Loss Of Radiogenic 40Ar Research Articles

A Kinetic Explanation for Combined Potassium Gains and Radiogenic 40Argon Losses of Diagenetic Illite-Rich Clay Separates

In this study, a kinetic model sketches the concomitant K gain and radiogenic 40Ar loss of clay separates recovered from progressively buried sediments. Published K-Ar ages of clay separates from the Mahakam Delta, the Texas Gulf Coast, and the North Sea were used to constrain the modeling. As compared with analytical results, the model simulates changes relative to depth or to deposition time of the K-Ar ages from fine- and coarse-grained clay crystals. The decrease in the K-Ar ages of detrital-rich coarse-grained fractions (>2 μm) is bracketed with depth by K addition rates between 0.2 and 3.5%/Ma and slightly higher 40Ar release rates between 0.5 and 4.5%/Ma. The former rate varies from 3.5 to 6%/Ma in the fine-grained fractions (<0.4 μm) and the latter rate varies from 0.7 to 6%/Ma in the same fractions. In fact, the K addition and the radiogenic 40Ar release rates record independent processes on different material sizes and types. Small K addition and 40Ar escape rates of about 1%/Ma also simulate analytical cases in which the mean K-Ar ages of the finer grained fractions remain about constant with increasing depth, confirming that the relationship between K-Ar age and stratigraphic depth cannot represent a temporary “steady state”, but a continuing dynamic process at a smaller rate. In turn, the modeled results help quantifying the illitization reactions in size separates consisting of authigenic and detrital clay materials from sediments covering stratigraphic intervals from 10 to 1000 Ma.

Petrogenesis and geodynamics of Eocene alkaline intrusions in the pre-salt sedimentary sequence of Santos Basin, Brazil

Lamprophyres and phonolite intrusions were sampled in the so-called Pre-Salt sequence of well 1-BRSA-905-RJS in northern Santos Basin nearby the Cabo Frio Structural High in SE Brazil. These alkaline rocks indicate the existence of a strongly silica-undersaturated trend. However, fractional crystallization modeling as well as Sr-Nd-Pb-Hf isotope data shows that the alkaline lamprophyres are not suitable parental compositions to the evolved felsic phonolitic magmas. The Sr-Nd-Pb-Hf isotope compositions of the lamprophyric magmas represent mixing of EM1 and Trindade plume mantle components. The EM1 is possibly related with the melt- and/or fluid-induced metasomatic processes on the subcontinental lithospheric mantle (SCLM) that occurred beneath the South American Platform during the Neo-Proterozoic-Cambrian subduction processes associated with the final stages of the Gondwana amalgamation, as recorded by the TDM ages (601-499 Ma) of the studied intrusions in Santos. 40Ar/39Ar analysis presented integrated age of 41.06±0.02Ma and 38.62±0.02Ma for phonolite and lamprophyre samples, respectively. A “plateau” age assignment for the true crystallization age of the sample could not be obtained due to the discordance in the ages of individual steps. This discordance was attributed to loss of radiogenic 40Ar that can happen in nature due to reheating events or metasomatism. An interaction between the Trindade plume head and SCLM is proposed for the geodynamics of Santos Basin at 40 Ma. The presence of an anomalously hot mantle represented by the deflected Trindade plume head beneath Santos at the Eocene is supported by the petrological and geochronological data. Therefore, it may be relevant to consider high geothemal flow regimes in basin evolution models proposed for Santos during the Eocene.

Studying the Stability of the K/Ar Isotopic System of Phlogopites in Conditions of High T, P: 40Ar/39Ar Dating, Laboratory Experiment, Numerical Simulation

Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produces estimates that suggest much older ages than those when these pipes were intruded. High-pressure (3 GPa) laboratory experiments enabled the authors to explore the behaviour of argon in the phlogopite structure under the conditions that correspond to the mantle, at the temperatures (from 700 to 1000 °С), far exceeding closure temperature of the K/Ar isotopic system. “Volume diffusion” remains foremost for describing the mobility of argon in phlogopite at high pressures. The mantle material age can be estimated through the dating of the phlogopites from deep-seated xenoliths of kimberlites, employing the 40Ar/39Ar method, subject to correction for a partial loss of radiogenic 40Ar when xenolith moves upwards to the Earth’s surface. The obtained data served as the basis for proposing the behaviour model of the K/Ar isotopic system of minerals in conditions of great depths (lower crust, mantle), and when transporting xenoliths in the kimberlite melt.

