Zircon, titanite, and apatite (U‐Th)/He ages and age‐eU correlations from the Fennoscandian Shield, southern Sweden

Abstract

Craton cores far from plate boundaries have traditionally been viewed as stable features that experience minimal vertical motion over 100–1000 Ma time scales. Here we show that the Fennoscandian Shield in southeastern Sweden experienced several episodes of burial and exhumation from ~1800 Ma to the present. Apatite, titanite, and zircon (U‐Th)/He ages from surface samples and drill cores constrain the long‐term, low‐temperature history of the Laxemar region. Single grain titanite and zircon (U‐Th)/He ages are negatively correlated (104–838 Ma for zircon and 160–945 Ma for titanite) with effective uranium (eU = U + 0.235 × Th), a measurement proportional to radiation damage. Apatite ages are 102–258 Ma and are positively correlated with eU. These correlations are interpreted with damage‐diffusivity models, and the modeled zircon He age‐eU correlations constrain multiple episodes of heating and cooling from 1800 Ma to the present, which we interpret in the context of foreland basin systems related to the Neoproterozoic Sveconorwegian and Paleozoic Caledonian orogens. Inverse time‐temperature models constrain an average burial temperature of ~217°C during the Sveconorwegian, achieved between 944 Ma and 851 Ma, and ~154°C during the Caledonian, achieved between 366 Ma and 224 Ma. Subsequent cooling to near‐surface temperatures in both cases could be related to long‐term exhumation caused by either postorogenic collapse or mantle dynamics related to the final assembly of Rodinia and Pangaea. Our titanite He age‐eU correlations cannot currently be interpreted in the same fashion; however, this study represents one of the first examples of a damage‐diffusivity relationship in this system, which deserves further research attention.

Thermochronological Evidence of Early Orogenesis, Eastern Pyrenees, France

“…Similar age variability to that in our ZHe data set is reported in other data sets from the Pyrenees and from other similar orogenic settings (e.g., Guenthner et al, , , ; Vacherat et al, , ). It has been demonstrated that this results from the inability of a single set of kinematic parameters for He diffusion to describe the natural thermochronometric behavior of zircon (Cherniak et al, ; Guenthner et al, ; Ketcham et al, ; Reiners et al, , ).…”

“…These observations suggest that heterogeneous distribution of U and Th in our zircons, albeit likely contributing to some of the observed age irreproducibility in each sample, cannot alone explain the large spread in our ZHe ages. The remainder of this results section, and more generally this study, will therefore be mainly focused on the third main source of age variation in our ZHe data set: radiation damage (Guenthner et al, , 2014(Guenthner et al, , 2015(Guenthner et al, , 2017.…”

“…Recent studies (e.g., Anderson et al, 2017;Guenthner et al, 2014Guenthner et al, , 2015Guenthner et al, , 2017Johnson et al, 2017;Powell et al, 2016;Vacherat et al, 2014Vacherat et al, , 2016 have shown that numerical modeling, incorporating apatite and zircon radiation-damage and annealing models for both the fission track and (U-Th)/He systems, is extremely valuable for understanding and interpreting data. It allows exploration of the response of different thermochronometers to various thermal histories and ultimately provides more quantitative constraints on the thermal history of a sample.…”

Innovations in (U–Th)/He, Fission Track, and Trapped Charge Thermochronometry with Applications to Earthquakes, Weathering, Surface‐Mantle Connections, and the Growth and Decay of Mountains

“…However, mean (U–Th)/He dates are commonly overdispersed, with standard deviations of the means exceeding analytical precision. Overdispersion is due to various factors that control He diffusivity and thus influence T c and dates, including diffusion‐domain length scale and cooling rate (Dodson, ; Farley, ; Jensen et al, ; Reiners & Farley, ), radiation damage accumulation (Baughman et al, ; Flowers et al, ; Gautheron et al, ; Guenthner et al, , ; Shuster & Farley, ), grain chemistry (Gautheron, Barbarand, et al, ), and grain breakage (Brown et al, ). Additional variables that cause He data dispersion include implantation of parentless He by adjacent U–Th‐rich phases (Gautheron et al, ; Murray et al, ; Spiegel et al, ), parent isotope zonation (Ault & Flowers, ; Farley et al, ; Gautheron, Barbarand, et al, ; Guenthner et al, ; Hourigan et al, ; Meesters & Dunai, ; Orme et al, ), microvoids or crystal defects (McDannell, Zeitler, Janes, et al, ; Zeitler et al, ), and inaccuracies in the alpha ejection correction factor owing to grain geometry or zoning (Farley et al, , ; Glotzbach et al, ).…”

“…High eU zircon He dates can be younger than apatite He dates reflecting low T c . Titanite He dates similarly yield an inverse date‐eU relationship and older dates owing to lower eU values and higher T c (Baughman et al, ; Guenthner et al, ).…”