Scott M. Kuehner

Otolith chemistry reflects migratory characteristics of Pacific salmonids:: Using otolith core chemistry to distinguish maternal associations with sea and freshwaters

Abstract Experimental crosses with coho, sockeye, and chinook salmon showed that otolith core strontium concentrations reflected maternal associations with fresh and sea water. Fry spawned by females maturing in sea water had otolith Sr/Ca values roughly four times greater than those from their freshwater counterparts, indicating that otolith core strontium is an effective natural marker for captive brood stock programs where fish are raised to maturity in freshwater. Anadromous and freshwater resident salmonid populations were also distinguishable on the basis of otolith core strontium levels, but case studies illustrate that some freshwater systems may have strontium levels high enough to blur the distinction between sympatric progeny from anadromous and non-anadromous salmonids. Among a number of wild and hatchery anadromous salmon populations, a trend in otolith core Sr/Ca reflected the typical time of entry to freshwater and stage of egg development for migrating females.

Complex zoning in apatite from the Idaho Batholith; a record of magma mixing and intracrystalline trace element diffusion

Abstract Apatite crystals in a sample of biotite granodiorite from the Idaho batholith display complex chemical zoning, characterized by abrupt changes in REE+Y+Si content, more subtle variations in S, Na, La/Yb, and Mn contents, and petrographic evidence for multiple episodes of partial resorption. The zoning is attributed to changes in melt composition, resulting from magma mixing and differentiation, although the possibility that some rounded cores may be inherited cannot be disproved. High S contents in apatite cores and the presence of an included anhydrite grain indicate crystallization from an oxidized host magma. Divalent cations that occupy the apatite Ca sites (Sr, Mn, Fe) show evidence of having been redistributed between zones by intracrystalline diffusion, whereas cations that participate in coupled substitutions involving the tetrahedral site (Si, Y, REE, Na, S) were not readily redistributed. The main REE substitution in this case is REE3+ + Si4+ ↔ Ca2+ + P5+, and REE diffusion is rate-limited by slow Si diffusion. However, exchange of LREE (e.g., LaSm-1) on the Ca sites does not involve Si and proceeds more rapidly, resulting in homogenization of La/Sm between zones within individual crystals. Relative diffusion rates inferred from zoning profiles in this study are: Mn, Sr, Fe, and LaSm-1 are faster than Na, S, and LaYb-1, which are faster than Si. These data imply that REE patterns can be decoupled from REE abundances during diffusion, and that even apatite zones or cores that appear sharply bounded in backscattered electron images may not retain their original chemical or Sr-Nd isotopic traits.

Implications of ferrous and ferric iron in antigorite

Abstract Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe3+ is not simply related to fO₂. For Si = 1.95 apfu estimated Fe3+ = 0.032 apfu (or 0.95 wt% Fe2O3). Such estimates of Fe3+ require high analytical accuracy and precision, and assume a fixed polysomatic formula (e.g., m = 17) and freedom from interlayer sheet-silicate impurities. In many cases the estimates appear to be high. An alternative measure of Fe3+ is provided by the partitioning of total Fe and Mg between antigorite and olivine in well-equilibrated natural antigorite-olivine-magnetite parageneses. Extrapolation of Nernst and Roozeboom partition plots to Fe-free olivine permits an estimate of the Fe3+ content of the average antigorite in this paragenesis, namely 0.42 or 0.64 wt% Fe2O3. The partition estimates are in good agreement with the results of Mössbauer spectroscopy performed here on 14 antigorites from metaperidotites, together with four from the literature. These spectra reveal a range of 0.16 to 1.94 in wt% Fe2O3 in metaperidotite antigorite, with an average of 0.83. In two olivine-bearing rocks, antigorite has Fe3+/ΣFe ratios of 0.13 and 0.15, which corresponds to wt% Fe2O3 = 0.47 and 0.54, respectively. Larger amounts of Fe2O3 occur in some, but not all, vein antigorites. The prograde formation of antigorite in serpentinite from lizardite is accompanied by loss of some cronstedtite component and the precipitation of additional magnetite. The Roozeboom Mg/Fe partition plot is concave down rather than up; in other words the partition coefficient KD is a function of the XMg of olivine. This behavior has been found in other olivinemineral pairs. It can be interpreted to reflect strongly non-ideal solution behavior of MgFe-olivine at low temperatures, viz. WG ≈ 8.5 kJ assuming a symmetrical solution. MgFe-brucite appears to be similarly non-ideal.

