Thermal Stability of Assemblages in the Cu--Fe--S System

The phase relations in the Cu-Fe-S system were determined from700 C to approximately 200 C in most portions of the systemand below 100 C in restricted areas. Approximate solid solutionlimits for bornite, chalcopyrite, and pyrrhotite were determinedat elevated temperatures. At low temperatures emphasis was placedon establishing the stable assemblages and less on determiningthe compositions of coexisting phases. At 700 C two extensiveternary solid solutions dominate the phase relations in thissystem. One of these solid solutions (bornite) includes thecompositions Cu₂S, Cu₁₈S, and Cu₅FeS₄and the other (chalcopyrite)lies with in the area bounded by the compositions CuFeS₂ CuFe₂S₃,and CU₃Fe₄S₄. The two fields are separated by approximately10 weight per cent copper at 700 C. The chalcopyrite volume,as seen in a trigonal prism representing temperature and composition,is intersected by a miscibility gap below approximately 600C.Below this temperature the two one-phase volumes are referredto as chalcopyrite and cubanite. Chalcopyrite is tetragonalat low temperature but isometric above approximately 550C.The temperature of the transformation is a function of composition.Cubanite is isometric above 252C, tetragonal from 252 to atleast 213C, and orthorhombic at lower temperature. The temperatureof the second transformation is unknown because the tetragonal-to-orthorhombictransformation has not been achieved in the laboratory. Borniteand pyrite become stable together at 568C and coexist downto 228C. Covellite appears with lowering temperature at 507C,and idaite at 501C. Idaite—pyrite and idaite—borniteare stable assemblages below 501 C. The composition of bornitecoexisting with idaite changes gradually towards digenite withdecreasing temperature, thus permitting the change from thebornite—pyrite tie-line to the digenite—chalcopyritetie-line at 228C. Other major tie-line changes are bornite—ironto pyrrhotite—copper below 475C and cubanite—pyriteto chalcopyrite—pyrrhotite below 334C. A new syntheticphase, x-bornite, which has a composition close to bornite (Cu₅FeS₄)but contains about 04 weight per cent more sulfur, forms whensulfur-rich bornite synthesized at high temperature is annealedbetween 62 and 140C. Optically this new phase is very similarto bornite, and their X-ray powder diffraction patterns aregiven for comparison. o The determined phase relations are applicable to numerous deposits.The tie-line changes involving bornitepyrite reacting to producedigenitechalcopyrite below 228 C and cubanite (isometric)pyritegoing to chalcopyritepyrrhotite below 334 C are of considerablegeological interest. The rates of these reactions are sufficientlyslow to allow the higher temperature assemblages to be observedin some ores. The cubic—tetragonal inversion in chalcopyriteis often deduced in ores by inversion twins. However, twinningis also commonly produced through deformation. Geological applicationof the inversion therefore depends on correct interpretationof the twinning. Because of the considerable solubility of copperin pyrrhotite the pyrrhotite—pyrite solvus of the pureFe—S system cannot be applied indiscriminately to oresthat also contain chalcopyrite or cubanite, or both. The newx-bornite phase was identified with the natural ‘anomalousbornites’, which when heated exsolve chalcopyrite and,depending on their composition, also digenite. The experimental results indicate that the mineral commonlyidentified as chalcopyrrhotite is in reality tetragonal or evenisometric cubanite. Experimental evidence could not be obtainedfor the existence of a phase of Cu₂Fe₄S₇ or Cu₂Fe₄S₇ composition,the older formulae given foor valleriite. The thermal breakdownof natural material supports the idea that valleriite is a low-temperaturepolymorph of chalcopyrite. The relatively uncommon occurrenceof idaite in comparison to covellite is attributed to the greaterdifficulty in nucleating idaite. The possibility of stable coexistenceof chalcocite and pyrite was investigated but was found to beprohibited by tie-lines between bornite and digenite even aslow as 100 C.     

