We present THe and ZHe data for basement samples across the Kaapvaal craton to investigate the cratonic burial and erosion history during the assembly and breakup of the Rodinia supercontinent. The oldest THe dates of 1.0-1.2 Ga, combined with volcanic rocks that document the basement was at the surface at ~1.4 Ga, require heating to ≥130°C at ~1.0-1.2 Ga to cause complete He loss from the titanites.
We attribute this Mesoproterozoic heating event to widespread burial of the craton during Rodinia supercontinent assembly by sediments shed from the Namaqua-Natal orogen on the craton’s margin. Later Neoproterozoic-Paleozoic erosion re-exhumed the basement to the surface. Spatial variability in ZHe data patterns can be explained by geologically-reasonable differences in Phanerozoic Karoo basin burial and erosion. The Mesoproterozoic and Karoo burial phases are analogous in that their detritus was derived from similarly-located orogens during two different supercontinent cycles. Mesoproterozoic deposition across the Kaapvaal craton may be part of a more global signal of burial and erosion during Rodinia assembly and breakup. Our dataset illustrates the power of THe and ZHe thermochronology for detecting missing Proterozoic depositional events in the rock record. |
Estimated alpha dose versus Tc for our results and published THe diffusion data document Tc values of ∼150–210 °C at alpha doses <50 × 10^16 α/g, with a sharp decrease in He retentivity above this threshold. This damage-diffusivity pattern is similar to zircon, but the damage level at which the reduction in retentivity occurs appears to be lower for titanite. Because titanite typically has substantially lower eU than zircon, for the same protracted thermal history THe dates are likely to record higher temperature portions of the history than zircon (U–Th)/He dates. The results demonstrate that THe dating can access a much larger temperature range and potentially be used to decipher more detailed time–temperature paths than previously thought. |
Titanite is a common U-Th bearing accessory mineral that previous study suggested has a (U-Th)/He closure temperature (Tc) of 210–175 °C. Although it has been recognized that radiation damage influences the He retentivity of apatite and zircon, this effect has not been documented in titanite. We acquired 51 THe analyses from 11 Archean basement samples across the Kaapvaal craton, South Africa. The THe dates range from ∼20 to 1200 Ma and are negatively correlated with their ∼10–970 ppm span of effective uranium concentration (eU). Radiation damage effects in zircon often manifest as a negative correlation between date and eU, which suggests increased damage leads to a decrease in He retentivity, resulting in younger He dates. Raman spectra acquired for 4 of these samples display peak broadening with increasing eU, an independent assessment of increased damage associated with eU. 4He diffusion experiments on the same 4 samples suggest Tc values that vary by ∼175 °C and is also negatively correlated with eU. |
There is great potential to refine our knowledge of well studied phases such as apatite, zircon and titanite and better develop other U-Th bearing phases for (U-Th)/He work. Doing so would make additional lithologies amenable to thermochronologic analysis, expand the temperature range accessible by the (U-Th)/He technique, and enable interpretation of more detailed time-temperature histories. Previous work has documented the strong effect of radiation damage on the He retentivity of apatite, zircon and titanite. The differences in He diffusion kinetics between and within different He thermochronometers are amplified by protracted thermal settings, which can lead to spans of He dates correlated with effective U concentration (eU) that allow for better thermal history interpretation.
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We are conducting a multi-thermochronometer (baddeleyite, rutile, titanite, zircon, and apatite) study of the 2.06 Ga Phalaborwa carbonatite complex, associated syenite plugs, and nearby Archean basement in the Kaapvaal Craton, South Africa. Our goals are to better constrain the temperature sensitivities of several non-traditional He thermo-chronometers, compare and contrast the apatite, titnaite and zircon systems, and unravel the thermal evolution of the eastern Kaapvaal Craton. Baddeleyite dates record Phalaborwa emplacement at ~2060 Ma. Rutile and titanite dates overlap between 1200-800 Ma. Phalaborwa zircon dates are younger than THe dates (as predicted by an earlier study) and older than Archean zircon dates with greater eU. Apatite dates are reproducible with a mean of 107±7 Ma.
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