Host Grains Research Articles

The role of strain accommodation during the variant selection of primary twins in magnesium

Samples of magnesium alloys AM30 and AZ31 were deformed in tension at room temperature and a strain rate of 0.1 s −1 to strains of 0.08 and 0.15. Of the numerous contraction twins that formed, the orientations of 977 were determined by electron backscatter diffraction techniques. The orientations of their host grains were also measured, so that the Schmid factors (SFs) applicable to each of the six contraction twins that could potentially form in each grain could also be calculated. About half of the observed twins were of the “high SF” (0.3–0.5) type, while nearly half had “low” SFs (0.15–0.30). Furthermore, 5% of the observed twins had associated Schmid factors of only 0.03–0.15, i.e. these were of the “very low SF” type. Of particular interest is the observation that many potential “high Schmid factor” twins did not form. The presence of the low and very low SF twins and the absence of many potential high SF twins are explained in terms of the accommodation strains that are or would be required to permit their formation. These were calculated by rotating the twinning shear displacement gradient tensor into the crystallographic reference frame of the neighboring grain. It is shown that the very high plastic anisotropy of Mg grains permits the “easy” accommodations to take place but conversely prevents accommodation of the potential twinning shears when these are “difficult” (when these involve high critical resolved shear stresses). The twins that appear require little or no “difficult” accommodation.

Dust grain growth inρ-Ophiuchi protoplanetary disks

We present new ATCA observations at 3.3 mm of 27 young stellar objects in the rho-Oph young cluster. 25 of these sources have been detected. We analyze the sub-millimeter and millimeter SED for a subsample of 17 isolated class II protoplanetary disks and derive constraints on the grain growth and total dust mass in the disk outer regions. All the disks in our sample show a mm slope of the SED which is significantly shallower than the one observed for the ISM at these long wavelengths. This indicates that 1) class II disks in Ophiuchus host grains grown to mm/cm-sizes in their outer regions, 2) formation of mm/cm-sized pebbles is a fast process and 3) a mechanism halting or slowing down the inward radial drift of solid particles is required to explain the data. These findings are consistent with previous results in other star forming regions. We compare the dust properties of this sample with those of a uniformly selected sample in Taurus-Auriga and find no statistical evidence of any difference in terms of grain growth between the two regions. Finally, in our sample the mm slope of the SED is not found to correlate with indicators of grain growth to micron sizes in the surface layers of the inner disk.

Nucleation and growth of new grains in recrystallized quartz vein: An example from banded iron formation in Iron Quadrangle, Brazil

Intracrystalline microcracks developed in quartz single crystals deformed in greenschist metamorphic conditions. A detailed study of samples collected in tabular to lens shape quartz vein was carried out to investigate how the microcracks initiated and how the microstructures evolved with the progressive deformation. A combination of light and EBSD (electron backscatter diffraction) techniques was used to analyze the microstructures and determine the crystallographic orientation of quartz grains. The crystallographic orientations of microcracks indicate that they might have initiated parallel to the direction of one of the rhombohedral planes of the host crystals. It is suggested that new grains nucleated by rotation of broken fragments from the host grains. c-axes the of host are distributed in a small-circle close to the foliation plane while the c-axes of the new grains in microcracks are more scattered when compared with the host orientations. New grains grew with their c-axes approximately perpendicular to the shortening direction.

Evolution of cataclastic faulting in high-porosity sandstone, Bassin du Sud-Est, Provence, France

Cataclastic deformation structures in Cretaceous high-porosity sands in the Bassin du Sud-Est, SE France were surveyed by scan-lines to examine: (i) the role of tectonic loading path on cataclastic deformation band (CDB) network development, and (ii) the development of larger ultracataclastic faults as strain increases. Deformation during Pyrenean–Provençal shortening resulted in a persistent high density (∼10/m 2) of conjugate reverse-sense CDB zones (displacements up to ∼30 cm), with no generation of larger faults. High–low-density undulations occur for each pair of the conjugate set in an alternating manner, suggestive of network hardening, with a wavelength of several tens of metres being in the order of mechanical bed thickness. For two study areas which experienced significant Oligocene–Miocene extension, a moderate, undulating background density (∼4/m 2) of normal-offset CDBs was recorded, which became focussed in places into clusters (∼50/m 2) a few metres wide. Thus tectonic loading path may strongly influence strain distribution. CDB zones develop by the addition of successive bands at the edges until, at a thickness of around 5 cm, new bands tend to stray further away from the zone edges. Coarser sands have thicker CDB zones, suggesting that host grain size, along with mechanical bed thickness, could be an important contributor to the scale limit in CDB zone growth. Larger ultracataclastic faults and discrete slip zones localised within or at the edges of some clusters of CDB zones, post-date cluster development rather than inducing it. This stage of deformation evolution is only reached in extension, not in shortening, suggesting the infeasibility of achieving the critical state line during horizontal compression.

