δ18O and chemical composition of Libyan Desert Glass,country rocks,and sands: New considerations on target material

Oxygen isotope and chemical measurements were carried out on 25 samples of Libyan Desert Glass (LDG), 21 samples of sandstone, and 3 of sand from the same area. The δ¹⁸O of LDG samples range from 9.0‰ to 11.9‰ (Vienna Standard Mean Ocean Water [VSMOW]); some correlations between isotope data and typological features of the LDG samples are pointed out. The initial δ¹⁸O of a bulk parent material may be slightly increased by fusion due to the loss of isotopically light pore water with no isotope exchange with oxygen containing minerals. Accordingly, the δ¹⁸O of the bulk parent material of LDG may have been about 9.0 ± 1‰ (VSMOW). The measured bulk sandstone and sand samples have δ¹⁸O values ranging from 12.6‰ to 19.5‰ and are consequently ruled out as parent materials, matching the results of previous studies. However, separated quartz fractions have δ¹⁸O values compatible with the LDG values suggesting that the modern surface sand inherited quartz from the target material. This hypothesis fits previous findings of lechatelierite and baddeleyite in these materials. As the age of the parent material reported in previous studies is Pan‐African, we measured the δ¹⁸O values of bulk rock and quartz from intrusives of Pan‐African age and the results obtained were compatible with the LDG values. The main element abundances (Fe, Mg, Ca, K, Na) in our LDG samples conform to previous estimates; Fe, Mg, and K tend to be higher in heterogeneous samples with dark layers. The hypothesis of a low‐altitude airburst involving silica‐rich surface materials deriving from weathered intrusives of Pan‐African age, partially melted and blown over a huge surface by supersonic winds matches the results obtained.     

Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications

The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO₂relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H₂O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H₂O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H₂O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO₂ lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H₂O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H₂O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO₂ at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H₂O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria     

Flood and Shield Basalts from Ethiopia: Magmas from the African Superswell

The Ethiopian plateau is made up of several distinct volcaniccentres of different ages and magmatic affinities. In the NE,a thick sequence of 30 Ma flood basalts is overlain by the 30Ma Simien shield volcano. The flood basalts and most of thisshield volcano, except for a thin veneer of alkali basalt, aretholeiitic. In the centre of the province, a far thinner sequenceof flood basalt is overlain by the 22 Ma Choke and Guguftu shieldvolcanoes. Like the underlying flood basalts, these shieldsare composed of alkaline lavas. A third type of magma, whichalso erupted at 30 Ma, is more magnesian, alkaline and stronglyenriched in incompatible trace elements. Eruption of this magmawas confined to the NE of the province, a region where the lavaflows are steeply tilted as a result of deformation contemporaneouswith their emplacement. Younger shields (e.g. Mt Guna, 10·7Ma) are composed of Si-undersaturated lavas. The three maintypes of magma have very different major and trace element characteristicsranging from compositions low in incompatible elements in thetholeiites [e.g. 10 ppm La at 7 wt % MgO (=La₇), La/Yb = 4·2],moderate in the alkali basalts (La₇ = 24, La/Yb = 9·2),and very high in the magnesian alkaline magmas (La₇ = 43, La/Yb= 17). Although their Nd and Sr isotope compositions are similar,Pb isotopic compositions vary considerably; ²⁰⁶Pb/²⁰⁴Pb variesin the range of     

Comparison of 40Ar-39Ar and Rb-Sr Data on Phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): Implications for Dating White Mica

