Unravelling the effects of melt depletion and secondary infiltration on mantle Re-Os isotopes beneath the French Massif Central

Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events.Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic ¹⁸⁷Os/¹⁸⁸Os, and (ii) primary sulfides with high [Os] and unradiogenic ¹⁸⁷Os/¹⁸⁸Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).     

Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A.

Geomorphic, stratigraphic, geotechnical, and biogeographic evidence indicate that failure of a Pleistocene ice dam between 15.5 and 26 ka generated a megaflood from Glacial Lake Atna down the Matanuska Valley. While it has long been recognized that Lake Atna occupied ≥ 9000 km² of south-central Alaska's Copper River Basin, little attention has focused on the lake's discharge locations and behaviors. Digital elevation model and geomorphic analyses suggest that progressive lowering of the lake level by decanting over spillways exposed during glacial retreat led to sequential discharges down the Matanuska, Susitna, Tok, and Copper river valleys. Lake Atna's size, ∼ 50 ka duration, and sequential connection to four major drainages likely made it a regionally important late Pleistocene freshwater refugium. We estimate a catastrophic Matanuska megaflood would have released 500–1400 km³ at a maximum rate of ≥ 3 × 10⁶ m³ s^{− 1}. Volumes for the other outlets ranged from 200 to 2600 km³ and estimated maximum discharges ranged from 0.8 to 11.3 × 10⁶ m³ s^{− 1}, making Lake Atna a serial generator of some of the largest known freshwater megafloods.     

Research progress in volcanology in the Neapolitan area, southern Italy: a review and some alternative views

Comprehensive reviews are given for the major volcanic systems that occur in the greater metropolitan area of Naples, southern Italy; Mt. Somma-Vesuvius to the east and the Campi Flegrei volcanic system to the west. Also included in the review is a detailed discussion of the large, highly explosive Campania Volcanic Zone (CVZ) ignimbrite events. These volcanic areas have been studied for more than 100 years, yet significant differences of opinion exist related to fundamental issues of origin and distribution. We present some alternative views related to petrogenesis on some issues based on more than 25 years of research. The relationship between risk assessment and management that impacts the threatened society or culture and the past and ongoing fundamental volcanological research is an essential part of the science. Countries with limited resources may be forced to accept an increased risk but even highly industrialized societies may not be able to completely eliminate deaths from volcanic eruptions. Scientific studies of the hazardous regions should be comprehensive and include reasonable alternative interpretations as this information reveals the level of confidence that must be conveyed to the public officials. The authors review the state of the art of risk assessment and management of the volcanic hazards in the Neapolitan region in light of the review of research.     

Nondestructive determination of the physical properties of Antarctic meteorites: Importance for the meteorite—parent body connection

Photogrammetry is a low-cost, nondestructive approach for producing 3-D models of meteorites for the purpose of determining sample bulk density. Coupled with the use of a nondestructive magnetic susceptibility/electrical conductivity field probe, we present measurements for the interrogation of several physical properties, on a set of Antarctic meteorites. Photogrammetry is an effective technique over a range of sample sizes, with meteorite bulk density results that are closely comparable with literature values, determined using Archimedean glass bead or laser scanning techniques. The technique is completely noncontaminating and suitable for the analysis of rare or fragile samples, although there are limitations for analyzing reflective samples. It is also flexible, and, with variations in equipment setup, may be appropriate for samples of a wide range of sizes. X-ray computed tomography analyses of the same meteorite samples yielded slightly different bulk density results, predominantly for samples below 10 g, although the reason for this is unclear. Such analyses are expensive and potentially damaging to certain features of the sample (e.g., organic compounds), but may be useful in expanding the measurements to accommodate an understanding of internal voids within the sample, lending itself to measurement of grain density. Measurements of bulk density are valuable for comparisons with estimates of the bulk densities of asteroids that are suggested as meteorite parent bodies.     

Realizable accessibility: evaluating the reliability of public transit accessibility using high-resolution real-time data

Journal of Geographical Systems - The widespread availability of high spatial and temporal resolution public transit data is improving the measurement and analysis of public transit-based...     

