Weathering geochronology permits delimiting the ages of weathering profiles, determining rates of weathering and landscape evolution and inferring palaeoclimatic and environmental conditions that control the surficial evolution of continents. It also provides insights into the timing and rates of supergene enrichment of metal and precious mineral deposits. Until recently, weathering geochronology was primarily based on K-Ar and 40Ar/39Ar dating of supergene minerals. Recent advances by U-Th series dating of pedogenic carbonate (Sharp et aI., 2003), in situ U-Th series dating of iron hydroxides (Bernal et aI., 2006), U-Pb SHRIMP dating of opals (Nemchin et aI., 2006), and (U-Th)/He and 4HePHe analysis of supergene goethite (Shuster et aI., 2005) expand the number of minerals and the time range where weathering geochronology can be applied. Weathering profiles blanket more than one third of the Australian continent and are purportedly among the oldest weathering profiles on earth. Unravelling the history of weathering and landscape evolution in these areas requires techniques capable of dating supergene minerals at all time scales, but particularly at the > I Ma scale. Currently, the K-Ar, U-Pb, and U-Th/He methods are the only techniques suitable to directly date supergene minerals on such time-scales.
In this study, I show how the combination of 40Ar/39Ar and (U-Th)/He geochronology can be used to unravel the weathering and landscape evolution history of continents. The application of radiogenic isotope-dating techniques and the proper interpretation of geochronological results require understanding of the dated mineralogy and the presence of potentially contaminating phases. The successful application of 40Ar/39Ar weathering geochronology was made possible through detailed understanding of the hollandite-group Mn oxide and alunite-group sulfate crystal chemistry and mineral physics. Similarly, in this study, I have refined the application of (U-Th)/He dating of goethite. The successful application of this technique relies on the identification and selection of suitable goethitebearing samples in the field; the characterization of mineral paragenesis by optical microscopy; the determination of fine-scale mineral chemistry and paragenesis through electron microscopy and microprobe analysis; the determination of physico-chemical properties (mineral structure, crystallinity) of the various types of supergene goethite through x-ray (bench-top and synchrotron) diffraction techniques; and application to noble gas diffusion experiments in ultra-high vacuum techniques to quantify the crystallochemical controls on the helium (natural radiogenic 4He and artificial spallogenic 3He) diffusion properties and retentivity of goethite. Finally, to test the reliability of the (U-Th)/He dating method, I apply the methodology to weathering profiles in a range of geological environments where stratigraphic and paragenetic relationships provide tight constraints on the possible history of mineral precipitation.
For the applied component of this study, I (with a group of collaborators) chose the Hamersley Province as the key study site for the application of combined 40ArP9Ar and (UTh)/He geochronology, for several reasons: the Hamersley Province is one of the longest lived landscapes on Earth and is extensively blanketed by thick weathering profiles that contain a plethora of crystalline goethite suitable for (U-Th)/He dating; the goethite in these weathering profiles coexists with K-bearing Mn oxides suitable for 40Ar/39Ar dating; and mining operations provide exposure and access to near complete weathering profiles, enabling the sampling of goethite and Mn oxides for geochronology.
Deep (50 to 100 metres on average and up to 400 metres) lateritic weathering profiles in the Hamersley Province outcrop over 80, 000 km2 on ridges and plateaus, ranging in height from II 00 m to 400 m. These lateritic weathering profiles are developed on Archaean banded-iron formation and host some of the world's largest iron ore deposits. Some authors have proposed that the lateritic weathering profiles represent the remnants of a continuous Mesozoic land surface now partially eroded. Surrounding the plateaus and ridges, ferruginized detrital sediments on valley slopes and floors and ferruginized detritus in paleochannel deposits (channel iron deposits or CID), also hosting high-grade iron ore, reveal evidence of widespread erosion and re-deposition of former weathering profiles. They also display evidence of post-depositional weathering and ferruginization, suggesting a complex interplay between weathering and erosion during landscape evolution in the region.
