Metasomatism and sulfide mobility in lithospheric mantle beneath eastern Australia: Implications for mantle Re–Os chronology

Spinel lherzolite xenoliths from Tertiary basaltic host magmas at Allyn River, eastern Australia reveal two distinct petrographic and geochemical types. One group is distinguished by xenoliths with undeformed, equilibrated microstructures and interstitial melt patches; The second group shows deformation and contains abundant fluid inclusions but no melt patches. Trace-element signatures of clinopyroxene in these xenoliths provide evidence for metasomatism by a silicate agent with hydrous component and by a carbonate-rich agent respectively.

Melt patches in the undeformed xenoliths contain secondary minerals including clinopyroxene, olivine, feldspar, Mg- and Ca-rich carbonate, apatite, ilmenite and spinel. They are interpreted to represent volatile-rich melt captured shortly prior to entrainment in the host basalt. Sulfide globules, now recrystallised to discrete sulfide phases but inferred to be molten at lithospheric mantle T and P, are closely associated with the melt patches. The close association between sulfide and highly mobile, volatile-bearing fluid has important implications for the mobility of Re and Os, the use of their isotopes in dating mantle events, and the possible effect of volatile-bearing metasomatic agents on their composition.     

富铌玄武岩：板片熔体交代的地幔楔橄榄岩部分熔融产物

富铌玄武岩是一类具有特殊地球化学特征的岛弧玄武岩。与正常岛弧玄武岩相比，它具有硅饱和并富钠的特征；同时具有相对高的Nb（一般>7×10-6）、TiO2（1%～2%）和P含量，以及低的LILE/HFSE和HREE/HFSE比值，并富集高场强元素；它的原始地幔标准化微量元素图显示了弱的Nb、Ta负异常（有时出现弱的正异常），原始地幔标准化La/Nb比值小于2（但很少小于0.7），它是由受埃达克质熔体交代过的地幔橄榄岩部分熔融形成的。由于富铌玄武岩与埃达克岩是大洋板片俯冲作用的直接产物，因此，通过对该岩石组合及与俯冲作用有关的流体和熔体的研究，不仅可以查明洋壳俯冲作用过程中的岩浆活动特征，还可以阐明洋壳俯冲及壳幔相互作用，具十分重要的地质意义。     

Viscosity measurements of hydrous rhyolitic melts using the fiber elongation method

We have devised a new, simple and easy technique to measure the viscosity of hydrous silicate melts by combining an autoclave for melt hydration and the fiber elongation method for viscosity measurement. Using this, we measured the viscosity of hydrous rhyolitic melts whose water content ranges from 0.02 to 0.58 wt%. We observed a drastic decrease in viscosity against water content: 0.1 wt% water decreases the viscosity about an order of magnitude. Even when the water content is only 0.02 wt%, the viscosity decreased about half an order of magnitude. These results clearly demonstrate that the effect of water on viscosity should not be ignored even when it occurs as a trace constituent. We compared our experimental data with those derived from a non-Arrhenian viscosity model, which is considered to be applicable to calc-alkaline samples. This model succeeded in expressing the viscosity variation against water content but was unable to accurately predict the measured viscosity of liquids.Editorial responsibility: D. Dingwell     

Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°-10°N

Measurements of the seafloor deformation under ocean waves (compliance) reveal an asymmetric lower crustal partial melt zone (shear velocity less than 1.8 km/s) beneath the East Pacific Rise axis between 9° and 10°N. At 9°48′N, the zone is less than 8 km wide and is centered beneath the rise axis. The zone shifts west of the rise axis as the rise approaches the westward-stepping 9°N overlapping spreading center discontinuity and is anomalously wide at the northern tip of the discontinuity. The ratio of the compliance determined shear velocity to the compressional velocities (estimated by seismic tomography) suggests that the melt is well-connected in high-aspect ratio cracks rather than in isolated sills. The shear and compressional velocities indicate less than 18% melt in the lower crust on average. The compliance measurements also reveal a separate lower crustal partial melt zone 10 km east of the rise axis at 9°48′N and isolated melt bodies near the Moho beneath four of the 39 measurement sites (three on-axis and one off-axis). The offset of the central melt zone from the rise axis correlates strongly with the offset of the overlying axial melt lens and the inferred center of mantle melting, but its shape appears to be controlled by crustal processes.     

