Zircon as a provenance tracer: Coupling Raman spectroscopy and U Pb geochronology in source-to-sink studies

Abstract

U-Pb zircon geochronology is one of the most widely used techniques in sedimentary provenance analysis. Unfortunately, the ability of this method to identify sediment sources is often degraded by sediment recycling and mixing of detritus from different source rocks sharing similar age signatures. These processes create non-unique zircon U-Pb age signatures and thereby obscure the provenance signal. We here address this problem by combining detrital zircon U-Pb geochronology with Raman spectroscopy. The position and width of the Raman signal in zircon scales with its degree of metamictization, which in turn is sensitive to temperature. Thus, combined U-Pb + Raman datasets encode information about the crystallisation history of detrital zircons as well as their thermal history. Using three borehole samples from Mozambique as part of a source-to-sink study of interest for hydrocarbon exploration, we show that zircon populations with similar U-Pb age distributions can exhibit different Raman signatures. The joint U-Pb + Raman analysis allowed us to identify three different annealing trends, which were linked to specific thermal events. Thus we were able to differentiate a dominant Pan-African U-Pb age peak into several sub-populations and highlight the major effect of Karoo tectono-magmatic events. In our case study, we used Raman *Revised manuscript with no changes marked Click here to view linked References

The closure temperature(s) of zircon Raman dating

“…During stage I annealing, the unit cell shrinks anisotropically, reducing a more than c, causing a further increase of c/a and lowering of the O-Si-O angle. The anisotropic shrinkage is thought to be due to the preferential diffusion of point defects in the basal plane of zircon during recovery (Ríos et al, 2000;Colombo and Chrosch, 1998a). We interpret the further decrease of the O-Si-O angle to cause the decrease of ω 2 during stage I.…”

“…Stage II is interpreted as representing a state of the zircon lattice that is independent of the damage accumulation history. We assume, based on the interpretation of the successive annealing stages of Colombo and Chrosch (1998a), Ríos et al (2000), and Geisler et al (2001), that stage II describes zircons in which the lattice has lost most of its point defects and is predominantly strained by the amorphous domains caused by α recoils. In this case, the position of a zircon along the stage II trend represents the remaining amorphous fraction.…”

“…Härtel et al: The closure temperature(s) of zircon Raman dating more intense. This loss of damage with temperature and time is a problem for the interpretation of zircon Raman dates as crystallization ages (Nasdala et al, 2001(Nasdala et al, , 2002 but unlocks the potential for determining cooling ages and analyzing the thermal histories of natural zircon samples (Resentini et al, 2020). Annealing of radiation damage also affects He diffusion and is thus a process that needs to be taken into account in the interpretation of (U-Th) / He dates of zircon (Ginster et al, 2019;Anderson et al, 2020).…”

“…A sharp break separates the steep stage (I) and the flat stage (II), but a more gradual transition occurs between stage II and a stage III assumed by Geisler (2002). Stage I is dominated by elimination of point defects in the basal plane of the crystal structure by tilting and twisting of the ZrO 8 polyhedra (Ríos et al, 2000); stage II is ascribed to crystallization of amorphous domains (Colombo and Chrosch, 1998a;Capitani et al, 2000;Geisler et al, 2001;Geisler, 2002;Ginster et al, 2019); stage III is related to diffusion of point defects that form due to extension in the c direction. These point defects are assumed to be more stable due to the strong linkage of the SiO 4 and ZrO 8 polyhedra in the c direction, and their elimination includes a re-alignment of tilted SiO 4 tetrahedra (Ríos et al, 2000;Geisler, 2002).…”

The closure temperature(s) of zircon Raman dating

“…The Raman bands shift to higher wavenumbers, towards the band positions of well-ordered zircon, and become narrower and more intense. This loss of damage with temperature and time is a problem for the interpretation of zircon Raman ages as crystallization ages (Nasdala et al, 2001(Nasdala et al, , 2002 but unlocks the potential for determining cooling ages and analyzing the thermal histories of natural zircon samples (Resentini et al, 2020). Annealing of radiation damage also affects Hediffusion and is thus a process that needs to be taken into account in the interpretation of (U-Th)/He data of zircon (Ginster et al, 2019;Anderson et al, 2020).…”