‘Born to Run’: Remembering C.P. Pow

C.P. Pow was my confidant, colleague and (occasional) collaborator for more than 20 years. He had fought a valiant and dignified battle with an aggressive cancer but ultimately succumbed to it in July 2021. I will reminisce a few of the little known traits of Pow which had shaped him to become the conscientious, obliging and humble academic that he was known for.     

Results from the Greenland Search for Meteorites expedition

Abstract— Following discoveries of blue ice areas in Greenland resembling meteorite‐bearing blue ice fields in Antarctica, a surface search of several of the most promising sites was carried out in August 2003. The ice fields are located in Kong Christian X Land, in northeastern Greenland around 74°N at elevations between 2100 and 2400 m. No meteorites were found in any of the localities that were searched. Evidence of occasional significant melting (filled crevasses and melt sheets) suggest that summer temperatures are sometimes high enough that dark rocks, like meteorites, can melt through the upper layers of ice. Small terrestrial rocks and cryogenite were found down to 50 cm below the ice surface. Meter‐sized terrestrial rocks were found on top of the ice downstream from nunataks. These rocks shade the ice below, and since they were apparently too massive to warm up during warm days, they remained at the surface as the surrounding ice ablated away. Our findings strongly suggest that Greenland is currently unlikely to harbor significant meteorite concentrations on blue ice fields.     

The photospheric magnetic flux budget

The ensemble of bipolar regions and the magnetic network both contain a substantial and strongly variable part of the photospheric magnetic flux at any phase in the solar cycle. The time-dependent distribution of the magnetic flux over and within these components reflects the action of the dynamo operating in the solar interior. We perform a quantitative comparison of the flux emerging in the ensemble of magnetic bipoles with the observed flux content of the solar photosphere. We discuss the photospheric flux budget in terms of flux appearance and disappearance, and argue that a nonlinear dependence exists between the flux present in the photosphere and the rate of flux appearance and disappearance. In this context, we discuss the problem of making quantitative statements about dynamos in cool stars other than the Sun. This paper evolved out of a more comprehensive version which appeared in Harvey (1993).     

Phase Equilibria Constraints on the Chemical and Physical Evolution of the Campanian Ignimbrite

The Campanian Ignimbrite is a > 200 km³ trachyte–phonolitepyroclastic deposit that erupted at 39·3 ± 0·1ka within the Campi Flegrei west of Naples, Italy. Here we testthe hypothesis that Campanian Ignimbrite magma was derived byisobaric crystal fractionation of a parental basaltic trachyandesiticmelt that reacted and came into local equilibrium with smallamounts (5–10 wt%) of crustal rock (skarns and foid-syenites)during crystallization. Comparison of observed crystal and magmacompositions with results of phase equilibria assimilation–fractionationsimulations (MELTS) is generally very good. Oxygen fugacitywas approximately buffered along QFM + 1 (where QFM is the quartz–fayalite–magnetitebuffer) during isobaric fractionation at 0·15 GPa ( 6km depth). The parental melt, reconstructed from melt inclusionand host clinopyroxene compositions, is found to be basaltictrachyandesite liquid (51·1 wt% SiO₂, 9·3 wt%MgO, 3 wt% H₂O). A significant feature of phase equilibria simulationsis the existence of a pseudo-invariant temperature, 883 °C,at which the fraction of melt remaining in the system decreasesabruptly from 0·5 to < 0·1. Crystallizationat the pseudo-invariant point leads to abrupt changes in thecomposition, properties (density, dissolved water content),and physical state (viscosity, volume fraction fluid) of meltand magma. A dramatic decrease in melt viscosity (from 1700Pa s to 200 Pa s), coupled with a change in the volume fractionof water in magma (from 0·1 to 0·8) and a dramaticdecrease in melt and magma density acted as a destabilizingeruption trigger. Thermal models suggest a timescale of 200kyr from the beginning of fractionation until eruption, leadingto an apparent rate of evolved magma generation of about 10^–3km³/year. In situ crystallization and crystal settling in density-stratifiedregions, as well as in convectively mixed, less evolved subjacentmagma, operate rapidly enough to match this apparent volumetricrate of evolved magma production. KEY WORDS: assimilation; Campanian Ignimbrite; fractional crystallization; magma dynamics; phase equilibria