The origin and significance of groundwater-seepage gypsum from Bristol Dry Lake, California, USA

Gypsum and anhydrite fabrics observed in trenches and deep (500 m) cores from Bristol Dry Lake, California, USA, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentological evidence indicate that the gypsum formed in Bristol Dry Lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater-seepage gypsum). Evidence for displacive growth of gypsum is: (i) the geometry of the deposit, (ii) stable isotopic data and the water chemistry of the brine, and (iii) inclusions of matrix which follow twin planes and completely surround crystals as they grow. The bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake. Sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin. Basinward of this zone, sulphate concentrations decrease sharply to trace element levels in the basin centre brine. Authigenic gypsum is rare in the centre of the playa. Stable (δ¹⁸O values measured for gypsum waters of crystallization (GWC) are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments (?– 6%0), whereas measured brine δ¹⁸O values range from + 0·5 to + 3·7%0. Deuterium values measured for groundwater are ?– 70%0, GWC are ?– 60 to – 65%0 and brine values are ?– 57%0. The geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine. However, some mixing between groundwater and brine is likely. Within 100 m of the surface, the gypsum dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process. The similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in Bristol Dry Lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained.     

Anaerobic diagenesis within Recent, Pleistocene, and Eocene marine carbonate frameworks

Porewaters from a variety of Recent, Pleistocene, and Eocene lithified marine carbonate frameworks displayed similar chemical characteristics: highly depleted concentrations of dissolved oxygen (>20 μM), elevated levels of dissolved methane (25-5000 nM), and near-seawater sulphate levels. These porewaters also had low pH values (7·5-7·9), and contained elevated concentrations of sulphide (4–10 μM), dissolved inorganic carbon (2·05–2·46 mM), and inorganic nutrients. Hydrocarbon composition data indicate that the methane is biogenic, whereas the methane δ¹³C values (–47·4 ± 2·7%0) suggest that it has been subject to oxidation. The porewater dissolved inorganic carbon δ¹³C values varied from –0·6 to –39%0, suggesting input of carbon dioxide from organic matter oxidation. We conclude that anaerobic diagenesis involving bacterial degradation of organic matter is a common process in lithified marine carbonates and hypothesize that it may be an important factor controlling their carbonate geochemistry.     

An Ion and Electron Microprobe Study of the Mineralogy of Enclaves and Host Syenites of the Red Hill Complex, New Hampshire, USA

The Red Hill complex of New Hampshire is unusual for the WhiteMountain Magma Series of northern New England because it consistsof both silica-undersaturated and -saturated to -oversaturatedsyenites. Amphibole, pyroxene, and apatite in two of the saturatedunits, the Outer Coarse Syenite (OCS) and the Garland Peak Syenite(GPS), and in the undersaturated Nepheline Sodalite Syenite(NSS), were analyzed to determine the relationship between coexistingunder-saturated and saturated magmas. Mafic enclaves in theNSS and the GPS were also studied to elucidate their relationshipswith the host syenites. In addition to mafic enclaves, the NSS contains later emplacedcamptonitic dikes and associated pipe-like benmoreites. Thebenmoreites contain amphibole that is compositionally continuouswith amphibole in the NSS. However, REE and other trace elementabundances in apatite from the benmoreites and the NSS are notcompatible with a genetic relationship between the two. Maficenclaves within the NSS contain amphibole and pyroxene thatare compositionally continuous with the NSS. Bulk-rock compositionsof the enclaves plot along trends defined by the NSS. Furthermore,chondrite-normalized REE patterns for apatite in both the enclavesand the NSS are parallel, and REE abundances increase systematicallyfrom the enclaves to the NSS. We therefore suggest that theenclaves represent magmas similar to the NSS parent that intrudedup into its daughter products. These magmas appear to have beentephritic to phonotephritic in composition. Abundances of REE in apatite in the Nepheline Sodalite Syenite(NSS) are distinct from those in apatite in the silica-saturatedOCS. OCS apatites have LREE abundances up to 26 000 times chondritesand La/Yb ratios of 16–27. NSS apatites have comparableLREE concentrations, but HREE abundances are considerably lowerthan those of the OCS; La/Yb ratios range from 68 to 104. Theseobserved differences in both the REE and other trace elementabundances between apatite in the two rocks present difficultieswith a common parental magma hypothesis for the NSS and OCS.Hence it is suggested that, although the OCS and NSS are contemporaneousin time and space, they are probably not consanguineous. The silica-saturated GPS is a fine-grained syenite containingstrongly zoned amphiboles with kaersutite to hastingsite coresrimmed by hastingsitic hornblende and ferro-hornblende. Discretegrains of hastingsitic hornblende and ferro-hornblende occurin a feldspar-quartz groundmass. Coarser-grained, quartz-richpatches, containing feldspars and ferro-hornblende and ferroedenite,are also found in the GPS. The kaersutite cores are identicalto the amphibole in the GPS enclaves and the NSS suite. TheseGPS enclaves are silica undersaturated; the kaersutite coresin the GPS host rocks are probably xenocrysts derived from disaggregatedundersaturated magmas similar to that represented by the enclaves.     