Diffusion and fluid interaction in Itrongay pegmatite (Madagascar): Evidence from in situ 40Ar/39Ar dating of gem-quality alkali feldspar and U[sbnd]Pb dating of protogenetic apatite inclusions

Alkali feldspar 40Ar/39Ar and apatite UPb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40Ar and 206-208Pb. Some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures; others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration. These two end-member interpretations have fundamentally different implications for tectonic models derived from geochronological data, and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes. Here, we revisit the mechanisms of 40Ar loss in the famous gem-quality alkali feldspar from the Itrongay pegmatite in Madagascar. Previous studies have suggested that volume diffusion is the dominant mechanism of 40Ar loss, providing key evidence to support the use of 40Ar/39Ar dating of alkali feldspar for thermochronology. We attempted to verify these results by obtaining time-temperature paths from petrologically characterised cogenetic feldspar and apatite from the Itrongay pegmatite and comparing them with each other. However, our results suggest that only a minor component in the variability of 40Ar/39Ar dates of Itrongay feldspar is related to the diffusive loss of 40Ar, and that this loss was not compatible with the majority of previously proposed models, which hinders quantitative interpretations. The crystal studied here grew in five episodes related to the influx initially of co-existing dense SiO2-rich solution and CO2-dominated fluid (the first and supposedly the following two episodes) and subsequently of H2O-rich fluid (supposedly the final two episodes). Much greater component in the variability of the acquired 40Ar/39Ar dates is interpreted to reflect the differences in the ages of these growth episodes, which we estimate to span from 477 Ma to 176 Ma (the first four episodes). Apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the Itrongay pegmatite. These yield older UPb dates than the estimated age of their host feldspar and have apparently experienced diffusive loss of 206,207Pb prior to entrapment. Our 40Ar/39Ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40Ar from alkali feldspar that can be readily interpreted for thermochronological purposes. However, in situ 40Ar/39Ar dating of alkali feldspar appears to be a promising tool for tracking fluid-flow events in the Earth's crust whose applicability is not restricted to sedimentary rocks. Our UPb results corroborate previous suggestions that UPb dating of apatite can be used for thermochronology.

40Ar-39Ar geochronology of the active phonolitic Cadamosto Seamount, Cape Verde

Cadamosto Seamount is located in the SW of the Cape Verde Archipelago in the central Atlantic Ocean off the west coast of Africa. Many radiometric dates exist for the islands in the archipelago, however, no geochronological information has been obtained from the numerous seamounts. The timescales for igneous processes in the submarine realm are thus poorly understood.In this study, we investigated five lavas that were sampled by dredging and ROV (remotely operated vehicle) from the flanks and summit areas of the largely phonolitic Cadamosto Seamount during two different research cruises. Chemical zonation patterns of minerals were determined by electron microprobe, and radiometric ages were obtained from single-crystal total-fusion and single−/multi-grain step-heating 40Ar-39Ar analyses of sanidine, nepheline and sodalite-group minerals.Our 40Ar-39Ar results reveal young sanidine eruption ages (all <100 ka) at Cadamosto Seamount: (1) Three western flank/summit lavas have a relatively simple petrology dominated by phenocrysts, and overlap with mean sanidine ages of 20.98 ± 0.87 ka, 21.44 ± 0.80 ka and 22.3 ± 2.0 ka, with a combined mean age of 21.04 ± 0.62 ka from the three samples (all uncertainties are quoted at 2σ). (2) The remaining two samples from the summit/NE flank are dominated by complex zoned sanidines with resorbed antecrystic cores and phenocrystic rims. These samples yield older sanidine ages of 51.8 ± 2.4 ka and 97 ± 14 ka, which are interpreted to be maximum eruption ages. This is due to the dominance of antecrysts in these two samples and the possibility that the analyzed sanidine grains may be a mixture of older antecrystic cores and younger phenocrystic rims.The older 40Ar-39Ar ages of many sanidine and nepheline antecrysts also give us clues regarding older magmatic events at Cadamosto Seamount, despite these grains having undergone resorption and phenocrystic rim overgrowths, resulting in some radiogenic 40Ar loss during entrainment in the subsequent magmas. The antecrysts minimum ages extend back to 1.5215 ± 0.0083 Ma, which supports the age progression of magmatism observed in the southern islands chain of the Cape Verde Archipelago.The youngest volcanic eruption period (21.14 ± 0.62 ka) occurred during the Last Glacial Maximum, a period of global sea level lowstands. We suggest that the comparatively rapid crust unloading leading up to the lowstand may have reduced pressure conditions within the Cadamosto Seamount magma plumbing system, and thus led to enhanced submarine eruption activity.

