The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints

This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl₂O₃ and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl₂O₃. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H₂O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr₂O₃ content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.     

Corrosion of metal roof materials related to volcanic ash interactions

Metal roofing material is commonly used for residential and industrial roofs in volcanically active areas. Increased corrosion of metal roofing from chemically reactive volcanic ash following ash deposition post-eruption is a major concern due to decreasing the function and stability of roofs. Currently, assessment of ash-induced corrosion is anecdotal, and quantitative data are lacking. Here, we systematically evaluate the corrosive effects of volcanic ash, specifically ash leachates, on a variety of metal roofing materials (i.e. weathered steel, zinc, galvanized steel, and Colorsteel©) utilizing weathering chamber experiments and direct acid treatments. Weathering chamber tests were carried out for up to 30 days, and visual, chemical, and surface analyses did not definitively identify significant corrosion in any of the test roofing metal samples. Direct concentrated acid treatments with hydrochloric (HCl), sulphuric (H₂SO₄), and hydrofluoric (HF) acids demonstrate that roofing materials are chemically resilient. Our experimental results suggest that ash-leachate-related corrosion is a longer-term process (>1 month), potentially related to a multitude of factors including increased ash leachate concentrations, the dissolution of the glass matrix of the ash, moisture retention at the ash-surface boundary, and potential reactions involving photo-oxidation. Overall, corrosion is not a simple process related to the short-term release of acid and/or salt leachates from the ash surface, but a product of dynamic interactions involving ash and water at the surface of metal roofing material for extended periods.     

Spatial Distribution of Solar Energetic Particles in the Inner Heliosphere

We study the spatial distribution of solar energetic particles (SEPs) throughout the inner heliosphere during six large SEP events from the period 1977 through 1979, as deduced from observations on the Helios 1 and 2, IMP 7 and 8, ISEE 3, and Voyager 1 and 2 spacecraft. Evidence of intensity maxima associated with the expanding shock wave is commonly seen along its central and western flanks, although the region of peak acceleration or “nose” of the shock is sometimes highly localized in longitude. In one event (1 January 1978) a sharp peak in 20?–?30 MeV proton intensities is seen more strongly by Voyager at ～?2 AU than it is by spacecraft at nearby longitudes at ～?1 AU. Large spatial regions, or “reservoirs,” often exist behind the shocks with spatially uniform SEP intensities and invariant spectra that decrease adiabatically with time as their containment volume expands. Reservoirs are seen to sweep past 0.3 AU and can extend out many AU. Boundaries of the reservoirs can vary with time and with particle velocity, rather than rigidity. In one case, a second shock wave from the Sun reaccelerates protons that retain the same hard spectrum as protons in the reservoir from the preceding SEP event. Thus reservoirs can provide not only seed particles but also a “seed spectrum” with a spectral shape that is unchanged by a weaker second shock.     

New lunar meteorite Northwest Africa 2996: A window into farside lithologies and petrogenesis

The Northwest Africa (NWA) 2996 meteorite is a lunar regolith breccia with a “mingled” bulk composition and slightly elevated incompatible element content. NWA 2996 is dominated by clasts of coarse‐grained noritic and troctolitic anorthosite containing calcic plagioclase (An#~98) and magnesian mafic minerals (Mg#~75), distinguishing it from Apollo ferroan anorthosites and magnesian‐suite rocks. This meteorite lacks basalt, and owes its mingled composition to a significant proportion of coarse‐grained mafic clasts. One group of mafic clasts has pyroxenes similar to anorthosites, but contains more sodic plagioclase (An#~94) distinguishing it as a separate lithology. Another group contains Mg‐rich, very low‐titanium pyroxenes, and could represent an intrusion parental to regional basalts. Other clasts include granophyric K‐feldspar, disaggregated phosphate‐bearing quartz monzodiorites, and alkali‐suite fragments (An#~65). These evolved lithics are a minor component, but contain minerals rich in incompatible elements. Several anorthosite clasts contain clusters of apatite, suggesting that the anorthosites either assimilated evolved rocks or were metasomatized by a liquid rich in incompatible elements. We used Lunar Prospector gamma‐ray spectrometer remote sensing data to show that NWA 2996 is most similar to regoliths in and around the South Pole Aitken (SPA) basin, peripheral regions of eastern mare, Nectaris, Crisium, and southern areas of Mare Humorum. However, the mineralogy of NWA 2996 is distinctive compared with Apollo and Luna mission samples, and is likely consistent with an origin near the SPA basin: anorthosite clasts could represent local crustal material, mafic clasts could represent intrusions beneath basalt flows, and apatite‐bearing rocks could carry the SPA KREEP signature.     