A new location of Holocene dolomite formation, Lake Hayward, Western Australia

Holocene dolomite forms in the sediment of Lake Hayward, a small permanent hypersaline lake in the Clifton-Preston Lakeland System, Western Australia. The geomorphological setting of dolomite formation in Lake Hayward is similar to the Coorong region in South Australia. Unlike in the Coorong region, dolomite in Lake Hayward does not form as a direct precipitate from the lake water, but is of diagenetic origin. This can be deduced from the following features: (1) the dolomite occurs only below 60–70 cm from the sediment-water interface, (2) dolomite occurs as luminescing cement, and (3) dolomite has pristine well-formed rhomb-shaped crystals. The source of magnesium for dolomitization is probably from the concentration on inflowing groundwater by evaporation and the selective removal of calcium by chemical and biological aragonite/calcite precipitation.     

A collection of papers by Björn Kurtén

A collection of papers by Bjorn Kurth, CHRISTINE JANIS
BJORN KURTEN 1924 11 19-1988 1228     

Metamorphic Evolution of Iron-rich Mafic Cumulates from the Otztal-Stubai Crystalline Complex, Eastern Alps, Austria

Olivine-rich rocks containing olivine + orthopyroxene + spinel+ Ca-amphibole ± clinopyroxene ± garnet are presentin the central Ötztal–Stubai crystalline basementassociated with eclogites of tholeiitic affinity. These rockscontain centimetre-sized garnet layers and lenses with garnet+ clinopyroxene ± corundum. Protoliths of the olivine-richrocks are thought to be olivine + orthopyroxene + spinel dominatedcumulates generated from an already differentiated Fe-rich () tholeiitic magma that was emplaced into shallowcontinental crust. Protoliths of the garnet-rich rocks are interpretedas layers enriched in plagioclase and spinel intercalated ina cumulate rock sequence that is devoid of, or poor in, plagioclase.U–Pb sensitive high-resolution ion microprobe dating ofzircons from a garnet layer indicates that emplacement of thecumulates took place no later than 517 ± 7 Myr ago. Aftertheir emplacement, the cumulates were subjected to progressivemetamorphism, reaching eclogite-facies conditions around 800°Cand >2 GPa during a Variscan metamorphic event between 350and 360 Ma. Progressive high-P metamorphism induced breakdownof spinel to form garnet in the olivine-rich rocks and of plagioclase+ spinel to form garnet + clinopyroxene ± corundum inthe garnet layers. Retrogressive metamorphism at T 650–680°Cled to the formation of Ca-amphibole, chlorite and talc in theolivine-rich rocks. In the garnet layers, högbomite formedfrom corundum + spinel along with Al-rich spinel, Ca-amphibole,chlorite, aspidolite–preiswerkite, magnetite, ilmeniteand apatite at the interface between olivine-rich rocks andgarnet layers at P < 0·8 GPa. Progressive desiccationof retrogade fluids through crystallization of hydrous phasesled to a local formation of saline brines in the garnet layers.The presence of these brines resulted in a late-stage formationof Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterizedby extremely high Cl contents of up to 3·5 and 6·5wt % Cl, respectively. KEY WORDS: cumulates; Variscan metamorphism; SHRIMP dating; högbomite; saline brines     

Corundum-bearing residues produced through the evaporation of natural and synthetic hibonite

Abstract— We have produced corundum-bearing residues through the evaporation of natural and synthetic hibonite samples. The sequence of major element losses as well as volatility related trace element fractionations in these residues are similar to those previously observed in residues from the evaporation of chondritic starting material, which suggests that the processes by which these fractionations occur may be largely independent of the starting material used. However, the mineralogy of the residues does depend on the composition of the starting material and, to some extent, on the conditions under which evaporation took place. Similarly, the degree of isotopic mass fractionation observed in the residues is composition-dependent. This observation means that it may be possible to use isotopic data for several elements to constrain the compositions of precursor materials of Ca-Al-rich inclusions, which have an evaporation origin. Although corundum-bearing inclusions are known, their origins are complex and variable, and the scarcity of such inclusions indicates that melting of hibonite, with or without concomitant evaporation, must have been a rare process in the solar nebula. By evaporating mixtures of synthetic oxides of the rare earth elements, we have reproduced the patterns of Group III inclusions and some of the characteristics of ultrarefractory patterns. However, the extreme conditions required to do so indicate that refractory inclusions with these patterns probably have a condensation rather than evaporation origin.