Investigation of ion beam techniques for the analysis and exposure of particles encapsulated by silica aerogel: Applicability for Stardust

Abstract— In 2006, the Stardust spacecraft will return to Earth with cometary and perhaps interstellar dust particles embedded in silica aerogel collectors for analysis in terrestrial laboratories. These particles will be the first sample return from a solid planetary body since the Apollo missions. In preparation for the return, analogue particles were implanted into a keystone of silica aerogel that had been extracted from bulk silica aerogel using the optical technique described in Westphal et al. (2004). These particles were subsequently analyzed using analytical techniques associated with the use of a nuclear microprobe. The particles have been analyzed using: a) scanning transmission ion microscopy (STIM) that enables quantitative density imaging; b) proton elastic scattering analysis (PESA) and proton backscattering (PBS) for the detection of light elements including hydrogen; and c) proton‐induced X‐ray emission (PIXE) for elements with Z > 11. These analytical techniques have enabled us to quantify the composition of the encapsulated particles. A significant observation from the study is the variable column density of the silica aerogel. We also observed organic contamination within the silica aerogel. The implanted particles were then subjected to focused ion beam (FIB) milling using a 30 keV gallium ion beam to ablate silica aerogel in site‐specific areas to expose embedded particles. An ion polished flat surface of one of the particles was also prepared using the FIB. Here, we show that ion beam techniques have great potential in assisting with the analysis and exposure of Stardust particles.     

The lost world of the olenid trilobites

Trilobites of the Family Olenidae (Upper Cambrian to Upper Ordovidan) were closely tied to an oxygen-poor environment. They show a remarkable range of adaptations, testifying to a singular evolutionary plasticity.     

Lithological and Structural Controls on Regional 3-D Fluid Flow Patterns during Greenschist Facies Metamorphism of the Dalradian of the SW Scottish Highlands

Greenschist facies metabasite sills enclosed within phyllitesand psammites of the Dalradian Supergroup of the SW ScottishHighlands have been used as quantitative sensors of fluid flow,from which ‘fossil’ fluid flow-paths have been mapped.Infiltration of H₂O–CO₂ fluid caused carbonation of themetabasite assemblage. Consequently, metabasite sills have developeda characteristic asymmetric zonation of reacted (carbonate-bearing)margins and unreacted (carbonate-absent) interiors. Reactionfront advection theory has been used to constrain the time-integratedfluid flux and flow direction in one dimension, for each sill,from the widths of the reacted margins. Three-dimensional fluxesand flow directions have been constrained from one-dimensionalfluxes by geometrical computation, and regional fluid flow patternshave been deduced. On a regional scale, fluid flow was channelled through phyllitesand focused towards the axial zone of major antiformal foldstructures. Fluid flow was channelled through the 6 km thickArdrishaig Phyllites, within which fluid fluxes were found toincrease exponentially with proximity to the axial surface ofthe Ardrishaig Anticline. Fluid flow through the psammites waslimited to narrow zones of intense deformation associated withmetabasic or phyllitic intercalations. Although time-integratedfluid fluxes were locally high (4 10² m³/m²), average fluxes(100 m¹/m²) are compatible with availability of fluid from underlyingdevolatilizing rocks. Complete isotopic homogenization of phyllites,psammites and metabasites within the axial region of the ArdrishaigAnticline is predicted by these high measured fluxes. KEY WORDS: 3-D flow; Dalradian; SW Scotland; greenschist facies; metabasites ^*Corresponding author. Telephone: 0131-650-5885 Fax: 0131-668-3184 E-mail: askelton{at}glg.ed.ac.uk     

Temporal Evolution of Boron Flux in the NE Japan and Izu Arcs Measured by Ion Microprobe from the Forearc Tephra Record

