Fractional melting and smelting on the ureilite parent body

We investigate petrologic and physical aspects of melt extraction on the parent asteroid of the ureilite meteorites (UPB). We first develop a petrologic model for simultaneous melting and smelting (reduction of FeO by C) at various depths. For a model starting composition, determined from petrologic constraints to have been CV-like except for elevated Ca/Al (2.5 × CI), we determine (1) degree of melting, (2) the evolution of mg, (3) production of CO + CO₂ gas and (4) the evolution of mineralogy in the residue as a function of temperature and pressure. We then use these relationships to examine implications of fractional vs. batch melt extraction.In the shallowest source regions (∼30 bars), melting and smelting begin simultaneously at ∼1050 °C, so that mg and the abundance of low-Ca pyroxene (initially pigeonite, ultimately pigeonite + orthopyroxene) begin to increase immediately. However, in the deepest source regions (∼100 bars), smelting does not begin until ∼1200 °C, so that mg begins to increase and low-Ca pyroxene (pigeonite) appears only after ∼21% melting. The final residues in these two cases, obtained just after the demise of augite, match the end-members of the ureilite mg range (∼94-76) in pyroxene abundance and type. In all source regions, production of CO + CO₂ by smelting varies over the course of melting. The onset of smelting results in a burst of gas production and very high incremental gas/melt ratios (up to ∼2.5 by mass); after a few % (s)melting, however, these values drastically decline (to <0.05 in the final increments).Physical modelling based on these relationships indicates that melts would begin to migrate upwards after only ∼1-2% melting, and thereafter would migrate continuously (fractionally) and rapidly (reaching the surface in < a year) in a network of veins/dikes. All melts produced during the smelting stage in each source region have gas contents sufficient to cause them to erupt explosively and be lost. However, since in all but the shallowest source regions part of the melting sequence occurs without smelting, fractional melting implies that a significant fraction of UPB melts may have erupted more placidly to form a thin crust (∼3.3 km thick for a 100 km radius body).Our calculations suggest that melt extraction was so rapid that equilibrium trace element partitioning may not have been attained. We present a model for disequilibrium fractional melting (in which REE partitioning is limited by diffusion) on the UPB, and demonstrate that it produces a good match to the ureilite data. The disequilibrium model may also apply to trace siderophile elements, and might help explain the “overabundance” of these elements in ureilites relative to predictions from the smelting model.Our results suggest that melt extraction on the UPB was a rapid, fractional process, which can explain the preservation of a primitive oxygen isotopic signature on the UPB.     

TWENTIETH-CENTURY CLIMATE ANOMALY PATTERNS OVER THE SOUTHWESTERN UNITED STATES

Winter precipitation anomaly patterns between 1911 and 1977 over the southwestern United States are investigated by empirical orthogonal function (EOF) analysis. The first four EOFs explain 74% of the total variance in the original data assemblages. The first three were statistically significant at the 95% confidence level. The spatial distribution of the coefficients and the time plots of the amplitudes of the first two EOFs indicate that the first three decades of the study period, identified as encompassing the most anomalously warm decades in the western Untied States this century, was a period when negative precipitation anomalies predominated in the Great Basin and in the upper Colorado and Green River basins, but positive anomalies predominated in the lower Colorado and the Colorado Plateau region of Arizona. In the early 1950s not only was there a hiatus in winter precipitation distribution, but the earlier anomaly patterns reversed to a predominantly wet northern section and a predominantly drier south. This hiatus in anomaly patterns coincided with the change to increased meridionality in upper tropospheric circulation patterns. These results, when viewed in the light of those from Euler et al. (1979), Wigley, Jones and Kelly (1980), Williams (1980) and Jäger and Kellogg (1983), support a strong case for an anomalously warm Southwest during strong hemispheric and hence Arctic warming and therefore a predominance of the anomaly patterns exemplified by the first two EOFs. The third EOF depicts the influence of physiography and the nature and trajectory of precipitation-inducing systems in winter on the variance pattern. The spatial distributions of the EOFs raise questions about the inferential use of surrogate data for environmental reconstruction in the presence of anomaly patterns that lead to nonsynchroneity in the response of samples in time and space. Power spectra of the amplitudes of the first three EOFs show the biennial oscillation and oscillations with periodicities greater than 30 years to be significant at the 95% confidence level on a white noise continuum.     

CLIMATIC VARIATION IN CALIFORNIA: LOOKING AHEAD TO 2000 AD

Temperature scenarios to 2000 AD for three regions in Califronia are given together with rationales for their realization. It is shown that urban and rural areas have had, and will continue to have, different time traces of temperature. Further, the time traces of temperature to the year 2000 AD will be a composite of natural causative factors and human impact. The implications of changing temperatures on selected economic activities, and the changing variability in precipitation are discussed. It is concluded that the probability of occurrence of severely dry and severely wet rainy seasons is sufficiently high to warrant an explicit incorporation of climate in future planning within the state.     

