Stratospheric aerosol measurements in the Arctic winter of 1996/1997 with the M‐55 Geophysika high‐altitude research aircraft

In‐situ aerosol measurements were performed in the northern hemispheric stratosphere up to altitudes of 21 km between 13 November 1996 and 14 January 1997, inside and outside of the polar vortex during the Airborne Polar Experiment (APE) field campaign. These are measurements of particle size distributions with a laser optical particle counter of the FSSP‐300 type operated during 9 flights on the Russian M‐55 high‐altitude research aircraft Geophysika. For specific flights, the FSSP‐300 measurements are compared with balloon‐borne data (launched from Kiruna, Sweden). It was found that the stratospheric aerosol content reached levels well below the background concentrations measured by the NASA operated ER‐2 in 1988/89 in the northern hemisphere. During the APE campaign, no PSC particle formation was observed at flight altitudes although the temperatures were below the NAT condensation point during one flight. The measured correlations between ozone and aerosol give an indication of the subsidence inside the 1996/97 polar vortex. Despite the lower aerosol content in the winter 1996/97 compared to the 1989 background, the heterogeneous reactivity of the aerosol (as calculated from the measured data with additional model input) is comparable. This is due to the dependency of the reactive uptake coefficients on the atmospheric water vapor content. Under the described assumptions the reaction rates on the background aerosol are significantly smaller than for competing gas phase chlorine activation, as can be expected for stratospheric background conditions especially inside the polar vortex.     

Recent Volcanism in the Puyehue--Cordon Caulle Region, Southern Andes, Chile (40?5?S): Petrogenesis of Evolved Lavas

Puyehue Volcano (40?5?S) in the southern volcanic zone (33?–46?)of the Andes is a largely basaltic stratovolcano constructedon a highly eroded, dominantly andesitic volcanic center. Duringgrowth of Puyehue Volcano there was a trend from basaltic tomore siliceous lavas, and the most recent eruptions (1921–22,1960) are Cordon Caulle rhyodacites and rhyolites erupted fromfissures northwest of the volcano. These basaltic through rhyoliticlavas define a medium-K₂O suite of tholeiitic affinity withtrace element and Pb-isotopic signatures typical of volcanicrocks associated with subduction zones. Most of the evolved lavas, ranging from andesite to rhyolite,formed by low to moderate pressure ( 5 kb) fractional crystallizationof a plagioclase-dominated anhydrous assemblage. Magma mixingproduced aphyric basaltic andesites with anomalously high incompatibleelement contents and latestage andesites with disequilibriumphenocryst assemblages. The age progression from abundant basaltto younger, less voluminous, more silicic lavas reflects increasinglygreater degrees of fractional crystallization which caused theapparent compositional gap between mixing end members to widen. There is no evidence in the silicic lavas for assimilation ofgeochemically distinctive continental crust. Puyehue basaltsare surprisingly more heterogeneous in ⁸⁷Sr/⁸⁶Sr (0?70378–0?70416)and incompatible element abundance ratios (e.g., La/Sm, Ba/Nb)than the more evolved lavas. This geochemical variability mayreflect subcrustal source heterogeneities or contamination bylower crust. The older basaltic andesites and andesites underlyingthe Puyehue edifice have Sr and Nd isotopic ratios and incompatibleelement abundance ratios within the range of Puyehue basalts.Apparently, similar sources and processes were involved in theirgenesis.     

Geochemistry of the Active Azufre--Planchon--Peteroa Volcanic Complex, Chile (3515'S): Evidence for Multiple Sources and Processes in a Cordilleran Arc Magmatic System