Wild horses and the political ecology of nature restoration in the Missouri Ozarks

From 1990 to 1996, the National Park Service and residents living near the Ozark National Scenic Riverways in south-central Missouri clashed over the federal agency’s intention to remove 25-30 wild horses from the protected area. The struggle was carried out in various legal and legislative arenas, the media, and in community protests and meetings. The dispute ended only with Congressional approval of the 1996 Omnibus Parks and Public Lands Management Act, which included an amendment ordering an end to any removal efforts.This article focuses on the contested social constructions of the horses themselves. To government scientists and managers, the animals represented a feral and exotic species with no legitimate place in agency-mandated ecosystem management and restoration scenarios. To many local members of the Missouri Wild Horse League, which contested the removal, the horses had critical historical and cultural importance as icons of regional identity, history and personal experience, and as core symbols of communities increasingly politically and economically marginalized.Local disputes with environmental groups and agencies concerned with Ozark ecosystem preservation and restoration have become more pronounced and numerous over the past two decades. This article approaches citizen opposition to environmental agendas not as an anti-environmental movement, but as a contemporary effort of marginalized groups to identify sources of economic, political, and social loss, and symbols of local identity and power. The wild horse issue reveals wider structural divides, and thus speaks to the question of which social groups shall have the power to impose their visions of the landscape and political economy.     

SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200°C

Trace element partition coefficients (D's) for up to 13 REE, Nb, Ta, Zr, Hf, Sr and Y have been determined by SIMS analysis of seven garnets, four clinopyroxenes, one orthopyroxene and one phlogopite crystallized from an undoped basanite and a lightly doped (200 ppm Nb, Ta and Hf) quartz tholeiite. Experiments were conducted at 2–7.5 GPa, achieving near-liquidus crystallization at relatively low temperatures of 1080–1200°C under strongly hydrous conditions (5–27 wt.% added water). Garnet and pyroxene D_REE show a parabolic pattern when plotted against ionic radius, and conform closely to the lattice strain model of Blundy and Wood (Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454). Comparison, at constant pressure, between hydrous and anhydrous values of the strain-free partition coefficient (D₀) for the large cation sites of garnet and clinopyroxene reveals the relative importance of temperature and melt water content on partitioning. In the case of garnet, the effect of lower temperature, which serves to increase D₀, and higher water content, which serves to decrease D₀, counteract each other to the extent that water has little effect on garnet–melt D₀ values. In contrast, the effect of water on clinopyroxene–melt D₀ overwhelms the effect of temperature, such that D₀ is significantly lower under hydrous conditions. For both minerals, however, the lower temperature of the hydrous experiments tends to tighten the partitioning parabolas, increasing fractionation of light from heavy REE compared to anhydrous experiments.

Three sets of near-liquidus clinopyroxene–garnet two-mineral D values increase the range of published experimental determinations, but show significant differences from natural two-mineral D's determined for subsolidus mineral pairs. Similar behaviour is observed for the first experimental data for orthopyroxene–clinopyroxene two-mineral D's when compared with natural data. These differences are in large part of a consequence of the subsolidus equilibration temperatures and compositions of natural mineral pairs. Great care should therefore be taken when using natural mineral–mineral partition coefficients to interpret magmatic processes.

The new data for strongly hydrous compositions suggest that fractionation of Zr–Hf–Sm by garnet decreases with increasing depth. Thus, melts leaving a garnet-dominated residuum at depths of about 200 km or greater may preserve source Zr/Hf and Hf/Sm. This contrasts with melting at shallower depths where both garnet and clinopyroxene will cause Zr–Hf–Sm fractionation. Also, at shallower depths, clinopyroxene-dominated fractionation may produce a positive Sr spike in melts from spinel lherzolite, but for garnet lherzolite melting, no Sr spike will result. Conversely, clinopyroxene megacrysts with negative Sr spikes may crystallize from magmas without anomalous Sr contents when plotted on mantle compatibility diagrams. Because the characteristics of strongly hydrous silicate melt and solute-rich aqueous fluid converge at high pressure, the hydrous data presented here are particularly pertinent to modelling processes in subduction zones, where aqueous fluids may have an important metasomatic role.     