Scaling impact melting and crater dimensions: Implications for the lunar cratering record

Abstract— The dimensions of large craters formed by impact are controlled to a large extent by gravity, whereas the volume of impact melt created during the same event is essentially independent of gravity. This “differential scaling” fosters size-dependent changes in the dynamics of impact-crater and basin formation as well as in the final morphologies of the resulting structures. A variety of such effects can be observed in the lunar cratering record, and some predictions can be made on the basis of calculations of impact melting and crater dimensions. Among them are the following: (1) as event magnitude increases, the volume of melt created relative to that of the crater will grow, and more will be retained inside the rim of the crater or basin. (2) The depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth-diameter trend and the simple-to-complex transition. (3) The volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the Moon; this would arguably correspond to a Moon-melting event. (4) Small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts, increasing the melt's viscosity and chilling it rapidly. Larger craters and basins should suffer little from such a process. (5) Deep melting near the projectile's axis of penetration during larger events will yield a progression in central-structure morphology; with growing event magnitude, this sequence should range from single peaks through multiple peaks to peak rings. (6) The minimum depth of origin of central-peak material should coincide with the maximum depth of melting; the main central peak in a crater the size of Tycho should have had a preimpact depth of close to 15 km.     

A Strontium Isotopic Investigation of the Bjerkreim--Sokndal Layered Instrusion, Southwest Norway

The 230 km² Proterozoic Bjerkreim—Sokndal layered instrusionhas a monzonoritic bulk composition and comprises a 6 km thick,broadly leuconoritic Layered Series (LS) overlain by unlayeredmangerite and quartz mangerite. In the Bjerkreim lobe the LScomprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV)in a syncline. This lobe is surrounded by migmatitic gneisses;the roof to the instrusion is missing. The mg-number opx, An% and whole-rock initial ⁸⁷Sr/⁸⁶Sr isotope ratios (Sr₀) displaybroadly parallel trends through MCUs II and III, and into MCUIV, with decreasing mg-number (75–58) and An % (50–40)accompanying increasing Sr₀ (0.7050–0.7085). This correlationbreaks down in the upper part of MCU IV and mangerite; the quartzmangerite has Sr₀ values of 0.7085. Abrupt reversals occur acrossthe MCU boundaries. The LS crystallized on the floor of a periodically replenishedmagma chamber that was continually assimilating country rockgneisses. Strong compositional zoning of the magma developedas a result of repeated replenishments with relatively dense,primitive ferrobasaltic magma (Sr₀ = 0.7049) along the floorand the development of extensive buoyant roof melts. Assimilationtook place on a massive scale (up to 50%) in the upper partsof the chamber. KEY WORDS: assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isolopes ^*Corresponding author.     

An Experimental Study of Nepheline-Kalsilite Exsolution

The nepheline-kalsilite exsolution reaction was studied isothermallybetween 400 and 700°C. Under nonaqueous conditions the mechanisminvolves nucleation of kalsilite and growth by diffusion ofthe alkalis. As predicted by simple nucleation theory, the nucleationrate and hence the over-all exsolution rate are strongly dependenton the supersaturation of the nepheline. A decrease in temperatureat constant composition increases the supersaturation and therebythe nucleation rate. This increased nucleation rate is opposedby the decrease in the growth rate due to slower volume diffusion.At a supersaturation of more than 8–10 mole per cent thenumber of nuclei is large and the over-all exsolution rate isdetermined primarily by the growth rate. The activation energyfor growth is 28 kcal/mole. An increase of two kilobars in thehydrostatic pressure has little effect on the kinetics of thereaction. Under nonhydrostatic conditions the exsolution rateincreases significantly because the nucleation rate is faster. Under hydrothermal conditions the ‘exsolution’ rateis approximately two orders of magnitude faster due to a modificationin the mechanism. Partial dissolution of the original solidsolution in distilled water creates a condition of nonequilibriumin which the fluid is sodium-rich. Rapid alkali exchange eliminatesthis condition but produces the equilibrium compositions ofthe solids because kalsilite nucleates and grows in contactwith the fluid. The experimental evidence for this mechanismincludes X-ray diffraction data showing a gradual change inthe composition of the initial supersaturated solid, essentiallyidentical activation energies for growth under aqueous and nonaqueousconditions, and a lower percentage of oxygen isotope exchangethan ‘exsolution’ in the same experiment.     

