Fluvial-aeolian interactions: Part I, modern systems

Two modern fluvial-aeolian depositional systems (Great Sand Dunes National Monument, Colorado and the Mojave River Wash, California) are remarkably similar in spite of different climates, sizes, fluvial sediment textures, and relative directions of aeolian and fluvial transport. Dune growth and migration, and deflation of blowouts create 8–10 m of local relief in unflooded aeolian landscapes. There are six prominent fluvial-aeolian interactions. (1) Fluvial flow extends into the aeolian system until it is dammed by aeolian landforms; (2) interdune areas (overbank-interdunes) upstream of aeolian dams, and alongside channels are flooded; (3) water erodes dunes alongside channels and interdunes; (4) flood waters deposit sediment in interdune areas; (5) fluvially derived groundwater floods interdunes (interdune-playas); (6) wind erodes fluvial sediment and redeposits it in the aeolian system. Unique and characteristic sediments are deposited in overbank-interdunes and in interdune-playas, reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions. These fluvial-aeolian interdune deposits are characterized by irregular, concave-up bases and flat upper surfaces containing mudcracks or evaporite cement. Characteristic low-relief surfaces form in aeolian systems as an effect of flooding. Fluvial deposits are resistant to aeolian deflation. Aeolian sand is preserved when flood sediments are deposited around the bases of the dunes. Thus repetitive fluvial and aeolian aggradation tends to be ‘stepwise’ as interdune floors are suddenly raised during floods. The effects of flooding should be easy to recognize in ancient aeolianites, even beyond the area covered with overbank muds.     

The organic geochemistry of ancient sediments—Part II

Several shales and oils ranging in age from 3 million to 2·7 billion years have been investigated for their hydrocarbon content using gas chromatography and mass spectrometry as primary analytical tools. From the Soudan Shale from Minnesota (2·7 × 10⁹yr) the C₁₈, C₁₉, C₂₀ and C₂₁ isoprenoid-alkanes were obtained. The Antrim shale from Michigan (about 265 × 10⁶ yr) yielded the C₁₆, C₁₈, C₁₉, C₂₀ and C₂₁ isoprenoids, as well as a C₁₆ iso-alkane and the C₁₈ and C₁₉ cyclohexyl n-alkanes. The San Joaquin Oil (30 × 10⁶ yr) and the Abbott Rock Oil (3 × 10⁶ yr) contained the C₁₆, C₁₈, C₁₉, C₂₀ and the C₁₈, C₁₉, C₂₀ and C₂₁ isoprenoids respectively. In addition, a series of iso-alkanes (C₁₆−C₁₈), anteiso-alkanes (C₁₆−C₁₈) and n-alkylcyclohexanes (C₁₆−C₁₉) as well as a C₂₁ isoprenoid were obtained from the Nonesuch Seep Oil (1 × 10⁹ yr). This analysis provides a comprehensive picture of the types of biogenic hydrocarbons found in oils and shales of widely differing ages, and in particular, the finding of isoprenoid alkanes in the Soudan Shale furnishes evidence for life processes at that period of geological time.     

