rich lamproites. Most trace elements display an incompatible behaviour in potassic-richterites; only Sr, Ti, Sc and V show strong positive anomalies in the partitioning pattern. When S/L D for potassic-richterites are compared with those for calcic amphiboles (pargasites and kaersutites) several differences become evident. In general, S/L D are lower in potassic-richterites; also, different partitioning patterns are apparent for RE and LIL elements. These differences are discussed in terms of the distinct crystal-chemical behaviour of the involved amphibole end-members, with particular emphasis to the available charge-balance mechanisms and to the site dimensional constraints ruling incorporation of trace elements in the various sites.
The sulfur isotopic compositions of barite (BaSO 4 ), anhydrite (CaSO 4 ), gypsum (CaSO 4 ·2H 2 O... more The sulfur isotopic compositions of barite (BaSO 4 ), anhydrite (CaSO 4 ), gypsum (CaSO 4 ·2H 2 O), mascagnite ((NH 4 ) 2 SO 4 ), thenardite (Na 2 SO 4 ), boetite (K 2 SO 4 ), epsomite (MgSO 4 ·7H 2 O), magnesium sulfate (MgSO 4 ·xH 2 O) and cysteine (an amino acid) were determined with a Cameca NanoSIMS 50 ion microprobe employing a Cs + primary ion beam and measuring negative secondary ions. This ion microprobe permits the analysis of sulfur isotope ratios in sulfates on 0.001-0.5 ng of sample material, enabling the analysis of individual S-bearing particles with diameters as small as 500 nm. The grain-to-grain reproducibility of measurements is typically 5‰ (1σ) for micron-sized grains, <5‰ for submicron-sized grains down to roughly 500 nm, and <2‰ for polished thin sections and ultra microtome sections which were studied for comparison. The role of chemical composition (matrix effect) and sample preparation technique on the instrumental mass fractionation (IMF) of the 34 S/ 32 S ratio in the NanoSIMS has been investigated for different sulfates and one amino acid. The IMF varies by ∼15‰ between the standards studied here, underlining the importance of a good understanding of the matrix-specific IMF correction in order to get precise S isotope data for very small samples such as aerosol particles. A good correlation between IMF and ionic radius of the cations in sulfates was found, permitting inference of IMF corrections for sulfates for which no isotope standards are available.
Contributions to Mineralogy and Petrology, Jan 1, 2000
Liquidus phase relations in the system diop-side±kalsilite±forsterite±quartz with 3 wt% F were ex... more Liquidus phase relations in the system diop-side±kalsilite±forsterite±quartz with 3 wt% F were examined at 1 bar and the locations of important invariant points were determined at 18 kbar. At all pressures within this range a large liquidus ®eld for¯uorphlogopite (Phl) exists, and has a large in¯uence on both melting and fractionation processes. One eutectic point was found to the silica-rich side of the plane Lc±Fo±Di at Di 1 Ks 30 Fo 2 Qz 67 , where a melt coexists with San, Qz, Phl and Di at 840°C and 1 bar. Another eutectic point must exist in the silica-poor part of the system because the phase topology determines that thermal barriers must exist. At this point a feldspathoid, either Lc or Ks, must coexist with Fo, Phl and a Ca-bearing phase such as Di. The exact location and phase assemblage were not determined, but the equilibrium melt must have a composition rich in Di (>29 wt%) and extremely poor in Qz (<8 wt%). The composition of the ®rst eutectic moves towards lower SiO 2 contents with increasing pressure (Di 3 Ks 40 Fo 1 Qz 56 at 18 kbar), whereas the second does not exist at 18 kbar due to the disappearance of Lc as a stable liquidus phase. Liquids which coexist with ma®c minerals such as En, Fo, Phl and Di are important for the genesis of potassium-rich ma®c rocks by partial melting in the mantle and for the early stages of fractional crystallisation. The equilibrium melt at the invariant point Fo + En + Phl + Di + L at 1125°C is very poor in Fo and Di components at atmospheric pressure (Di 5 Ks 37 Fo 5 Qz 53 ), whereas at 18 kbar the melt contains large amounts of Fo and Di (Di 19 Ks 31 -Fo 28 Qz 21 ), and has a composition close to that of natural lamproites. Kamafugites do not correspond to melts in this system under any of the studied conditions, and appear to require CO 2 in the source. Fractionation processes from primitive potassic basanite melts are controlled principally by the size (and not the mere presence) of the liquidus phase ®eld for phlogopite: at high pressures where the Phl ®eld is large, olivine is eliminated early from the fractionating assemblage and Cpx + Phl fractionation may lead to relatively silicarich rock dierentiates such as trachytes. At low pressures, extensive olivine and restricted Phl crystallisation prevents silica enrichment in the melt, resulting in phonolitic dierentiates. Later crystallisation of alkali feldspar accentuates the trends laid down in the early stages of fractionation.
When Did Plate Tectonics Begin on Planet Earth?, Jan 1, 2008
The Geological Society of America Special Paper 440 2008 A trace element perspective onArchean cr... more The Geological Society of America Special Paper 440 2008 A trace element perspective onArchean crust formation and on the presence or absence ofArchean subduction Stephen Foley* Institute of Geosciences, University of Mainz, Becherweg 21, 55099 Mainz, Germany ABSTRACT ...
Partition coefficients for compatible and incompatible trace elements for use in modelling the or... more Partition coefficients for compatible and incompatible trace elements for use in modelling the origin and evolution of lamproitic and other potassic rocks are reported. These are based on values obtained by in situ determination using laser ablation microprobe-inductively ...
II materials (AGV-2, GSP-2, JG-1a) usually agree within 10 % with published values. Similar accor... more II materials (AGV-2, GSP-2, JG-1a) usually agree within 10 % with published values. Similar accordance was observed between data from solution ICP-MS and laser ablation-ICP-MS of natural rock samples. Advantages of the fusion method are lower detection limits compared to XRF analyses and avoidance of wet-chemical processing and use of strong acids as in solution ICP-MS as well as smaller sample volumes compared to the other methods. Thus, the method is an user-friendly analytical tool that can be easily adopted wherever laser ablation-ICP-MS is used for data acquisition.
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