Combined oxygen-isotope and U-Pb zoning studies of titanite: New criteria for age preservation

Abstract

Titanite is an important U-Pb chronometer for dating geologic events, but its high-temperature applicability depends upon its retention of radiogenic lead (Pb). Experimental data predict similar rates of diffusion for lead (Pb) and oxygen (O) in titanite at granulite-facies metamorphic conditions (T = 650-800°C). This study therefore investigates the utility of O-isotope zoning as an indicator for U-Pb zoning in natural titanite samples from the Carthage-Colton Mylonite Zone of the Adirondack Mountains, New York. Based on previous field, textural, and microanalytical work, there are four generations (types) of titanite in the study area, at least two of which preserve diffusion-related δ 18 O zoning. U-Th-Pb was analyzed by SIMS along traverses across three grains of type-2 titanite, which show well-developed diffusional δ 18 O zoning, and one representative grain from each of the other titanite generations. Type-2 and type-4 titanites show broadly core-to-rim decreasing 206 Pb/ 238 U zoning, consistent with Pb diffusion at higher temperatures, and uniform or even slightly increasing 206 Pb/ 238 U near grain rims, indicating subsequent recrystallization and/or new growth below the Pb blocking temperature. Type-2 and type-4 grain cores preserve ca. 1160 Ma ages that correlate with the anorthositemangerite-charnockite-granite magmatic phase of the Grenville orogeny, whereas grain rims give ca. 1050 Ma 206 Pb/ 238 U ages that coincide with the culminating Ottawan phase. The type-3 titanite grain was sampled from a vein and yields 206 Pb/ 238 U dates older than the syenite into which the vein was emplaced; accordingly, its 206 Pb/ 238 U dates are interpreted as indicating excess uncorrected common Pb. Type-2 grains with recrystallized or shear-eroded margins show truncated or reversed 206 Pb/ 238 U zoning but retain symmetrically decreasing δ 18 O zoning, consistent with grain margin modification following arrest of Pb diffusion but before arrest of O diffusion. It is concluded that O diffusion was slightly faster than Pb diffusion in Adirondack titanites at the conditions of (local) peak Ottawan metamorphism, making δ 18 O zoning a useful discriminator of closed-system age domains that did not suffer Pb loss. In addition, the small offset in the O and Pb partial retention zones constrains the timing and temperature of oblique-slip deformation along the Carthage-Colton Mylonite Zone: the details of porphyroclast microstructure and zoning data show that the oblique-slip shear zones were active at ca. 1050 Ma, with deformation initiating near the peak of Ottawan metamorphism at~700°C and continuing through the O blocking temperature at~550°C.Published by Elsevier B.V.

Natural Titanite Reference Materials forIn SituU‐Pb and Sm‐Nd Isotopic Measurements byLA‐(MC)‐ICP‐MS

“…In situ U‐Pb analytical techniques such as secondary ion mass spectrometry (SIMS; Heaman , Bonamici et al . ) and laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS; Willigers et al . , Simonetti et al .…”

Oxygen isotope thermometry using quartz inclusions in garnet

“…This contrast in Δ 18 O(MQ–PQI) values supports the conclusion of Bonamici et al . (, , ) that locations where periods of rapid cooling occurred may be spatially limited.…”

“…This contrasts with cooling rate estimates for the Carthage–Colton shear zone (~25 km east of sample 44; Fig. ) of 30–70 °C Ma −1 from ~675 to 500 °C based on intragrain zonation of δ 18 O in titanite (Bonamici et al ., , , ). Contrasting cooling rates in the Lowlands and the Carthage–Colton shear zone can be explained by independent cooling histories, which could have resulted from northwest displacement of the Lowlands during exhumation of the Highlands along the Carthage–Colton shear zone (Mezger et al ., ; Streepey et al ., ; Johnson et al ., ; Selleck et al ., ; McLelland et al ., ).…”

“…Bonamici et al . (, , ) proposed a segmented cooling history where rapid changes on a time‐scale of <50 Ma are followed by periods of slow cooling. A multi‐stage cooling model is also proposed for the southern Highlands based on modelling of diffusion profiles in garnet and variably sized biotite inclusions in garnet (Storm & Spear, ), which yields a slow–fast–slow cooling history where slow periods cooling at ~8 °C Ma −1 and a fast period cooling at rates greater than 200 °C Ma −1 (Storm & Spear, ).…”

Polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh‐pressure metagranite in the Sulu orogen

“…Li et al ., ) and orogenic evolution (e.g. Essex & Gromet, ; Rubatto & Hermann, ; Bonamici et al ., , , ; Kohn & Corrie, ; Warren et al ., ; Spencer et al ., ).…”