Feldspar in type 4–6 ordinary chondrites: Metamorphic processing on the H and LL chondrite parent bodies

Abstract– We have carried out a study of feldspar compositions in a suite of H and LL ordinary chondrites, of petrologic types 4, 5, and 6, in order to examine the process of recrystallization and equilibration of feldspar as the degree of metamorphism increases. In the H chondrites, there is little variation in feldspar compositions among the petrologic types, suggesting that homogenization of chondrule mesostasis, from which feldspar is presumed to have crystallized, occurred before feldspar crystallization began. The LL chondrites we studied are more complex. In Bjurböle (L/LL4), plagioclase in individual relict chondrules has distinct compositions, with a range of An/Ab ratios and low Or contents. This heterogeneity is most likely attributable to original compositional heterogeneity among chondrule mesostases: localized recrystallization of mesostasis must have occurred before diffusional equilibration took place. In Tuxtuac (LL5), the An/Ab ratio of plagioclase is more homogeneous, and plagioclase includes a significant Or component. In addition, we observe what appears to be exsolution of K‐feldspar from albitic host grains. In Saint Séverin (LL6), the An/Ab ratio of plagioclase is homogeneous, but plagioclase compositions show a range of Or contents, corresponding to a patchy distribution of K in individual feldspar grains. The observations in these LL chondrites are difficult to interpret with a simple model of progressive equilibration with increasing petrologic type. We suggest that the current criteria for assigning petrologic types are poorly defined: it is possible that the assigned petrologic types of these chondrites do not correlate with their peak temperatures. We propose that feldspar compositions might record conditions during the heating stage of metamorphism, and that the early stages of metamorphism may have occurred in the presence of fluids, rather than under the dry conditions that are commonly assumed.

Zircon Behaviour during Low-temperature Metamorphism

Zircon in greenschist-facies metasedimentary rocks from the Scottish Highlands displays a range of complex textures that reflect low-temperature alteration of original detrital grains. In situ back-scattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous, weakly luminescent, enriched in non-formula elements such as Al and Fe, and is associated with fractures within the host zircon. The low-temperature zircon appears to be nano-crystalline and to replace U-rich zircon via modification of whole grains or selective alteration of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is abundant in slates and other mica-rich samples and its formation is linked to a dissolution–reprecipitation mechanism. Zircon within quartz-rich host rocks typically shows evidence of deformation and the resulting fractures enhance its dissolution, creating rounded embayed morphologies. In contrast, zircon from phyllosilicate-rich rocks contains more new low-temperature growth. Zircon alters during both prograde and retrograde metamorphic events and its development is controlled by both the progressive accumulation of radiation damage in the host grain and the access of metamorphic fluids to the metamict zircon.

Variant selection during nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys

This work deals with the mechanisms of nucleation and growth of γ-massive phase in TiAl-based intermetallic alloys. In particular, it focuses on the process of variant selection operating at both stages of the transformation. Small γ-massive domains produced by rapid cooling are extensively characterized by high-resolution electron backscatter diffraction. This large data set allows a statistical analysis of nucleation sites, according to different crystallographic configurations. It is established that, whatever the nucleation sites, i.e., grain boundaries or triple junctions, a coherent facet is always found presenting a Blackburn orientation relationship (BOR) between the γ nucleus and the α parent grain. Moreover, some γ nuclei can additionally present another semicoherent facet with an approximate BOR with the α host grain. A new nucleation mechanism, called “co-nucleation”, is highlighted for this type of double-faceted nucleus. Variant selection during nucleation is discussed for both types of nuclei, in terms of minimization of interface energy. In addition, it is shown that growth of γ-massive domains from their initial nucleus always involves successive {1 1 1} twinning. Variant selection also occurs either at the nucleus growth stage or during the development of successive twin generations, and is discussed in terms of interphase boundary mobility.

