Lead isotopic measurements in archeological objects

Pb isotopic analyses on metal artifacts help constrain their provenance. Useful and reliable data require analytical protocols that ensure accuracy and monitor precision. The interpretation can sometimes be ambiguous because natural ore deposits frequently have overlapping Pb isotopic compositions. In most cases, however, provenances can be reliably narrowed down. For the Alpine region, a pattern of small, strongly localized mine exploitations in pre-Roman times, supplemented by some long-distance trading, is taking shape

Dating deformation: the role of atomic-scale processes

Dating deformation is difficult, as textures and petrogenesis of deformed rocks are complex. Moreover, geochronometer categories are pursued by communities that often do not communicate. Hygrochronology dates the retrograde metasomatic/metamorphic reactions caused by aqueous fluid circulation events. Thermochronology models time-temperature histories by assuming that mineral ages can be uniquely assigned to a "closure temperature Tc", the only process occurring in rocks being Fick's Law diffusion. Diffusion by definition produces a bell-shaped concentration profile. In contrast, patchy intra-grain isotope concentration profiles denounce aqueous retrogression, whose rate is orders of magnitude faster than diffusion. Petrochronology is based on opposite assumptions, as the mobility of structure-forming major cations is higher than that of radiogenic Pb, Ar, and Sr. Whenever the formation of a mineral occurs at T<Tc, its apparent age dates its formation. Nanochronology analyzes samples at the nm-scale. These analyses illuminate atomic-scale processes, e.g. open-system transport of soluble ions along self-sealing networks of nanopores. The key to dating deformation and producing correct, regional-sized (up to 100s of km) tectonic models is the realization that minerals consist of atoms, whose behavior is only firmly constrained by nm-scale analyses

The in vacuo release of Ar from minerals: 1. hydrous minerals

The systematics of Ar release during in vacuo stepwise heating of hydrous minerals is examined. By means of common-denominator three-isotope correlation diagrams, in use for 60 years, it is possible to recognize the presence of polymineralic mixtures in a step-heated mineral separate and characterize their components. This data presentation, when applied to literature studies proposing Fick's Law diffusion during in vacuo stepwise heating, shows that in actual fact these studies were based on partly retrogressed micas that were intergrown with their alteration products, probably at a scale < 20 µm so as to escape recognition during handpicking. The degassing of hydrous minerals in vacuo does not follow Fick's Law in an inert, homogeneous matrix, and instead proceeds from the structural collapse during dehydration. The data presentation discussed here quantifies the differential release of the Ar isotopes. Their release follows the structural collapse of their host phases and gives rise to remarkably well resolved patterns even in polymineralic samples

Copper supply during the Final Neolithic at the Saint-Blaise/Bains des Dames site (Neuchâtel, Switzerland)

The Saint-Blaise/Bains des Dames stratified site in Neuchâtel, Switzerland, contains several occupations that span the Late through Final Neolithic, including the Horgen, Lüscherz, and Auvernier-Cordé periods. As part of a study on prehistoric metallurgy in western Switzerland, we compare the lead isotope ratios (multicollector inductively coupled plasma mass spectrometer) and elemental compositions (instrumental neutron activation analysis) of the site's numerous copper finds to a database of corresponding measurements for copper ores throughout Europe. The results show a considerable variation in copper compositions present at the site, suggesting complex economic relationships and multiple chaînes opératoires during the time in question. Specifically, during the Final Neolithic, we distinguished ten coherent clusters, confirmed by both the elemental compositions and lead isotope ratios. When compared to the Europe-wide database of copper ores, we observed significant changes in the provenance of the copper through time that reflect equally significant changes in social, cultural, and economic interactions

"Excess Ar" by laboratory alteration of biotite

Many biotite phenocrysts from marine tephra layers have substoichiometric potassium concentrations and alkali occupation << 2.0 atoms per formula unit. Diagenetic alteration is an expected effect of exposure of fresh magmatic minerals to interstitial water and brine intrusions after the deposition and burial of sediments. To test the effect of diagenetic alteration on potassium-argon ages, we irradiated and step heated untreated Fish Canyon biotite (t = 28.2 Ma) and several aliquots leached to various extents in strong and weak acids. Laboratory alteration caused loss of K, age spectrum discordance, high step ages and total gas ages, Ar release at lower furnace temperature, higher Cl/K and Ca/K, and a slight decrease in 36Ar concentration. Potassium loss was always higher than 40Ar* loss. Electron microprobe element maps document that acids preferentially penetrate in phyllosilicate interlayers, removing K (and Na). Because Ar* is removed to a lesser extent than K, we propose that natural 40K decay partly implants radiogenic Ar* into the tetrahedral-octahedral-tetrahedral (T-O-T) phyllosilicate layer, where Ar is shielded from interlayer leaching. The recoiled 39Ar, which was produced by irradiation after the leaching, also partitioned between T-O-T and the interlayer; age spectrum discordance was probably enhanced by the heterogeneous partition of 39Ar and 40Ar* in leached samples