The enrichment of boron relative to similarly incompatible elements,such as Be, in arc volcanic rocks has been used as a proxy forthe involvement of slab flux in petrogenesis. New ion microprobeanalyses of single glass shards in tephra layers recovered bythe Ocean Drilling Program (ODP) in the Izu and NE Japan forearcbasins now allow the temporal variation in slab flux to be chartedsince 7 and 5 Ma, respectively. B/Be ratios are typically <70in NE Japan and <130 in Izu, with no single grain exceeding200. Although moderate to high for modern arcs, these valuesare much less than those recorded in the Marianas and Tongaat 3–4 Ma, shortly after the start of rifting of theirback-arc basins. This observation suggests that the peak B/Bevalues seen in Tonga and the Marianas are related to the tectonicsof slab roll-back and basin opening, rather than changes inthe dynamics of the Pacific Plate. There is no temporal trendto enrichment in the high field strength elements (HFSE) orrare earth elements (REE) in either Izu or NE Japan since 7Ma, although the two elemental groups do show clear positivecorrelation. A lack of correlation between REE, HFSE and B/Besuggests that slab flux is not the only control on melting inthese arcs.     

西秦岭温泉钼矿床成矿作用时限及其对斑岩型钼矿床系统分类制约

斑岩系统是一个涉及岩浆和热液作用的复杂系统,建立精细的斑岩系统成因模型对于寻找更为丰富的金属矿产尤为重要,成矿作用时限是建立成因模型和指导矿产勘查的关键。温泉钼矿床是西秦岭造山带内与晚三叠世花岗岩有关的斑岩型钼矿床,其在西秦岭造山带的独特发育蕴含印支期斑岩成矿作用、大陆地壳演化及矿产勘查关键科学问题。钼矿体主要赋存于温泉复式岩体Ⅱ单元和Ⅲ单元的黑云母二长花岗斑岩和似斑状二长花岗岩中,钼以细脉和浸染状矿化形式产出。赋矿岩石单元锆石U-Pb年龄为224.6±2.5Ma到216.2±1.7Ma,Ⅱ和Ⅲ单元分别侵位于~223Ma和~217Ma,持续约8Myr。辉钼矿Re-Os年龄为212.7±2.6Ma到215.1±2.6Ma,暗示晚三叠世钼成矿作用与花岗质岩浆作用密切时空关系,且成矿年龄稍晚,反映钼矿化主要发生在岩浆作用晚期阶段。成岩、成矿作用发生于华北板块与华南板块全面对接后秦岭造山带构造体制由碰撞到后碰撞的转折阶段,响应南秦岭变质变形、勉-略洋盆闭合及大别-苏鲁超高压岩石板片折返统一地质事件。黑云母K-Ar年龄为207~226Ma,可能反映~223Ma和~208Ma的岩体冷却事件和~216Ma的岩浆-热液成矿作用。锆石U-Pb、辉钼矿Re-Os和黑云母K-Ar多元同位素定年系统准确刻画岩体侵位、热液成矿与冷却事件上有所重叠,岩浆-热液分异演化充分,且具有较高的冷却速率,精确厘定温泉斑岩系统岩浆活动的"多期性"(复式岩体)、成矿事件的"瞬时性"(~214Ma)和成矿作用的"持续性"(~8Myr)。同时,系统对比全球典型斑岩钼(铜)矿床成矿动力学背景,细化分类方案,即产于挤压背景的大洋俯冲和大陆碰撞环境矿床及产于伸展背景的后碰撞、陆缘弧后和板内裂谷环境矿床。明确在大洋俯冲→大陆碰撞→后碰撞→板内裂谷旋回的四个阶段均可以产生规模的斑岩型钼(铜)矿床,且挤压向伸展过渡的构造体制转换尤其是大型矿床形成的有利环境。     

Comparisons of the four Miller Range nakhlites,MIL 03346, 090030, 090032 and 090136: Textural and compositional observations of primary and secondary mineral assemblages