Magnetic petrology of the Devonian Achala Batholith, Argentina: titanohaematite as an indicator of highly oxidized magma during crystallization and cooling

SUMMARY Devonian magmatism in the Eastern Sierras Pampeanas (Cordoba, Argentina) is represented by elliptical, porphyritic, batholithic, late- to post-orogenic monzogranites emplaced in a metamorphic–plutonic basement. Two groups of Devonian granitoids are distinguished: (1) metaaluminous to weakly peraluminous, hornblende-biotite bearing granitoids, and (2) peraluminous, biotite-(muscovite) bearing granitoids. Groups 1 and 2 are characterized by accessory phases of sphene-magnetite with high susceptibility, and ilmenite-(monazite-uraninite) with low susceptibility, respectively. The magnetic assemblage of the Group 2 Achala Batholith, predominantly paramagnetic, is the subject of this paper. Forty-three sites were sampled in the Achala monzogranite. Overall, it is weakly magnetic, with a mean magnetic susceptibility of 1.5 × 10−4 SI. Twenty-one sites showed titanohaematite as the magnetic carrier of a stable remanence. The titanohaematite is an accessory mineral and contains exsolved disc-shaped intergrowths of ilmenite. The weak magnetism of Achala monzogranite facies contrasts with the magnetic character of other Devonian porphyritic, batholithic monzogranites in the region. The virtual absence of magnetite, together with the presence of exsolved titanohaematite and Mn-ilmenite indicate crystallization and cooling under more oxidizing conditions. Oxidizing conditions continued into the subsolidus, causing late-stage reactions of ilmenite host grains to ilmenite + rutile+ haematite, and titanohaematite to ilmenite-poorer haematite + rutile. These reactions resulted in low susceptibility values and a strong reduction of natural remanent magnetization (NRM) intensities. Thermal demagnetization, up to 620–640 °C, isolated steeply dipping, dual-polarity remanence directions. The coincidence of the Achala paleomagnetic pole with the 380–360 Myr segment of the Gondwana Apparent Polar Wander Path and the U-Pb zircon age implies that the remanence was acquired soon after emplacement.

Evolution of quartz microstructure and c-axis crystallographic preferred orientation within ductilely deformed granitoids (Arolla unit, Western Alps)

We have studied quartz microstructures and the c-axis crystallographic preferred orientations (CPOs) in four granitoid samples representative of increasing ductile shear deformation, from a weakly deformed granitoid (stage 1) to a mylonitic granitoid (stage 4). The quartz c-axis CPO measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area. All the samples belong to the Austroalpine Arolla unit (Western Alps) and were deformed at greenschist facies conditions. The quartz c-axis CPO was analyzed using a U-stage and the optical orientation imaging technique. The magmatic plagioclase, forming more than 50% of the volume of the granitoid, is extensively replaced by a mica-rich aggregate even in weakly deformed samples of stage 1. These aggregates flow to form an interconnected weak matrix with increasing deformation, wrapping relatively less strained quartz grains that undergo dominantly coaxial strain. Recrystallization of quartz ranges from less than 1% in the weakly deformed granitoid to up to 85% in the mylonitic granitoid, with average grain strain of 41% and 64%, respectively. With increasing strain and recrystallization, quartz grains in the granitoids show a sequence of transient microstructures and CPOs. Crystal plastic deformation is initially accomplished by dislocation glide with limited recovery, and at 50% grain strain it results in a CPO consistent with dominantly basal 〈 a〉 slip. At 60% grain strain, recrystallization is preferentially localized along shear bands, which appear to develop along former intragranular cracks, and the recrystallized grains develop a strong c-axis CPO with maxima orthogonal to the shear band boundaries and independent of the host grain orientation. Within the granitoid mylonite, at an average quartz grain strain of 64%, recrystallization is extensive and the c-axis CPO of new grains displays maxima overlapping the host c-axis orientation and, therefore, unrelated to the bulk sense of shear. The host-controlled CPO is inferred to reflect pervasive recrystallization by progressive subgrain rotation. The switch from ‘shear band-control’ to ‘host-control’ on c-axis CPO occurred between 40% and 70% of recrystallization. In the quartz mylonite, the quartz c-axis CPO develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions. This study indicates that the CPO evolution of quartz may significantly differ in cases of polymineralic vs. monomineralic rocks under the same deformation conditions, if quartz in the polymineralic rock behaves as a ‘strong’ phase.