The effect of sub-seismic fault slip processes on the isotopic signature of clay minerals – Implications for K-Ar dating of fault zones

Clay minerals are abundant in fault gouges and isotope dating of the radiogenic noble gases that they contain may be used to constrain timing of fault initiation, reactivation and to estimate recurrence intervals of earthquakes. The potential influence of fault slip and ambient temperature on the isotope signature of authigenic clay minerals, however, remains unknown. This study reports measurements of the K-Ar isotope signature of samples that have been sheared experimentally at sub-seismic slip rates within a range of ambient temperatures, as well as of samples undergoing standardised clay heating degas experiments in the investigation and comparison of the influence of sub-seismic frictional slip and external heating on clay gouge. The age data confirm that the sub-seismic frictional slip of clay gouge affects the clay microstructures and releases radiogenic 40Ar. The experimental data indicate that radiogenic 40Ar release due to external heating can be disregarded until ~350 °C, which is supported by Ar diffusion modelling. Frictional slip at sub-seismic slip rates and at room temperature results in a significant radiogenic 40Ar loss of ~56% and the remaining clay will inherit ~44% of radiogenic 40Ar. Sub-seismic frictional slip experiments at imposed elevated temperatures from 150 to 450 °C increase the amount of radiogenic 40Ar loss and decrease the amount of inherited radiogenic 40Ar. At a relatively high temperature of 450 °C, about 16% of radiogenic 40Ar will be present in the remaining sample portion, reinforcing the importance of rigorous clay grain size separation processes required prior to fault gouge dating.

Paleotemperatures at the lunar surfaces from open system behavior of cosmogenic 38Ar and radiogenic 40Ar

The simultaneous diffusion of both cosmogenic 38Ar and radiogenic 40Ar from solid phases is controlled by the thermal conditions of rocks while residing near planetary surfaces. Combined observations of 38Ar/37Ar and 40Ar/39Ar ratios during stepwise degassing analyses of neutron-irradiated Apollo samples can distinguish between diffusive loss of Ar due to solar heating of the rocks and that associated with elevated temperatures during or following impact events; the data provide quantitative constraints on the durations and temperatures of each process. From sequentially degassed 38Ar/37Ar ratios can be calculated a spectrum of apparent 38Ar exposure ages versus the cumulative release fraction of 37Ar, which is particularly sensitive to conditions at the lunar surface typically over ∼106–108 year timescales. Due to variable proportions of K- and Ca-bearing glass, plagioclase and pyroxene, with variability in the grain sizes of these phases, each sample will have distinct sensitivity to, and therefore different resolving power on, past near-surface thermal conditions. We present the underlying assumptions, and the analytical and numerical methods used to quantify the Ar diffusion kinetics in multi-phase whole-rock analyses that provide these constraints.For Apollo 15 samples 15016, 15556, and 15596 we find apparent 40Ar/39Ar plateau ages between 3.21 and 3.28Ga and evidence for diffusive loss of radiogenic 40Ar primarily from K-bearing glass. From 38Ar/37Ar spectra normalized to the apparent Ca/K ratios, we also find evidence of diffusive loss of cosmogenic 38Ar that requires elevated temperatures either during or after surface exposure. Using 39Ar and 37Ar, we construct multiple-phase–multiple diffusion domain (MP-MDD) models to quantify the diffusion kinetics of Ar from a range of macroscopic grain sizes of each phase. While diffusive loss of 40Ar can be explained by brief reheating conditions after crystallization (e.g., during an impact event), we find that both the radiogenic 40Ar and cosmogenic 38Ar abundances can be explained by internally consistent thermal conditions expected for solar heating of the rocks at the lunar surface. These conditions correspond to effective diffusion temperatures (EDT, i.e., the temperature corresponding to the mean diffusivity over temperature variability) between 65 and 81°C, with an error-weighted mean of 77.0±1.3°C, despite differences in diffusion kinetics and large differences in surface exposure duration between the three samples (∼56–621Ma). This EDT corresponds to a maximum daytime temperature of ∼96°C, which is in excellent agreement direct measurements of temperature at the Apollo 15 landing site. The open system behavior in both a radiogenic and a cosmogenic nuclide provides tests for internal consistency in best-fitting solutions of time-integrated thermal conditions of rocks collected from planetary surfaces. This thermal information is important for the study of habitable conditions at planetary surfaces, and has implications for Ar-based geochronology and paleomagnetic observations applied to planetary materials.

