ALBERT

Description: The satellite instruments Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) provide unique empirical data about the frequency of lightning flashes around the globe (OTD), and the tropics (LIS), which 5 has been used before to compile a well received global climatology of flash rate densities. Here we present a statistical analysis of various additional lightning properties derived from OTD/LIS, i.e. the number of so-called "events" and "groups" per flash, as well as 10 the mean flash duration, footprint and radiance. These normalized quantities, which can be associated with the flash "strength", show consistent spatial patterns; most strikingly, oceanic flashes show higher values than continental flashes for all properties. Over land, regions with high (Eastern US) 15 and low (India) flash strength can be clearly identified. We discuss possible causes and implications of the observed regional differences. Although a direct quantitative interpretation of the investigated flash properties is difficult, the observed spatial patterns provide valuable information for the 20 interpretation and application of climatological flash rates. Due to the systematic regional variations of physical flash characteristics, viewing conditions, and/or measurement sensitivities, parametrisations of lightning NOx based on total flash rate densities alone are probably affected by regional biases.

Description: The Lightning Imaging Sensor LIS aboard the TRMM satellite provides unmatched empirical data of the global lightning distribution (up to approx.35deg S/N) since end of 1997. Climatological flash rate densities derived from LIS are standard references, e.g. for flash rate parameterizations used in GCMs. It is known that flash characteristics are quite variable, and that various quantities (like the flash energy or the NOx production per flash) vary considerably, statistically as well as systematically on regional and seasonal scales. LIS provides information beyond flash counts, in particular radiance and flash footprint. Here we present an analysis of global patterns of various lightning properties derived from LIS, in relation to the number of flashes. These normalized flash characteristics show consistent spatial patterns of regions with "strong" versus regions with "weak" lightning. Most striking is a clear land-ocean contrast, with oceanic flashes being "stronger" than continental flashes. But also over continents, flash strength shows systematic variations. Highest continental values are found over the US, while values over South America and India are quite low. These regional variations cannot be simply parameterized as function of latitude. Information on spatial patterns of mean flash "strength", though rather qualitative up to now, is potentially a valuable input for improving empirical parameterizations based on flash counts (like precipitation or lightning NOx). Further investigation is in progress to come to a more physical and quantitative understanding of the spatial patterns of the different LIS properties. In particular, it has to be checked how far they could be related to established lightning properties (like energy or the fraction of intra-cloud to cloud-to-ground flashes) or to meteorological quantities (like CAPE).

Description: Abstract. Metabasites in the high-pressure unit of the southern Dabie Shan (eastern China) contain quartz veins with high-pressure mineral assemblages. Two veins with the parageneses quartz-paragonite-ankerite-rutile and quartz-kyanite-talc-zoisite-rutile-calcite (pseudomorph after aragonite) as well as quartz-kyanite-paragonite-garnet-omphacite were investigated in detail. Host rocks of these veins are garnet amphibolites with eclogitic relics and quartz eclogites, respectively. The oldest phase of deformation (D1) is recorded in the eclogite, which displays a schistosity with shape alignment of high-pressure minerals such as omphacite, phengite and garnet. An estimate of the metamorphic P-T conditions yields 19-21 kbar at 570-620 °C for the eclogites. Fluid inclusion studies reveal a primary, low-salinity aqueous fluid phase responsible for the formation of the quartz veins. This is in correspondence with calculations of phase equilibrium curves on minerals of the vein paragenesis, which yield P-T conditions of 19.4 kbar and 591 °C in the presence of an aqueous fluid phase. Geochronology using the U-Pb system of rutile gives evidence for a Triassic minimum age of 207-221 Ma for the vein formation, which fits in the geotectonic framework of the orogen. This rather wide age range arises from the heterogeneous nature of the initial lead isotopic composition, reflecting the heterogeneous sources of the lead and the fluid phase. Part of the fluid was probably generated by prograde dehydration reactions in the country rocks of the veins, whereas strong evidence is also given for the derivation from an older basement, which also underwent subduction. The veins may have formed by extensive hydraulic fracturing or by volume reduction during eclogitisation of the crust. Subsequent to the formation of the veins, great parts of the high-pressure rocks experienced an intensive, syn-tectonic (D2) amphibolite facies metamorphism at 8.5-10.5 kbar and 590-645 °C, indicating isothermal decompression during exhumation. In the course of this exhumation, pseudosecondary CO2 inclusions were trapped in quartz of the veins. The last recorded stage of the retrograde overprint took place under static, greenschist facies conditions (450-480 °C) in the stability field of kyanite. The shape of the metamorphic P-T path for this stage is reflected by the fluid phase, which either re-equilibrated during this event or penetrated the rock on fractures. These findings show that fluid flow during subduction-related high-pressure metamorphism and subsequent uplift is rather complex; the fluid phase may originate from different sources and may interact with country rocks in the form of a chromatographic column transforming mineral assemblages, and also changing its own character.

Description: Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources or fluid histories. The former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, and the latter allows the reconstruction of precipitation and redox processes. These processes take place during ore formation or alteration. The aim of this contribution is to investigate the suitability of this new isotope method as a probe of ore-related processes. For this purpose 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, the ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300 °C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (〈100 °C). Measured iron isotope compositions are variable and cover a range in δ56Fe between -2.3‰ and +1.3‰. Primary hematite (δ56Fe: -0.5‰ to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: -0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: -1.4 to -0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitating from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethites from oolitic sedimentary iron ores were also analyzed. Their compositional range appears to indicate oxidative precipitation from relatively uniform Fe dissolved in coastal water. This comprehensive iron isotope study illustrates the potential of the new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.