Renald N. Guillemette

Oxygen minimum zone and internal waves as potential controls on location and growth of Waulsortian mounds (Mississippian, Sacramento Mountains, New Mexico)

SummaryLower Mississippian Waulsortian mounds in the Sacramento Mountains grew on a south-dipping homoclinal ramp at depths ranging from approximately 110 to 250 m, a setting in which nutrient sources and pathways are poorly understood in ancient carbonate depositional systems. Lithologic, biotic, and chemical data suggest that the mounds grew in an area on the ramp within a dysoxic oxygen minimum zone (OMZ) resulting from the concentration of organic matter below a thermocline. The abundance of organic matter there, perhaps a consequence of upwelling, enhanced the productivity of the major sediment contributors to the mounds—sponges, pelmatozoans, bryozoans, and heterotrophic carbonate-producing microbes.The siting and growth of individual mounds within the OMZ is best explained by two factors. One is the positive low-relief intra-ramp topography on and around which the mounds grew. The other is that nutrient supply for carbonate producing microbes was concentrated at the sites of mounds growth by internal waves on the pycnocline coincident with the thermocline. The internal waves mixed the nutrient-rich water of the OMZ with the better-oxygenated adjacent water mass, stirred the substrate and resuspended organic matter so that it was more available for the primarily suspension-feeding macrofauna both above and below the pycnocline, and generated local vertical mixing. Because of initial intra-ramp topography and subsequently increased relief as the mounds grew, internal waves focused and localized these processes, thus enhancing carbonate production at individual mound sites in a positive feed-back loop.

Carbonate accretionary lapilli in distal deposits of the Chicxulub impact event

The petrography and chemical composition of carbonate accretionary particles of Chicxulub impact origin are described from Cretaceous-Paleogene boundary deposits at Brazos River, Texas, and Bass River borehole, New Jersey. The particles consist of lapilli and lapilli fragments ranging in size from 0.05 to 0.3 cm; they are white in color, have an accretionary fabric at several scales, and are composed of micrometer-sized microspar of low-Mg calcite with an elevated sulfur content. The internal aggregate microfabric indicates that they formed by accretion of small solid particles, suggesting an origin from carbonate crystals generated within the vapor plume of the Chicxulub impact. Carbonate accretionary lapilli occur with altered glass spherules in Cretaceous-Paleogene boundary deposits at sites in Texas and northern Mexico and in the spherule layer in New Jersey, indicating that a large amount of particulate carbonate was present within the impact plume.

Mineral Preservatives in the Wood of Stradivari and Guarneri

Following the futile efforts of generations to reach the high standard of excellence achieved by the luthiers in Cremona, Italy, by variations of design and plate tuning, current interest is being focused on differences in material properties. The long-standing question whether the wood of Stradivari and Guarneri were treated with wood preservative materials could be answered only by the examination of wood specimens from the precious antique instruments. In a recent communication (Nature, 2006), we reported about the degradation of the wood polymers in instruments of Stradivari and Guarneri, which could be explained only by chemical manipulations, possibly by preservatives. The aim of the current work was to identify the minerals from the small samples of the maple wood which were available to us from the antique instruments. The ashes of wood from one violin and one cello by Stradivari, two violins by Guarneri, one viola by H. Jay, one violin by Gand-Bernardel were analyzed and compared with a variety of commercial tone woods. The methods of analysis were the following: back-scattered electron imaging, X-ray fluorescence maps for individual elements, wave-length dispersive spectroscopy, energy dispersive X-ray spectroscopy and quantitative microprobe analysis. All four Cremonese instruments showed the unmistakable signs of chemical treatments in the form of chemicals which are not present in natural woods, such as BaSO4, CaF2, borate, and ZrSiO4. In addition to these, there were also changes in the common wood minerals. Statistical evaluation of 12 minerals by discriminant analysis revealed: a. a difference among all four Cremona instruments, b. the difference of the Cremonese instruments from the French and English antiques, and c. only the Cremonese instruments differed from all commercial woods. These findings may provide the answer why all attempts to recreate the Stradivarius from natural wood have failed. There are many obvious implications with regard to how the green tone wood should be treated, which chould lead to changes in the practice of violin-making. This research should inspire others to analyze more antique violins for their chemical contents.

Determination of Fe3+/Fe using the electron microprobe: A calibration for amphiboles

