Jorullo Volcano,Michoacán,Mexico (1759–1774): The earliest stages of fractionation in calc-alkaline magmas

Between 1759 and 1774, Jorullo Volcano and four associated cinder cones erupted an estimated 2 km³ of magma which evolved progressively with time from early, hypersthene-normative, primitive basalts to late-stage, quartz-normative, basaltic andesites. All lavas contain <6 vol% phenocrysts of magnesian olivine (Fo_90-70) with Cr-Al-Mg-spinel inclusions, and microphenocrysts of plagioclase and augite; late-stage basaltic andesites also carry phenocrysts of plagioclase, augite, and rare orthopyroxene, hornblende pseudomorphs, and microphenocrysts of titanomagnetite. Olivine-melt compositions indicate liquidus temperatures ranging from 1,230° C to 1,070° C in the early- and late-stage lavas, respectively; \(f_{{\text{O}}_{\text{2}} } \) was about 0.6 log units above the Ni-NiO buffer in the early lavas but increased to 2.5 log units above Ni-NiO in the late lavas, perhaps through groundwater-magma interaction. Smooth major and trace element compositional trends in the lavas can be largely modeled by simple crystal fractionation of olivine, augite, plagioclase, and minor spinel. La, Ce, and other incompatible elements (Rb, Sr, Ba, Hf, Th, Ta), however, are anomalously enriched in the latestage lavas, whereas the heavy rare earth elements (Dy, Yb, Lu) are anomalously depleted. The modeled crystal fractionation event must have occurred at lower-crustal to upper-mantle pressures (8–15 kb), although the crystals actually present in the Jorullo lavas appear to have formed at low pressures. Thus, a two-stage crystallization history is implied. Despite the presence of granitic xenoliths in middle-stage lavas from Jorullo, bulk crustal assimilation appears to have played an insignificant role in generating the compositional trends among the lavas. As MgO decreases from 9.3 to 4.3 wt% through the suite, Al₂O₃ increases from 16.4 to 19.1 wt%. Most highalumina basalts reported in the literature have 18 to 21 wt% Al₂O₃, but are too depleted in MgO, Ni, and Cr to have been generated directly through mantle partial melting. These high-alumina basalts have probably undergone significant fractionation of olivine, augite, plagioclase, and spinel from primitive parental basalts similar to the early Jorullo lavas. Such primitive basalts are rarely erupted in mature arcs and may be completely absent from mature stratovolcanoes. Cerro La Pilita is a late-Quaternary cinder and lava cone centered just 3 km south of Jorullo. The primitive trachybasalts of Cerro La Pilita, however, are radically different from the Jorullo basalts. They are nepheline normative with high concentrations of K₂O (>2.5 wt%), P₂O₅ (>0.9 wt%), Ba (1,200 ppm), Sr (>2,000 ppm), and many other incompatible elements, and contain crystals of hornblende and apatite in addition to olivine, spinel, augite, and plagioclase. The magmas of these two neighboring volcanoes cannot be related to one another by any simple mechanism, and must represent fundamentally different partial melting events in the mantle. The contrasts between Jorullo and Cerro La Pilita demonstrate the difficulty in defining simple relationships between magma type and distance from the trench in the Mexican Volcanic Belt.     

The vertical structure of summertime local winds in the Wright Valley,Antarctica

Intensive observations of summertime up- and down-valley winds in a dry valley utilising airsondes, pilot balloons and a monostatic acoustic sounder are described. Both circulations show a distinctive layered vertical wind and temperature structure. Westerly down-valley flow is typically neutral and is characterised by strong surface winds overlain by light variable winds extending to an inversion between 2000 and 4000m in depth. Above this inversion, gradient winds prevail. This structure is similar to that of downslope winds observed elsewhere. The thermally-induced up-valley easterly flow is shown to be extremely well-developed in terms of its strength, depth and persistence. The strong surface easterly may reach 800 m in depth and usually undercuts the warmer westerly. The boundary between the two regimes is marked by an inversion. During easterly flow a surface-based, super-adiabatic layer of 100–200 m in depth is evident and is associated with weak convective activity. An intriguing aspect of the wind regime is the interaction between the easterly and westerly circulations in the valley. These are separated at the surface by a frontal zone which migrates up and down the valley. Further observational and modelling studies are recommended.     