Magnetite-free, yellow lizardite serpentinization of olivine websterite, Canyon Mountain complex, N.E. Oregon

Abstract We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite (XFe = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite’s identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of Fe3+ exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables T, fH₂O, fH₂, and aSiO₂ attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization

Geochemistry of Mafic Enclaves and Host Granitoids from the Chilliwack Batholith, Washington: Chemical Exchange Processes between Coexisting Mafic and Felsic Magmas and Implications for the Interpretation of Enclave Chemical Traits

Mafic enclaves from three plutons in the Chilliwack batholith have been compared with contemporaneous mafic stocks in order to determine (1) the processes by which mafic and felsic magmas hybridize in the plutonic environment and (2) whether analysis of early‐formed enclave minerals, particularly apatite, can provide a means of seeing through hybridization effects and deciphering the original trace element characteristics of enclave magmas. Whole rock and mineral chemistry data reveal a two‐stage history of enclave hybridization. Stage 1, a diffusive exchange of trace elements between coexisting liquids, produced enclaves with distinctive concave‐upward rare earth element patterns that parallel those of the host granitoids but had minimal impact on the major elements, whose transfer is rate limited by the slow diffusion of Si. This stage probably occurred at a mafic‐felsic interface in a stratified magmatic system. Stage 2, a partial reequilibration of enclave minerals with a differentiated and hybridized interstitial melt, occurred after the enclaves were entrained in the host and partially crystallized. This process caused enclave and host minerals (amphibole, biotite, apatite) from each pluton to have similar major oxide chemistries but did not reequilibrate the trace elements. As a result of these hybridization processes, even early‐formed apatite crystals do not preserve information about the original trace element characteristics of enclave magmas in this case. However, the results of this study illustrate the potential of using enclave chemistry to constrain the nature and timing of mafic magma inputs into felsic magma bodies.

A simple empirical method for high-quality electron microprobe analysis of fluorine at trace levels in Fe-bearing minerals and glasses

Abstract We present a new, high-quality electron microprobe method for analyzing F in Fe-bearing minerals and glasses down to trace levels (<1000 ppm F). This method is empirically based and corrects for the problem of overlap of the FKα peak onto the “shoulder” of the FeLα1 peak that arises when using synthetic multi-layered diffraction crystals. It also achieves high precision and accuracy while maintaining low detection limits and a small beam diameter. Analytical conditions for F using the new method and a synthetic W/Si diffraction crystal are: 10 kV accelerating voltage, 180 nA beam current, 8 μm beam diameter, and 400 second total peak count time. An iterative beam exposure method is employed to minimize potential damage to the sample due to beam heating. The method presented here can be used to address problems related to degassing of F at active volcanoes, the reservoirs of F in the mantle, and the partitioning of F between minerals and silicate liquid.

Precision of rock-varnish chemical analyses and cation-ratio ages

New data and statistical analyses indicate that the precision of the mean rock-varnish cation ratio determined for a geomorphic surface and the uncertainty of the age calculated from that ratio are controlled by the area of varnish included in each chemical analysis, the precision with which each cation ratio is determined, and the number of independent chemical analyses. This study has implications for the interpretation of published cation-ratio ages and the future use of cation-ratio dating.