GRAINS OF Pt-Fe ALLOY AND INCLUSIONS IN A Pt-Fe ALLOY FROM FLORENCE CREEK, YUKON, CANADA: EVIDENCE FOR MOBILITY OF Os IN A Na-H2O-Cl-RICH FLUID

We report results of detailed electron-microprobe analyses (EMP) of fourteen grains of Pt–Fe alloy, ca. 0.4 to 1 mm, principally droplet-shaped or roundish, from Florence Creek, Yukon. We also describe new occurrences of Pt–(Pd)–Fe alloy from Arch Creek (up to 11 wt.% Pd) and Canadian Creek, Yukon. An extensive compositional series is observed at Florence, in which values of ∑PGE/(Fe + Cu + Ni) vary from 2.2 to 5.4. Minor Ir and Os display a contrasting behavior in this series. The observed levels of Ir are generally greater than those of Os. The Ir correlates negatively with Pt (the correlation coefficient, R, is −0.81; 140 EMP point-analyses); in contrast, the correlation between Os and Pt is slightly positive (R = 0.64). Probably, at a higher temperature of crystallization, Ir readily replaces Pt in the structure of a Pt–Fe alloy. As a result, Ir did not attain its maximum, and the observed inclusions of exsolution-induced Ir-dominant alloy, hosted by the Pt–Fe alloy, are moderately enriched in Ir (52–54 at.%). An inclusion of erlichmanite (4.2 wt.% Rh) and a partial rim of Au–Ag alloy (Au 0.80 Ag 0.18 ) were observed. Droplet-like inclusions of highly aluminous silicate minerals, hosted by grains of Pt–Fe alloy, are rich in Na, K and in Cl (up to 2 wt.%). Veinlets and micrograins of an Os-rich alloy (86–92 wt.% Os) are intimately associated with micrograins of nearly pure albite, Ab 97.8 An 1.7 Or 0.5 , and with an intermediate member of the chamosite–clinochlore series. The observed association chamosite + albite + native osmium, infilling a cavity in a host grain of Pt–Fe alloy, was likely deposited from a late Na–H 2 O–(Cl)-bearing fluid phase at a postmagmatic–hydrothermal stage, at a temperature less than 700°C. The Os and Na were possibly mobilized and transported as Na–(Os)-rich complexes related to the sodium hydroxy- or oxo-osmates. The precipitation of the metallic Os phase could be related to a change in physicochemical conditions, e.g., in fluid pressure or redox potential, leading to instability of these Na–(Os)-rich hydroxy- or oxo-osmate complexes. We suggest that a mineralized zone rich in chromite–magnesiochromite and hosted by an Alaskan–Uralian-type complex is the likely provenance for grains of Pt–Fe alloy recovered in Florence Creek.

Empirical thermometer of TiO 2 in quartz for ultrahigh-temperature granulites of East Antarctica

Abstract Two preliminary experiments, heating of rutilated quartz grains with 0.082 wt% TiO 2 on average from Bunt Island, Napier Complex, East Antarctica and a synthetic TiO 2 –SiO 2 (rutile–cristobalite) system in air at 1300 °C for 39 days, showed increasing solubility of TiO 2 in silica minerals with temperature. Bunt quartz was converted to cristobalite and traversed by many transparent seams with the disappearence of needles and spots of rutile. Unreacted host grains retained many fine needles of rutile. The seams are homogeneous and slightly enriched in TiO 2 up to 0.149 wt% on average, which is about one-fifth lower than that of the synthesized TiO 2 –SiO 2 cristobalite (0.767 wt% on average). Area analyses with an electron beam in the raster mode at a magnification of ×5000 gave 0.308 wt% TiO 2 for the bulk composition of the Bunt quartz. This indicates that needles of rutile exsolved from the TiO 2 -saturated quartz at the cooling stage, or during retrograde metamorphism. Natural examples of quartz in geologically and petrologically well-characterized metamorphic rocks were chemically analysed to examine the temperature controls on the Ti saturation level in quartz. The TiO 2 content of quartz in equilibrium with rutile increases sensitively with the metamorphic temperature, which can be expressed as T ( ° C ) = − 5895 / ( ln X TiO 2 Qtx + 1.729 ) − 273 where X TiO 2 Qtz is the mole fraction of TiO 2 , or the number of Ti atoms per formula unit based on a two-oxygen atom normalization. This empirical equation is very useful to evaluate the metamorphic temperatures for ultrahigh-temperature granulites. The temperatures calculated by the existing Ti-in-quartz thermometer are about 200 °C higher than those estimated by the present thermometer, potentially because of underestimates of Ti solubility in quartz in the previous calibration.