Mineralogical, chemical and K–Ar isotopic changes in Kreyenhagen Shale whole rocks and

Progressive maturation of the Eocene Kreyenhagen Shale from the San Joaquin Basin of California was studied by combining mineralogical and chemical analyses with K–Ar dating of whole rocks and <2μm clay fractions from naturally buried samples and laboratory induced maturation by hydrous pyrolysis of an immature outcrop sample. The K–Ar age decreases from 89.9±3.9 and 72.4±4.2Ma for the outcrop whole rock and its <2μm fraction, respectively, to 29.7±1.5 and 21.0±0.7Ma for the equivalent materials buried to 5167m. The natural maturation does not produce K–Ar ages in the historical sense, but rather K/Ar ratios of relative K and radiogenic 40Ar amounts resulting from a combined crystallization of authigenic and alteration of initial detrital K-bearing minerals of the rocks. The Al/K ratio of the naturally matured rocks is essentially constant for the entire depth sequence, indicating that there is no detectable variation in the crystallo-chemical organization of the K-bearing alumino-silicates with depth. No supply of K from outside of the rock volumes occurred, which indicates a closed-system behavior for it. Conversely, the content of the total organic carbon (TOC) content decreases significantly with burial, based on the progressive increasing Al/TOC ratio of the whole rocks. The initial varied mineralogy and chemistry of the rocks and their <2μm fractions resulting from differences in detrital sources and depositional settings give scattered results that homogenize progressively during burial due to increased authigenesis, and concomitant increased alteration of the detrital material.Hydrous pyrolysis was intended to alleviate the problem of mineral and chemical variations in initially deposited rocks of naturally matured sequences. However, experiments on aliquots from thermally immature Kreyenhagen Shale outcrop sample did not mimic the results from naturally buried samples. Experiments conducted for 72h at temperatures from 270 to 365°C did not induce significant changes at temperatures above 310°C in the mineralogical composition and K–Ar ages of the rock and <2μm fraction. The K–Ar ages of the <2μm fraction range from 72.4±4.2Ma in the outcrop sample to 62.4±3.4Ma in the sample heated the most at 365°C for 216h. This slight decrease in age outlines some loss of radiogenic 40Ar, together with losses of organic matter as oil, gas, and aqueous organic species.Large amounts of smectite layers in the illite–smectite mixed layers of the pyrolyzed outcrop <2μm fraction remain during thermal experiments, especially above 310°C. With no illitization detected above 310°C, smectite appears to have inhibited rather than promoted generation of expelled oil from decomposition of bitumen. This hindrance is interpreted to result from bitumen impregnating the smectite interlayer sites and rock matrix. Bitumen remains in the <2μm fraction despite leaching with H2O2. Its presence in the smectite interlayers is apparent by the inability of the clay fraction to fully expand or collapse once bitumen generation from the thermal decomposition of the kerogen is completed, and by almost invariable K–Ar ages confirming for the lack of any K supply and/or radiogenic 40Ar removal. This suggests that once bitumen impregnates the porosity of a progressively maturing source rock, the pore system is no longer wetted by water and smectite to illite conversion ceases. Experimental attempts to evaluate the smectite conversion to illite should preferentially use low-TOC rocks to avoid inhibition of the reaction by bitumen impregnation.

40Ar/39Ar Ages of the Da Lien Granite Related to the Nui Phao W Mineralization in Northern Vietnam

Abstract 40Ar/39Ar dating was conducted on the Da Lien granite related to greisen‐skarn type polymetallic (W‐CaF2‐Cu‐Bi‐Au) mineralization in Nui Phao, northern part of Vietnam in the South China Plate. Biotite and muscovite separates from the biotite‐muscovite granite and greisenized granite indicate four plateau ages: 82.2 ± 0.4 Ma, 82.8 ± 0.3 Ma, 81.5 ± 0.3 Ma and 82.5 ± 0.4 Ma. The plateau ages were not significantly influenced by excess 40Ar in dated minerals or by loss of radiogenic 40Ar due to hydrothermal activities. The results indicate that solidification of granite related to the polymetallic mineralization occurred in the Late Cretaceous between 82.8 Ma and 81.5 Ma.