Comment on “Parent body depth‐pressure‐temperature relationships and the style of the ureilite anatexis” by P. H. Warren (MAPS 47:209–227)

Ureilites are carbon‐rich ultramafic (olivine + dominantly low‐Ca pyroxene) achondrites with poorly understood petrogenesis. One major problem concerns the origin of extensive variation in FeO content (olivine core Fo values ranging from approximately 75 to 95) among the individual ureilites. The two main competing hypotheses to explain this variation are: (1) equilibrium smelting, in which ureilite Fo values were established by pressure‐dependent (depth‐linked) carbon redox reactions on the ureilite parent body during partial melting; or (2) nebular inheritance, in which the variation in FeO contents was derived from ureilite precursors and was preserved during partial melting. The paper “Parent body depth‐pressure‐temperature relationships and the style of the ureilite anatexis” by Warren (2012) discusses a series of topics related to ureilite petrogenesis. In each case, an argument is presented within the context of smelting versus nonsmelting models. Collectively, these arguments create the impression that there are many valid arguments against smelting. The purpose of this comment is to point out flaws in some of these arguments, and/or to show that the issues they address are independent of smelting versus nonsmelting models. Both equilibrium smelting and nebular inheritance (simple anatexis) models face challenges in explaining all the properties of ureilites, but both remain viable.     

Regional landslide risk to the Cairns community

A GIS-based regional reconnaissance-level assessment of landslide risk to the Cairns community has been carried out to provide information to the Cairns City Council for planning and emergency management purposes. Magnitude recurrence relations were tentatively established for the two main slope processes: landslides on the hill slopes; and large debris flows extending out from the gully systems on to the plains. From the recurrence relations, landslide hazard (H) was estimated as the annual probability of a point being impacted by a landslide. The nature, number (E) and geographic distribution of the elements at risk were obtained by interrogating the GIS, and their vulnerabilities (V) to destruction by the two main landslide slope processes were assessed. From this information, specific risk (= H × V) and total risk (= H × V× E) maps were produced.Although total landslide risk is relatively low at present, it will increase as development extends further into the hill slopes, unless adequate mitigation measures are taken. Large debris flows, while considerably less frequent than landslides on cut slopes, could impact on subdivisions at the base of the slopes. Blockage by landslides of roads and railways providing access to Cairns can cause isolation of the community. Flash flooding in Freshwater Creek, or debris flows, have the potential to disrupt the Cairns water supply by blocking the intake or destroying sections of the pipeline.     

The impact of artificially impounded, residential headwater lakes on downstream water temperature

In the southeastern United States, which has very few natural lakes, developers often impound headwater streams to create esthetic lakes as focal points in the residential landscape. Given the prevalence and increasing abundance of these water features, it is important to assess and quantify the spatial and temporal impacts these lakes have on headwater stream temperatures. Any changes in the downstream thermal regime may influence not only the biological functioning of a stream, but also important physical and chemical water quality characteristics. The purpose of this study was to quantify the magnitude and extent of the downstream temperature disturbance associated with three different, artificially impounded, residential headwater lakes in Greenville, South Carolina. Water temperature loggers were installed upstream, in-lake, and downstream of three surface-release residential lakes and monitored at 5-min intervals from July 2007 to April 2008. In July/August 2007, longitudinal stream temperature profiles were measured at 50-m intervals both upstream and downstream of each lake in order to assess the spatial extent of the lake’s temperature disturbance. The lakes altered the downstream thermal regime at all three sites, increasing temperature by as much as 8.4 °C and decreasing diurnal variability by as much as 3.9 °C within the period of record. Furthermore, the longitudinal profiles of all three stream-lake systems showed no significant signs of downstream recovery to the upstream temperatures. The ecological effects of such temperature disturbances on stream biotic communities were not quantified in this study, but are likely significant. This study considered lakes only in South Carolina, but the observed changes to the downstream thermal regime are presumably similar for impounded residential headwater lakes across the US Southeast given the similar climate regime and comparable design and construction of these lakes across the region.     

Climate Change and Cryptosporidiosis: A Qualitative Analysis

The effects of climate change on drinking-waterborne cryptosporidiosis transmission in the United States are analyzed using an influence diagram representation of epidemic development. Results from a systematic qualitative analysis indicate that climate change will have little effect on cryptosporidiosis incidence if the United States continues to be wealthy and maintains its commitment to public health. The major impact will, instead, be the additional costs of adapting to new climate regimes in order to avoid drinking-waterborne disease risk. These costs, for the most part, will be from improved monitoring and treatment of drinking water. The consequences of disaster scenarios are also considered. These, too, suggest that climate change per se will be a poor predictor of waterborne cryptosporidiosis in countries with high standards of living. Rather, the risk of epidemics will depend on the interplay between population, public health investment, infrastructure maintenance, emergency planning/response capabilities, water-treatment technologies, drinking-water regulations, and climate.