Geothermal systems ancient and modern: a geochemical review

Geothermal systems occur in a range of crustal settings. The emphasis of this review is on those occurring in regions of active or recently active volcanism, where magmatic heat at depths up to 8 km leads to convection of groundwater in the upper crust. Hot water (and steam) flows are controlled by the permeability of the crust and recent data have emphasised the dominance of secondary permeability, especially fractures. Drilling to depths of up to 3 km in these systems encounters near-neutral pH alkali chloride waters with temperatures up to about 350°C and chloride contents generally in the range 500 to 15,000 mg kg^?1 although much higher salinities are encountered in some systems such as in the Imperial Valley, California. Stable isotope studies indicate the predominance of a meteoric source in the majority of geothermal systems although seawater predominates in some regions, such as Reykjanes, Iceland. Mixing of waters from both sources also occurs in some systems and some magmatic fluid may also be present.The major element geochemistry of geothermal fluids is determined by a set of temperature-dependent mineral-fluid equilibria although chloride and rare gas contents appear to be independent variables reflecting the sources of these components (sedimentary or volcanic rocks, seawater, magmatic fluids, etc).Boiling in the upper portion of geothermal systems is accompanied by the transfer of acidic gases (CO₂ and H₂S) to the resultant steam which may penetrate the surface as fumarolic activity or become condensed into shallow groundwaters giving rise, with oxidation, to distinctive low pH sulphate bicarbonate water.Fluid inclusion, stable isotope and mineral alteration studies have led to the recognition in many Tertiary hydrothermal ore deposits of physical and chemical environments analogous to those encountered in the present-day systems. The vein-type gold-silver, Carlin-type gold and porphyry-type copper-molybdenum deposits of the western United States are particularly well studied examples. Sub-ocean floor equivalents of the terrestrial geothermal systems have been recognized in ocean floor spreading centres such as the East Pacific Rise and deep-sea submersible vehicles have allowed visual observation of sea floor hot springs actively depositing metal sulphides. These environments may parallel those of the Cyprus-type massive sulphide depositing systems, while sub-sea floor systems of the type responsible for Kuroko-type massive sulphide deposits may eventually be encountered in island are settings.     

Disjunctive kriging revisited: Part II

In order to find distributions other than infinitely divisible distributions which are suitable for disjunctive kriging, infinitesimal generators are used. In addition to distributions developed in Part I, this leads to development of suitable models for the beta (), hypergeometric, and binomial distributions.     

The Colima volcanic complex,Mexico: Part II. Late-quaternary cinder cones

Since the late Pleistocene, eleven cinder and lava cones have erupted on the floor of the southern Colima graben, NE and NW of the large, active, andesitic volcano Colima. Scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites (9), leucite-basanites (3), and minettes (15), the commonest variety of mica lamprophyre. These samples represent primitive, high temperature magmas with 47.6–50.3% SiO₂, 7.4–15.3% MgO, 2.5–4.4% K₂O, and 2.2–9.9% normative nepheline. All members of this basic alkalic suite contain Mg-olivine (Fo_75–94), chromite, augite, and late plagioclase and titanomagnetite. The petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile-bearing phases phlogopite, apatite, and analcime during late stages of crystallization. As these phases increase in abundance, presumably reflecting a rise in magmatic volatile content, there are corresponding increases in the whole rock concentrations of 16 incompatible elements. Although these incompatible elements are relatively abundant even in the basanites, many are highly concentrated in the minettes: Ba 4,200 ppm, Sr3,100 ppm, Zr 550 ppm, Ce190 ppm, Hf 18 ppm. Among the incompatible elements, the degrees of enrichment in the minettes relative to the basanites decrease in the order: H, Th, Ce, La, Nd, Zr, Hf, U, Ba, Sm, Eu, Pb, P, Nb, Sr, Ti. These enrichments may reflect the increasing importance of minor, incompatible element rich mantle phases during partial melting. The concentrations of alkali metals K, Na, Rb, and Cs do not correlate with these other elemental enrichments. The leucite-basanties have similar incompatible element contents to many minettes, differing from them only in the presence of leucite rather than analcime, and Ti-F-rich groundmass phlogopite rather than hydrous phlogopite phenocrysts; thus the leucite-basanites represent relatively dry equivalents of minettes.Two of the eleven cinder cones are calc-alkaline in nature and do not belong to the basanite-minette group; the easternmost cone is constructed of high-Al basalt, and the northernmost of basaltic andesite. The high-Al basalt (49.5% SiO₂, 9.3% MgO, 221 ppm Ni) closely approximates a parental magma to the post-caldera andesitic suite of V. Colima (56.5–61.6% SiO₂). The basaltic-andesite is relatively enriched in incompatible elements compared to the high-Al basalt — V. Colima trend.The ne-normative basanite-minette samples are highly enriched in incompatible elements, while the contemporaneous hy — qz-normative calc-alkaline suite, encompassing the high-Al basalt and V. Colima's andesites, is characterized by relatively low abundances of these elements. No likely mineral assemblage can relate the alkaline and calc-alkaline suites through crystal fractionation; they probably represent fundamentally different melting events.During the Quaternary, the main focus of andesitic volcanism in the southern Colima graben has migrated southward with time. Volcán Colima marks its present position, 5 km south of the Pleistocene volcano Nevado de Colima, and another 15 km from the still older Volcán Cantaro. The eruptions of basic alkalic magma probably occurred during the late stages of Nevado's life and through the life of V. Colima. They generally migrated from west to east with time, towards V. Cantaro. The most recent cone, V. Apaxtepec, is the only one east of the andesitic Colima-Cantaro axis. The oldest and the two youngest cones produced basanites, while minettes dominated at cones of intermediate ages. The cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of V. Colima. The southern end of the Colima graben can be viewed, then, as the volcanic analog of many classical, post-plutonic, hypabyssal lamprophyre localities.     