The in vacuo release of Ar from minerals: 2. The role of structural modifications of K-feldspar during heating revealed by Raman microprobe analyses

The release of Ar during stepwise heating 39Ar-40Ar dating experiments may be controlled by Fick's Law diffusion in an inert matrix, and/or by structural modifications of the host mineral. A ca. 1 mm3 irradiated cleavage fragment of a low sanidine crystal from Itrongay, Madagascar was degassed isothermally at 888 ± 2 °C for 264 h, acquiring 67 stepwise 39Ar release data. The 39Ar release was observed to follow a smooth sigmoid curve and not a line as would be predicted by Fick's Law. If the 39Ar release is controlled by crystallographic changes, the implication is that these changes undergo time-dependent variations. The long-term bulk degassing, being the sum of various structural modifications, approximates a Fickian behavior that is not verified in detail in short-term experiments, as it averages over different 39Ar release regimes. This would make the downslope extrapolation of laboratory data to geological conditions highly underconstrained. In order to constrain the behavior of the crystal structure of K-feldspar during laboratory heating, we measured the Raman spectra of a different, ca. 1 mm3 cleavage fragment of the same irradiated sanidine crystal. The sample was heated in air in a Linkam heating stage, and Raman spectra were acquired at temperatures increasing from 300 to 1000 °C, including a 6-hour isothermal run at 900 ± 1 °C. Raman modes between 50 and 1200 cm-1 were observed to record two kinds of change, defining robust trends. The positions of most peaks were shifted towards lower wavenumbers (lower energies) and broadened; in addition, the relative heights of different peaks showed robust variation trends. The larger changes coincide with discrete temperature increases, but all changes also progressed over time at constant temperature. These peaks mirror the excitation of phonon modes, which are associated with interatomic bond stretching and deformation, with Si,Al ordering and with deformation and rotation of SiO2 tetrahedra. Most, but by far not all, of the change is reversible (such as e.g. the differential activation of phonon modes), but irreversible structure modifications (such as e.g. Al,Si disordering) are also recorded. We conclude that the K-feldspar structure violates mathematical requirements of matrix inertness during laboratory heating

Petrochronology and hygrochronology of tectono-metamorphic events

U-Th-Pb petrochronology is based on the incontrovertible fact that the diffusion of radiogenic Pb is negligibly small relative to retrograde reaction rates. Multi-element maps demonstrate that patchy textures tightly correspond to (U+Th)-Pb age variations, requiring that fluid-induced dissolution/ reprecipitation is the principal cause of Pb mobility. Attempts to model intracrystalline core-rim Pb zonations as diffusive transport are not legitimate unless genuine bell-shaped diffusion profiles in minerals can be documented, which happens only exceptionally. Monazite and zircon intra-grain age maps confirm that coupled dissolution-reprecipitation and retrogression reactions assisted by fluids control (Th+U)-Pb ages, not temperature. The chemical zonations observed in many (Th+U)-bearing mineral chronometers (e.g. monazite, allanite, xenotime, zircon) provide petrological constraints. Linking petrology with textures and the isotope record allows reconstructing entire segments of the P-T-A-X-D-t history of a rock and its geodynamic environment. The dearth of mathematically sound diffusion profiles equally applies to the isotope record of micas and feldspars. The tight link between petrology, microtextures, chemical composition and geochronology also pertains to Rb-Sr and K-Ar. Overdetermined multi-mineral Rb-Sr isochrons with excess scatter, and spatially resolved/stepwise release 39Ar-40Ar results, demonstrate ubiquitous correspondence between relict phases and isotopic inheritance. Many rock-forming minerals are highly retentive of Sr and Ar, unless they are obliterated by retrograde reactions. The rates of dissolution in fluid-controlled reactions are several orders of magnitude faster at upper and mid-crustal levels than diffusive reequilibration rates. Thus, as a rule Rb-Sr and K-Ar chronometers date their own formation. Accurately establishing P-T paths of monometamorphic rocks requires assessing petrologic equilibrium using multivariate thermodynamic software. Dating complex parageneses of polymetamorphic, unequilibrated rocks requires labor-intensive disentangling by: (i) qualitative identification of relicts, retrogression reactions, and chemically open systems by imaging techniques (e.g. cathodoluminescence, element maps, etc.); (ii) microchemical analyses at the µm-scale quantifying heterochemical disequilibrium phases and assigning them to a P-T-A-X segment; (iii) spatially resolved/stepwise release, relating the chemical signature of the analyzed mineral to its age. K-Ar and Rb-Sr usually provide a different perspective on the P-T evolution of a rock than does (Th+U)-Pb, as K+Rb-rich minerals (phyllosilicates and especially feldspars) mostly form later and react/dissolve faster in the retrograde path than U-rich accessory phases. The present paper reviews these general principles by means of well-understood examples, both successful and insuccessful in matching the independently known external constraints

Tracing wedge-internal deformation by means of strontium isotope systematics of vein carbonates

Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events

Tracing wedge-internal deformation by means of strontium isotope systematics of vein carbonates

Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events. © The Author(s), 2022. Published by Cambridge University Press