积雪对南极冰盖自动气象站气温观测影响的研究

随着冰盖表面雪的累积或消融,自动气象站(AWS)传感器相对地表的高度随之发生变化,故所记录的观测资料不能直接反映相对地表固定高度上的气象参数。为了使南极冰盖上AWS所获取的气象资料具有可靠性,在积累率对AWS观测气温影响的基础上,将东南极冰盖上中山站至DomeA断面3个AWS的连续观测气温修正到相对于雪表面的某一真实高度上。结果表明:(1)DomeA,Eagle和LGB69年平均1m气温分别为-53.19℃,-41.33℃和-26.29℃,年平均积累率分别为0.11m、0.30m和0.49m,对应的1m气温年平均修正量分别为0.34℃、0.29℃和0.35℃,2m和4m气温的年平均修正量均小于0.1℃;(2)积累率变化对离地表最近层次上的气温影响最大,越往上层影响越小;(3)气温的修正量大小与积累率并非成简单的正比关系,它与气温本身的季节变化特征以及局地近地表逆温强度有很大的关系。气温的平均修正量冬季为正,夏季修正量的正负由局地是否存在逆温决定,修正量值的大小主要由逆温强度和积累率决定。     

Relating Radar Remote Sensing of Biomass to Modelling of Forest Carbon Budgets

This paper addresses the use of radar remote sensing to map forest above-ground biomass, and discusses the use of biomass maps to test a dynamic vegetation model that identifies carbon sources and sinks and predicts their variation over time. For current radar satellite data, only the biomass of young/sparse forests or regrowth after disturbances can be recovered. An example from central Siberia illustrates that biomass can be measured by radar at a continental scale, and that a significant proportion of the Siberian forests have biomass values less than 50 tonnes/ha. Comparison between the radar map and calculations by the Sheffield Dynamic Global Vegetation Model (SDGVM) indicates that the model considerably overestimates biomass; under-representation of managed areas, disturbed areas and areas of low site quality in the model are suggested reasons for this effect. A case study carried out at the Büdingen plantation forest in Germany supports the argument that inadequate representations of site quality and forest management may cause model overestimates of biomass. Comparison of the calculated biomass of stands planted after 1990 with biomass estimates by radar allows identification of forest stands where the growth conditions assumed by the model are not valid. This allows a quality check on model calculations of carbon fluxes: only calculations for stands where there is good agreement between the data and the model predictions should be accepted. Although the paper only uses the SDGVM model, similar effects are likely in other dynamic vegetation models, and the results show that model calculations attempting to quantify the role of forests as carbon sources or sinks could be qualified and potentially improved by exploiting remotely sensed measurements of biomass.     

Assessing characteristics of Mediterranean explosive cyclones for different data resolution

A comparison of two objective climatologies of explosive cyclones in the Mediterranean region is performed. The results are derived from two different mean sea-level pressure reanalysis data resolutions, but from the same assimilation model, in order to quantify the pure impact of higher resolution on the identification and characteristics of explosive cyclones, when the assimilation model is the same. The explosive cyclones were identified with the aid of the Melbourne University automatic cyclone finding and tracking scheme over a 40-year period, using the 6-hourly analyses of ERA-40 mean sea-level pressure (MSLP) on: (a) 2.5?×?2.5 and (b) 1?×?1 latitude–longitude grid. The comparison of the two datasets revealed the significant role of the increase in spatial resolution of MSLP data on the identification and tracking process, and the number of the explosive cyclones in the high-resolution dataset is almost four times greater than the respective one in the lower resolution dataset. However, the comparison of explosive cyclone characteristics, including spatial and temporal variations of explosive deepening, revealed differences in the geographical distribution of the location of the maximum explosive deepening and average explosive cyclone Laplacian of the central pressure. These differences are due to the identification in the higher resolution set of smaller scale and secondary explosives along the strongly baroclinic northern Mediterranean boundaries, south of the Alps and the Pyrenees. Explosive deepening appears a bias to the daytime period from 12 to 18 Coordinated Universal Time (UTC) for both datasets, which is more prominent in the LR dataset. Statistically significant difference of pressure tendency between the two datasets appear for the daytime period from 06 to 12 UTC, accounting for better representation of orographic forcing in the HR dataset.     