The Geochemistry of Scapolite: Part II. Trace Elements, Petrology, and General Geochemistry

Fifty scapolites have been analysed spectrographically for numerouselements. Average concentrations (p.p.m.) were as follows: B25, Be 9–3, Ga 33, Ti 82, Li 56, Cu 4–4, Zr 59,Mn 57, Sr 1,800, Pb 45, Ba 120, Rb 20. The following were seldomor never detected: Cr, Ni, Co, Mo, Sn, V, Sc, Ag, Y, La. Themajor elements Ca, Na, K were also determined. The distributionof the trace elements can be explained by isomorphous substitution,but no detailed correlation of trace elements with each otheror with major elements was found. Refractive indices were determined and the relation betweenaverage index and per cent Me was examined: correlation waspoor, which may in part be attributed to analytical error. Examination of scapolite parageneses shows that scapolite characteristicallyoccurs in the upper amphibolite facies or the pyroxene hornfelsfacies: it is not restricted to these and may occur in any faciesfrom zeolitic to granulitic and in any hornfels facies. Theelements generally concentrated in scapolite include Ca, Na,C, Cl, S, H, B, Be, Li, Sr, Pb. The presence of C, Cl, S, Htestify to genesis in the presence of high partial pressureof CO₂, Cl₂, SO₃, H₂O (or related compounds), that is in pneumatolytic,pegmatitic, or hydrothermal environments. The concentrationof B, Be, Li can also be attributed to these conditions. The source of the elements concentrated in scapolite must inpart be common rocks. In a limited contact zone, the nearbymagma supplied some elements, but where regional scapolitizationhas taken place the presence of magma is less clear. Many commonrocks or rock series contain all the necessary constituents,but some particular conjunction of conditions is necessary forscapolite to form, or it would be more common.     

Anatomy of ancient passive margin slope systems: Aptian gravity-driven deposition on the Vocontian palaeomargin, western Alps, south-east France

The Aptian succession on the Vocontian palaeomargin (south-east France) consists of marl and marly calcareous pelagic slope facies together with a range of gravity-driven deposits (slumps, debris-flow deposits, turbidite packages and massive sandstones). The massive sandstones were emplaced by high-density turbidity currents and are associated with extensive clastic sills and dykes. The sedimentology is constrained by a high-resolution bio- and lithostratigraphic framework and permits a detailed analysis of the slope succession including: (1) a sequence stratigraphical analysis of the slope deposits; and (2) lateral tracing of individual sedimentary packages downslope. The resulting model for the Vocontian slope represents an alternative to the ‘classic’ Exxon delta-fed, mud-rich model. Key elements of the Vocontian model are: (1) an emphasis on lowstand slope erosion and complex slope morphology controlled by contemporary tectonism and salt diapirism; (2) slope deposition in confined erosional and structurally controlled conduits rather than the buildout of slope fans/channel-levee complexes; (3) a dominance of large-volume muddy slump and transitional debris-flow deposits, with subordinate sandy turbidites, including significant massive sandstone facies; (4) common sand injections (sills and dykes) associated with the massive sandstone facies; and (5) minimal downslope evolution of the flows, with the nature of the source sediment being the over-riding factor determining flow behaviour and deposit character. The Vocontian system is a rare instance in which large sections of a ‘fossil’ passive margin slope are preserved in the geological record. The slope deposits differ from the classic models of turbidite systems that have mainly been built from peripheral foreland basins, and the new insight makes it possible to compare ancient and present-day passive margin slope systems.     