(U–Th)/He dating of volcanic phenocrysts with high-U–Th inclusions, Jemez Volcanic Field, New Mexico

We measured (U–Th)/He ages of fayalite and hedenbergite phenocrysts with britholite and chevkinite inclusions from the ∼1.6 Ma Otowi (Lower) Member of the Bandelier Tuff, Jemez Volcanic Field. U and Th, the major parent nuclides of radiogenic 4He, are concentrated in britholite and chevkinite inclusions and are essentially absent from the host grains. We analyzed 36 fayalite (21 non-abraded + 15 abraded) and 13 hedenbergite (7 non-abraded + 6 abraded) phenocrysts, and applied alpha-ejection corrections assuming uniform intracrystalline distributions of inclusions and parent nuclides in the phenocrysts. A subset of phenocrysts was subjected to abrasion to remove the alpha-ejection affected regions. The alpha-ejection correction factors ( F T ) of the non-abraded samples range between 0.90 and 0.96, whereas those of the abraded samples are virtually unity. The fayalite and hedenbergite crystals yield mean ages of 1.60 ± 0.10 Ma (S.D.) and 1.69 ± 0.13 Ma, respectively. The overall mean age of 1.62 ± 0.11 Ma (S.D.) is indistinguishable from previously published 40Ar/ 39Ar ages of 1.61 ± 0.01 Ma (1 σ). Our results suggest that other inclusion-bearing volcanic phenocrysts and their host units may be dateable by the (U–Th)/He method.

NATURAL RADIATION-INDUCED DAMAGE IN QUARTZ. I. CORRELATIONS BETWEEN CATHODOLUMINENCE COLORS AND PARAMAGNETIC DEFECTS

Quartz grains from the McArthur River uranium deposit of the Athabasca Basin, Saskatchewan, are characterized by three distinct types of cathodoluminescence (CL): 1) halos surrounding U- and Th-bearing mineral inclusions, and 2) patches and 3) continuous rims along grain boundaries and fractures. These three types of CL have a constant width of ~35 to 45 m, consistent with the maximum depth of penetration of alpha particles, and therefore they record alpha-particle-induced radiation damage. Relative to the host grains, the radiation-damaged areas are characterized by pronounced but broad CL bands in the ultraviolet (~350 nm) and red (~620–650 nm) regions. Isochronal annealing experiments reveal that the ultraviolet CL persists to 500°C but is annealed out at 600°C, whereas the red CL persists to at least 800°C. Electron paramagnetic resonance (EPR) spectroscopy, including detailed measurements on saturation behavior and thermal properties, revealed six paramagnetic defects: one oxygen vacancy center (E1’), three silicon vacancy hole centers [O2 3– /H + (I), O2 3– /H + (II) and O2 3– /M + ] and two O2 – peroxy centers. Spectral simulations confi rm the presence of these centers. Moreover, EPR spectra of HF-treated samples show that the silicon vacancy-hole centers and the peroxy centers are concentrated in the radiation-damaged rims and fractures. Center E1’ appears to occur throughout the quartz grains and is annealed out at ~500°C; it thus cannot be responsible for the ultraviolet or red CL. The silicon vacancy-hole centers are all annealed out between 550° and 600°C, similar to the annealing temperature of the ultraviolet CL. The peroxy centers are the only paramagnetic defects stable above 600°C, corresponding to the preservation of red CL in radiation-damaged areas at high temperatures. Therefore, the silicon vacancy-hole centers and the peroxy centers are probably responsible for the characteristic ultraviolet and red CL, respectively, associated with radiation-damaged halos, patches and rims in quartz.

