The behavior of light lithophile and halogen elements in felsic magma: geochemistry of the post-caldera Valles Rhyolites, Jemez Mountains Volcanic Field, New Mexico

Post-collapse rhyolite lava domes, lava flows and pyroclastic rocks from Valles caldera (1140 ka), erupted from 1133 ka to approximately 520-60 ka, have been sampled to study variations of light lithophile (Li, Be, B) and halogen (F, Cl) elements. Our principal objectives were: (1) to examine the mobility of these elements during post-eruptive devitrification and hydration; and (2) to study their behavior during magma differentiation. Compared to fresh glassy samples, devitrified rocks from the same dome are depleted in B, Li, F and Cl, but not in Be. During devitrification, Be was immobile while the other elements were progressively more mobile in the order B < Li < Cl < F, fluorine being the most mobile element. Considering only fresh glassy samples, Li, Be and B were enriched in residual liquids and behaved incompatibly during differentiation of successive magma batches at 973-787 ka and 557-521 ka. The rhyolites have low B/Be ratios of 2–3 which decrease slightly with increasing Be; these values suggest a small fractionation of B from Be during evolution of the magmas. While F behaves like the light lithophile elements, Cl shows (1) much smaller temporal enrichment during differentiation at 973-787 ka and (2) depletion with time from 557 to 521 ka. At the same time, the Cl/Be ratio declines progressively from ˜250 in the oldest rhyolites to ˜ 100 in the youngest rhyolites. These data suggest that (1) a magmatic fluid phase continuously extracted Cl from fluid-saturated magmas and (2) some of the magmatic Cl lost could have been incorporated into the Valles hydrothermal system.     

The petrology of basalts from Surtla (Surtsey), Iceland

Surtla is the site of a short-lived submarine vent which built basaltic elastic deposits almost to sea level, in 1963, early in the eruption of Surtsey. Since then wave and current activity have eroded the volcanic pile such that in July 1981 its top was a fairly level plateau 45 m below sea level, and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules. This winnowed layer was underlain by a nonreworked, poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles, steam explosions and quench brecciation. During the eruption, the explosion violence and associated comminution increased as the pile built up to shallower water depths. It is argued that at times of continuous effusion a cupola of steam was situated over the vent, as indicated by scoriaceous spatter which shows agglutination and “bread-crust” features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption.     

The growth of large cities in India 1961–1971

The general characteristics and the growth 1961–1971 of class 1 cities (those with more than 100,000 population in 1971) in India are examined. An unexpected correlation shows that the larger cities are more specialized, and growing faster. A factor analysis reduces the original variables to apparently significant factors, but these fail to classify the individual cities convincingly. An alternative simple classification of cities in terms of growth rates and degree of specialization shows strong regional components, dividing India into an emerging system of dynamic interdependent cities, and areas of stagnation and lack of specialization. The only counter to this pattern appears to be cities which are the subject of heavy government investment. Some speculative thoughts are offered on the effects of such investment in backward regions.     

Tectonically deformed pillow lava as an indicator of bedding and way-up

Often the outlines of tectonically deformed lava pillows cannot be used to give directly the true bedding and way-up directions. Where the pillows are strained homogeneously, with moderate total strains, certain procedures allow the determination of the true bedding and way-up directions. Where the strain history has been coaxial, these are: the use of pillows fortuitously parallel with a principal extension direction and graphical restoration of pre-tectonic pillow shapes to fix true bedding directions. Furthermore, use may be made of special pillow shapes with flat bases or multiple cusps, even where the strain history has been noncoaxial. With total strain ratios > 4.5, heterogeneous strain often occurs and it may not be possible to determine the true bedding and way-up directions.     

