Zircon has long been used as the principal mineral for determining the crystallization ages of granitic rocks. However, in leucogranites, accurate age determination is often problematic due to radiation damage from high uranium contents. U–Pb dating of monazite, which is more resistant to metamictization and Pb loss, thus offers a more reliable alternative. Here, I present a case study applying this approach to the Imwon and tourmaline-bearing leucogranites in the northeastern Yeongnam Massif, Korea, which experienced high-temperature metamorphism up to the amphibolite facies. To evaluate the reliability of U–Pb chronometers, I compared monazite and zircon ages from two leucogranites. In the Imwon leucogranite, monazite showing magmatic zoning yields an upper intercept age of 1905 ± 12 Ma, which is interpreted as the magmatic crystallization age. In contrast, the coexisting zircon records ca. 1.86 Ga ages, consistent with the regional metamorphic age indicated by monazite weighted-mean ²⁰⁷Pb/²⁰⁶Pb age of 1865 ± 7 Ma from adjacent psammitic schists. In the tourmaline-bearing leucogranite, magmatically zoned monazite yields a weighted-mean ²⁰⁷Pb/²⁰⁶Pb age of 1860 ± 3 Ma, indicating magmatic crystallization during this metamorphic event. In contrast, zircon from this granite yielded highly scattered U–Pb data and did not define a coherent age because of severe Pb loss. These results demonstrate that, in leucogranites affected by or formed during ca. 1.86 Ga regional metamorphism in the Yeongnam Massif, monazite more faithfully records magmatic crystallization ages than zircon, which is more susceptible to isotopic disturbance and Pb loss. Collectively, our results demonstrate that the Yeongnam Massif experienced two discrete Paleoproterozoic leucogranite-forming events at ca. 1.90 Ga and ca. 1.86 Ga, as revealed by monazite rather than zircon U–Pb geochronology.

The ⁵³Mn–⁵³Cr chronometer applied to carbonates in chondrites and asteroidal samples provides key insights into aqueous alteration processes on planetesimals in the early Solar System. In this study, we evaluated the useful yields of secondary ions and relative sensitivity factors (RSFs) of Cr and Mn for calcite and dolomite in Mn–Cr isotope measurements by secondary ion mass spectrometry (SIMS) with two different primary ion beam species, ¹⁶O^– and ¹⁶O₂^–. The choice of primary beam species exerts only minor effects on the chemical composition dependences of the ⁵⁵Mn/⁵²Cr RSFs among carbonate standards. In contrast, the ¹⁶O^– beam gives ~42% and ~6–11% higher useful yields of ⁵²Cr⁺ than the ¹⁶O₂^– beam for calcite and dolomite, respectively. Because higher useful yields directly improve the analytical precision of Cr-isotope ratios and the resulting formation ages, the use of the ¹⁶O^– primary beam is advantageous for ⁵³Mn–⁵³Cr dating of carbonates by SIMS.

To identify the chemical structure of quenched nitrogen-included carbonaceous composite (QNCC), synthesized as analogues of organic dust formed around novae, we conducted high-temperature vacuum Temperature-Programmed Desorption (TPD) analysis and X-ray photoelectron spectroscopy (XPS) on QNCC and filmy quenched carbonaceous composite (QCC) for comparison. QNCC is a laboratory organic dust synthesized by quenching and condensing plasma generated from hydrocarbon dust, such as polycyclic aromatic hydrocarbons (PAHs) and filmy QCC, and nitrogen gas using a 2.45 GHz microwave source. The infrared spectrum of QNCC closely matches that of the unidentified infrared (UIR) bands observed around classical novae, making QNCC the best laboratory analogue at present for organic dust formed in these circumstellar environments. The analysis revealed that both filmy QCC and QNCC are primarily composed of sp³-bonded carbon (i.e., diamond-like carbon), predominantly with hydrogen-terminated structures and small amounts of alkyl functional groups. The low desorption levels of N₂ and relatively high amounts of HCN and NH₃ from QNCC indicated in the TPD analysis suggest that QNCC is characterized by minor inclusion of graphitic nitrogen and major inclusion of pyridinic and pyrrolic nitrogen. Additionally, comparison of the desorption profile of HCN in the TPD experiment with temperature for QNCC, nanodiamond, and amine-modified nanodiamond indicates that amine structures attached to sp³ carbon are also present in QNCC.