Rb-Sr and K-Ar ages on minerals at temperatures of 300°–400° C from deep wells in the Larderello geothermal field (Italy)

Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete ⁴⁰Ar and ⁸⁷Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL)     

Localized climatic responses during the 11 July 1991 eclipse: Phoenix AZ

Measurements taken from a variety of land use environments in and around the Phoenix AZ metropolitan area were compared to determine the effects of various land use characteristics on the thermal response to a reduction of 68% in solar radiation during the solar eclipse of 11 July 1991. The results include: (a) The magnitude of the change in ambient air temperature during the eclipse is related to land use. Heavily asphalted military sites and irrigated golf courses recorded small decreases in temperature while natural desert terrain experienced the largest decreases. Variations in thermal response are more closely related to the albedo of the specific land type than to other factors such as thermal admittance but this high correlation is probably an artifact of the combination of other variables such as moisture. (b) Due to the complex mix of land uses and timing of eclipse, marked wind speed changes associated with the eclipse do not appear to be linked directly to land use changes, and, (c) Temperature minima at residential/commercial stations occurred in general, before the minima at stations in agricultural/golf terrains. Because typical albedo differences are not significant between these two land uses, land use characteristics other than albedo are most likely controlling the timing of the temperature minima. Opportunities to empirically examine concepts previously testable only through numerical modeling such as solar reduction experiments should continue to be exploited.     

Ar/Ar dating of amphiboles from Zabargad Island (Red Sea) is precluded by interaction with fluids

Stepwise heating data on four amphiboles separated from various Zabargad rocks (a dolerite dike, a peridotite, a gneiss and a pyroxenite) invariably show saddle-shaped age spectra. The age information in the spectrum is generally masked by 1–2 nl/g excess ⁴⁰Ar. The spectrum least affected by excess Ar constrains the emplacement age of the dolerite dike (Z-2137) to be 20 Ma. The greatest concentration of excess Ar is found in the peridotite, implying that the ultramafic rocks were metasomatized by a very ⁴⁰Ar-rich fluid. A complex history of subsolidus reactions with hydrothermal fluids is suggested by the irregularities in the ³⁷Ar/³⁹Ar spectra.     

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen,northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.     

Late quaternary tephra layers along the Cilento coastline (southern Italy)

The results of stratigraphic and tephrostratigraphic analyses of several pyroclastic levels, collected along the coastal sector of the Cilento region (southern Italy), are presented. Some of these levels are here described for the fist time, others, already known in literature, were reconsidered in order to better understand their stratigraphic position and to point out the possible volcanic sources.     

Isotopic fingerprints of Milankovitch cycles in Pennsylvanian carbonate platform‐top deposits: the Valdorria record,Northern Spain

Sedimentary cyclic sequences deposited during the Late Palaeozoic Ice Age are widespread. Glacio‐eustatic control of the cyclic patterns is commonly accepted, and the durations of the cyclothems generally match the short‐ and long‐eccentricity Milankovitch orbital parameters. Nevertheless, geochemical fingerprints of orbital parameters are poorly documented in deep‐time sedimentary records. Here, we report on well‐exposed Bashkirian cyclothems of c. 123 ka and c. 400 ka durations from the Valdorria platform. The shorter‐term cyclothems can be grouped into longer‐term composite sequences that are consistent with generally accepted durations of c. 125 ka and c. 400 ka for Milankovitch eccentricity cycles. The stratigraphic pattern is mirrored by the isotope geochemical signals, which show distinct recurring trends. These trends are confirmed by statistical tests. Whereas intrinsic factors and/or subaerial exposure related to sea‐level lowstands might have truncated cycle patterns in tectonically stable basins, rapid subsidence of the Valdorria platform's foreland basin appears to have contributed to a faithful recording of cyclothems of different orders. The patterns and biostratigraphic constraint revealed in this study demonstrate the power of orbital forcing in imprinting sedimentary and geochemical signals in the rock record.     

Measurements of Beam Pattern Perturbation in Corrugated Feed Horn Arrays for CMB Observations

An experimental study on the perturbation of the angular response of a W-band corrugated horn produced by nearby feeds, reproducing a typical situation of multi-feed arrays commonly used in high sensitivity Cosmic Microwave Background instruments, is reported in this paper. The effects for different positions of the scattering horn in the two principal polarisation planes were measured. We analyze the effects on both the side-lobe level and on the main beam. In particular, these results allowed us to validate design criterion for the multi-frequency focal array of the ESA PLANCK mission.This revised version was published online in July 2005. The first author’s e-mail address was removed.     

Isotopic closure

The closure of isotopic chronometers to loss or exchange of radiogenic daughters is best analysed in terms of phenomena that induce recrystallization, i.e. fluid circulation and strain. In the presence of these fast phenomena, temperature is not a rate-controlling parameter for isotope transport. Only in those rare instances where no fast mechanism can be petrographically documented, do minerals survive very high temperatures without appreciable modification of their isotopic record. For these (admittedly unlikely) rocks, a self-consistent upward recalibration of the 'closure temperature' is defined.