Is (U-Th)/He dating of pyromorphite a new chronometer of weathering?

Pyromorphite [Pb₅(PO₄)₃Cl] is an end-member phase of the apatite-group minerals. It commonly forms during supergene weathering of Pb-rich ore deposits. Owing to its chemical stability and low solubility under natural environmental conditions, pyromorphite preserves a record of supergene weathering events. The U concentration of pyromorphite is typically high (up to several thousand ppm) making it a potential as a (U-Th)/He and/or (U-Th)/Ne chronometer of paleoenvironmental change.  (U-Th)/He ages of nine pyromorphite specimens from around the globe are less than 5 Ma. The near complete absence of fission tracks in all samples, despite the relatively high U concentration (1-30 ppm), is consistent with the young He ages. Complete helium extraction requires considerably shorter heating at lower temperature than similar sized Durango apatite fragments implying that He closure temperature in pyromorphite is lower than apatite.

A detailed study of an inclusion-free pyromorphite crystal from Daoping mine, Guangxi (China) has been undertaken to determine its suitability as a chronometer of supergene mineralisation.  Incremental extraction of He from 250-500 mm fragments display good linearity on an Arrhenius diagram for temperature steps less than 220°C.  The kinetic parameters (E and ln(D₀/a²)) display a range of values that we use to determine a helium closure temperature in the range -30°C to -84°C for a nominal cooling rate of 10°C/Myr.  This range may reflect the presence of varying size sub-grains that are evident from EBSD.  Density functional theory modelling shows that the substitution of Ca²⁺ (0.99 Å) by Pb²⁺ (1.19 Å) expands the interstitial sites in the pyromorphite lattice compared to apatite, likely lowering the energy barrier for helium diffusion and changing the global minimum location, thus changing the diffusion pathway of He in pyromorphite. EBSD shows that lattice distortion is ubiquitous, perhaps due to the off-centre position of the electron pair of Pb²⁺ within its coordination environment or the substitution of V.  The dislocations may function as sinks for He and impede helium diffusion. This leads us to conclude that pyromorphite is unlikely to find use as He thermo/geochronometer but further study may aid understanding He diffusion in apatite group minerals.