Apophyllite 40 Ar/ 39 Ar and Rb‐Sr geochronology: Potential utility and application to the timing of secondary mineralization of the Kirkpatrick Basalt, Antarctica

Apophyllite (KCa 4 Si 8 O 20 (F, OH)·8H 2 O), which formed as a low‐temperature secondary mineral in the Kirkpatrick Basalt, has been examined to evaluate its potential for geochronology using the K‐Ar (by the 40 Ar/ 39 Ar technique) and Rb‐Sr methods. The 40 Ar/ 39 Ar total‐gas ages for apophyllites from the Kirkpatrick Basalt are 114–133 Ma for the central Transantarctic Mountains, 95–114 Ma for south Victoria Land, and 76–100 Ma for north Victoria Land. Within individual hand samples, apophyllite 40 Ar/ 39 Ar apparent ages show variations of up to 24 m.y., and within single large (up to ∼2 cm) crystals, differences of up to 14 m.y. are observed. The 40 Ar/ 39 Ar incremental‐heating spectra are generally flat but have lowand high‐temperature discordances attributed to 39 Ar recoil. Rb‐Sr model ages for the apophyllites range from 94 to 144 Ma and vary from concordant with the 40 Ar/ 39 Ar dates to as much as 14 m.y. older. The dates are consistent with other temporal indicators of low‐temperature alteration in the basalts and are interpreted to broadly reflect the time of apophyllite precipitation. They document a geographically widespread early to middle Cretaceous secondary mineralization which is inferred to be caused by a major period of groundwater movement associated with the initiation of uplift and denudation of the Transantarctic Mountains. The results suggest that apophyllite can produce geologically meaningful ages by both Rb‐Sr and K‐Ar methods, although further work is required to fully understand the variations in age that may be attributed to Ar loss, alkali mobility, or an extended history of mineral precipitation.