The chemistry of lava-seawater interactions II: the elemental signature

The flow of lava into the ocean at the shoreline of Kilauea Volcano during the ongoing Pu’u O’o eruption has allowed a detailed study of the geochemical interaction between lava and seawater. This paper focuses on the chemistry of the major and minor elements in the fluids that resulted from this interaction. The elemental enrichments in these fluids are dominated by three processes: (1) evaporation of water from seawater, which creates solutions enriched in the major elements found in seawater, (2) congruent dissolution of the basalt glass matrix, which is limited by the solubility of some of the elements in seawater, and (3) removal of volatile phases from the lava on contact with seawater.Using a simple model of volatile emanation (using published emanation coefficients) and congruent dissolution, we are able to explain the concentrations observed for the majority of elements in precipitation from the steam plume at the shoreline lava entry and in water allowed to interact with molten lava in controlled experiments. Fe, Al, Ti, and some of the rare earth elements (REEs) in precipitation samples from the steam plume at the lava entry were > 10,000-times enriched over their ambient seawater concentrations, suggesting that these elements may be useful for identifying submarine eruptions. The flux of elements from the Kilauea ocean lava entry is greater than that from a typical midocean ridge hydrothermal vent field for Al, Cd, Co, and the REEs, whereas the opposite is true for the remainder of the elements studied.     

The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden. Part II: chemostratigraphy and alteration

The Kristineberg massive sulfide deposit is hosted by metamorphosed volcanic and subvolcanic rocks of the Palaeoproterozoic Skellefte Group. The deposit consists of: (1) two main massive sulfide horizons, the A-ores and B-ores, which dip steeply southwards and are separated by 100–150 m; and (2) the Einarsson Zone, a complex interval of Cu–Au-rich ‘stockwork‘ sulfides and small massive sulfide lenses in altered and deformed rocks near the 1,000 m level. The Einarsson Zone occurs some 20–100 m south of the B-ores. There are no definite younging indicators in the mine sequence. In many areas of the mine, the original host rocks are impossible to identify petrographically due to the abundance of secondary minerals such as quartz, chlorite, muscovite, cordierite, andalusite, phlogopite, pyrite and talc, combined with variably schistose fabrics. Application of immobile-element methods to 600 recent whole-rock chemical analyses has, however, allowed the original rock types to be identified and correlated. Rhyolite X lies immediately north of the A-ore, while andesitic to dacitic to rhyodacitic rocks make up the 100–150 m interval between the A-ore and B-ore, and massive rhyolite A lies immediately south of the B-ore. The felsic rocks are mostly of calc-alkaline affinity, excluding rhyolite X, which is transitional. The mine porphyry, which lies north of the A-ore and forms the marginal phase of the synvolcanic Viterliden Intrusive Complex, is compositionally similar to dacite and rhyodacite. Mass changes calculated for all rock types indicate that most of the volcanic rocks in the mine area are strongly depleted in Na and Ca, and have gained variable amounts of Mg and Fe, whereas Si changes range from negative to positive. Gains in Fe and changes in Si are largest within 5–10 m of the massive sulfide lenses. Cordierite-bearing schists of andesitic to felsic compositions that lie between massive sulfide lenses A and B are not as altered. The Einarsson Zone commonly shows large gains in Fe and Mg, while Si shows large gains to large losses. Immobile-element ratios indicate that very different secondary assemblages in the mine, e.g. andalusite–quartz–muscovite and cordierite–chlorite–talc, can be produced from the same precursor volcanic unit, e.g., rhyolite. Conversely, the same secondary mineral assemblage can be produced from different rocks, e.g. weakly altered andesite and strongly altered rhyolite. The common presence of cordierite + andalusite in the mine area, without anthophyllite, is unusual in the alteration systems of volcanic-hosted massive sulfide deposits, and is proposed to have formed by the metamorphic reaction of the synvolcanic alteration minerals kaolinite and chlorite to produce cordierite. Where kaolinite was in excess of chlorite, andalusite was also formed. We propose that highly acidic alteration fluids locally produced high-Al minerals such as kaolinite that either overprinted, or occurred in place of, a more typical sericite–chlorite–quartz alteration assemblage that otherwise formed near the massive sulfide lenses. Application of lithogeochemical methods to the altered, deformed and metamorphosed Kristineberg rocks has identified specific volcanic contacts with massive sulfide potential, and quantified the effects of synvolcanic hydrothermal alteration. Such an approach can increase the effectiveness of mineral exploration in metamorphosed terrains.     