Calpionellid zones of the Tithonian–Berriasian exotic limestone clasts from the Outer Carpathians,Poland

Calpionellids and chitinoidellids were identified from exotic limestone clasts occurring in flysch deposits of the Early Cretaceous to Oligocene age from the Silesian and Subsilesian nappes of the Polish Outer Carpathians. They represent the remnants of carbonate sedimentation along the northern and southern margin of the Severin-Moldavidic Basin, areas which are no longer preserved on the surface. Chitinoidellid and calpionellid assemblages characterize the Chitinoidella, Crassicollaria and Calpionella zones (latest early Tithonian–early Berriasian). The calpionellid zonation for the Western Carpathians was used for the present study, but some difficulties were encountered resulting from the application of this zonation. Cathodoluminescence imaging (CL) was useful for the identification of these microfossils in samples containing sparse and poorly preserved specimens, especially in the studied shallow-water limestone (the so-called Štramberk-type limestones).     

An exotic insect Aethiocarenus burmanicus gen. et sp. nov. (Aethiocarenodea ord. nov., Aethiocarenidae fam. nov.) from mid-Cretaceous Myanmar amber

An exotic wingless female insect in mid-Cretaceous Myanmar amber is described as Aethiocarenus burmanicus sp. et gen. nov. in the order Aethiocarenodea ord. nov. and family Aethiocarenidae fam. nov. The distinguishing feature of Aethiocarenus burmanicus sp. et gen. nov. is its unique head, the dorsum of which is shaped like an isosceles right triangle with the hypotenuse at the top and vertex positioned at the base of the neck. While insects with triangular-shaped heads are common today, the hypotenuse of the triangle is always located at the base of the head and attached to the neck, with the vertex at the apex of the head. Other features of the fossil are the long narrow, flat body, long slender legs, especially the hind pair that are twice the length of the abdomen, lack of wings, protruding eyes, paired ocelli, secretory glands located on the dorsum of the neck and swollen abdomen bearing paired segmented cerci.     

Crystallization of melts, pegmatite intrusion and the Inverian retrogression of the Scourian complex, north-west Scotland

Abstract Partial melting of tonalitic gneisses in the 2.7 Ga Badcallian granulite facies metamorphic episode in the Scourian complex of north-west Scotland produced a suite of granitic to trondhjemitic liquids. On cooling and excavation of the complex, these melts underwent fractional crystallization and the residual liquids eventually became water saturated. Comparison with experimental data suggests that water saturation would have occurred in these melts at around 620–700°C. From the retrograde P–T -time path followed by the complex it is estimated that H₂O-dominated fluids were exsolved from these melts at c. 2.5 Ga. It is proposed that these fluids were the cause of the 2.5 Ga Inverian retrogression of the Scourian complex and that water-saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites. The timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates, timing, distribution and nature of the Inverian phase of metamorphism. It is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains, such as West Greenland and Australia.     

西南天山东德沟橄榄岩的地球化学特征及其地质意义

东德沟橄榄岩出露于中国新疆伊犁—中天山板块和塔里木板块之间西南天山造山带的北缘,含大量角闪石、金云母等含水矿物,显示全部遭受了不同程度的交代变质作用.岩石主量元素含量变化较大,其后期蚀变基本向富铝和富钙两个方向变化,其Nb/Ta和Zr/Hf值显示难熔地幔残余的特征.该区橄榄岩轻重稀土分馏较明显,稀土元素配分曲线总体为LREE富集的右倾型,且富集大离子亲石元素Rb、Ba、Sr和放射性元素Th,但岩石呈现不一致的解耦现象,即主量元素表现难熔而稀土元素、不相容微量元素体现富集,说明主量元素和微量元素具有各自的演化特点.研究认为东德沟橄榄岩是经历部分熔融的楔形地幔残余,微量元素呈现的特征是俯冲板块析出熔体或流体交代的结果.与其岩石矿物学研究结果一致,对东德沟橄榄岩地球化学特征的深入研究证明,它们来源于SSZ环境俯冲上板片地幔楔橄榄岩残片.