Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern

Age and Origin of Monazite Symplectite in an Iron Oxide-Apatite Deposit in the Adirondack Mountains, New York, USA: Implications for Tracking Fluid Conditions

Monazite crystals, intergrown with allanite, fluorapatite, and quartz from the Cheever Mine iron oxide-apatite (IOA-type) deposit in Essex County, New York, USA, display rare symplectite textures. Electron probe wavelength-dispersive spectrometry (WDS) mapping and major and trace element characterization of these features reveal a natural experiment in fluid-mediated monazite recrystallization. Two types of monazite with symplectite intergrowths have been recognized (Type I and II). Both types of symplectite development are associated with a decrease in HREE, Si, Ca, Th, and Y, but an increase in both La and Ce in monazite. Electron microprobe Th-U-total Pb analysis of Type I monazite with suitable ThO2 concentrations yielded a weighted mean age of 980 ± 5.8 Ma (MSWD: 3.3), which is interpreted as the age of monazite formation and the onset of symplectite development. Both types of monazite formed during a series of reactions from fluorapatite, and possibly britholite, to produce the final assemblage of monazite, allanite, and fluorapatite. Monazite formation was likely a response to evolving fluid conditions, which favored monazite stability over fluorapatite at ca. 980 Ma, possibly a NaCl brine. A subsequent transition to a Ca-dominated fluid may have then promoted the consumption of monazite to produce another generation of allanite and fluorapatite. Our results indicate that recrystallized monazite formed during fluid-mediated processes that, over time, trended towards an increasingly pure end-member composition. Regionally, these data are consistent with a magmatic-origin followed by fluid-mediated remobilization of select phases at subsolidus conditions for the Adirondack IOA deposits

MINERAL FINE STRUCTURE OF THE AMERICAN LOBSTER CUTICLE

ABSTRACT A major role of lobster integument is protection from microbes. Calcite and amorphous calcium carbonate are the most abundant and most acid vulnerable of the cuticle minerals. We propose that calcite is invested in neutralizing an acidifying environment modulated by the epicuticle. A minor cuticle component is carbonate apatite (CAP), proposed to play critical roles in the integumentÕs structural protective function. The CAP of lobster exhibits a flexible composition; its least soluble forms line the cuticular canals most exposed to the environment. A trabecular CAP structure illustrates efficient use of a sparse phosphate resource, cooperating in the hardness of the inner exocuticle. A schematic model of the cuticle emphasizes structural and chemical diversity. A thin outer calcite layer provides a dense microbial barrier that dissolves slowly through the epicuticle, providing an external, alkaline, unstirred layer that would be inhibitory to bacterial movement and metabolism. Injury to the epicuticle covering this mineralized surface unleashes an immediate efflux of carbonate, accentuating the normal alkalinity of an antimicrobial unstirred layer. The trabecular CAP inner exocuticle provides rigidity to prevent bending and cracking of the calcite outer exocuticle. The combined mineral fine structure of lobster cuticle supports antimicrobial function as well as plays a structural protective role

EXTRAPOLATION OF NUCLEAR WASTER GLASS AGING

Increased confidence is provided to the extrapolation of long-term waste form behavior by comparing the alteration of experimentally aged natural basaltic glass to the condition of the same glass as it has been geologically aged. The similarity between the laboratory and geologic alterations indicates that important aging variables have been identified and incorporated into the laboratory experiments. This provides credibility to the long-term predictions made for waste form borosilicate glasses using similar experimental procedures. In addition, these experiments have demonstrated that the aging processes for natural basaltic glass are relevant to the alteration of nuclear waste glasses, as both appear to react via similar processes. The alteration of a synthetic basaltic glass was measured in MCC-1 tests done at 90/sup 0/C, a SA/V of 0.1 cm/sup -1/ and time periods up to 182 days. Tests were also done using (1) MCC-2 procedures at 190/sup 0/C, a SA/V of 0.1 cm/sup -1/ and time periods up to 91 days and (2) hydration tests in saturated water vapor at 240/sup 0/C, a SA/V of approx. 10/sup 6/ cm/sup -1/, and time periods up to 63 days. These results are compared to alteration observed in natural basaltic glasses of great age. 6 references, 6 figures, 1 table