Abstract Iron is a common constituent in minerals from the Earth’s crust and upper mantle and often occurs in minerals as mixtures of two valence states, Fe3+ or Fe2+. Quantification of the values of Fe3+/FeTotal, where FeTotal = Fe3++Fe2+, in minerals may be necessary to accurately apply certain mineral equilibria to determine equilibrium values of important variables such as temperature (T), pressure (P), and oxygen fugacity (ƒO₂). Most useful would be an analytical technique that permits determination of values of Fe3+/FeTotal within a single mineral grain that is contained within a standard petrographic thin section, and the excellent spatial resolution and relative accessibility of the electron microprobe (EMP) have resulted in various attempts to use this instrument to determine values of Fe3+/FeTotal. These efforts have typically involved quantifying characteristics of the FeLα and/or FeLβ peaks. In this paper, we employ the method of Fialin et al. (2001), who have shown that the location of the FeLα peak changes as a function of Fe content and values of Fe3+/ FeTotal, to determine values of Fe3+/FeTotal in amphiboles. We have characterized the FeLα peak in several amphiboles with known values of Fe3+/FeTotal using the electron microprobe at Texas A&M University. Initial analyses employed a beam current of 20 nA in an effort to avoid Fe-oxidation due to electron beam generated H-loss (Wagner et al. 2008). Subsequent analyses were conducted at 100 nA, and the results are consistent with the 20 nA data only when relatively short duration analytical times were used. The position of the FeLα peak was determined for three suites of amphiboles that have been experimentally treated such that grains in any one of these mineral suites are chemically identical except for differences in the values of Fe3+/FeTotal. A linear relation between the FeLα peak location and value of Fe3+/FeTotal was observed for each of these three amphibole suites. These three lines differ from one another in both their slope and intercept and these differences vary as a function of Fe content. Thus, these amphiboles served as the basis for the derivation of a relation between Fe content and FeLα peak location, both measured with the EMP, and the value of Fe3+/FeTotal as originally determined with 57Fe Mössbauer spectroscopy. The relation between the relative peak position (RPP = hematite standard FeLα peak position - amphibole FeLα peak position), Fe content, and Fe3+/FeTotal is Fe3+/FeTotal = RPP - RPP(0)/RPP(1) - RPP(0), where RPP(0) = -1.37 × FeO2 + 19.59 × FeO - 3.85, RPP(1) = -1.25 × FeO2 + 21.39 × FeO + 13.05, and FeO refers to the wt%FeO. This relation reproduces the measured values of Fe3+/FeTotal to within ±0.07 and, therefore, should permit determination values of Fe3+/FeTotal in amphiboles with Fe contents from 7 to 13 wt% FeO with similar precision. The amphiboles that were used in this study were kaersutites, Ti-bearing pargasites, and pargasitic hornblendes. The calibration presented here should, at the very least, be applicable to amphiboles with similar compositions, and although further verification is necessary, this calibration may be useful for determining values of Fe3+/FeTotal in amphiboles with distinctly different compositions and may even be more universally applicable.

Composition and Provenance of Volcanic Glass in Late Eocene Manning Formation, East-Central Texas

ABSTRACT Airfall volcanic ash beds in the Late Eocene Manning Formation, Brazos River Valley, Texas, contain large quantities of hydrated, but otherwise unaltered and non-corroded, rhyolitic glass shards, along with mica, sanidine, and quartz crystals. Enclosing deposits contain well preserved siliceous microfossils (diatoms, sponge spicules, phytoliths). Unaltered glass occurs only where volcanic ash was deposited in nonmarine, non-swamp environments: lacustrine, fluvial, and forested soil surface. Composition of glass shards from the upper part and top of the Manning Formation was determined by broad-beam electron microprobe analysis. Glasses from four ash beds analyzed to date are very similar in major and minor element composition. The mean and standard deviation in weight% oxides for 398 shards are: 72.7% SiO2 (Standard deviation 0.7%), 11.6% Al2O3 (0.2),5.7% K20 (0.5),2.4% Na2O (0.3),0.7% FeO (0.2), 0.4% CaO (0.1), and 6.5% H2O (by difference). Na and possibly K have likely been substantially leached from glass shards during hydration. MgO, MnO, TiO2, P2O5, and SO3 are all less than 0.2% (below detection limit). The youngest ash layer (top Manning) contains 0.9% FeO compared to 0.7% FeO for two ash layers in the middle Manning and 0.5% FeO for the oldest ash layer in the lower Manning. A possible source for these glasses would be the Sierra Madre Occidental in west Mexico. Age (34.9 and 34.4 Ma for two of the ash beds) and composition is dissimilar to most rhyolites in the Davis Mountains of west Texas.

Eocene–Oligocene chronostratigraphy of ignimbrite flareup volcanic ash beds on the Gulf of Mexico coastal plains

Sanidine 40Ar/39Ar dates and zircon U-Pb dates of middle Eocene to late Oligocene volcanic ash beds provide high-resolution geochronology for the northern Gulf of Mexico. The dates coincide with silicic volcanism generated as North America moved closer to the Pacific spreading axis. Ten new dates are reported, five for upper Eocene Jackson Group strata and five for Oligocene Catahoula Group strata. Dating is extended to south Texas and the Rio Grande Valley. Clusters of radiometric dates at ca. 34.1–34.5 Ma and at ca. 35.7–35.8 Ma indicate times of greater volcanic eruptive activity during the late Eocene. The ca. 34.1–34.5 Ma cluster occurs across the northern Gulf of Mexico from south Texas to Mississippi. Airborne volcanic ash plumes carried sanidine grains as coarse as 500 μm as much as 650 km away from the closest eruptive center, and grains to 150–200 μm occur in ash beds 600–800 km from the closest source. Volcanic ash bed dates do not correspond with any dated calderas in the southwestern United States. Long-distance transport of relatively coarse crystals has important implications for use of detrital mineral geochronology in paleodrainage studies. Early Rupelian dates of lower Gueydan Formation strata in the Catahoula Group are coeval to lower Vicksburg Group in the subsurface. The previous interpretation of a long-duration early Oligocene depositional hiatus in Texas is not supported and is replaced with the interpretation of early Oligocene nonmarine fan deposition in south Texas.