Now showing1 - 2 of 2
- Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Thermally activated multistep alteration of Fe2+-bearing fluorophlogopite revealed by in situ Raman spectroscopy(Copernicus Publications, 2026-07-22); ; ; Elucidating the temperature-induced structural and crystallochemical transformations in phlogopite mineral species with a partial substitution of Fe 2+ for Mg at the M(1,2) sites and OH-/F-occupancy disorder at the X site can help in better understanding the transport phenomena in the lithosphere and mantle metasomatism. Here we present the results from in situ temperature-dependent Raman spectroscopy on fluorophlogopite with Fe 2+ content of ∼0.15 atoms per formula unit (apfu) and hydroxyl-group content of ∼0.40 apfu. A few heating–cooling runs were conducted in air up to different temperatures, with the highest temperature achieved being 1450 K. The chemical composition and crystal structure before and after cooling down from 1450 K to room temperature were probed by wavelength-dispersive electron microprobe analysis (WD-EMPA) and single-crystal X-ray diffraction (XRD), respectively. The anomalies in the temperature dependencies of phonons reveal a sequence of heating-induced changes in phlogopite: (1) near 550–650 K structural instability related to interlayer interactions occurs, which activates the mobility of interlayer K + cations. This process is reversible up to ∼1100 K, and, hence, diffusion of K + can potentially contribute to phlogopite electrical conductivity between ∼650–1100 K. (2) At ∼1150 K all H + cations delocalize, including those from OH groups linked to MgMgMg chemical configurations, and therefore can also act as charge carriers. (3) Between 1300 and 1450 K irreversible Fe 2+ →Fe 3+ oxidation develops, confirmed by a permanent resonance Raman-scattering signal after cooling down to room temperature, along with a subtle decrease in the unit-cell volume. Simultaneously, partial dehydrogenation and dehydroxylation take place as only 65 % of the hydroxyl groups recover at room temperature. The Raman-scattering data also suggest a minor loss of K + ( ∼0.02 apfu) from the mica structure, whereas WD-EMPA indicates no change in the content of F − within uncertainties. A partial thermal decomposition of fluorophlogopite occurs above 1300 K, leading to the formation of a minor amount of nano-sized forsterite ( ∼1 % in volume), which nucleates mainly on the sample surface parallel to the cleavage plane.Publicationtype: Journal Article20 12 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Highly siderophile element, triple oxygen-, ε¹⁸²W, and Re-Os isotopic composition of early Archean impact spherules from South Africa and the spherule–matrix complementarity(University of Arkansa, 2026-03-19); ; ; ; Archean impact spherule layers represent exceptional archives of extraterrestrial (ET) material, containing large amounts of ET highly siderophile elements (HSE) that dominate the bulk content of these elements. This enrichment makes them prime targets for testing additional impact tracers, such as ε¹⁸²W and triple oxygen isotopes. We investigated samples from the Paleoarchean BARB5 drill core (Barberton Mountain Land, South Africa), which preserves four spherule layers with chondritic HSE contents and ¹⁸⁷Os/¹⁸⁸Os signatures. Tungsten isotope data from bulk spherule layer samples yield ε¹⁸²W values indistinguishable from the bulk silicate Earth, most likely reflecting the limited sensitivity of the ε¹⁸²W composition to detect meteoritic admixture. If present, such a component must lie within analytical uncertainties, limiting contributions to ≤6% for a chondritic endmember or ≤3% for an iron-meteorite endmember, unless a larger signal was erased by postimpact hydrothermal overprint. In addition, bulk triple oxygen data fall within Archean shale fields and do not show resolvable ET signatures, consistent with a chondritic contribution of at most ~5% given analytical uncertainties; elevated ¹⁸O values most likely reflect seawater alteration of glass spherules. Thus, despite clear HSE–Os isotope evidence for admixture of ET components, ε¹⁸²W and oxygen isotopes yield no such information. This can be explained by plume condensation models predicting temporally separated fallout of refractory and volatile element carriers. To test this, we separated spherules, matrix, and mixed fractions from one of the four BARB5 beds. While the matrix hosts the highest HSE contents and least radiogenic ¹⁸⁷Os/¹⁸⁸Os, spherules have the lowest HSE contents and slightly more radiogenic ¹⁸⁷Os/¹⁸⁸Os signatures, with mixed fractions being intermediate. Together with highly siderophile interelement trends, these results most likely highlight stepwise condensation followed by early syn-depositional to diagenetic alteration, establishing Archean spherule beds as unique probes of early plume dynamics and impact processes.Publicationtype: Journal Article21