DAM IMPACTS ON AND RESTORATION OF AN ALLUVIAL RIVER-RIO GRANDE, NEW MEXICO

1 INTRODUCTION The construction of more than 75,000 dams and reservoirs on rivers in the United States (Graf, 1999) has resulted in alteration of the hydrology, geometry, and sediment flow in many of the river channels downstream of dams. Additionally, hydrologic and geomorphic impacts lead to changes in the physical habitat affecting both the flora and fauna of the riparian and aquatic environments. Legislation for protection of endangered species as well as heightened interest in ma…     

Archean High-Mg Granodiorite: A Derivative of Light Rare Earth Element-enriched Monzodiorite of Mantle Origin

The Roaring River Complex, Superior Province, Canada, containsrocks varying from diorite and monzodiorite to granodioritewhich are characterized by high mg-numbers (0.43–0.62),high abundances of Cr (150 ppm), Sr (500–2000 ppm), Ba(1000–2500ppm), and P2O5 (0.5 wt.%), low Rb/Sr ratios (001–0.02),and steeply fractionated, subparallel REE patterns (Ce_n =65–170,Yb_n = 3–6) without Eu anomalies. The continuous compositionalvariation of the rock suite provides a basis for testing thevarious processes thought to have been important in the extractionof granodiorite magmas from the mantle during the Archean. Weconsider (1) the relative roles of partial melting, crystallizationfractionation, and other processes; (2) the role of garnet orother phases in controlling the steep REE patterns of the rocks;and (3) the chemical and isotopic composition of the sourceregion. The subparallel and decreasing REE patterns with increasingsilica, and the ten-fold variation and high abundances of Crand Ni within the diorite-granodiorite series are not consistentwith different extents of melting of basic crust. The scatterin bivariate plots for closely spaced samples does not supportsimple two-component mixing or liquid immiscibility. The compositionalvariation can be explained by crystallization differentiation(from 0 to 90%) of monzodioritic magma through separation ofdioritic cumulates containing clinopyroxene, hornblende, biotite,plagioclase, K-feldspar, and accessories. The compatibilityof the REEs resulted principally from crystallization of spheneand apatite. The parental monzodioritic magmas with their high mg-numbers,Ni, and Cr contents were derived from peridotitic source rocks(mg-numbers>0.80) with low Rb/Sr ratios (<0.02) and light-REEenrichment relative to chondrites. The differences in the REEpatterns of monzodiorite samples do not support, nor rule out,garnet in the residue for melting. If the monzodioritic meltswere derivatives of other melts, the parent melts would havebeen similar to high-Mg monzodiorites (‘sanukitoids’)recognized as components of other diorite-granodiorite bodiesin the region. An Rb-Sr whole-rock isochron (n = 25) yields a minimum crystallizationage of 2623 Ma (?19) with initial ⁸⁷Sr/⁸⁶Sr = 070134 (?000004;MSWD=l.8). Sm-Nd isotope data for six rocks yield _Nd (2623)=+0.8 ?0.3. The isotope data indicate a source region with long-termRb/Sr of 0.02, similar to depleted mantle, and light-REE depletionrelative to chondrites. The peridotite source to the diorite-granodioriteseries became light-REE enriched before melting through theaddition of a light-REE component of a fluid or melt. In generating Archean granodiorite with suitably high mg-numbers,and Ni, Cr, Sr, Ba, P2O5, and light-REE contents, these dataindicate: (1) the importance of crystallization differentiationof high-Mg monzodioritic parent magmas, (2) that the steep REEpatterns may be a characteristic of the source rocks, and (3)light-REE-enriched, peridotitic sources were melting and contributingsiliceous material directly to the Archean crust.     

Atmospheric Effects of the Emerging Mainland Chinese Transportation System at and Beyond the Regional Scale

