Strengths and limitations of in situ U–Pb titanite petrochronology in polymetamorphic rocks: An example from western Maine, USA

Abstract

Titanite is a potentially powerful U–Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, which may lead to inaccurate and geologically ambiguous U–Pb dates if a proper correction is not or cannot be applied. Here, we present laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS)‐derived titanite U–Pb dates and trace element concentrations for two banded calcsilicate gneisses from south‐central Maine, USA (SSP18‐1A and SSP18‐1B). Single spot common Pb‐corrected dates range from 400 to 280 Ma with ±12–20 Ma propagated 2SE. Titanite grains in sample SSP18‐1B exhibit regular core‐to‐rim variations in texture, composition, and date. We identify four titanite populations: (1) 397 ± 5 Ma (95% CL) low Y + HREE cores and mottled grains, (2) 370 ± 7 Ma high Y + REE mantles and cores, (3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and (4) 295 ± 6 Ma LREE‐depleted rims. We interpret the increase in titanite Y + HREE between ca. 397 and ca. 370 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr‐in‐titanite temperatures (803 ± 36°C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 370 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (>80 vol%), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite dates the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well‐defined date–composition–texture relationships observed for titanite from SSP18‐1B, titanite grains from sample SSP18‐1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed dates spanning 380–330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U–Pb data along with chemical and textural data to screen for inherited radiogenic Pb.

Titanite Petrochronology Records Secular Temperature and Fluid Evolution During Ductile Deformation: An Example From Late Cretaceous Shear Zones in the Eastern Transverse Ranges

“…Mineral abbreviations are the same as Figure 3. textures have been observed in numerous other titanite grains interpreted to have recrystallized via dissolutionreprecipitation (Garber et al, 2017;Moser et al, 2022;Walters et al, 2022). The dates from the lobate-cuspate rims are identical (within uncertainty) to the timing of deformation recorded by the synkinematic titanite aggregates (Figure 5).…”

“…Thermally mediated volume diffusion of radiogenic Pb in titanite is negligible at temperatures <750°C (Holder, Hacker, Seward, & Kylander, 2019; Kohn, 2017; Stearns et al., 2016) and there are few clear examples of Pb volume diffusion in natural titanite data sets (Bonamici et al., 2015; Garber et al., 2017). Titanite further recrystallizes via dissolution–reprecipitation recrystallization, wherein a fluid reacts with a crystal, replaces the original grain with the same or similar phase, alters the trace‐element composition, and variably resets the U‐Pb dates (Holder & Hacker, 2019; Moser et al., 2022; Walters et al., 2022). The development of crystal‐plastic deformation microstructures can also affect U‐Pb dates in titanite and other minerals, with dislocations and/or subgrain boundaries variably trapping or serving as fast diffusion pathways for Pb and other trace‐elements, or forming efficient conduits for fluids to recrystallize grain interiors (Bonamici et al., 2015; Fougerouse et al., 2021; Gordon et al., 2021; Kavanagh‐Lepage et al., 2023; Moser et al., 2022; Odlum et al., 2022; Piazolo et al., 2016; Reddy et al., 2006).…”

“…Each of these processes produces diagnostic intragrain spatial patterns among U‐Pb dates, trace‐element concentrations, and microstructures that can be used to identify the process responsible for the U‐Pb date and trace‐element distributions (Bonamici et al., 2015; Cherniak, 1993; Garber et al., 2017; Holder, Hacker, Seward, Kylander‐Clark, & Holder, 2019; Holder & Hacker, 2019; Kohn, 2017; Moser et al., 2022; Spencer et al., 2013; Stearns et al., 2016; Walters et al., 2022; Walters & Kohn, 2017). We combine backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD) data, laser‐ablation split‐stream ICP‐MS (LASS) analyses of U‐Pb dates and trace‐element concentrations together with electron probe microanalysis (EPMA) measurements of F contents to relate titanite U‐Pb dates to zoning, microstructures, and trace‐element compositions to differentiate among the above processes.…”

“…Each of these processes produces diagnostic intragrain spatial patterns among U-Pb dates, trace-element concentrations, and microstructures that can be used to identify the process responsible for the U-Pb date and trace-element distributions (Bonamici et al, 2015;Cherniak, 1993;Garber et al, 2017;Holder, Hacker, Seward, Kylander-Clark, & Holder, 2019;Kohn, 2017;Moser et al, 2022;Spencer et al, 2013;Stearns et al, 2016;Walters et al, 2022;Walters & Kohn, 2017). We combine backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD) data, laser-ablation split-stream ICP-MS (LASS) analyses of Figure 1.…”

MinPlot: A mineral formula recalculation and plotting program for electron probe microanalysis

“…The values of 0.035 for ilmenite-bearing rocks (XHem < 0.10) and 0.070 for hematite-ilmenite rocks (XHem > Walters et al, 2019;, and phengite (green diamonds) and paragonite (purple squares) from an ultrahigh pressure metapelite from the Tian Shan, western China (Xu et al, 2022). Biotite analyses, plotted in C and D, were conducted on grains in a garnet mica schist from western Maine, USA (red circles; Walters et al, 2022), granulite facies para-and orthogneiss (orange diamonds; Spreitzer et al, 2021), and calc silicate rocks from central Nepal (yellow triangles; Walters and Kohn, 2017). 0.10) following Holdaway et al (1991) are recommended.…”