Investigation of the martensitic-like transformation from Mg2GeO4 olivine to its spinel structure polymorph

A coherent mechanism for the transformation of olivine to its spinel structure polymorph has been investigated in Mg 2 GeO 4 olivine at conditions ranging from 6 GPa to 18 GPa at 300 to 1250 °C using a multi-anvil apparatus. This transformation mechanism, which produces plates of spinel lying parallel to the (100) planes in olivine, becomes increasingly important above 10 GPa at temperatures between 300 and 400 °C. The primary effect of temperature above 400 °C is to increase the length of the lamellae. The primary effect of pressure is to increase the number of lamellae that have nucleated. The distribution of transformation is very inhomogeneous. Lamellae first appear in patches associated with grain edges and corners. As the degree of transformation increases, patches become more extensive and more heavily populated until the majority, if not all grains, contain large numbers of lamellae that span large portions of their host grains. The mechanism that produces the lamellae requires three factors, high pressure overstep of the equilibrium boundary, some level of differential stress (either microscopic or macroscopic), and high enough temperature to facilitate rearrangement of the cations. It is most consistent with a martensitic-like transformation mechanism. The results of this study suggest that olivine may exhibit similar behavior in cold, rapidly subducting lithospheric slabs. If conditions are too cold for olivine to transform by reconstructive nucleation and growth, it could instead progressively transform by the martensitic-like mechanism as conditions deviate further and further from the equilibrium phase boundary.

A non-terrestrial 16O-rich isotopic composition for the protosolar nebula

The discovery in primitive components of meteorites of large oxygen isotopic variations that could not be attributed to mass-dependent fractionation effects has raised a fundamental question: what is the composition of the protosolar gas from which the host grains formed? This composition is probably preserved in the outer layers of the Sun, but the resolution of astronomical spectroscopic measurements is still too poor to be useful for comparison with planetary material. Here we report a precise determination of the oxygen isotopic composition of the solar wind from particles implanted in the outer hundreds of nanometres of metallic grains in the lunar regolith. These layers of the grains are enriched in 16O by >20 +/- 4 per thousand relative to the Earth, Mars and bulk meteorites, which implies the existence in the solar accretion disk of reactions--as yet unknown--that were able to change the 17O/16O and 18O/16O ratios in a way that was not dependent strictly on the mass of the isotope. Photochemical self-shielding of the CO gas irradiated by ultraviolet light may be one of these key processes, because it depends on the abundance of the isotopes, rather than their masses.

Oxygen-isotopic compositions of relict and host grains in chondrules in the Yamato 81020 CO3.0 chondrite

We report the oxygen-isotope compositions of relict and host olivine grains in six high-FeO porphyritic olivine chondrules in one of the most primitive carbonaceous chondrites, CO3.0 Yamato 81020. Because the relict grains predate the host phenocrysts, microscale in situ analyses of O-isotope compositions can help assess the degree of heterogeneity among chondrule precursors and constrain the nebular processes that caused these isotopic differences. In five of six chondrules studied, the Δ 17O (=δ 17O −0.52 · δ 18O) compositions of host phenocrysts are higher than those in low-FeO relict grains; the one exception is for a chondrule with a moderately high-FeO relict. Both the fayalite compositions as well as the O-isotope data support the view that the low-FeO relict grains formed in a previous generation of low-FeO porphyritic chondrules that were subsequently fragmented. It appears that most low-FeO porphyritic chondrules formed earlier than most high-FeO porphyritic chondrules, although there were probably some low-FeO chondrules that formed during the period when most high-FeO chondrules were forming.

Isotopic and chemical alteration of zircon by metamorphic fluids: U-Pb age depth-profiling of zircon crystals from Barrow’s garnet zone, northeast Scotland

