Fe‐Mn systematics of type IIA chondrules in unequilibrated CO,CR, and ordinary chondrites     

Jana BERLIN  Rhian H. JONES  Adrian J. BREARLEY 《Meteoritics & planetary science》2011,46(4):513-533

Abstract– We have examined Fe/Mn systematics of 34 type IIA chondrules in eight highly unequilibrated CO, CR, and ordinary chondrites using new data from this study and prior studies from our laboratory. Olivine grains from type IIA chondrules in CO chondrites and unequilibrated ordinary chondrites (UOC) have significantly different Fe/Mn ratios, with mean molar Fe/Mn = 99 and 44, respectively. Olivine analyses from both these chondrite groups show well‐defined trends in Mn versus Fe (afu) and molar Fe/Mn versus Fe/Mg diagrams. In general, type IIA chondrules in CR chondrites have properties intermediate between those in UOC and CO chondrites. In most UOC and CR type IIA chondrules, the Fe/Mn ratio of olivine decreases during crystallization, whereas in CO chondrites the Fe/Mn ratio does not appear to change. It is difficult to interpret the observed Fe/Mn trends in terms of differing moderately volatile element depletions inherited from precursor materials. Instead, we suggest that significant differences in the abundances of silicates and sulfides ± metals in the precursor material, as well as open‐system behavior during chondrule formation, were responsible for establishing the different Fe/Mn trends. Using Fe‐Mn‐Mg systematics, we are able to identify relict grains in type IIA chondrules, which could be derived from previous generations of chondrules, including chondrules from other chondrite groups, and possibly chondritic reservoirs that have not been sampled previously.  相似文献   

Alteration of platinum-group minerals and dispersion of platinum-group elements during progressive weathering of the Aguablanca Ni–Cu deposit, SW Spain     

Saioa Suárez  Hazel M. Prichard  Francisco Velasco  Peter C. Fisher  Iain McDonald 《Mineralium Deposita》2010,45(4):331-350

The distribution, mineralogy and mobility of the platinum-group elements (PGE) in the surface environment are poorly understood. This study of the lower, less altered and upper, more altered gossan, overlying the Aguablanca Ni–Cu-(PGE) magmatic deposit (Spain), has shown that the platinum-group minerals (PGM) are progressively oxidised and dispersed into iron oxides that form the gossan. A combination of the characterization of PGE in host PGM, using a scanning electron microscope, and measurement of PGE at lower concentrations in host iron oxides, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), has for the first time allowed the total distribution of the PGE within a gossan to be documented. This study has revealed a complete in situ alteration and dispersion sequence of the PGM including (1) breakdown of both the more stable Pt-arsenides, Pt/Pd-tellurides and the less stable bismuthotellurides, (2) formation of partially oxidised PGM, (3) development of a wide range of oxidised Pt- and Pd-bearing phases, (4) subsequent formation of Fe–PGE-oxides and PGE-hydroxides, (5) incorporation of PGE into ferruginous supergene products and lastly (6) concentration of PGE at the edges of veins and iron oxides. Dispersion of Pd is greater than for the other PGE with Pd being widely distributed throughout the iron oxides. This oxidising environment produced PGE-oxides rather than PGE-alloys, also commonly found in the surface environment, especially in placers. These results provide critical evidence for the stages of mineralogical change from PGE host mineralogy in magmatic ores to surface weathering producing PGE-oxides.  相似文献   

Timing and distribution of alluvial fan sedimentation in response to strengthening of late Holocene ENSO variability in the Sonoran Desert,southwestern Arizona,USA     

Steven N. Bacon  Eric V. McDonald  Todd G. Caldwell  Graham K. Dalldorf 《Quaternary Research》2010,73(3):425-438

The integration of geomorphic mapping, soil stratigraphy, and radiocarbon dating of alluvial deposits offers insight to the timing, magnitude, and paleoclimatic context of Holocene fan sedimentation near Yuma, Arizona. Mapping of 3400 km² indicates about 10% of the area aggraded in the late Holocene and formed regionally extensive alluvial fan and alluvial plain cut-and-fill terraces. Fan deposits have weakly developed gravelly soils and yielded a date of 3200–2950 cal yr BP from carbonized wood. Alluvial plain deposits have weakly developed buried sandy soils and provided a date of 2460–2300 cal yr BP from a terrestrial snail shell. Precipitation records were analyzed to form historical analogues to the late Holocene aggradation and to consider the role of climatic variability and extreme hydrologic events as drivers of the sedimentation. The historical precipitation record indicates numerous above-average events correlated to the Southern Oscillation Index (SOI) in the region, but lacks any significant reactivation of alluvial fan surfaces. The timing of aggradation from 3200 to 2300 cal yr BP correlates well with other paleoclimatic proxy records in the southwestern U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period.  相似文献   