Apparent partial loss 40Ar/39Ar age spectra of hornblende from the Palaeoproterozoic Lapland-Kola orogen (arctic European Russia): insights from numerical modelling and multi-method in-situ micro-sampling geochronology

40Ar/39Ar age spectra with progressively increasing step ages are well known for metamorphic hornblende and have been classically interpreted by partial loss of radiogenic argon by diffusion processes during younger thermo-tectonic reworking. Application of a number of numerical modelling tools based on diffusion theory and that assume thermally activated loss of radiogenic 40Ar by solid-state volume diffusion suggests that staircase-shaped age spectra of Neoarchaean tschermakitic hornblende from the Lapland-Kola Orogen are due to argon losses of 40–50% during reheating to 450 ± 25 °C in Palaeoproterozoic time. However, in hornblende samples that yielded staircase-type age spectra, biotite occurs in the matrix, as well as intimately and abundantly intergrown with the amphibole along grain boundaries, cleavages, fractures and other defects. Drilling of 1.5 mm diameter discs from carefully selected hornblende grains in petrographic thin sections permitted to minimise the effects of contaminant biotite inclusions and/or compositional zoning of the amphibole. 40Ar/39Ar laser probe step-heating of drilled biotite-free hornblende discs yielded flat age spectra, suggesting absence of thermally activated radiogenic 40Ar loss. This would imply unrealistically contrasting temperature histories for neighbouring grains. Apparent-loss age spectra, thus, result from differential gas release of hornblende and an included, earlier degassing minor contamination of much younger biotite, which had apparently not been completely eliminated from the amphibole separate, despite careful handpicking. This is confirmed by the Ca/K ratio spectra — a proxy for 37ArCa/39ArK — of hornblende that are flat for drilled biotite-free hornblende grains, but initially low for hornblende separates. A drilled disc and a separate of hornblende from a biotite-free amphibolite did not yield apparent loss spectra, but flat age and Ca/K ratio spectra, confirming the interpretation of the role of biotite.

Early Yellowstone hotspot magmatism and gold metallogeny

High-grade epithermal gold deposits in the Northern Great Basin have long been associated with regional Miocene basaltic to rhyolitic volcanism. Previous models for the low-sulfidation epithermal gold ores in this region have generally portrayed the bimodal magmas as a source of heat to drive large-scale convection of meteoritic water that leached gold from crustal sources and deposited it in hydrothermal vein systems, or required that the gold evolve from fractionated silicic magmas. New data of the present study indicate a more direct genetic link to the plume-related basaltic magmas of the region. Laser 40Ar/ 39Ar incremental heating plateau ages for single crystals of adularia from several of these low-sulfidation epithermal gold deposits range from 16.6 Ma to 15.5 Ma. Adularia from the Jumbo deposit yields three concordant plateau ages with a combined statistical result of 16.54 ± 0.04 Ma (95% confidence level, MSWD = 0.23). Plateau ages for adularia from other deposits in the region, and from gold-bearing veins in the Owyhee Mountains of southwestern Idaho, yield similar ages up to ~16.5 Ma, however some veins are as young as ca. 15.5 Ma and the grain-to-grain ages for a given sample can vary by up to ca. 0.5 Ma. Observed variations in age among the adularia crystals of a given rock sample indicate varying amounts of extraneous argon, and also loss of radiogenic 40Ar, among the population of grains for a particular sample. The single-crystal results are interpreted to indicate a 16.5–15.5 Ma interval for formation of gold-bearing adularia veins in the region. The initiation and duration of this gold-forming event appears contemporaneous (within uncertainties) with the basaltic volcanism at the Steens Mountain section and an ensuing one-million-year episode of basaltic volcanism from multiple centers in the region ( Brueseke et al., 2007). Trace amounts of lead are alloyed with gold in the deposits studied. The isotopic compositions of this lead are not compatible with regional crustal units that host the gold ores, or the silicic igneous lithologies of the region, but have the same lead isotopic composition as basalts of the earliest Yellowstone plume (represented by the earliest lavas of the Columbia River basalt province, the Steens basalts, and Stonyford Volcanic Complex; Hanan et al., 2008). We propose that the gold studied and its traces of alloyed lead were derived together from the mantle, released from basaltic magma chambers of the province, and carried by low-density fluids into shallow geothermal systems during the earliest stages of Yellowstone hotspot magmatism.

Radioactive production and diffusional loss of radiogenic 40Ar in clays in relation to its flux to the atmosphere