Wind-Direction Effects on Urban-Type Flows

Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector.     

Housing the Urban Poor in Twenty-First Century Sub-Saharan Africa: Policy Mismatch and a Way Forward for Ghana

The debate on housing the urban poor has become more sophisticated since Turner’s original ideas of self-help and self-building by the poor were introduced in the 1970s. Today, the emphasis in housing the poor is on a pluralistic approach that stresses enabling housing provision for the poor by expanding the range of providers to include government, the private sector, the poor themselves, non-governmental agencies, and cooperatives. Official housing policy in Ghana does not reflect the pluralistic approach that prevails in practitioner and academic circles. Using ethnography, this paper presents the housing experience of a typical poor family in Ghana to determine what the poor build. It highlights the obstacles the poor have to overcome to acquire the housing they want. The ethnography provides a basis upon which the mismatch between the pluralistic approach and Ghana’s housing policy can be bridged thus providing a way forward. The paper concludes by emphasizing the importance of the housing industry in poverty alleviation in Ghana.     

Hydrogeochemistry of the Goulburn Valley region of the Murray Basin, Australia: implications for flow paths and resource vulnerability

Groundwater in the Goulburn region of the Murray Basin (Australia) contains solutes derived mainly from evapotranspiration of rainfall-derived marine aerosols, silicate dissolution, and ion exchange. ¹⁴C data indicate that groundwater in the Shepparton Formation recharges vertically across the region, whereas groundwater in the Calivil–Renmark Formation shows a greater component of lateral flow. The overall pattern of geochemical and stable isotope variations implies that long-term vertical groundwater flow into the Calivil–Renmark Formation has occurred over thousands of years. Elevated C, N, and F concentrations, together with variable Cl/Br ratios and ¹⁸O values, suggest that short-term (years to decades) flow of surface water into the Calivil–Renmark Formation also occurs locally. The high degree of vertical flow implies that the high-quality groundwater resources of the Calivil–Renmark Formation are vulnerable to surface contamination. Groundwater in both the Shepparton and Calivil–Renmark Formations yields ¹⁴C ages of up to 20 ka that imply that, overall, recharge rates are low and that, consequently, the groundwater resource in both formations could be impacted by over abstraction.

---

Resumen Las aguas subterráneas de la región de Goulburn, en la cuenca del río Murray (Australia), contienen solutos procedentes principalmente de la evaporación de agua de lluvia, disolución de silicatos e intercambio iónico. Aunque la variabilidad espacial de la hidrogeoquímica es reducida, el contenido total de sales disueltas en las aguas subterráneas de la formación no confinada de Shepparton es generalmente mayor que el del acuífero inferior confinado de la Formación Calivil–Renmark. Los datos de ¹⁴C, ³H y geoquímica sugieren que la recarga del acuífero de la Formación Shepparton es por infiltración (vertical) en toda la región, mientras que la Formación Calivil–Renmark tiene una mayor proporción de aportes laterales. Sin embargo, a escala local se produce recarga vertical en la Formación Calivil–Renmark, hecho que indica que este recurso es vulnerable a la contaminación. Las aguas subterráneas de ambas formaciones tienen edades de hasta 20.000 años, según datos de ¹⁴C, lo que implica que las tasas de recarga son bajas y, en consecuencia, que podría haber impactos asociados a la sobreexplotación del recurso. Las aguas subterráneas recientes de la región se caracterizan por relaciones molares muy variables de Cl/Br (de 50 a 1.200), reflejando la química de una precipitación semiárida que se ve modificada posteriormente por disolución de halita, asociada con terrenos secos y riego con aguas salinas. Las aguas subterráneas más profundas presentan relaciones más uniformes de Cl/Br (entre 500 y 800), y probablemente reflejan que la recarga se produjo en condiciones climáticas más húmedas que las actuales.

