Donald S. Miller

Enhanced tint fission track densities in low spontaneous track density apatites using 252Cf-derived fission fragment tracks: A model and experimental observations

Abstract Two types of confined etched fission tracks are commonly observed in apatite, TINCLEs (tracks-in-cleavage) and TINTs (tracks-in-track). The TINCLE fission tracks are imaged by the chemical etchant via a crack that intersects the etched surface. TINT fission tracks are etched via intersecting fission tracks that cross the etched surface. We present a simple mathematical model that predicts the TINT fission track density (tracks per unit area) in terms of the confined and surface etchable fission track length distributions and the surface fission track density. For an apatite surface with 4π track registration geometry containing only natural (or induced) fission tracks, the TINT fission track density is predicted to be approximately proportional to (a) the arithmetic mean confined etchable fission track length, and (b) the square of the fission track density on the etched surface. Furthermore, the model predicts that the TINT fission track density can be significantly enhanced by adding charged-particle tracks from an external source to the apatite surface prior to chemical etching. To verify this latter prediction, eight polished crystals of Durango apatite, containing only natural fission tracks from the spontaneous fission of 238U, were placed in close proximity (1–2 mm distant) to a planar 252Cf fission fragment source for times ranging from 0 to approximately 226 h. The TINT fission track density is observed to increase approximately linearly with increasing 252Cf fission track density, a result predicted by the model.

Etchable length reduction of induced fission tracks in apatite at room temperature (≈ 23°C): Crystallographic orientation effects and “initial” mean lengths

Abstract Etchable length reduction of thermal-neutron induced fission tracks is observed in four apatites that experienced temperatures no greater than approximately 23°C subsequent to track formation. In each apatite, the mean etchable fission track lengths parallel ( l c ) and perpendicular ( l a ) to the crystallographic c -axis as well as the arithmetic (conventional) mean ( l m ) decrease by approximately 0.5 μm during the time interval ≈ 10 min-≈ 3 weeks after irradiation. Two crystallographic orientation dependencies of mean etchable fission track length at room temperature are identified: (a) l c > l a in all cases (tracks etched for 25 s at 23°C in 5 M HNO 3 ) and (b) l a decreases approximately twice as fast as l c . These measurements strongly support previous suggestions that natural charged-particle tracks in crystalline materials, including fission tracks, undergo slow annealing at low ambient temperatures. The observed sensitivity of induced fission tracks in apatite to low-temperature annealing requires that care be taken to avoid heating apatites before and during track etching for so-called “initial” mean track length measurements.

Early cretaceous uplift and erosion of the northern Appalachian Basin, New York, based on apatite fission track analysis

Abstract The thermal history of outcropping Devonian sediments of the northern Appalachian Basin, New York, has been investigated using fission track analysis of detrital apatites from 57 sandstone samples. Based on lengths and apparent age measurements using fission tracks in apatite it is concluded that Lower Devonian sediments presently at the surface in the Catskill region were cooled rapidly from temperatures higher than about 110°C during Early Cretaceous times (120–140 Ma ago). In the western part of New York (Wellsville-Buffalo) data from late Devonian sediments are consistent with cooling at the same time as that identified for the Catskill region but from lower temperatures, in the range of approximately 80–110°C, the maximum temperature these sediments experienced since deposition. For a pre-uplift paleogeothermal gradient of 25–35°C/km, the confined track length data indicates uplift and erosion of ∼ 2–3 km for western New York and greater than ∼ 3–4 km for the Catskill region, a differential uplift pattern which is consistent with the historical stratigraphic data from the region. This conclusion is at variance with earlier interpretations put forth by others. Rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern Appalachian Basin during the Early Cretaceous. This timing for the dominant post-Devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the Appalachian Orogen but is compatible with the change from carbonate to siliciclastic deposition in the Atlantic coastal plain. It is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with, and subsequent to, continental rifting. Apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems.

Tertiary extension in the Old Woman Mountains Area, California: Evidence from apatite fission track analysis

Apatite fission track ages and confined track length distributions from the Old Woman Mountains area, southeastern California, record evidence for periods of rapid cooling during the Late Cretaceous following Mesozoic orogenesis and during Miocene crustal extension. Apatites from the Clipper, Marble, and Turtle Mountains give fission track ages between ∼62 and ∼69 Ma as a result of cooling of these upper crustal rocks after regional Late Cretaceous plutonism and metamorphism. In the Piute Mountains, apatite fission track ages decrease from >70 Ma with mean track lengths of ∼13.5 μm to 14 μm from west to east. Intermediate aged samples from the Piute Mountains have mean track lengths between 11 and 13 μm due to their extended residence in the apatite annealing zone during the early Tertiary. This pattern is the result of 20°–30° of westward tilting of the Piute Mountains block as indicated by the dip of Tertiary strata. The apatite data indicate that tilting occurred after ∼18 Ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range. Knowing the amount of tilting and the apparent paleotemperatures across the Piute Mountains allows us to estimate that the synextensional geothermal gradient was 50° ± 20°C/km. Apatites from the northeastern Old Woman Mountains yield fission track ages of ∼68 Ma with mean track lengths of 13.8 μm. These samples record cooling after batholith emplacement at 73 Ma. In the northwestern Old Woman Mountains, apatite fission track ages decrease to <20 Ma and have mean track lengths ∼15 μm indicating very rapid cooling. We interpret this age pattern to be the result of minor eastward tilting of the Old Woman Mountains block after 18–19 Ma or flexural unroofing of the northwestern Old Woman Mountains due to movement on the Ship Mountains fault. Extension in the Old Woman Mountains area occurred between 19 and 16 Ma, which is coincident with extension in the Colorado River Extensional Corridor and later than major extension in the Mojave Extensional Belt.

