Sources and sinks for atmospheric H2: A current analysis with projections for the influence of anthropogenic activity

Oxidation of CH₄ provides the major source for atmospheric H₂ which is removed mainly by reaction with OH. Biological activity at the Earth's surface appears to represent at most a minor sink for H₂. Anthropogenic activity is a significant source for both H₂ and CO in the present atmosphere and may be expected to exert a growing influence in the future. Models are presented which suggest a rise in the mixing ratio of H₂ from its present value of 5.6 × 10^?7 to about 1.8 × 10^?6 by the year 2100. The mixing ratio of CO should grow from 9.7 × 10^?8 to 2.3 × 10^?7 over the same time period and there should be a rise in CH₄ by about a factor of 1.5 associated with anthropogenically induced reductions in tropospheric OH.     

Phase relationships in Buchan facies series pelitic assemblages: calculations with application to andalusite-staurolite parageneses in the Mount Lofty Ranges,South Australia

Low-pressure, medium- to high-temperature (Buchan-type) regional metamorphism of pelitic rocks in the Mount Lofty Ranges, South Australia, is defined by the development of biotite, staurolite-andalusite, fibrolite, prismatic sillimanite and migmatite zones. K-feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite, concentrating particularly on well-developed andalusitestaurolite parageneses. In general, the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and P-T and T-X _Fe pseudo-sections constructed from the internally consistent data set of Holland and Powell (1990) in the system KFMASH, assuming a(H₂O) 1, although analysed white mica compositions are systematically more aluminous than predicted. Importantly, the stability ranges of most critical assemblages predicted by these grids and pseudo-sections coincide closely with P-T estimates calculated using the data set of Holland and Powell (1990) from the Mount Lofty Ranges assemblages. With the exception of Mn in garnet and Zn in one staurolite-cordierite-muscovite assemblage non-KFMASH components do not significantly appear to have affected the stability ranges of the observed assemblages. An apparent local reversal in isograd zonation in which andalusite first appears down-grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and, together with evidence for essentially isobaric heating of individual rocks, is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels.Mineral abbreviations used in figures als Al₂SiO₅ phase - bi biotite - chl chlorite - ky kyanite - ph phengite - sill sillimanite - and andalusite - cd cordieritc - gt garnet - mu muscovite - q quartz - st staurolite     

Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

An ⁴⁰Ar/³⁹Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental ⁴⁰Ar/³⁹Ar release spectra indicate that substantial differential unroofing occurred in the CMB between 1000 and 600 Ma. A consistent pattern of significantly older hornblende and phlogopite ⁴⁰Ar/³Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende ⁴⁰Ar/³⁹Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite ⁴⁰Ar/³⁹Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield ⁴⁰Ar/³⁹Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before 600 Ma as required by the deposition of overlying undeformed Cambrian and/or Ordovician sedimentary rocks.Contribution No. 481 from the Mineralogical Laboratory, University of Michigan     

Guide and examples for users of the ‘SPACEKAP’ style file

Guide and examples for users of the SPACEKAP style fileBasic instructions     

Spectral-compositional variations in the constituent minerals of mafic and ultramafic assemblages and remote sensing implications

The 0.3–2.6 m reflectance spectra of most mafic and ultramafic assemblages can best be interpreted by considering the spectra as being composed of mafic silicate spectra modified by the presence of opaques, such as ilmenite or magnetite, and plagioclase feldspar. The systematic spectral-compositional relationships for olivine, orthopyroxene, and clinopyroxene have been examined and it has been determined that absorption band wavelength positions are correlated with ferrous iron content. Binary mafic silicate mixtures are generally less well understood, but certain spectral features such as reflectance maxima and minima wavelength positions and absorption band areas can be used to quantify or at least constrain end member abundances and compositions. The addition of opaques to a mafic silicate assemblage lowers overall reflectance and band depths. This differs from the effects of increasing grain size which are to lower overall reflectance but increase band depths. Plagioclase is relatively transparent compared to mafic silicates and must be present in appreciable amounts (tens of percent) to be spectrally detectable. The reflectance spectra of most mafic and ultramafic assemblages are dominated by mafic silicate absorption features and analysis of their spectra on this basis allows constraints to be placed on properties such as end member abundances and compositions.     

Structure of the marine boundary layer over north western Indian Ocean during 1983 summer monsoon

The spatial variability of the structure of the lower troposphere over the northwestern Indian Ocean for the period 12th July to 2nd September, 1983 has been studied using upper air data collected during the first scientific cruise of ORV Sagar Kanya.An analysis of thermodynamic structure and kinematics of the marine boundary layer for different zonal and meridional sections revealed the following features: (a) Temperature and humidity inversions were generally absent over the study area except over a few locations in the western region; (b) Large-scale subsidence was found over the central equatorial Indian Ocean; (c) The convective activity over the western Indian Ocean was found to be moderately suppressed as compared to the eastern region; (d) The zonal and meridional components of winds along the equator and 10° N zonal section exhibited a mirror-image-like distribution.     

Magmatism in the eastern Aleutian Arc: temporal characteristic of igneous activity on Akutan Island

Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO₂). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO₂), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO₂) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An_92?45), olivine (Fo_88?51), orthopyroxene (En_69?56), and titanomagnetite (15%–21% TiO₂) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased⁸⁷Sr/⁸⁶Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.     

Chemical exchange during hydrothermal alteration of basalt by seawater—I. Experimental results for major and minor components of seawater

Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO₂, SO₄), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO₂, Ca, K, Ba, B and CO₂ were leached from basalt and enriched in solution. SiO₂ concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.