The IGS VTEC maps: a reliable source of ionospheric information since 1998

The International GNSS Service (IGS) Working Group on Ionosphere was created in 1998. Since then, the Scientific community behind IGS, in particular CODE, ESA, JPL and UPC, have been continuosly contributing to reliable IGS combined vertical total electron content (VTEC) maps in both rapid and final schedules. The details on how these products are being generated, performance numbers, proposed improvement as far as VTEC evolution trends during near one Solar Cycle, are summarized in this paper. The confirmation of (1) the good performance of the IGS combined VTEC maps, and (2) the characteristic VTEC variability periods, are two main results of this work.     

New insights into the size and timing of the Lawn Hill impact structure: relationship to the Century Zn – Pb deposit

The Lawn Hill circular structure in northwest Queensland contains unambiguous evidence of an extraterrestrial impact, including planar deformation features in quartz, impact diamonds, widespread shatter cone formation and impact melt breccia in the Mesoproterozoic basement. The question of its relevance to ore genesis is investigated because the world-class Century Zn – Pb deposit is situated at the conjunction of the 100+ km Termite Range Fault and the previously defined margin of the impact structure. The impact structure is considered to be a 19.5 km wide feature, this constrained in part by the outer margin of an annulus of brecciated and highly contorted limestone. New evidence is presented indicating impact into this Cambrian limestone, including: (i) ‘dykes’ of brecciated Cambrian limestone extending hundreds of metres into the Mesoproterozoic basement; (ii) highly contorted bedding in the limestone annulus compared with essentially undeformed limestone away from the impact site; as well as (iii) a 1 Mt megaclast of Mesoproterozoic Century-like ore suspended in the limestone. Through aerial photograph analysis, large-scale convoluted flow structures within the limestone are identified, and these are interpreted to indicate that parts of the Cambrian sequence may have been soft or only semi-consolidated at the time of impact. This highly contorted limestone bedding is suggested to represent slump-filling of an annular trough in response to impact-induced partial liquefaction of a sediment veneer. The age of impact is therefore considered to be concurrent with limestone formation during the Ordian to early Templetonian, at 520 – 510 Ma. Formation of the Century deposit is found to be unrelated to impact-generated hydrothermal activity, although some minor hydrothermal remobilisation of metals occurred. However, there was macro-scale remobilisation of gigantic ore fragments driven by impact-induced lateral and vertical injection of limestone into the Proterozoic sediments. The limestone-filled annular trough surrounds a 7.8 km diameter central uplift, consistent with formation of a complex crater morphology.     

Zircon geochronology and geochemistry to constrain the youngest eruption events and magma evolution of the Mid-Miocene ignimbrite flare-up in the Pannonian Basin,eastern central Europe

A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U–Pb (LA-ICP-MS and ID-TIMS) and (U–Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U–Pb data is supported by (U–Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (>10 km³) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U–Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16–14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase.     

Contrasting stable isotope behavior between calcite and dolomite marbles,Lone Mountain,Nevada

 Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions that can be divided into two groups, one with positive δ¹³C values from+3.1 to +1.4 ‰ (PDB) and high δ¹⁸O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ¹³C values from –3.3 to –3.6‰ and low δ¹⁸O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent, clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ¹⁸O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite marbles typically have low δ¹⁸O values from +2.0 to +9.3‰. The δ¹⁸O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and +14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ¹⁸O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and quartz overgrowths in dolomite marbles, low δD values of micas, negative δ¹³C values and low δ¹⁸O values of calcite and dolomite, and depleted δ¹⁸O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the Tertiary. Partial oxygen isotopic exchange between calcite and low ¹⁸O meteoric fluids lowered the δ¹⁸O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration. Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in ¹⁸O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the quartz as a result of its dissolution by meteoric fluids. The negative δ¹³C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western Nevada in Tertiary time. Received: 1 February 1994/Accepted: 12 September 1995     

GGR Biennial Review: Atomic Absorption,Inductively Coupled Plasma-Atomic Emission Spectrometry,Neutron Activation Analysis and X-Ray Fluorescence Spectrometry Review for 2008–2009

This review describes developments major and in trace element determination using atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), instrumental neutron activation analysis (INAA), and X-ray fluorescence (XRF) spectrometry that were reported in 2008 and 2009. Publication levels were comparable to those of previous years, except for XRF which showed an increase in the number of articles published in the past 2 years. In terms of number of publications and impact, the most active field was AAS, while INAA was the least active. As expected for well-established and mature methods, novel developments for all four analytical techniques were relatively sparse. For AAS, the most notable publications concerned developments in sample introduction, particularly focussing on solid samples, increasing analyte sensitivity, and higher sample throughput. For ICP-AES, publication of developments in sample introduction, remediation of matrix effects and calibration continues. Compared with past years, there was a clear focus on sample preconcentration methods, but very few publications reporting new hyphenated speciation methods. For INAA, there were several publications exploring the accuracy and robustness of the method, as well as the requirements for INAA to meet criteria for a primary method of measurement. Two other related techniques, delayed neutron activation analysis, and prompt gamma activation analysis, were also described. Hyphenated XRF techniques showed interesting developments in enabling XRF and XRD analysis on the same spot, and further work characterising and calibrating three-dimensional micro-XRF shows promising results for investigating sample heterogeneity.     

Partitioning of Ru, Rh, Pd, Re, Ir, and Au between Cr-bearing spinel, olivine, pyroxene and silicate melts

A series of high temperature experiments was undertaken to study partitioning of several highly siderophile elements (HSE; Ru, Rh, Pd, Re, Os, Ir, Pt and Au) between Cr-rich spinel, olivine, pyroxene and silicate melt. Runs were carried out on a Hawaiian ankaramite, a synthetic eucrite basalt, and a DiAn eutectic melt, at one bar, 19 kbar, and 20 kbar, respectively, in the temperature range of 1200 to 1300°C, at oxygen fugacities between the nickel-nickel oxide (NNO) and hematite-magnetite (HM) oxygen buffers. High oxygen fugacities were used to suppress the formation of HSE-rich “nuggets” in the silicate melts. The resulting oxide and silicate crystals (<100 μm) were analyzed using both SIMS and LA-ICP-MS, with a spatial resolution of 15 to 50 μm. Rhenium, Au and Pd were all found to be incompatible in Cr-rich spinel (D_Re^sp/melt = 0.0012-0.21, D_Au^sp/melt = 0.076, D_Pd^sp/melt = 0.14), whereas Rh, Ru and Ir were all found to be highly compatible (D_Rh^sp/melt = 41-530, D_Ru^sp/melt = 76-1143, D_Ir^sp/melt = 5-22000). Rhenium, Pd, Au and Ru were all found to be incompatible in olivine (D_Re^oliv/melt = 0.017-0.073, D_Pd^oliv/melt = 0.12, D_Au^oliv/melt = 0.12, D_Ru^oliv/melt = 0.23), Re is incompatible in orthopyroxene and clinopyroxene (D_Re^opx/melt = 0.013, D_Re^cpx/melt = 0.18-0.21), and Pt is compatible in clinopyroxene (D_Pt^cpx/melt = 1.5). The results are compared to and combined with previous work on HSE partitioning among spinel-structured oxides, and applied to some natural magmatic suites to demonstrate consistency.