Paleoenvironmental evolution of continental landscapes through combined high-resolution geochronology and d18O ion microprobe analysis of goethite

Abstract

Goethite (a-FeOOH), the most common product of weathering of Fe-bearing minerals, is an attractive target for (U-Th)/He geochronology because of its abundance and relative stability in the surficial environment. Combined (U-Th)/He4 He/3 He geochronology of 24 goethite samples representative of different source rocks, precipitation mechanisms, solid-solution compositions, purity, and porosity provides information on the influence of these factors on U-Th-He contents, distribution, and retentivity in natural goethites. Measured activation energies (E) for He diffusion from goethites range from 11.2 ± 0.5 to 74.5 ± 3.2 kcal.mol-1 , with an average value at 36.2 ± 13.5 (1s) kcal.mol-1 ; and frequency factors (lnD0/r0 2) range from -4.1 ± 0.5 to 62.8 ± 1.9 s-1 , with an average value of 24.1 ± 13 (1s) s-1. The average activation energy of ~ 35 kcal.mol-1 indicates that He retentivity in goethite is comparable to other phases dated by the (U-Th)/He method. A major distinction between goethite and other phases used in (U-Th)/He geochronology is its large frequency factor, a possible consequence of the fact that supergene goethite is composed of masses of cryptocrystalline aggregates and not single crystals.Armed with an improved understanding of goethite’s He retentivity, and its suitability to (U-Th)/He and cosmogenic 3He analyses, two field sites were investigated in detail to unravel the history of mineral precipitation, exhumation and erosion that created the characteristic plateau landscapes at the Quadrilatero Ferrifero and Carajas, Brazil. (U-Th)/He geochronology of goethite and cosmogenic 3He in goethite and hematite reveal mineral precipitation ages as old as 55 Ma and apparent exposure ages > 5 Ma for canga-cemented plateaus at the Quadrilatero Ferrifero. Pebbles of hypogene hematite-magnetite from colluvia or shallow creeks draining the canga-cemented plateaus record a much longer exposure history than in situ canga blocks, showing that even older duricrusts, now eroded, once blanketed these plateaus. The long-term erosion history obtained from cosmogenic 3He on BIF plateaus confirms that relic surfaces persist in the landscape for millions of years. Combined 3He and (U-Th)/He dating shows that cangas are preferentially goethite cemented by biogeochemical reactions in the subsurface. Physically stable but biogeochemically dynamic, cangas armor the landscape by pervasive and recurrent iron cycling and cementation, slowing erosion of weathered BIF and friable hematite-magnetite ore. At Carajas, (U-Th)/He geochronology reveals a longer history of weathering, spanning from ca. 80 Ma to the present. Cosmogenic 3He measurements for hematite blocks cemented onto the plateaus yield apparent exposure ages > 7 Ma and erosion rates as low as 0.08 m.Ma-1 , confirming that plateau surfaces are virtually immune to physical erosion for tens of millions of years. In contrast, (U-Th)/He geochronology of ferruginous duricrusts blanketing low elevation (250-100 m) plains surrounding the Carajas plateaus reveal that the low elevation plain is diachronous and range from ~ 10 Ma away from the plateau to ~1 Ma next to it, having evolved by scarp retreat. The virtual cessation of scarp retreat at some sites suggests that metamorphosed banded iron-formations provide effective barriers to retreating escarpments, helping to preserve some of the oldest continuously exposed landsurfaces on Earth.To derive paleoenvironmental information about these landsurfaces, new protocols were developed for high-spatial resolution δ18O analysis of goethite with the Sensitive High Mass Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI). A natural sample from the Capao Mine, Minas Gerais, Brazil, was extensively characterized and now provides a working goethite reference material (RM) for ion microprobe analyses. A laser fluorination value of -17.22 ± 0.03 ‰ (1 σ) was obtained for an aliquot of this RM. The repeatability of SHRIMP-SI measurements of the Capao L4 RM was often better than 1.5‰ (2 s). Using Capao L4 as a RM, several natural goethite samples were analysed to test the relationship between natural properties, preparation procedures, instrument conditions, and the overall reliability of the δ18OSIMS results.Finally, combined laser-heating (U-Th)/He geochronology, SHRIMP-SI δ18O measurements (δ18OSIMS-gth), and electron microprobe analysis of goethites from the Quadrilatero Ferrifero and Carajas sites produced two extensive Cenozoic paleoclimatic records for the continental interior of Brazil, spanning from ~ 70 Ma to 900 ka at Carajas and from ~ 40 Ma to 600 ka at the QF. The results identify major climatic shifts in continental Brazil, compatible with global climatic changesobserved in the δ18O record of oceanic sediments. Periods of optimum climatic conditions (Early to Mid-Eocene, Late Oligocene, and Mid-Miocene) correspond to more frequent precipitation of weathering minerals, stoichiometrically pure goethites, and δ18OSIMS-gth shifts towards lighter isotopic values. These shifts are consistent with warm monsoonal conditions in the continental interior. Abrupt shifts in the d18OSIMS-gth record towards more positive values occur during global glaciation (Oi-1 and Mi-1 glaciations), periods that also correspond to less frequent precipitation of Al-rich goethites suggestive of dryer continental environments. The combined goethite (U-Th)/He– δ18OSIMS-gth record provides a robust new way of investigating continental paleoclimates on Earth.