Different lithologies (impure marble, eclogite and graniticorthogneiss) sampled from a restricted area of the coesite-bearingBrossasco–Isasca Unit (Dora Maira Massif) have been investigatedto examine the behaviour of ⁴⁰Ar–³⁹Ar and Rb–Srsystems in phengites developed under ultrahigh-pressure (UHP)metamorphism. Mineralogical and petrological data indicate thatzoned phengites record distinct segments of the P–T path:prograde, peak to early retrograde in the marble, peak to earlyretrograde in the eclogite, and late retrograde in the orthogneiss.Besides major element zoning, ion microprobe analysis of phengitein the marble also reveals a pronounced zoning of trace elements(including Rb and Sr). ⁴⁰Ar–³⁹Ar apparent ages (35–62Ma, marble; 89–170 Ma, eclogite; 35–52 Ma, orthogneiss),determined through Ar laserprobe data on phengites (step-heatingand in situ techniques), show wide intra-sample and inter-samplevariations closely linked to within-sample microchemical variations:apparent ages decrease with decreasing celadonite contents.These data confirm previous reports on excess Ar and, more significantly,highlight that phengite acted as a closed system in the differentlithologies and that chemical exchange, not volume diffusion,was the main factor controlling the rate of Ar transport. Conversely,a Rb–Sr internal isochron from the same eclogite yieldsan age of 36 Ma, overlapping with the time of the UHP metamorphicpeak determined through U–Pb data and thereby corroboratingthe previous conclusion that UHP metamorphism and early retrogressionoccurred in close succession. Different phengite fractions ofthe marble yield calcite–phengite isochron ages of 36to 60 Ma. Although this time interval matches Ar ages from thesame sample, Rb–Sr data from phengite are not entirelyconsistent with the whole dataset. According to trace elementvariations in phengite, only Rb–Sr data from two wet-groundphengite separates, yielding ages of 36 and 41 Ma, are internallyconsistent. The oldest age obtained from a millimetre-sizedgrain fraction enriched in prograde–peak phengites mayrepresent a minimum age estimate for the prograde phengite relics.Results highlight the potential of the in situ ⁴⁰Ar–³⁹Arlaser technique in resolving discrete P–T stages experiencedby eclogite-facies rocks (provided that excess Ar is demonstrablya negligible factor), and confirm the potential of Rb–Srinternal mineral isochrons in providing precise crystallizationages for eclogite-facies mineral assemblages. KEY WORDS: ⁴⁰Ar–³⁹Ar dating; Rb–Sr dating; phengite; SIMS; UHP metamorphism     

Dolomite characteristics and diagenetic model of the Calcari Grigi Group (Asiago Plateau,Southern Alps – Italy): an example of multiphase dolomitization

A multidisciplinary study, conducted over the carbonate platform deposits of the Liassic Calcari Grigi Group (Southern Alps), highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks, to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history. This study was carried out in the Asiago Plateau (southernmost part of the eastern Southern Alps, northern Italy) which provides excellent outcrops of the Jurassic Calcari Grigi Group. The dolomitization of the Jurassic sequence is variable in terms of stratigraphic extension and geographic distribution. In the studied localities the dolomitization is generally limited to the Mount Zugna Formation and is characterized by an undulatory front, with ‘sub‐vertical dolomitic chimneys’ along the major faults. Within this unit, and often associated with faults, stacked high‐porosity and permeability bed‐parallel dolomitic bodies are developed that show excellent petrophysical properties. The dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals. Thin section, cathodoluminescence, isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities. The first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core. The porosity is thought to have increased after this stage, with dark blue luminescent dolomite accompanied by the corrosion of older crystals. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infilling of vugs and the remaining open pores with calcite cement. The diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores. Paragenetic and fluid‐inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation. The integration of the available data seem to indicate that the diagenetic evolution of the study area is related to: (i) the interplay between evolving fluids (from marine to brackish); (ii) the burial of the sequence (increasing temperature); and (iii) the evolution of the hydrogeological system (fault and fracture network, fluid mixing). This complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good, widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements.     

EROSION PROCESSES AT THE HILLSLOPE SCALE

The results are presented of an intensive monitoring programme to determine eroded volumes due to observed rainfall events on a hillslope surface. The surface investigated has been reproduced through a digital elevation model and analysed in terms of drainage network, contributing area and slope by performing planimetric and altimetric analysis. Erosion maps were derived from a comparison of consecutive digital elevation models relative to time. These maps reveal the spatial and temporal evolution of the erosion process at the hillslope scale. The erosion was uniform across the surface, supporting the assumption of randomness in the erosion process commonly used in surface drainage development models. The observed value of erosion has been estimated at approximately 0.11 m/m² per year, with almost 500 mm of total annual rainfall.     

Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley

Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO₂ conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalite–magnetite–quartz buffer)and at high fF₂ inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite [molar (Na₂O+ K₂O)/Al₂O₃ (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A     

Petrological and Experimental Constraints on the Pre-eruption Conditions of Holocene Dacite from Volcan San Pedro (36{degrees}S, Chilean Andes) and the Importance of Sulphur in Silicic Subduction-related Magmas

We present an experimental and petrological study aimed at estimatingthe pre-eruptive conditions of a Holocene dacitic lava fromVolcán San Pedro (36°S, Chilean Andes). Phase-equilibriumexperiments were performed at temperatures (T) from 800 to 950°C,and mainly at 200 MPa, but also at 55, 150, and 406 MPa. Oxygenfugacity (fO₂) ranged from the Ni–NiO buffer (NNO) to3·5 log units above (NNO + 3·5), and water contentsfrom