Evaluation of palagonite: crystallization, chemical changes and element budget

[1] The structural and chemical evolution of palagonite was studied as a function of glass composition, alteration environment, and time by applying a range of analytical methods (electron microprobe, infrared photometry, atomic force microscopy, X-ray fluorescence, and X-ray diffraction). Palagonitization of volcanic glass is a continuous process of glass dissolution, palagonite formation, and palagonite evolution, which can be subdivided into two different reaction stages with changing element mobilities. The first stage is characterized by congruent dissolution of glass and contemporaneous precipitation of “fresh,” gel-like, amorphous, optically isotropic, mainly yellowish palagonite. This stage is accompanied by loss of Si, Al, Mg, Ca, Na, and K, active enrichment of H2O, and the passive enrichment of Ti and Fe. The second stage is an aging process during which the thermodynamically unstable palagonite reacts with the surrounding fluid and crystallizes to smectite. This stage is accompanied by uptake of Si, Al, Mg, and K from solution and the loss of Ti and H2O. Ca and Na are still showing losses, whereas Fe reacts less consistently, remaining either unchanged or showing losses. The degree and direction of element mobility during palagonitization was found to vary mainly with palagonite aging, as soon as the first precipitation of palagonite occurs. This is indicated by the contrasting major element signatures of palagonites of different aging steps, by the changes in the direction of element mobility with palagonite aging, and by the general decrease of element loss with increasing formation of crystalline substances in the palagonite. Considering the overall element budget of a water-rock system, the conversion of glass to palagonite is accompanied by much larger element losses than the overall alteration process, which includes the formation of secondary phases and palagonite aging. The least evolved palagonitized mafic glass studied has undergone as much as 65 wt% loss of elements during palagonite formation, compared to ∼28 wt% element loss during bulk alteration. ABout 33 wt% element loss was calculated for one of the more evolved, in terms of the aging degree, rocks studied, compared to almost no loss for bulk alteration

Determining the Pyroxene Mineralogies of Vestoids

Bulk pyroxene compositions were calculated for a number of V-type asteroid spectra using formulae derived by Burbine et al. These formulae were derived by analyzing HED (howardite, eucrite, and diogenite) meteorites and calculate bulk Fs (mol%) and Wo (mol%) contents using derived band centers. Using HEDs with known bulk pyroxene compositions, the uncertainty in the predicted Fs contents was determined to be ±3 mol%, and the uncertainty in the predicted Wo contents was ±2 mol%. V-type asteroids tend to have interpreted pyroxene mineralogies consistent primarily with eucrites and howardites. We investigate why diogenitic mineralogies appear so rare among ∼5–10 km V-type asteroids but are much more commonly present among HED meteorites. One possibility is that diogenitic intrusions are extremely “thin” but widespread in Vesta’s eucritic crust. In this scenario, Vestoids (V-type asteroids thought to be derived from Vesta) would be expected to be solid fragments of Vesta. Another possibility is that Vesta’s upper crust has been significantly shattered and diogenitic material would be much less common than the eucritic material in the crust. Vestoids would then be expected to be rubble piles. The belief that most asteroid families were shattered at least twice would argue that Vesta’s crust is also shattered and that Vestoids are rubble piles

A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K-Feldspar Reference Materials

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with \u3c 2000 μg g-1 total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with \u3e 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified

The advantages of a salt/bentonite backfill for Waste Isolation Pilot Plant disposal rooms

A 70/30 wt% salt/bentonite mixture is shown to be preferable to pure crushed salt as backfill for disposal rooms in the Waste Isolation Pilot Plant (WIPP). This report discusses several selection criteria used to arrive at this conclusion: the need for low permeability and porosity after closure, chemical stability with the surroundings, adequate strength to avoid shear erosion from human intrusion, ease of emplacement, and sorption potential for brine and radionuclides. Both salt and salt/bentonite are expected to consolidate to a final state of impermeability (i.e., {le} 10{sup {minus}18}m{sup 2}) adequate for satisfying federal nuclear regulations. Any advantage of the salt/bentonite mixture is dependent upon bentonite's potential for sorbing brine and radionuclides. Estimates suggest that bentonite's sorption potential for water in brine is much less than for pure water. While no credit is presently taken for brine sorption in salt/bentonite backfill, the possibility that some amount of inflowing brine would be chemically bound is considered likely. Bentonite may also sorb much of the plutonium, americium, and neptunium within the disposal room inventory. Sorption would be effective only if a major portion of the backfill is in contact with radioactive brine. Brine flow from the waste out through highly localized channels in the backfill would negate sorption effectiveness. Although the sorption potentials of bentonite for both brine and radionuclides are not ideal, they are distinctly beneficial. Furthermore, no detrimental aspects of adding bentonite to the salt as a backfill have been identified. These two observations are the major reasons for selecting salt/bentonite as a backfill within the WIPP. 39 refs., 16 figs., 6 tabs