Thermochemistry of the tremolite-edenite amphiboles using fluorine analogues,and applications to amphibole-plagioclase-quartz equilibria

Heats of mixing of synthetic C2/m fluortremolite-fluoredenite amphiboles measured at 985 K show a systematic deviation from ideal mixing consistent with a subregular solution model. The deviations from ideal mixing are interpreted in terms of Na ordering in the A-site and Na-Al interactions in edenite-poor compositions. Enthalpies of edenite substitution reactions in amphiboles and in SiO₂-NaAlO₂ glasses and framework silicates are comparable. Gibbs free energies of formation of fluortremolite and fluoredenite at 298K are -2,821.07±3.34 kcal mol^–1 and -2,889.59±2.40 kcal mol^–1 respectively. The former value is in good agreement with values calculated from both F-OH exchange experiments and from a natural fluortremolite-bearing metamorphic rock. Least-squares fitted sub-regular heat-of-mixing parameters are poorly constrained and unrealistically high, but estimated subregular mixing parameters consistent with 95% confidence interval uncertainties in the calorimetric data and with TEM constraints give activity-composition relations in good agreement with the A-site compositions of natural metamorphic and igneous hornblendes. These relations predict unmixing in edenite-rich compositions over a wide range of temperature, but lend no support to the existence of a hornblende-actinolite miscibility gap. Calibration of the reaction tremolite+ albite=edenite+4 quartz as a function ofP,T andX _ed ^amph indicates negativedP/dT slopes and a limited range of X _ed ^amph (0.3 to 0.5) in equilibrium with plagioclase and quartz over a wide range of pressure and temperature, consistent with metamorphic hornblende-plagioclase assemblages. The energetics of this reaction suggest, however, that amphibole-plagioclase disequilibrium may be common.     

Composition and reactivity of some river sediments from the Mackenzie Valley,N.W.T., Canada

Sediments from the Mackenzie Valley — two suspended and one river bottom — were analyzed for major and minor elements, mineralogy, particle size, and ability to release trace metals when subjected to mild leaching. Copper and zinc in suspended sediments and cadmium in bottom sediment occurred largely (70 to 84 percent for Cu, 72 to 98 percent for Zn, 75 to 81 percent for Cd) in a form that could not easily be separated from the sediments by 0.1M HC1 and EDTA solutions. Cadmium was present in suspended sediments in low concentrations, and was present totally in sorbed leachable form. Iron, manganese, cobalt and chromium were separable from the particulate phase by mild leaching. It was found that approximately 70 hours were required for an apparent steady state concentration to be attained in such leachings. Reaction of these sediments with Beaufort Sea water (salinity 22.3 parts per thousand) indicated that both iron and manganese were released from sediments to sea water to a very small degree. Copper, lead, and zinc, however, were absorbed by sediments from sea water.     

Apollo 17 KREEPy basalt: A rock type intermediate between mare and KREEP basalts

The Apollo 17 KREEPy basalt is a unique lunar volcanic rock, observed only as clasts in the light friable breccia matrix (72275) of Boulder 1, Station 2 at Taurus-Littrow. Its status as a volcanic rock is confirmed by the absence of any meteoritic contamination, a lack of cognate inclusions or xenocrystal material, and low Ni contents in metal grains.The basalt was extruded 4.01 ± 0.04 b.y. ago, approximately contemporaneously with the high-alumina mare basalts at Fra Mauro; shortly afterwards it was disrupted, probably by the Serenitatis impact, and its fragments emplaced in the South Massif. The basalt, which is quartz-normative and aluminous, is chemically and mineralogically intermediate between the Apollo 15 KREEP basalts and the high-alumina mare basalts in most respects. It consists mainly of plagioclase and pigeonitic pyroxene in approximately equal amounts, and 10–30% of mesostatis. Minor phases outside of the mesostatis are chromite, a silica mineral, Fe-metal, and rare olivine; the mesostatis consists primarily of ilmenite, Fe-metal, troilite, and ferroaugite, set in a glassy or microcrystalline Si-rich base.Chemical and isotopic data indicate that an origin by partial melting of a distinct source region is more probable than hybridization or contamination of magmas, and is responsible for the transitional composition of the basalt. The moon did not produce two completely distinct volcanic groups, the KREEP basalts and the mare/mare-like basalts; some intermediate rock types were generated as well. A corresponding spectrum of source regions must exist in the interior of the moon.