This paper addresses the release mechanisms of radiogenic 40Ar from clays, for the purpose of estimating the 40Ar flux from the sedimentary crust to the atmosphere. Clays from sedimentary sequences of Cambrian to Neogene age show discrepancies between their depositional stratigraphic ages and the K–Ar apparent ages or dates of the different size fractions. The possible geochemical mechanisms that may be responsible for the K–Ar dates being either higher or lower than the stratigraphic age are discussed. In general, the known observation of a decrease in the K–Ar apparent age of clays with decreasing particle size is interpreted as due to a faster escape of radiogenic 40Ar from the smaller particles. In the younger, Neogene clays, the mass fractions of 40Ar lost are in a range from 13 to 34% in the fine-size particles that are smaller by a factor of 10 in their linear dimension than the bigger particles. In the older formation samples, Cambrian to Triassic, the 40Ar loss is smaller, from 7 to 19%, for the same particle-size range. This smaller decrease in the 40Ar/ 40K ratio in the stratigraphically older samples may be, at least in part, accounted for by the closure of the mineral system after the process of 40Ar escape went on for some time. In all sediments, a lowering of the K–Ar apparent age of the different size fractions, mainly in the smaller, is possibly due to authigenesis of mixed-layered illite–smectite and illite as a new generation of particles that result from, and are therefore better adapted to, the new diagenetic environment. In a Neogene basin sediments of depositional age < 18 Ma, the amount of the authigenic illite fraction increases with an increase of its crystallization age from about 22 to 38%. If the rate of new crystallization is linear with time, this change corresponds to about 1.5%/Ma of authigenic particles added to the smaller-size fractions. As an overall model, we present a mechanism that explains the lowering of the 40Ar/ 40K ratio and K–Ar apparent age in fine-clay size fractions based on the production of 40Ar in a closed system for some period of time, followed subsequently by the diffusional escape of 40Ar with its continuing production from 40K in the particles. This analysis gives 40Ar diffusion coefficients consistently in a range from 10 − 28 to 10 − 27 cm 2/s. From the results in this paper, the flux of radiogenic 40Ar from the sedimentary crust to the atmosphere is estimated as 5 to 13 × 10 6 mol/year, which is about 15 to 40% of the flux from the whole continental crust, as reported by other investigators.

High-temperature Mars-to-Earth transfer of meteorite ALH84001

Martian meteorites provide crucial insights into Martian evolution and interplanetary mass transfer, including the potential for exogenesis. ALH84001 is the oldest Martian meteorite discovered so far, and has been used to derive important conclusions about Martian surface temperatures and very low-temperature Mars-to-Earth transfer. To better constrain the thermal evolution and shock metamorphic history of ALH84001, we applied (U–Th)/He thermochronometry to single grains of phosphate (merrillite) from ALH84001. The (U–Th)/He ages of individual phosphate grains in ALH84001 range from 60 Ma to 1.8 Ga, with a weighted mean of ~830 Ma. This broad age distribution reflects multiple diffusion domains, and requires a relatively high-temperature resetting event younger than ∼ 60 Ma. These new data are combined with the published whole-rock (maskelynite as a main Ar reservoir) 40Ar/ 39Ar age spectra which show 5–8% fractional loss of radiogenic 40Ar since 4.0 Ga. He diffusion in both terrestrial and extraterrestrial apatite has a significantly higher activation energy (138 ∼ 184 kJ/mol) than Ar diffusion in maskelynite (75 kJ/mol), leading to an important “kinetic crossover” in fractional loss contours for these systems. Taken together, the phosphate (U–Th)/He and whole-rock 40Ar/ 39Ar ages require both very low surface temperatures on Mars, and one or more short-lived, high-temperature, shock events after 4.0 Ga. We suggest that the last shock event occurred with ejection of ALH84001 from Mars, and reached a peak temperature of approximately 400 °C. These results undermine the proposed low-temperature ejection hypothesis for ALH84001, but support long-lived extremely cold Martian surface temperatures.

Mineralogy, Mossbauer Characteristics, and K-Ar Isotopic Age of Glauconite from the Lower Cambrian Sediments, Western Lithuania

The paper presents results of lithological-mineralogical, Mossbauer, and isotope-geochronological study of globular phyllosilicates (glauconite and illite) from the Lower Cambrian Virbalis Formation in western Lithuania. K-Ar dates of these minerals (382 and 374 Ma) do not correspond to stratigraphic positions of the studied samples and indicate the loss of radiogenic 40Ar, probably, in Middle Devonian. Simulation of cation distribution in the mineral structure and comparison of the obtained data with Mossbauer spectral characteristics suggest that loss of 40Ar was caused by the structural rearrangement of globular phyllosilicates during the postdiagenetic substitution of Fe2+, Mg2+, and Fe3+ by Al3+, with the subsequent ordering of cation distribution in octahedral layers. These processes were presumably related to the same Caledonian tectonic events that were responsible for metasomatic dolomitization of Lower Paleozoic rocks in adjacent areas of the Baltic region at the terminal Silurian-Devonian. Diffraction analysis showed that globules consist of two micaceous phases with different Fe contents (illite and glauconite). Two alternative models are considered to explain the formation of two-phase globules: (1) disequilibrium conditions of diagenetic mineral formation; (2) transformation of glauconite into illite. Although the available data are insufficient to make an unambiguous choice between these models, the second mechanism seems to be more preferable.