Long-term brine impacted fly ash,Part II: Mobility of major species in the ash residues

The leaching of major species from fly ash is a function of the pH of the solution in contact with the fly ash. The aim of this study was to determine the effect of the pH of the leachant on the leaching of species from the ash residues recovered after the long-term fly ash–brine interactions. Acid neutralization capacity (ANC) tests using solutions of different pH values ranging from the initial pH of the ash residues (11–12) to pH 4 were employed in the leaching experiments. The ANC tests revealed that the release of major species from the ash residues depended on the pH of the leachants except for Na and Cl, where the significant concentrations leached were independent of the solution pH. The concentrations of Al and Si in the ANC leachates were very high at pH below 6 while Ca, K, Sr, Mg and B were immediately mobilized from the brine impacted fly ash when in contact with de-ionized water, and leaching increased as the pH decreased. The concentration of SO₄ leached from the brine impacted ash residues at high pH was high, and the leaching increased with decrease in the pH of the leachant. This study shows that most of the major elements captured in the ash residues could be mobilized when in contact with aqueous solutions of various pH. This reveals that the co-disposal of fly ash and brine may not be the best practice as the major elements captured in the ash residues could leach over time.     

P-T-t Evolution of Ultrahigh-Temperature Granulites from the Saxon Granulite Massif, Germany. Part II: Geochronology

Granulite-facies metamorphism in the structurally lower partof the Saxon Granulite Massif culminated at 1010–1060°Cand     

Hydrogeology of a coal-seam gas exploration area, southeastern British Columbia, Canada: Part 1. Groundwater flow systems

Discovery of high contents of methane gas in coals of the Mist Mountain Formation in the Elk River valley, southeastern British Columbia, Canada, has led to increased exploration activity for coal-seam gas (CSG). CSG production requires groundwater abstraction to depressurize the coal beds and to facilitate methane flow to the production wells. Groundwater abstraction will have hydrodynamic effects on the flow system, and an understanding of the groundwater flow system is needed to evaluate these effects. The purpose of this paper is to describe the groundwater flow system in the area by means of a groundwater flow model and interpretation of hydrochemical and isotopic analyses of groundwater and surface water. Groundwater flow for the Weary Creek exploration area is modeled in two vertical sections. The model domains, based on classic upland–lowland conceptual flow models, are approximately 10,000 m long and 4,000 m deep. Each consists of a fixed water-table boundary and no-flow boundaries along the traces of major faults. Steady-state groundwater flow is calibrated to hydraulic-head, streamflow, and groundwater-recharge data. Simulated steady-state velocity fields define regional and local flow components consistent with the conceptual model. The results are consistent with regional trends in δ²H, δ¹⁸O, tritium, and TDS, which define two distinct groundwater groups (A and B) and a third of intermediate composition. An active, shallow, local flow component (group A) is recharged in beds cropping out along subdued ridges; this component discharges as seeps along lower and mid-slope positions in the southern part of the study area. The waters are tritiated, relatively enriched in δ²H and δ¹⁸O, and have low TDS. A deeper regional flow component (group B), which originates at a higher altitude and which discharges to the Elk River valley bottom, is characterized by non-tritiated groundwater with relatively depleted δ²H and δ¹⁸O, and higher TDS. Groundwater contributes less than 10% of the total direct flow to the Elk River, as indicated by flow measurements and by the absence of group A and group B characteristics in the river water. Thus it is hypothesized that groundwater extraction during CSG production will have little impact on the river. The groundwater flow model developed in this work is used in a companion paper to further test this hypothesis. Electronic Publication     