Abstract– Petrological and geochemical analyses of Miller Range (MIL) 03346 indicate that this meteorite originated from the same augitic cumulate layer(s) as the nakhlite Martian meteorites, but underwent rapid cooling prior to complete crystallization. As with the other nakhlites, MIL 03346 contains a secondary alteration assemblage, in this case consisting of iddingsite‐like alteration veins in olivine phenocrysts, Fe‐oxide alteration veins associated with the mesostasis, and Ca‐ and K,Fe‐sulfate veins. We compared the textural and mineralogical compositions of MIL 090030, 090032, and 090136 with MIL 03346, focusing on the composition and Raman spectra of the alteration assemblages. These observations indicate that the meteorites are paired, and that the preterrestrial olivine‐bound alteration assemblages were produced by weakly acidic brine. Although these alteration assemblages resemble similar assemblages in Nakhla, the absence of siderite and halite in the Miller Range nakhlites indicates that the parental alteration brine was comparatively HCO₃^? depleted, and less concentrated, than that which altered Nakhla. This indicates that the Miller Range nakhlite alteration brine experienced a separate evolutionary pathway to that which altered Nakhla, and therefore represents a separate branch of the Lafayette‐Nakhla evaporation sequence. Thin‐sections cut from the internal portions of these meteorites (away from any fusion crust or terrestrially exposed edge), contain little Ca‐sulfate (identified as gypsum), and no jarosite, whereas thin‐sections with terrestrially exposed edges have much higher sulfate abundances. These observations suggest that at least the majority of sulfate within the Miller Range nakhlites is terrestrially derived.     

SOME ASPECTS OF THE INTERACTION OF BLOWING SNOW WITH THE ATMOSPHERIC BOUNDARY LAYER

Several possible effects of blowing snow on the atmospheric boundary layer are investigated, mostly within the general framework of the Prairie Blowing Snow Model (PBSM). The processes of snow saltation and suspension are first described. Variations to the drift density profile are tested and the effects of stratification and density variation calculations are evaluated. Despite high density gradients of blowing snow, stratification effects on turbulence and the velocity profiles can generally be neglected. However, with saltating or suspended snow in a constant shear stress layer, part of the shear stress is carried by the particles. A highly simplified, single-phase approach, based on the density variation of the air–snow mixture coupled to a simple turbulent stress–strain relationship, is used to illustrate this. Sublimation rates in a column of blowing snow are calculated using the PBSM and results are compared with those obtained with a modified formulation which incorporates a spectrum of sublimating particles of varying sizes at each height in a steady-state surface boundary layer and different specifications of the ventilation velocity.     

Petrology of the Younger Andesites and Dacites of Iztacc?huatl Volcano, Mexico: II. Chemical Stratigraphy, Magma Mixing, and the Composition of Basaltic Magma Influx

The Younger Andesites and Dacites of Iztacc?huatl volcano, Mexico,constitute a medium-K calcalkaline rock suite (58–66 wt.per cent SiO₂) characterized by high Mg-numbers (100Mg/(Mg+0?85Fe²⁺=55–66) and relatively high abundances of MgO (2?5–6?6wt. per cent), Ni(17–158 p.p.m.), and Cr (42–224p.p.m.). Chemical stratigraphy plots of eruptive sequences indicatethe existence of a plexus of long-lived dacite magma chambersperiodically replenished by influxes of basaltic magma ascendingfrom depth. Short-term geochemical evolution after batch influxwas dictated by magma mixing and eventual dilution of the basalticcomponent by ‘quasi-steady state’ hornblende dacitemagma. The chemical data support textural and mineralogicalevidence for rapid homogenization of originally diverse magmasby convective blending of residual liquids accompanied by dynamicfractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive thecomposition of basaltic magma influx incorporate analyticaluncertainties and the observed range of salic end-member compositions.Mafic end-members are basalts to basaltic andesites (52–54wt. per cent SiO₂) with Mg-numbers (73–76), MgO (9–11wt. per cent), Ni (250 p.p.m.), and Cr (340–510 p.p.m.)concentrations, and liquidus olivine compositions (Fo_90–88),appropriate for unfractionated partial melts of mantle peridotite.The majority of model compositions are Ol-Hy-normative, similarto those of primitive basaltic lavas on the flanks of Iztacc?huatland in the Valley of Mexico. However, calculated magma batchesrange from weakly Qz-normative to strongly Ne-normative. Bothcalculated and analyzed basaltic compositions are distinguishedby highly variable abundances of alkalies and incompatible traceelements, notably Rb, Ba, Sr, P, Zr, and Y. Initial ⁸⁷Sr/⁸⁶Sr ratios for Iztacc?huatl lavas (0?7040–0?7046;n=24) are comparable to those for primitive basaltic rocks (0?7037–0?7045;?=4) and indicate that (1) mantle source regions are isotopicallyheterogeneous; and (2) contamination of iztacc?huatl magma chambersby radiogenic crustal rocks was not a significant factor inthe evolution of calc-alkaline andesites and dacites. The replenishment of Iztacc?huatl dacite reservoirs by Ne-normativemagmas late in the history of cone growth precludes exhaustionof mantle source regions by progressive partial melting. Thewaning stages of volcanic activity at Iztacc?huatl appear toreflect the inability of dense basaltic influxes to successfullypenetrate a large high-level chamber of low density hornblendedacite magma.     