Losses of Radiogenic 40Ar in the Fine-Clay Size Fractions of Sediments

AbstractThe common observation that smaller particle-size fractions of sedimentary rocks yield younger K-Ar apparent ages than the larger particle-size fractions of the same stratigraphic age was analyzed with the aid of the 40Ar/40K ratio from 14 stratigraphically and regionally different sections. Estimation of the loss of radiogenic 40Ar from varied clay-rich size fractions was based on two models: a relationship between particle size and the 40Ar/40K ratio, and a theoretical diffusional loss from spherical particles. The differences between the two models and reconciliation of their results are discussed. For the smallest fractions (up to &lt;0.5 μm), percent-wise losses of 40Ar from the spherical particles model increase from Upper Carboniferous and Permian (38±10%), to Late Triassic (47±10%), and to Miocene and Late Neogene (65±8%). This trend suggests that escape of 40Ar from the smaller particles in older sediments decreased or even stopped after deposition of the sedimentary sections.The large 40Ar losses derived from small 40Ar/40K ratios in the younger Tertiary sediments, indicate that addition of K to the small fractions is, at least in part, responsible for the young K-Ar apparent ages in geologically different settings. In several 102–103 m thick sections, authigenic illite in the &lt;0.1 to &lt;2 μm fractions yields young K-Ar apparent ages resulting from simultaneous 40Ar production and release during clay authigenesis. In a production and loss model, a first-order escape-rate parameter (e) was estimated at 0.2 × 10−8 to 4 × 10−8 y−1, depending on the K-Ar apparent age of the size fractions and the stratigraphic age of the section. The limitations and uncertainties of the methods of evaluating diagenetic 40Ar losses from fine clay particles are discussed.

Postdepositional Evolution of Platform Claystones Based on a Simulation of Thermally Induced Diffusion of Radiogenic 40Ar from Diagenetic Illite

ABSTRACT The Lower Cambrian Lontova claystones contain well crystallized authigenic 1M illite mixed with detrital 2M illite. Part of the authigenic illite probably precipitated during sedimentation and subsequently experienced a temperature up to 140°C, despite a limited burial to 500-1000 m. The difference between the Rb-Sr and K-Ar dates of this authigenic illite are attributed to preferential loss of radiogenic 40Ar induced by a short-lasting thermal pulse. This interpretation is based on an Ar-diffusion model suggesting that the Lontova blue clays were heated to around 140°C for 2-5 Myr. This thermal pulse could have occurred in relation with hot-fluid migration induced by either tectonic dislocations in the basement rocks during the Late Silurian-Early Devonian, or rifting conditions during the Permian. By comparing Rb-Sr and K-Ar data, evaluation of radiogenic 40 Ar losses by small authigenic illite particles at low temperatures could be a powerful tool for reconstruction of evolutionary histories of sedimentary successions that were either shallowly buried, or percolated by migrating fluids released during tectono-thermal activities.

Magma flow regimes in sills deduced from Ar isotope systematics of host rocks

The best place to seek evidence of the style of past magma flow through a conduit is in the country rock. Heat flow has been studied in country rock adjacent to two Tertiary dolerite sills intruding the Caledonian schists and quartzites, on the Isle of Mull, Scotland. Radiogenic 40Ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the Ultra‐Violet Laser Ablation Micro‐Probe, to discriminate between a history of prolonged magma flow, a history of conductive cooling following laminar flow, and instantaneous emplacement of the intrusions. The 40Ar/39Ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3–5 months resulted in extensive argon loss from the micas, country rock melting, and mineral breakdown adjacent to a 6‐m sill. These features were absent from the wall rocks of a smaller 2.7‐m‐thick sill, which exhibited even less argon loss than might have been predicted for an instantaneous intrusion. If the heat loss from the 6‐m sill observed in one locality had been repeated along its length, it would have formed an important magma conduit to the Mull volcano, but dolerite is not a common flow composition on Mull. If on the other hand, the heat loss from the sill varies along strike, it constitutes strong evidence for channelling and heterogeneous flow within the sill.