---

Résumé Les eaux souterraines de la région de Goulburn dans le bassin de Murray (Australie) contiennent des solutés provenant essentiellement de lévaporation des pluies, de la dissolution de silicates et déchanges dions. Alors quil y a peu de variations spatiales du chimisme des eaux souterraines, les concentrations en sels dissous totaux des eaux souterraines de laquifère libre de la formation de Shepparton sont en général plus élevées que celles des eaux souterraines de laquifère captif sous-jacent de la formation de Calivil-Renmark. Des données de carbone-14, de tritium et de géochimie indiquent que laquifère de la formation de Shepparton est rechargé verticalement dans toute la région, tandis que laquifère de la formation de Calivil-Renmark présente une plus forte composante découlement latéral. Toutefois, localement, il se produit une recharge verticale de laquifère de la formation de Calivil-Renmark, ce qui indique que cette ressource est vulnérable à la contamination. Les eaux souterraines des formations de Shepparton et de Calivil-Renmark présentent toutes deux des âges carbone-14 de plus de 20 ka, ce qui implique des taux de recharge très faibles et, par conséquent, que leur ressource peut être affectée par des prélèvements excessifs. Les eaux souterraines récentes dans cette région sont caractérisées par un rapport molaire Cl/Br très variable (de 50 à 1200), qui reflète le chimisme de pluies en région semi-aride modifié en conséquence par la dissolution de la halite associée à la salinité due aux terres sèches et à lirrigation. Les eaux souterraines plus profondes possèdent des rapports Cl/Br plus uniformes (de lordre de 500 à 800) et reflètent probablement des eaux de recharge sous des conditions climatiques plus humides que les conditions actuelles.

     

SHRIMP U–Pb zircon geochronology of Neoproterozoic crustal granitoids (Southern Brazil): A case for discrimination of emplacement and inherited ages

SHRIMP U–Pb zircon studies on two post-collisional granitic plutons and reassessment of the data previously reported for two anatectic gneissic granites are used to assess the late Neoproterozoic history of the Florianópolis Batholith, southern Brazil. The results, supported by SEM backscattered and cathodoluminescence imagery, identify inherited zircon populations and confirm the long-lived, crustal recycling processes responsible for the accretion of the batholith. The study casts new lights on the timing of the processes involved in the generation and modification of the internal structure of distinct zircon populations, and enables discrimination to be made between inherited cores and melt-precipitated overgrowths. New dating of two post-tectonic plutons (samples 1 and 2) revealed crystals showing magmatic-textured cores sharply bounded by melt-precipitated overgrowths. The U/Pb isotopic results from both samples spread along concordia by ca. 40 m.y. (sample 1) to 100 m.y. (sample 2), clustering in two closely spaced (bimodal), partially overlapping peaks. Melt-precipitated rims and homogeneous new grains, dated at ca. 600 Ma, furnish the crystallisation age of the plutons. The magmatic textured cores and xenocrysts dated at ca. 630–620 Ma are interpreted as inherited restitic material from supposedly short-lived (meta)granitic sources. The reassessment of previous SHRIMP data of two banded anatectic granitoids (samples 3 and 4) revealed more complex morphological patterns, in which the overgrown inherited cores are sharply bounded against large melt-precipitated rims, dated at ca. 600 Ma and 592±2 Ma, respectively. Major populations of magmatic-textured inherited cores dated at 2006±3 Ma and 2175±13 Ma characterise samples 3 and 4, respectively. The latter additionally shows metamorphic and magmatic inherited cores with a large range of ages (ca. 2900–620 Ma), suggesting partial melting of metasedimentary components. The main magmatic Paleoproterozoic core populations are interpreted as inherited restite from partial melting of the adjacent (meta)tonalitic gneiss and amphibolitic country-rock (paleosome). The recognition of the (melt-precipitated) Neoproterozoic overgrowths and new crystals, and the restite provenance of the cores, supplants a previous interpretation of Paleoproterozoic magmatism (cores) and Neoproterozoic (solid-state) metamorphic overprint. As a major consequence of the former interpretation, the unit was mistakenly considered part of major Paleoproterozoic gneissic remnant within the Neoproterozoic Florianópolis Batholith/arc.