The Geochemistry of Scapolite Part II. Trace Elements, Petrology, and General Geochemistry

Fifty scapolites have been analysed spectrographically for numerouselements. Average concentrations (p.p.m.) were as follows: B25, Be 9-3, Ga 33, Ti 82, Li 56, Cu 4-4, Zr 59, Mn 57, Sr 1,800,Pb 45, Ba 120, Rb 20. The following were seldom or never detected:Cr, Ni, Co, Mo, Sn, V, Sc, Ag, Y, La. The major elements Ca,Na, K were also determined. The distribution of the trace elementscan be explained by isomorphous substitution, but no detailedcorrelation of trace elements with each other or with majorelements was found. Refractive indices were determined and the relation betweenaverage index and per cent Me was examined: correlation waspoor, which may in part be attributed to analytical error. Examination of scapolite parageneses shows that scapolite characteristicallyoccurs in the upper amphibolite facies or the pyroxene hornfelsfacies: it is not restricted to these and may occur in any faciesfrom zeolitic to granulitic and in any hornfels facies. Theelements generally concentrated in scapolite include Ca, Na,C, Cl, S, H, B, Be, Li, Sr, Pb. The presence of C, Cl, S, Htestify to genesis in the presence of high partial pressureof CO₂, Cl₂, SO₃, H₂O (or related compounds), that is in pneumatolytic,pegmatitic, or hydrothermal environments. The concentrationof B, Be, Li can also be attributed to these conditions. The source of the elements concentrated in scapolite must inpart be common rocks. In a limited contact zone, the nearbymagma supplied some elements, but where regional scapolitizationhas taken place the presence of magma is less clear. Many commonrocks or rock series contain all the necessary constituents,but some particular conjunction of conditions is necessary forscapolite to form, or it would be more common.     

Mononodal indicator variography—Part II: Application to a computer-simulated deposit

The theory of mononodal variography developed in the preceeding paper is checked against a simulated deposit consisting of 60,500 grade values, called Stanford II. In the case of this deposit at least, assumptions underlying the concept of mononodal variography are borne out accurately. In particular, a linear relationship does exist indeed between indicator and grade variogram values of Stanford II at corresponding lags. Furthermore, such grade-indicator plots, and the information deduced from them, are robust under reduction of data at the mononodal cutoff. The method thus has predictive potential for grade variograms of highly variant deposits. Forecasting a grade variogram from the associated mononodal indicator variogram and grade-indicator plot is illustrated. Agreement with the experimental variogram is shown to be excellent.     

Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part II: Siderophore-promoted dissolution