Sedimentary cycles and volcanic ash beds in the Lower Pliocene lacustrine succession of Ptolemais (NW Greece): discrepancy between 40Ar/ 39Ar and astronomical ages

A high-resolution cyclostratigraphy for the rhythmically bedded lignite–marl sequences of the Lower Pliocene Ptolemais Formation is combined with 40Ar/ 39Ar dating results of intercalated volcanic ash beds. Detailed field reconnaissance in three open-pit lignite mines reveals three end-member sediment types: lignites, composed primarily of organic material; grey marls, a mixture of carbonate and organic material; and beige marls, almost exclusively composed of carbonate. These lithologies are arranged in two basic types of sedimentary cycles: lignite–grey marl and lignite–beige marl cycles. A cyclostratigraphic composite section comprising 56 lignite–marl cycles is constructed which combines the consistent cycle patterns from three parallel sections. The concordant positions of 20 volcanic ash beds in these sections confirm the cyclostratigraphic correlations and indicate that the lignite–marl cycles result from regional, basin-wide forcing rather than lateral facies migrations. 40Ar/ 39Ar ages on sanidine and biotite separates from nine volcanic ash beds were obtained by multiple total fusion and incremental-heating experiments. The 40Ar/ 39Ar ages range between 5.00±0.05 and 4.04±0.04 Ma and are, in general, consistent with the stratigraphic order. A least-square linear regression using the measured 40Ar/ 39Ar ages gives an average duration of 21.8±0.8 kyr per lignite–marl cycle. Evidently, the lignite–marl cycles in the Ptolemais Formation are linked to the precessional variation in the Earth's orbit through its influence on Mediterranean climate. For the first time, 40Ar/ 39Ar dating results, totally independent from any other dating and or tuning technique, confirm the astronomical theory of climate change. The 40Ar/ 39Ar ages of the volcanic ash beds show a constant ∼200 kyr (∼4.5%) age discrepancy with the astronomical ages of the same ash beds. This inconsistency remains difficult to explain. The discrepancy is unlikely to have resulted from erroneous astronomical ages, through incorrectness in the astronomical tuning, inaccuracies of the magnetostratigraphic data or the orbital time-series used, and/or errors in the APTS. The 40Ar/ 39Ar dating results neither give clear indications for a possible source of error. From the excellent data set it is evident that neither loss of radiogenic 40Ar, nor an underestimation of the contribution of Ca- and K-derived Ar isotopes could have caused the discrepancy. Moreover, the discrepancy is also beyond the errors in the systematic variables, like the decay constants of 40K or the ages for the neutron-fluence monitors.

Apparent argon diffusive loss 40Ar/ 39Ar age spectra in amphiboles

Detailed investigations have been made of two amphibole samples where 40Ar/ 39Ar age spectra have been interpreted to show argon diffusive loss. Both samples display complex compositional zoning that reflects partial major and minor element chemical re-equilibration during later thermal events that is associated with the loss of radiogenic 40Ar. The apparent diffusive loss age spectra in these samples are an artefact of this chemical re-equilibration process. One sample additionally shows the effect of potassium gain and contamination with a potassium-rich phase (biotite). The resulting ages in this sample are geologically meaningless and the apparent fit to a theoretical diffusive loss curve is coincidental. This study shows that volume diffusive loss of argon appears to operate at slower rates than argon loss by chemical re-equilibration in amphiboles.

Formula omitted] thermochronology of isotopically zoned micas: Insights from the southwestern USA proterozoic orogen

We have used three different 40 Ar 39 Ar laser microprobe methods to explore the distribution of radiogenic 40Ar in 1.0–1.5-mm biotite crystals from the ca. 1680 Ma Horse Mountain monzogranite of central Arizona. Incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near-plateau ages of ∼1190 Ma, showing no indication of intracrystalline 40Ar inhomogeneity. In contrast, total fusion of twenty-five biotite fragments (∼ 100 pm) yielded apparent ages ranging from 1006.7 to 1212.0 Ma. Detailed age mapping in the {001} plane of two crystals, with the laser focused to a minimum spot size, confirms that the age dispersion in the fragment data reflects the existence of 200 m.y. age gradients in single crystals. The two mapped crystals display very different age distribution patterns that suggest radiogenic 40Ar loss through two mechanisms: volume diffusion on a scale comparable to that of the grain radius, and more rapid diffusion along discrete zones of high crystal defect density. Simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of ∼0.5 K/m.y. The Horse Mountain results, as well as previously published studies, indicate that conventional, incremental heating of hydrous phases can homogenize intracrystalline gradients in 40Ar, thereby masking important details of the thermal history of analyzed samples. In contrast, detailed isotopic mapping studies offer a wealth of information, and will become more powerful with continued improvement in the spatial resolution of 40 Ar 39 Ar laser microprobes. Total-fusion studies of crystal fragment populations can be readily automated, making them less labor-intensive than mapping studies. Our preliminary experiment on a limited Horse Mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research.