Naturally occurring ferric iron sanidine from the Leucite Hills lamproite

Abstract Sanidine crystals precipitating from lamproitic magmas characteristically contain appreciable amounts of Fe3+. Sanidine grains from the Leucite Hills lamproites (Wyoming) have compositions in which up to 70 mol% of the KAlSi3O8 molecule is replaced by KFe3+Si3O8, thus constituting a new natural feldspar component. Sanidine is among the last phases to crystallize in the groundmass of Leucite Hills lamproites and rarely exceeds 150 μm in length. Nearly all grains are zoned in Fe, ranging from 2.5 wt% Fe2O3 in the cores to 19.6 wt% Fe2O3 at rims adjacent to quenched glass. The Fe-rich rims are relatively narrow on (010) crystal faces in comparison with (001) faces (maximum width 10 μm), probably reflecting faster- (001) and slower-growing (010) crystallographic directions. A near-perfect correlation is obtained by combining Ti, Mg, and excess Si cations with Fe3+ in tetrahedral substitution for Al. Insufficient numbers of R2+ cations exist to balance the high concentrations of Mg through an R2+Mg2+R1+-1R3+-1 substitution, suggesting the exchange Mg2+Si4+Al3+-2 in 2KAlSi3O8, analogous to that found in synthetic leucite and kalsilite. Selected-area electron diffraction and convergent-beam electron diffraction show that the Fe-rich sanidine rims have a C2/m structure with cell dimensions of a = 8.68 ± 0.15, b = 13.14 ± 0.23, c = 7.31 ± 0.15 Å, α = 90.0 ± 0.54, β = 116.0 ± 0.46, γ = 90.0 ± 0.33°, and a calculated cell volume of 747 ± 21 Å3. These crystallographic data are in very close agreement to those obtained previously for synthetic KFe3+Si3O8. The unusually high concentrations of Fe3+, Ti, and Mg in the sanidine rims are likely the consequence of rapid crystallization during quenching of an unusually Si- and K-rich, yet Al-deficient residual magma.

Thermodynamic properties of tremolite: A correction and some comments

Abstract Microprobe analysis of tremolite from “St. Gotthard,” Switzerland, shows that it is close to endmember in composition (Ca = 1.97 apfu), and not hypercalcic (Ca = 2.16 apfu), as a previous wetchemical analysis suggested. The latter formula, and a corresponding thermodynamic activity (0.67), were used in the experimental database for the extraction of an optimal set of thermodynamic properties for tremolite (Chernosky et al. 1998). We derive a revised enthalpy of formation of tremolite (-12307.9 kJ/mol) based on bracketing experiments for the breakdown reactions of tremolite and tremolite + forsterite, and the assumption of time-averaged effective equilibrium compositions of phases in the experimental charges. Mg-cummingtonite substitution in tremolite is accommodated during free-energy minimization calculations by allowing the stoichiometry of the standard-state reaction to vary with the changing bulk composition of the amphibole. The breakdown temperature of tremolite is lowered with increasing cummingtonite component. Our procedure also allows continuous variation in the equilibrium compositions and activities of pyroxene and amphibole phasecomponents with P and T.

Accurate and precise measurement of rock varnish chemistry using SEM/EDS

Abstract We describe important refinements to the Harrington-Whitney technique by which an energy-dispersive spectrometer attached to a scanning electron microscope (SEM/EDS) is used to measure the chemical composition of rock varnish, a Si-/Al-/Fe-/Mn-rich material coating rock surfaces in arid regions. In particular, we have developed a calibration procedure which generates accurate, quantitative SEM/EDS analyses. This procedure differs from that typically followed for SEM/EDS because the reference spectrum for each element is calibrated using multiple standards. Analyses of 27 standards of varied composition, including three synthetic rock varnish standards, demonstrate the accuracy of our measurement technique. Repeated analyses of the same sample quantify the precision of our measurements and suggest that, if a high beam current and pulse processing rate are used, 1000 s is the optimal acquisition time for material similar in composition to rock varnish. Our data show that a spectral analysis program, using stored reference spectra, is capable of deconvoluting the severely overlapping Ba- L α and Ti- K α X-ray peaks and accurately determining the abundance of Ti and Ba, both of which are present in rock varnish. The accuracy and precision of SEM/EDS analyses of intentionally roughened synthetic varnish standards compare favorably with those of polished standards. This finding indicates that surface roughness does not introduce a significant error into SEM/EDS analyses of in situ rock varnish. Our data imply that carefully calibrated SEM/EDS can accurately measure rock varnish chemistry and determine varnish cation ratios, ( Ca + K ) Ti , which have been used as a calibrated geochronometer by previous investigators.