Along strike of the Quaternary magmatic arc in the SouthernVolcanic Zone of the Andes, there is a south to north increasein crustal thickness, and the lavas define systematic geochemicaltrends which have been attributed to variations in the proportionsand compositions of mantle-and crustal-derived components. Realisticinterpretations of these regional geochemical trends requiresan understanding of the sources and processes that control lavacompositions at individual volcanoes. Because it is in an importantgeophysical and geochemical transition zone, we studied theAzufre—Planchon—Peteroa volcanic complex, a nestedgroup of three volcanoes <055 m.y. in age located at 3515'Sin the Southern Volcanic Zone of the Andes. North of this complexat 33–35S the continental crust is thick, basalts areabsent, and there is abundant evidence for crustal componentsin the evolved lavas, but south of 37S, where the crust isrelatively thin, basaltic lavas are abundant and the contributionof continental crust to the lavas is less obvious. In additionto its location, this volcanic complex is important becausethere is a diversity of lava compositions, and it is the northernmostexposure of recent basaltic volcanism on the volcanic front.Therefore, the lavas of this complex can be used to identifythe relative roles of mantle, lower-crustal and upper-crustalsources and processes at a single location. Volcan Azufre is the oldest and largest volcano of the complex;it is a multi-cycle, bimodal, basaltic andesite–dacitestratovolcano. Volcan Planchon is the northernmost basalt-bearingvolcano along the volcanic front of the Southern Andes, andVolcan Peteroa, the youngest volcano of the complex, has eruptedmixed magmas of andesitic and dacitic composition. Most basalticandesite lavas at Azufre and Planchon are related by a plagioclase-poor,anhydrous mineral fractionating assemblage. High-alumina basaltis produced from a tholeiitic parent by an 4–8 kbar fractionatingassemblage. During this moderatepressure crystallization, themagmas also incorporated a crustal component with high La/Yband high abundances of Rb, Cs and Th. Based on the chemicalcharacteristics of the added component and the inferred depthof crystallization, the crustal source may have been garnetgranulite derived from solidified arc magmas in the lower tomiddle continental crust. At Planchon, the role of crustal assimilationhas increased with decreasing eruption age probably becausecrustal temperatures have increased during continued volcanism.Azufre dacite lavas formed at low pressures by fractionationof a plagioclase-rich assemblage. These dacite lavas containan upper-crustal component, probably derived in part from limestone,with high values of ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O. Thus two depths (upperand lower crust) of crystallization and associated crustal assimilationare evident in Planchon–Azufre lavas. Peteroa, the focusof recent volcanism, consists of calc-alkaline andesite anddacite eruptive products whose textures and compositions indicatean important role for magma mixing. Therefore, the volcanismevolved from a tholeiitic system of basalt and subordinate dacite(Planchon–Azufre) to a calc-alkaline system with abundantmixed lavas of intermediate composition (Peteroa). In additionto crustal thickness, two important parameters which controlledthe diversity of lava composition in this complex are magmasupply rate from the mantle and crustal temperature. Both parametersvaried with time, and they must be considered in broader interpretationsof along-strike geochemical trends. KEY WORDS: arc magmas; Andes; Peteroa; Planchan; geochemistry ^*Corresponding author. Present address: ENTRIX, Inc., 4II North Central Avenue, Glendale, CA 91203, USA     

A climatological study of polar stratospheric clouds (1989–1997) from LIDAR measurements over Dumont d'Urville (Antarctica)

Backscatter lidar data from the French Antarctic base in Dumont d'Urville (66.40°S, 140.01°E), including aerosol background and observations of polar stratospheric clouds (PSCs), have been collected since 1989. In the present work we present a climatological study of PSCs, using a data base consisting of almost 90 observations. The seasonal evolution of PSCs, their optical classification, and their relationship with the observation temperature were studied. The first PSC was observed on day number 175 (15 June) and the last on day number 260 (7 September). The characteristic mid‐cloud altitude decreases through the season at a rate of 2.5 km/month. Type Ia, Ib, and II PSCs — identified by their optical properties — have been observed. External mixtures of these types have also been observed. These observations have been related to the local temperature measured by radiosondes. The relationship between PSC type and the period of the winter season was also investigated. Mixed (solid and liquid) type I clouds are mostly observed at the beginning of the winter. Type II clouds are observed only during the coldest period around midwinter, although temperatures below the frost point begin earlier and persist longer than this. Type Ia, solid‐particle, clouds are observed mostly at the end of the winter.