Influence of deformation mechanisms on deformation-induced resetting of radiogenic isotope systems (U-Pb, Rb-Sr, Sm-Nd) and homogenization on the grain-scale (TP15)
Final Report Abstract
This project highlights the effects of deformation, fluids, and mineral reactions on monazite and titanite that are two widely used geo-chronometers. One particular aim was to develop a tool for dating fluid mediated processes in the Earth’s crust including meta-somatism along subduction interfaces and ductile deformation processes in quartz-feldspatic rocks. We combined in-situ U-Th-Pb, Rb-Sr isotope and mineral chemical analyses (Laser ablation split stream ICP MS; TIMS of microdrilled samples; FEG-EMP) with microfabric studies (SEM-EBSD). Diagnostic microfabrics allow for identifying dislocation creep and fluid-assisted dissolution precipitation creep (DPC), which are the most important ductile strain rate determining deformation mechanisms that control recycling and reprocessing of the Earth’s lithosphere on a larger scale. We focused on monazite of deformed quartz-feldspatic rocks. Our results demonstrate that the U-Th-Pb system of monazite behaves differently depending on deformation mechanism (DPC or dislocation creep) activated in the hosting metamorphic rocks. (i) Spatially decoupled dissolution and precipitation of monazite and feldspar occurs in quartz-feldspatic rocks deformed by DPC. Characteristic chemical changes of monazite, in particular its Eu anomaly and REE pattern, link the ages of newly precipitated monazite to feldspar that formed simultaneously by dissolution-precipitation reactions. We demonstrate that in rocks showing feldspar deformation by DPC, the U-Th-Pb age of different intragrain domains of monazite and monazite grains that formed in distinct microstructural positions have the potential to date deformation episodes reflected by feldspar microfabrics. This bears the potential to date duration and rate of deformation, as monazite, as well as feldspar, repeatedly dissolve and precipitate during ongoing deformation. (ii) By contrast, partially rehydrated (“dry”) high-grade quartz-feldspatic rocks that responded to deformation by dislocation creep of feldspar, only show localized dissolution of older, and re-precipitation of new monazite in relictic microlithons and new micro-shear zones. Hence, in such rocks the age record of pre-deformation metamorphic processes may be preserved to variable extent. In inhomogeneously deformed HP granulites, monazite of the same age may have different chemical compositions, i.e. inherit the local chemical context, due to the restricted length scale of fluid transport/fluid-mineral interaction (local subsystems) during deformation. In a second study, we focus on a large pre-UHP titanite crystal from a marble in the Dabie UHP unit. Titanite with crustal 87Sr/86Sr signatures (0.707) survived subduction to mantle depth and was replaced only at its margins during subduction and exhumation. This fluid-mediated replacement is characterized by very low 87Sr/86Sr signatures (as low as 0.704), both in the titanite reaction rim and the matrix, that are unusual for crustal carbonates. Our results show that dissolution-precipitation reactions efficiently support material transport and isotope chemical exchange between titanite and its meta-carbonate host rock. The Sr isotopic fingerprints of titanite core and rim and matrix allow to linking episodes of fluid-mineral interaction with the pro-to retrograde reaction history. Combining the results from our microfabric and isotope-chemical studies gives insight in the mechanisms of fluid-rock interaction, mineral replacement and mass transport on the grain scale. These processes of fluid-rock interaction eventually are responsible for metasomatism of large volumes of low 87Sr/86Sr UHP marbles.
Publications
- (2012): Interplay of fluid-rock interaction, deformation mechanisms and the U-Th-Pb system in monazite: dating crustal deformation episodes. 15th Symposium on Tectonics, Structural Geology and Geology of Crystalline Rocks (Potsdam, Germany)
Wawrzenitz, N., Krohe, A., Kylander-Clark, A.R.C., Romer, R.L., Rhede, D., Grasemann, B.
- (2012): The Potential of Titanite for Tracing and Dating Dissolution Precipitation Processes: Implications for Geochronology, European Mineralogical Conference EMC 2012 (Frankfurt/Main, Germany)
Wawrzenitz, N., Romer, R. L., Grasemann, B., Rhede, D.
- Dating rock deformation with monazite: the impact of dissolution precipitation creep. Lithos, Vol. 134–135. 2012, pp. 52-74.
Wawrzenitz [Hoymann], N., Krohe, A., Rhede, D., Romer, R. L.
(See online at https://doi.org/10.1016/j.lithos.2011.11.025) - Reaction zone between pre-UHP titanite and host rock: insights into fluid-rock interaction and deformation mechanisms during exhumation of deeply subducted continental crust (Dabie Shan UHP unit, China), (Geophysical Research Abstracts, Vol. 14, EGU 2012-11743), 9th General Assembly European Geosciences Union (Vienna, Austria)
Wawrzenitz, N., Romer, R. L., Grasemann, B., Morales, L. F. G.
- Two stages of fluid-rock interaction in UHP marbles (Dabie Shan, China): grain-scale processes and map-scale metasomatism, American Geophysical Union, Fall Meeting (San Francisco, USA), abstract #V43A-2817.
Wawrzenitz [Hoymann], N., Romer, R. L., Grasemann, B., Rhede, D.
- Dating fluid-mineral interaction in exhumed (U)HP rocks (Dabie Shan, China and Rhodope, Greece): processes and implications. Conference Abstract, 10th International Eclogite Conference, 2013,(Courmayeur, Italy).
Wawrzenitz [Hoymann], Nicole; Romer, Rolf L.; Grasemann, Bernhard, Kylander-Clark, Andrew R.C.; Krohe, Alexander; Mposkos, Evripidis
- LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems.
Lithos, Vol. 232. 2015, pp. 266-285.
Wawrzenitz [Hoymann], N., Krohe, A., Baziotis, I., Mposkos, E., Kylander-Clark, A. R., Romer, R. L.
(See online at https://doi.org/10.1016/j.lithos.2015.06.029) - Deformed monazite yields high-temperature tectonic ages: COMMENT.
Geology, Vol. 44. 2015, Issue 1: e377.
Wawrzenitz [Hoymann], N., Krohe, A.
(See online at https://dx.doi.org/10.1130/G37394C.1)