Local surface travel needs in the Peoples Republic of China (mainlandChina) have traditionally been met largely by nonpolluting bicycles. A majorautomobile manufacturing/importing effort has begun in the country over thelast decade, and planning documents indicate that the Chinese may strive toacquire more than 100 million vehicles early in the next century. By analogywith large automotive fleets already existing in the western world, bothregional and global scale pollution effects are to be expected from theincrease. The present work adopts the latest projections of Chinese automobilemanufacture and performs some quantitative assessments of the extent ofpollution generation.Focus for the investigation is placed upon the oxidant ozone. Emissions of theprecursor species nitrogen oxides and volatile organics are constructed basedon data for the current automotive sector in the eastern portion of the UnitedStates. Ozone production is first estimated from measured values forcontinental/oceanic scale yields relative to precursor oxidation. Theestimates are then corroborated through idealized two dimensional modeling ofthe photochemistry taking place in springtime air flow off the Asian land massand toward the Pacific Ocean. The projected fleet sizes could increase coastaland remote oceanic ozone concentrations by tens of parts per billion (ppb)in the lower troposphere. Influences on the tropospheric aerosol system andon the major greenhouse gas carbon dioxide are treated peripherally. Nitrogenoxides created during the vehicular internal combustion process willcontribute to nitrate pollution levels measured in the open Pacific. Thepotential for soot and fugitive dust increases should be considered as theautomotive infrastructure develops. Since the emerging Chinese automotivetransportation system will represent a substantial addition to the globalfleet and all the carbon in gasoline is eventually oxidized completely, asignificant rise in global carbon dioxide inputs will ensue as well.Some policy issues are treated preliminary. The assumption is made thatalterations to regional oxidant/aerosol systems and to terrestrial climate areconceivable. The likelihood that the Chinese can achieve the latest vehiclefleet goals is discussed, from the points of view of new production, positivepollution feedbacks from a growing automobile industry, and known petroleumreserves. Vehicular fuel and maintenance options lying before the Chinese areoutlines and compared. To provide some perspective on the magnitude of theenvironmental changes associated with an Asian automotive buildup, recentestimates of the effects of future air traffic over the Pacific Rim aredescribed.     

Style and timing of glaciation in the Lahul Himalaya,northern India: a framework for reconstructing late Quaternary palaeoclimatic change in the western Himalayas

This paper presents a revised glacial chronology for the Lahul Himalaya and provides the most detailed reconstruction of former glacier extents in the western Himalayas published to date. On the basis of detailed geomorphological mapping, morphostratigraphy, and absolute and relative dating, three glaciations and two glacial advances are constrained. The oldest glaciation (Chandra glacial stage) is represented by glacially eroded benches and drumlins (the first to be described from the Himalaya) at altitudes of >4300 m and indicates glaciation on a landscape of broad valleys that had minimal fluvial incision. The second glaciation (Batal glacial stage) is represented by highly weathered and disssected lateral moraines and drumlins representing two phases of glaciation within the Batal glacial stage (Batal I and Batal II). The Batal stage was an extensive valley glaciation interrupted by a readvance that produced superimposed bedforms. Optically stimulated luminescence (OSL) dating, indicates that glaciers probably started to retreat between 43400 ± 10300 and 36900 ± 8400 yr ago during the Batal stage. The Batal stage may be equivalent to marine Oxygen Isotope Stage 4 and early Oxygen Isotope Stage 3. The third glaciation (Kulti glacial stage), is represented by well-preserved moraines in the main tributary valleys that formed due to a less-extensive valley glaciation when ice advanced no more than 12 km from present ice margins. On the basis of an OSL age for deltaic sands and gravels that underlie tills of Kulti age, the Kulti glaciation is younger than 36900 ± 8400 yr ago. The development of peat bogs, having a basal age of 9160 ± 70 ¹⁴C yr BP possibly represents a phase of climatic amelioration coincident with post-Kulti deglaciation. The Kulti glaciation, therefore, is probably equivalent to all or parts of late Oxygen Isotope Stage 3, Stage 2 and early Stage 1. Two minor advances (Sonapani I and II) are represented by small sharp-crested moraines within a few kilometres of glacier termini. On the basis of relative weathering, the Sonapani advance is possibly of early mid-Holocene age, whereas the Sonapani II advance is historical. The change in style and extent of glaciation is attributed to topographic controls produced by fluvial incision and by increasing aridity during the Quaternary. © 1997 John Wiley & Sons, Ltd.     

Origin of Paleoproterozoic Komatiites at Jeesiorova, Kittila Greenstone Complex, Finnish Lapland

Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al₂O₃/TiO₂ ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial ¹⁸⁷Os/¹⁸⁸Os of 0·1131 ± 0·0006(2), _Os(I) = 0·1 ± 0·5. These data, coupledwith an initial _Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment