Abstract The petrogenesis of highly evolved, rare-metal-rich granites is globally contested between models of extreme fractional crystallization of a single magma and multi-stage crustal reworking. One key criterion for distinguishing between these models is the temporal structure of the magmatic record: the former is expected to produce a relatively restricted crystallization-age range, whereas the latter may generate multiple, temporally separated age populations spanning a much longer interval. The emplacement age of the Jiangjunshan blue-green microcline-bearing granite has long been disputed, with previously reported Permian and Jurassic ages differing by more than 100 Myr, making it a valuable case study for testing these contrasting models. Integrated zircon and monazite U–Pb geochronology reveals three temporally separated magmatic episodes at 290–273 Ma (Permian), 231–200 Ma (Triassic), and ∼150 Ma (Jurassic), spanning approximately 140 Myr. Despite its limited exposure, the Jiangjunshan pluton preserves a nearly complete record of Permian–Jurassic magmatism in the Chinese Altai, providing a remarkably condensed archive of regional magmatic evolution. This protracted, multi-stage geochronological record indicates episodic magma generation and reactivation of the magmatic system. To reconcile this protracted long-duration record with thermomechanical principles, we propose a novel, generalizable petrogenetic framework. We interpret the Jiangjunshan body not as a series of discrete intrusions, but as the product of a long-lived, crystal mush reservoir that was rooted in a lithospheric-scale mantle slope zone. This deep-seated tectonic discontinuity provided long-lived favourable structural conditions, enabling the periodic influx of heat and fluids from mantle/crustal sources during regional tectonic events. These influxes repeatedly reactivated the near-solidus crystal mush, driving localized melt extraction, pervasive fluid-rock interaction, and episodic mineral re-equilibration—processes recorded by the complex zircon textures and age domains. Zircon Hf isotopes (εHf(t) = –19.6 to +11.0) demonstrate pulsed open-system behavior, with each reactivation episode incorporating variable mixtures of juvenile material and ancient crust, including remobilized components from the mush itself. The Jiangjunshan granites exhibit extreme geochemical signatures, including pronounced M-type REE tetrad effects, exceptionally low K/Rb (36.7–97.6) and Zr/Hf (5.6–6.3) ratios, and high F concentrations. These whole-rock features indicate intense fractionation and fluid-mediated exchange within this stagnant, volatile-rich reservoir. Our mantle slope zone-hosted crystal mush model resolves the thermal paradox of prolonged magmatic histories and provides a universal mechanism to explain extreme geochemical fractionation and rare-metal enrichment in highly evolved granites worldwide. This work underscores the fundamental role of long-lived lithospheric discontinuities in localizing, sustaining, and chemically refining continental magmatic systems.
Abstract Force constant measurements employing NRIXS (nuclear resonant inelastic X-ray scattering) have seen increasing applications in isotope geochemistry, as it bypasses the need for equilibration experiments. This synchrotron technique measures the strength of chemical bonds, which are represented by force constants . The equilibrium isotope fractionation between any two materials is directly proportional to their difference in . While this method is promising, it is restricted to measuring Mössbauer-sensitive isotopes such as 57Fe. To expedite analyses, samples are typically doped with 57Fe, and it has not yet been tested whether this enrichment affects the measured . In this study, we systematically tested the effect of 57Fe enrichment to determine whether heavy isotope enrichment has a resolvable effect on . We enriched wüstite, fayalite, and bcc-Fe samples with varying 57Fe/ΣFe and measured their with NRIXS. Results show that while high levels of 57Fe enrichment do not affect , too little 57Fe is a concern. We establish a cut-off of 50 cps at the highest non-resonant peak to yield robust NRIXS results and recommend exercising caution while using NRIXS to measure on natural samples. Force constants obtained for bcc-Fe, wüstite (Fe0.94O), and fayalite are 182 ± 6 N/m, 162 ± 7 N/m, 175 ± 8 N/m, respectively.
The behavior of iron-bearing magnesium carbonate (Mg,Fe)CO3 at high pressures has significant implications for deep carbon cycling in Earth's mantle, as this composition more realistically reflects the carbonates expected in the Earth's deep interior. In this study, we have investigated high-pressure vibrational properties of ferromagnesite, a Mg-rich (Mg,Fe)CO3 phase, using synchrotron M & ouml;ssbauer, infrared (IR), and Raman spectroscopies in diamond-anvil cells up to similar to 71 GPa at room temperature. Above 29 GPa, Raman and IR spectra reveal signatures of pressure-induced lattice distortion, including splitting of lattice-coupled (T) and in-plane bending (nu 4) modes of internal CO32-$\mathrm{CO}_3<^>{2-}$vibrations, changes in the pressure dependence of the wavenumbers of both Raman and IR modes, and a variation in the L/nu 4 mode intensity ratio. While lattice distortion has been proposed in pure MgCO3, even minor Fe substitution alters the response of internal modes. This result suggests that low Fe content can modify the local bonding environment, which may influence the high-pressure and -temperature stability of the MgCO3-FeCO3 solid solution. M & ouml;ssbauer spectra show that a high-spin to low-spin transition occurs between similar to 45-50 GPa in ferromagnesite. Raman and IR measurements display discontinuous shifts in both lattice (L) and internal vibrational (nu 1 and nu 4) modes across the spin transition. Previous studies on the MgCO3 end-member have reported additional lattice distortion near 50 GPa, close to the pressure at which the spin transition occurs in Fe-rich compositions. The relative changes in the Gr & uuml;neisen parameters (Delta gamma) of the lattice mode across the spin transition for Fe-containing compositions and the further distortion for MgCO3 end-member exhibit a linear dependence on Fe content, whereas those of the internal modes show a second-order, nonlinear trend, particularly for the nu 4 mode. Both lattice and internal modes may reflect the combined effect of lattice distortion and the spin transition, but the two cannot be distinguished from the behavior of the lattice mode. While lattice distortions have been discussed for the Mg-end-member, they have not been reported for Fe-bearing or Fe-rich compositions, possibly because the lattice distortions may be masked by the dominant effects of the spin transition, particularly in Fe-rich compositions. Since lattice modes are expected to primarily govern elastic wave propagation and the contribution of internal-mode variations is limited, the Fe-content dependence of sound velocities after the spin transition is expected to be approximately linear. This suggests that Fe-bearing carbonates may lower seismic wave velocities in the deep mantle where they are sufficiently abundant.