U-Pb isotopic analyses were done using ion microprobe depth-profiling on zircon crystals from Dalradian rocks in Barrowʼs garnet zone, northeast Scotland, to determine the timing of metamorphic fluid infiltration and investigate fluid-zircon interactions. Zircon crystals collected from altered metasedimentary rock adjacent to a cross-cutting quartz vein and from relatively unaltered, continuously equivalent layers distal to the vein indicate that Paleozoic fluid influx caused isotopic and chemical alteration along zircon crystal surfaces and internal fractures. “Ion drilling” into natural zircon crystal faces yielded U-Pb depth profiles with 50-100 nm spatial resolution that permitted U-Pb isotopic age analysis of thin grain rims affected by fluid-mediated growth/recrystallization at 500-550 °C. We interpret the measurements to indicate that fluid influx at 462 ± 8.8 Ma caused the growth/recrystallization of new zircon along pitted crystal surfaces and fractures in Archean zircon grains. Our age estimate overlaps the accepted age range (ca. 467-464 Ma) for peak Barrovian garnet growth, confirming that metamorphism and fluid influx were contemporaneous. The new zircon was enriched in U, Th, and common Pb relative to host grains, suggesting that U, Th, and Pb were actively transported by upper greenschist-facies metamorphic fluids. We present the first field-based evidence that common Pb in zircon can serve as an indicator of metamorphic fluid infiltration. Archean ages for some detrital zircons suggest contributions from a Lewisian/Scourian provenance.

MORPHOLOGICAL AND CHEMICAL STUDY OF PLACER GOLD FROM THE SAN LUIS RANGE, ARGENTINA

The composition of placer gold from the San Luis Range, Argentina, is poorly documented, although these deposits have been known since the 18 th century. A detailed study of gold grains collected from different streams in the La Carolina gold district was carried out. We performed 502 analyses on 191 grains. The core composition of the gold grains is heterogeneous, with an average chemical composition of 58 to 74 wt.% Au, 25 to 40% Ag, up to 0.5% Hg, and less than 0.4% Cu. The rim of each grain is generally richer in gold than the core, and the contact between rim and core is very sharp but of irregular shape and complex texture. There are populations with different chemical composition, but the compositional differences among rounded grains and those having a more delicate morphology are not invariably either significant or consistent. All sources are restricted to deposits associated with Tertiary volcanic rocks in the area, and the variable and wide range of fineness points to low-sulfidation epithermal-type deposits as the most likely source. The sharpness of the core–rim contact may result from either electrochemical processes active in the stream or stream sediments, or Au and Ag dissolution processes followed by Au precipitation in surface or near-surface environments. The presence of overgrowths of gold on gold grains suggests that there was secondary precipitation of gold after the host grain was formed. Gold grains do grow in low-temperature geochemical settings, and we believe that their composition becomes more gold-rich as the grains move downstream or are buried in the sediments. Anthropogenic Hg was detected in some of the streams, and even where there are several sources for it, we believe that all of it is coming from the amalgamation processes used by local inhabitants to recover the panned gold.

Oxidation–decomposition facilitated reorientation of nanoparticles in reactively sintered (Ni 0.33Co 0.67) 1− δO polycrystals

(Ni 0.33Co 0.67) 1− δ O polycrystals with rock salt structure and a bimodal size distribution due to reactive sintering at 1000 °C were subject to annealing at 720 °C for 2–72 h in air and studied by analytical electron microscopy with regard to the effect of oxidation decomposition on the reorientation of nanoparticles in host grains. Upon annealing, the nanoparticles rapidly oxidized as spinel structure progressively Co-richer, whereas the host protoxide grains with rock salt-type structure progressively Ni-richer. The spinel particles less than 100 nm in size readily detached from grain boundaries and fell into parallel epitaxial relationship with respect to the host protoxide grains sharing a coherent interface. Such a Brownian-type reorientation process, in terms of anchorage release at interphase interface and driven by epitaxy energy cusp, at a rather low apparent homologous temperature ( T/ T m=0.45) was facilitated by oxidation decomposition process and nanometer-size effect.

Cementation of porous materials using calcite

A technique of cementing porous materials artificially using calcite is presented. The technique depends on flushing a mixture of chemical solutions through a porous medium, leading to precipitation of calcite due to reaction of the solution ingredients. The experimental procedures adopted to achieve the cementation are described. The response of different soils to the cementation process is analysed. The bond mechanism created by the calcite is illustrated, using optical and electron micrographs. The paper discusses the factors that appear to determine the response of geomaterials to calcite treatment. It has been found that the strength of a calcite-treated material increases with: (a) intrinsic strength of the individual grains, (b) density, (c) decreasing particle size of the host grains, (d) pre-coating of grains with calcite, and (e) roundness and non-angularity of soil grains. The cementation technique described in this paper has proved successful in reproducing the mechanical behaviour of natural calcarenite from the North West Shelf of Australia.