Ferrihydrite (Fh) coprecipitated with exopolymers of plants and microbes may differ in its geochemical reactivity from its abiotic counterpart. We synthesized Fh in the presence and absence of acid polysaccharides (polygalacturonic acid (PGA), alginate, xanthan) and characterized the physical and structural properties of the precipitates formed [Mikutta C., Mikutta R., Bonneville S., Wagner F., Voegelin A., Christl I. and Kretzschmar R. (2008) Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part I: Characterization. Geochim. Cosmochim. Acta]. In this paper, we focus on the reactivity of PGA and alginate coprecipitates and pure Fh, and studied their interaction with the microbial siderophore desferrioxamine B (DFOB) in the presence and absence of low molecular weight organic (LMWO) acid anions (malate, citrate). Batch adsorption and dissolution experiments were performed in the dark at pH 7 in 10 mM NaClO₄ background electrolyte. In the dissolution experiments, different modes of ligand addition were applied (single, simultaneous, stepwise). With an estimated Langmuir sorption maximum of 15 mmol/mol Fe, a PGA coprecipitate with 11% C_org sorbed about four times as much DFOB as pure Fh, and the amount of DFOB sorbed was ∼4-fold larger than estimated from the sum of DFOB sorption to pure Fh and PGA alone. The apparent initial dissolution rates, R_app-initial, and pseudo-first order rate coefficients, k, of the coprecipitates exceeded those of pure Fh by up to two orders of magnitude. Citrate and malate exerted a strong synergistic effect on the DFOB-promoted dissolution of pure Fh, whereas synergistic effects of both anions were absent or negligible for the coprecipitates. R_app-initial of the citrate and DFOB-promoted dissolution of PGA coprecipitates increased with increasing molar C/Fe ratio of the coprecipitates, independent of the charge of the LMWO ligand. Our results indicate that polyuronates stabilize Fh particles sterically and /or electrostatically, thus increasing the mineral surface area accessible to LMWO ligands. In contrast, pure Fh was coagulated at pH 7 (pH_iep of Fh = 7.1), and hence only a small fraction of the Fh surface underwent dissolution. The increase in ligand-accessible surface area of Fh upon coprecipitation with acid polysaccharides seems to primarily control the kinetics of the ligand-promoted dissolution at neutral pH. In pH environments where the solubility of Fe(III) is very low, dissolution rates of Fe(III) (hydr)oxides in such coprecipitates may therefore exceed those of pure minerals by several orders of magnitude, despite a similar crystallinity of the minerals.     

Partial melting of metagreywackes, Part II. Compositions of minerals and melts

A series of experiments on the fluid-absent melting of a quartz-rich aluminous metagreywacke has been carried out. In this paper, we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe. This analytical work includes biotite, plagioclase, orthopyroxene, garnet, cordierite, hercynite, staurolite, gedrite, oxide, and glass, over the range 100–1000 MPa, 780–1025 °C. Biotites are Na- and Mg-rich, with Ti contents increasing with temperature. The compositions of plagioclase range from An₁₇ to An₃₅, with a significant orthoclase component, and are always different from the starting minerals. At high temperature, plagioclase crystals correspond to ternary feldspars with Or contents in the range 11–20 mol%. Garnets are almandine pyrope grossular spessartine solid solutions, with a regular and significant increase of the grossular content with pressure. All glasses are silicic (SiO₂ = 67.6–74.4 wt%), peraluminous, and leucocratic (FeO + MgO = 0.9–2.9 wt%), with a bulk composition close to that of peraluminous leucogranites, even for degrees of melting as high as 60 vol.%. With increasing pressure, SiO₂ contents decrease while K₂O increases. At any pressure, the melt compositions are more potassic than the water-saturated granitic minima. The H₂O contents estimated by mass balance are in the range 2.5–5.6 wt%. These values are higher than those predicted by thermodynamic models. Modal compositions were estimated by mass balance calculations and by image processing of the SEM photographs. The positions of the 20 to 70% isotects (curves of equal proportion of melt) have been located in the pressure-temperature space between 100 MPa and 1000 MPa. With increasing pressure, the isotects shift toward lower temperature between 100 and 200 MPa, then bend back toward higher temperature. The melting interval increases with pressure; the difference in temperature between the 20% and the 70% isotects is 40 °C at 100 MPa, and 150 °C at 800 MPa. The position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process. A comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 MPa. The difference in fertility between pelites and greywackes decreases with decreasing pressure. A review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust (greywackes, pelites, or orthogneisses) produces only peraluminous leucogranites. More mafic granitic compositions such as the various types of calk-alkaline rocks, or mafic S-type rocks, have never been obtained during partial melting experiments. Thus, only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments. Other types of granites involve other components or processes, such as restite unmixing from the source region, and/or interaction with mafic mantle-derived materials. Received: 11 July 1995 / Accepted: 27 February 1997