Properties and origin of a very fine-grained kaolinitic lacustrine deposit

The Rosebrook Member of the Miocene Bunyan Formation is a highly kaolinitic clay-rock of lacustrine origin. It is ultra-fine grained, plastic and of high cation exchange capacity (CEC) of 20–33 c mol(+)kg^?1 of clay. Fabrics observed optically and by transmission electron microscopy (TEM) indicate significant post-depositional consolidation and localized shear failure. Sedimentation and TEM analyses confirm its very fine grain size which is consistent with the relatively high CEC. Tertiary soils and weathering profiles in the catchment of Lake Bunyan developed under relatively moist and cool conditions. Some were gibbsitic with kaolinite and smectite clay minerals. Highly weathered soils (oxisols) on basalt under modern high rainfall environments in the region are dominated by gibbsite and kaolinite. These and other highly weathered soils in the region, formed on a range of parent rocks, have high proportions of very fine grained kaolinite in their subsoils. Such materials are likely to have been abundant in the catchment of Lake Bunyan during the Miocene and would have contributed very fine grained kaolinite directly to the lake by sedimentation. However, the unusually high proportion of kaolinite in the Rosebrook Member and its euhedral crystal form suggest that some of its features developed diagenetically.     

THE INFLUENCE OF PERMEABILITY ON COMPRESSIONAL WAVE VELOCITY IN MARINE SEDIMENTS*

To investigate the effect of permeability on the propagation of seismo-acoustic waves through marine sediments, a theoretical model based on Biot's equations is established which relates the compressional wave velocity measured at a fixed frequency to computed velocities at zero and infinite frequencies in terms of sediment porosity and permeability. The model is examined experimentally in a standard soil mechanics consolidation test (itself dependent, among other things, on sediment porosity and permeability) which has been modified to include measurements of compressional wave velocity at 1 MHz and shear-wave velocity at 5 kHz. This test allows the elastic modulus of the sediment frame to be assessed under different load conditions simultaneous with the velocity determinations. From a number of tests on different samples, five samples are chosen to typify the range of sediment sizes. The results show that the difference between the measured velocity at 1 MHz and the model-derived velocity at zero frequency increases with increasing particle size (from clays to fine sand), with decreasing porosity, and with increasing permeability. For sediments coarser than fine sand the simple model breaks down, possibly because of the dominance of scattering/diffraction effects at the high frequency of the experiment. Within this limitation the model seems satisfactory to offer a capability of predicting the permeability of a sea floor sediment to an order of magnitude by the in situ measurement of seismic velocities over a wide range of frequencies; the prediction process requires a good in situ determination of sediment porosity such as that offered by electrical formation factor measurements.