40Ar/39Ar dating of 204 grains of supergene Mn oxides (mostly cryptomelane and hollandite) extracted from 70 samples from seven distinct weathering profiles at 7 field sites, up to 300 kilometres apart, yield precipitation ages ranging from 63.4 ± 0.9 to 1.5 ± 0.2 Ma. When combined with previous unpublished 40Ar/39Ar results, ranging from 81.1 ± 0.4 to 11.6 ± 0.3 Ma, the geochronology indicates a prolonged (Late Cretaceous to Recent) and episodic weathering record for the Hamersley Province, where periods of intense dissolution-reprecipitation of Mn oxides (51-41, 24-16 Ma) alternate with periods of relatively subdued mineral precipitation. The intense periods of mineral dissolution-precipitation correlate with maj or global climatic events.
The goethite precipitation record confirms the longevity of weathering processes identified in the Mn oxide record. (U-Th)/He dating of 85 grains of goethite from 39 samples (20 hand specimens) from six sampling sites (5 sites sampled for 40Ar/39Ar geochronology) yield reliable precipitation ages, ranging from 84.3 ± 12.2 to 3.0 ± 0.2 Ma.
The deep (≥100 m) lateritic weathering profile overlying banded iron-formation in the Hamersley Province record weathering process already ongoing in Late Cretaceous and spanning the Paleogene and Neogene. The geochronological results also reveal that the lateritic profiles in the Metawandy Valley (50-2 Ma), Mt Wall (60-30 Ma), Mt Tom Price (81-12 Ma) and Marandoo (52-12 Ma) regions had already reached great depths (70-100 m on average and up to 220 m below present land surfaces) by at least the Late Cretaceous or Early Paleogene. The results also show that weathering has been less effective at promoting the advancement of the weathering front during the Late Paleogene and Neogene.
The geochronological results for authigenic supergene Mn-oxides and goethite III ferruginized detrital deposits (canga) in the Rhodes Ridge (41 to 7 Ma) area indicate that former land surfaces blanketing the Hamersley Province plateaus and ridges had been partially or nearly completely eroded by at least the Eocene. Geochronological results for the channel iron deposits reveal a similar scenario. 40Ar/39Ar dating of Mn oxides (ranging from 32 to 17 Ma at the Lynn Peak Cm) and (U-Th)/He dating of goethite (ranging from 18 to 5 Ma at the Yandicoogina cm, and from 43 to 28 Ma at the Lynn Peak Cm) in late-stage authigenic cements indicate that the channel iron deposits had completely aggraded with ironrich sediments and were undergoing goethite cementation (ferruginization) by, at least, the Late Middle Eocene.
Age versus depth distributions in channel iron deposits indicate that ferruginization of the channel sediments becomes progressively younger with depth in the profile, strongly suggesting that ferruginization occurred at the groundwater-atmosphere interface and the process moved downwards through progressive deepening of the water table. I interpret that this process was driven by the overall transition towards aridity of northwestern Australia in the Neogene. Excursion towards more humid climates in the Early-Middle Miocene has promoted the partial dissolution and secondary precipitation of channel cements in the upper parts of the profiles or near surface environments.
Correlation between the weathering record and independent climatic and environmental indicators suggests that the formation of lateritic weathering profiles on banded iron-formation can be linked to warm and humid climatic conditions in the Late Cretaceous and Early Paleogene, when Australia lay with Gondwana at low latitudes. Climate change at the end of Paleogene, a consequence of the break-up of Gondwana and Australia's accelerated drift away from Antarctica, is identified at this stage as the causal event that promoted the erosion and deposition of former weathering profiles and the formation of extensive detrital and paleochannel deposits. Amid the Neogene aridification of northwestern Australia, a brief excursion towards more humid climatic conditions at the Early Miocene has promoted extensive re-crystallization of supergene minerals in weathering profiles throughout the region. The Neogene aridification of northwestern Australia may also explain the decelerating rates of weathering and weathering front propagation in lateritic profiles of this regIOn.
A comparison of the weathering record obtained for the Hamersley Province with the results of similar studies from the Carajas and Quadrilatero Ferrifero Regions, Brazil, and Burkina Faso and Gabon, West Africa, reveals that intense weathering and enrichment ofMn oxides within the weathering profiles occurred at the 50-40 Ma interval, but particularly at 47-45 Ma. The remarkably similar weathering history obtained for the three southern Hemisphere continents suggests that weathering in these ancient landscapes may be controlled by global (greenhouse) climatic conditions.