Watershed science: Linking hydrological science with sustainable management of river basins

Over the past decades, a number of water sciences and management programs have been developed to better understand and manage the water cycles at multiple temporal and spatial scales for various purposes, such as ecohydrology,global hydrology, sociohydrology, supply management, demand management, and integrated water resources management(IWRM). At the same time, rapid advancements have also been taking place in tracing, mapping, remote sensing, machine learning, and modelling technologies in hydrological research. Despite those programs and advancements, a water crisis is intensifying globally. The missing link is effective interactions between the hydrological research and water resource management to support implementation of the UN Sustainable Development Goals(SDGs) at multiple spatial scales. Since the watershed is the natural unit for water resources management, watershed science offers the potential to bridge this missing link.This study first reviews the advances in hydrological research and water resources management, and then discusses issues and challenges facing the global water community. Subsequently, it describes the core components of watershed science:(1)hydrological analysis;(2) water-operation policies;(3) governance;(4) management and feedback. The framework takes into account water availability, water uses, and water quality; explicitly focuses on the storage, fluxes, and quality of the hydrological cycle; defines appropriate local water resource thresholds through incorporating the planetary boundary framework; and identifies specific actionable measures for water resources management. It provides a complementary approach to the existing water management programs in addressing the current global water crisis and achieving the UN SDGs.     

铂族元素在地壳中的富集:以布什维尔德杂岩为例

地幔是地壳铂族元素富集的主要源库。铂族元素迁移主要有两个途径:(1)地幔部分熔融物质侵入地壳;(2)地幔板片就位于俯冲/碰撞带。前一途径比后一途径重要得多。地幔物质进入地壳造成铂族元素富集并成为可供开采的主矿产而非副产品,这一过程可包含许多成矿作用机制:(i)基性侵入体中Ni-Cu硫化物矿浆的发育,岩浆冷却与分离结晶作用导致富含Cu,Pt,Pd的硫化物矿浆的形成;(ii)层状侵入体一定层位形成高品位的铂族元素硫化物层,伴生或不伴生铬铁岩;(iii)富铂族元素及硫化物的岩浆沿着层状侵入体的边缘就位;(iv)直至层状侵入体结晶分异作用晚期的硫化物不混溶滞后分离;(v)不发育硫化物不混溶作用的铬铁矿结晶作用;(vi)低程度硫化物浸染带中的热液作用与铂族元素富集;(vii)乌拉尔-阿拉斯加型侵入体重结晶过程中的铂族元素与铬铁矿的次生富集作用,岩体在风化过程中形成砂矿床;(viii)黑色页岩形成过程中Pt的富集。南非布什维尔德火成杂岩蕴藏世界Pt资源的75%,Pd资源的54%,Rh资源的82%,并具有(ii)、(iii)、(iv)、(v)、(vi)成矿作用的实例。在这些作用中,作用(ii)形成的现有经济储量和资源量占90%,作用(iii)占9%。Merensky矿层(占总资源量30%)是一个铂族元素富集层位,它含1~3铬铁矿薄层,在可采宽度内硫化物平均含量为1%~3%(质量分数)。硫化物一般被认为是铂族元素的主要聚集体。该矿层由两个或两个以上含硫化物的基性热岩浆上升汇聚而成。这些岩浆的汇聚造成超镁铁质堆晶岩的厚度(主要是斜方辉石岩,某些地区包括橄榄岩)变化于50cm至数米之间。开采通常集中在厚度不到1m的地带。矿层的成因至今仍存在争议,一些观点认为铂族元素来自下部上升的热液流体,另一些观点认为铂族元素来自上部岩浆的硫化物沉降作用,并形成了Merensky辉石岩。已经知道矿层上覆的辉石岩、苏长岩和斜长岩中矿物来自两种岩浆类型:一种富含MgO(12%,质量分数)和Cr,而贫Al2O3(12%);另一种含典型的粒玄岩成分。UG-2铬铁岩含有全部经济资源量的58%,由一0.6~1m厚的铬铁岩层(有时见辉石岩夹层)和上覆的1~3层由铬铁矿所构成的薄层。虽然硫化物被认为至少是某些情况下对铂族元素的富集起作用,但UG-2的硫化物含量(0.5%~1.5%)显著低于Merensky矿层。UG-2层之下共有13个铬铁岩层位,所有的都含铂族元素,虽然铂族元素总含量和(Pt+Pd)/(Ru+Ir+Os)比值远低于UG-2。UG-2内所含的辉石岩"夹层"具高的87Sr/86Sr比值,说明与顶部熔融岩石的混合促进了铬铁岩和硫化物的形成。作用(iii)的主要实例是Platreef。目前它占总资源量的9%。不过,沿该带正积极开展找矿勘探工作,这一比例将来还会提高。这一矿层的厚度比Merensky和UG-2都要大,目前开采厚度达50多米。Platreef呈带状,上部为斜方辉石岩的堆晶岩;下部为辉石岩、长石辉石岩和苏长岩,它们与页岩、铁矿层和白云岩强烈相互作用,直接形成了底盘岩石。笔者认为Platreef是不同期次岩浆作用的结果,这些作用形成了不同的单元产物,包括布什维尔德主岩浆房的UG-2和Merensky矿层。新的岩浆进入主岩浆房会造成先存岩浆移位、岩浆错动并会冲破岩浆房的壁。圆筒状、带状岩管中的超镁铁岩含极高的Pt品位,在布什维尔德杂岩的下部切穿堆晶层,被认为是热液再活化的产物。它们现在未被开采,只是构成存封的铂族元素资源,对整个杂岩体资源没有重要的贡献。     

Trace element geochemistry of high alumina basalt - Andesite - Dacite - Rhyodacite lavas of the Main Volcanic Series of Santorini Volcano,Greece

Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.