Fission track ages on apatite of Bergell rocks from Central Alps and Bergell boulders in Oligocene sediments

Previous radiometric dating studies indicated that the Bergell region, in contrast to other regions of the Central Alps, experienced an early, rapid uplift, but with decreasing rate. Furthermore, there is also a geological record of the early uplift history of the Bergell granite by the existence of boulders which were derived from this granite and which occur in the Late Oligocene sediments of the Po plain. In this work the uplift history of the Bergell is studied in more detail by fission track dating of additional apatites from the Bergell region. Secondly, by determining apatite fission track ages the granitic boulders of the Po plain can be re-assigned to their original vertical position within the Bergell intrusive before erosion removed them in Late Oligocene time. A rather conservative estimate replaces them 6 km above the present morphology of the Bergell massif. Thus, the thickness of the Bergell granite must have been at least 8 km. Generally, fission track studies on boulders may become an important tool to study the vertical extent of mountain chains during the geological past.

Results of interlaboratory comparison of fission-track age standards: Fission-track workshop—1984

Abstract Five samples were made available as standards for the 1984 Fission Track Workshop held in the summer of 1984 (Rensselaer Polytechnic Institute, Troy, New York). Two zircons, two apatites and a sphene were distributed prior to the meeting to 40 different laboratories. To date, 24 different analysts have reported results. The isotopic ages of the standards ranged from 16.8 to 98.7 Myr. Only the statement that the age of each sample was less than 200 Myr was provided with the set of standards distributed. Consequently, each laboratory was required to use their laboratorys accepted treatment (irradiation level, etching conditions, counting conditions, etc.) for these samples. The results show that some workers have serious problems in achieving accurate age determinations. This emphasizes the need to calibrate experimental techniques and counting procedures against age standards before unknown ages are determined. Any fission-track age determination published or submitted for publication can only be considered reliable if it is supported by evidence of consistent determinations on age standards. Only this can provide the scientific community with the background to build up confidence concerning the validity of the fission-track method.

Apatite fission-track thermochronology of the Pennsylvania Appalachian Basin

Thirty-four apatite fission-track apparent ages and twenty-four track length distributions for ash bed samples from the Valley and Ridge Province and Upper Devonian to Upper Pennsylvania sedimentary samples from the Allegheny Front and Allegheny Plateau of Pennsylvania suggest that these regions represent different thermal (uplift) regimes as well as different structural provinces. The Valley and Ridge Province Tioga and Kalkberg ash bed samples yield apatite fission-track apparent ages and track length distributions that indicate early post-Alleghanian (285-270 Ma) cooling and unroofing that began at ∼250 Ma. Assuming a geothermal gradient of 25°C km−1, a burial depth of at least 3.4 km can be estimated for all the Pennsylvania samples. At the Allegheny structural front and on the western Allegheny Plateau, the apatite fission-track apparent ages (<150 Ma) and track length measurements indicate a Late Jurassic-Early Cretaceous thermal event for these samples possibly resulting from a higher geothermal gradient coinciding with kimberlite intrusion at this time along the Greene-Potter Fault Zone. In northeast Pennsylvania on the Allegheny Plateau, the Upper Paleozoic sedimentary samples yield apatite fission-track apparent ages ≤180 Ma. Narrow track length distributions with long mean lengths (13–14 μm) and small standard deviations (1.3 μm) suggest rapid cooling from temperatures >110°C during the Middle Jurassic-Early Cretaceous for this part of Pennsylvania. This is consistent with the suggested uplift history of the Catskill Mountain region in adjacent New York State.

Control and distribution of uranium in coral reefs during diagenesis

ABSTRACT The concentration of about 2 ppm of uranium in the aragonitic skeletons of modern scleractinian corals which we studied is a constant value, regardless of occurrence, anatomy, or taxonomy. The presence of cement of aragonite or high-magnesian calcite usually raises the concentration of bulk samples to about 3 ppm. Modern corals may contain up to 50% of cementing minerals. Organisms, such as corals and coralline algae, while secreting their skeleton, discriminate against the uptake of uranium, whereas the uptake of uranium by mineral cements is less restrained. Aragonite cement contains about 3.6 ppm and high-magnesian calcite cement 2.6 ppm uranium. During leaching by freshwater, the aragonite of the skeletons of corals dissolves out. This creates hollow molds which fill with drusy low-magnesian calcite. In emergent reefs from the shores of the Red Sea which display the effects of progressive diagenesis this calcite is enriched in uranium (3.9 ppm) beyond that found in marine cements. Second-generation calcite which fills original voids in the corals from the emergent reefs contains a lower level of uranium concentration (1.3 ppm). The level of concentration of uranium in low-magnesian calcite of diagenetically altered corals is a function of the availability of uranium in meteoric waters. In aragonite as well as in high- and low-magnesian calcite uranium replaces calcium or occupies lattice vacancies in the crystal lattice.

The Absolute Age of the Eifelian Tioga Ash Bed, Pennsylvania

The Tioga ash bed, bed B of the Zeigler pit, Union County, Pennsylvania, lies in the Polygnathus costatus costatus Zone of the Eifelian Stage of the Middle Devonian Series. The 27.5-cm-thick bed contains igneous zircon and monazite crystals that have been dated by precise U-Pb techniques. Zircons are characterized by inherited Pb that occurs as barely perceptible interior regions enriched in inclusions. The zircons are discordant and do not reveal the igneous age of the bed. In contrast, high quality monazite crystals lack any evidence of inheritance or loss of Pb, and multiple analyses have overlapping