Recovery and interpretation of the 18O/16O of Miocene oolitic goethites in multi-generational mixtures of Fe (III) oxides from a channel iron deposit of Western Australia

Abstract Published (U-Th)/He ages indicate that an oolitic Miocene channel iron deposit (CID) on Mesa J in the Hamersley province of Western Australia (∼22 °S latitude) contains three predominant generations of Fe (III) oxides – two generations of older hematite and one of younger goethite. Selective dissolution and sequential atom balance calculations were combined with chemical and oxygen isotope analyses to determine the δ18O values of hematite, goethite, and quartz in eight samples from a core drilled in that CID. For the quartz (Q), δ18OQ = +21.7(±0.9)‰. For the younger hematite (Hm1), δ18OHm1 ≥ +3.6(±0.6)‰, whereas for the older hematite (Hm2), δ18OHm2 ≤ +2.1(±0.8)‰. The oolitic CID goethite (G) crystallized in the late Miocene [7(±1) Ma] and is a solid solution represented as Fe(1−Y)AlY(CO3)XO(1−X)OH with values of δ18OG that range from −0.4‰ to +0.7‰. A mixing model for the solid solution expresses the value of δ18OG in terms of Y, X, relevant fractionation factors, and the δ18O of the endmember FeOOH (δ18Ogt). Mixing model-derived values of δ18Ogt range from −2.4‰ to −0.7‰. Paired values of δDgt and δ18Ogt in seven of eight late Miocene oolitic goethites from Mesa J indicate crystallization from meteoric waters at an average temperature of 20(±5) °C. In the late Miocene, Mesa J was in the subtropics at a paleolatitude of ∼29 °S, which invites comparison with the climate of the near-coastal community of Mingenew in Western Australia (presently at 29 °S latitude). The modern MAT at Mingenew is 20 °C – i.e., indistinguishable from the late Miocene Mesa J average of 20 °C. However, this similarity is not matched by the mean annual precipitation (MAP). The modern MAP at Mingenew is about 400 mm, but the lateritic weathering that produces abundant pedogenic Fe (III) oxides appears to require a seasonally contrasting climate with rainfall totals of at least 1300 mm/yr. In the late Miocene, southward-shifted summer season tropical cyclones might have delivered such large amounts of rain (with relatively low δD and δ18O values). If so, the δDgt and δ18Ogt of the oolitic goethites of Mesa J support the idea of a rainfall-driven, possibly microbially mediated, summer season bias in the timing of crystallization. Thermal buffering in the regolith can reconcile summer-dominated goethite crystallization with a temperature that would be analytically indistinguishable from the MAT. Comparison of the δ18Ogt values of the Mesa J goethites with published δ18O values of goethites of similar age from two other sites in the Hamersley province suggest a possible increase in δ18O of late Miocene rainfall with increasing distance from the coast.