A review of seawater intrusion and its management in Australia   总被引：5，自引：3，他引：2  

Adrian D. Werner 《Hydrogeology Journal》2010,18(1):281-285

Extended periods of below-average rainfall combined with a rising population density in the Australian coastal margin have led to higher stresses on coastal water resources, and the risk of seawater intrusion has increased. Despite reports of seawater intrusion in the majority of states and evidence that some Australian coastal aquifers are seriously depleted, comprehensive seawater intrusion investigations have only been completed for coastal systems in Queensland and to a lesser degree in Western Australia and South Australia. The degree of assessment appears to be linked to the perceived economic value of the groundwater resource. The most detailed studies include those of the Pioneer Valley and Burnett basins in Queensland, for which conceptual and mathematical models have been developed at the regional scale, and have been used to underpin trigger-level management approaches to protect against further seawater intrusion. Historical responses to seawater intrusion include the establishment of artificial recharge schemes; the most prominent being that of the Lower Burdekin aquifers in Queensland. Recommendations for future solutions include enhanced fit-for-purpose seawater intrusion monitoring, continuing research into investigation methods, and improved knowledge-sharing through education programs and the development of national guidelines for seawater intrusion assessment and management.  相似文献   

Impact melt rocks from the Late Paleocene Hiawatha impact structure,northwest Greenland     

William R. Hyde  Adam A. Garde  Nynke Keulen  Sebastian N. Malkki  Steven J. Jaret  Tod Waight  Pierre Beck  Iain McDonald  Nicolaj K. Larsen 《Meteoritics & planetary science》2023,58(6):789-814

Impact melt rocks formed during hypervelocity impact events are ideal for studying impact structures. Here, we describe impact melt rock samples collected proximal to the 31 km wide 58 Ma Hiawatha impact structure, northwest Greenland, which is completely covered by the Greenland Ice Sheet. The melt rocks contain diagnostic shock indicators (e.g., planar deformation features [PDF] in quartz and shocked zircon) and form three groups based on melt textures and chemistry: (i) hypocrystalline, (ii) glassy, and (iii) carbonate-based melt rocks. The exposed foreland directly in front of the structure consists of metasedimentary successions and igneous plutons; however, the carbonate-based impactites indicate a mixed target sequence with a significant carbonate-rich component. Well-preserved organic material in some melt rocks indicates that North Greenland at the time of impact was host to abundant organic material, likely a dense high-latitude temperate forest. Geochemical signatures of platinum-group elements in selected samples indicate an extraterrestrial component and support previous identification of a highly fractionated iron impactor in glaciofluvial sand. Our results illustrate the possibility to study impact structures hidden beneath a thick ice sheet based on transported samples and this opens a new avenue for identifying other potential impact craters in Greenland and Antarctica.  相似文献   

Palaeomagnetic,geochronological and geological constraints on the tectonic evolution of the Mejillones Peninsula,northern Chile     

Adrian J. Hartley  Peter Turner  David C. Rex  Stephen Flint 《Geological Journal》1992,27(1):59-74

Palaeomagnetic and geochronological data from an Early Palaeozoic high grade metamorphic complex (Jorgina Formation) and Jurassic layered basic intrusion (Moreno Complex) are reported from the Mejillones Peninsula of northern Chile (23–23°30'S). ⁴⁰Ar–³⁹Ar dates from the Lower Palaeozoic Jorgina Formation and the Moreno Complex are between 170 and 158 Ma, coincident with a phase of emplacement of the north Chilean coastal batholith. This suggests that intrusion and magnetization of the Moreno Complex and the metamorphism and remagnetization of the Jorgina Formation were related to batholith emplacement. Extracted stable components of magnetization from all units (17 sites) define site-mean directions with a scattered distribution. The scatter in site-mean directions is interpreted as being due to minor, localized, non-uniform, block-fault related (normal or strike-slip, or both) rotation after 158 Ma. The palaeomagnetic and geochronological data indicate that no significant large-scale latitudinal translation of crustal blocks has taken place in this part of northern Chile since the Late Jurassic. In addition, they indicate that the uniform clockwise rotation after the mid-Cretaceous which affected the adjacent Cordillera de la Costa either did not extend into the Mejillones Peninsula or took the form of localized block-fault rotations. The restriction of palaeomagnetically defined styles of rotation to discrete areas within the north Chilean forearc indicates that forearc wide block-fault rotation models are not applicable to the Pacific margin of northern Chile.  相似文献