A trio of laser ablation in concert with two ICP‐MSs: Simultaneous, pulse‐by‐pulse determination of U‐Pb discordant ages and a single spot Hf isotope ratio analysis in complex zircons from petrographic thin sections

Abstract

We have developed a technique for the simultaneous in situ determination of U‐Pb ages and Hf isotope ratios from a single spot in complex,discordantzircons by combining both a single‐collector and a multicollector sector field inductively coupled plasma–mass spectrometry (ICP‐MS) with a 193 nm excimer laser ablation system. With a suite of zircon standards of various ages, we first show that U‐Pb ages can be determined accurately to within 0.3–2.5% (2σ) compared to the nominal value, while the internal errors are better than 0.4–0.7%; hafnium isotope ratios are accurate, relative to solution analyses, within one epsilon unit, and internal errors are typically <0.008%. We then apply the technique to complex, discordant zircons with variable 206Pb/238U and 207Pb/235U ratios, commonly discarded previously as “un‐reducible data,” to construct a Discordia in U‐Pb Concordia plot, using every scan, every laser pulse as individual data points from a single laser ablation spot (typically > 200–250 data points). We show that the upper and lower intercept ages from the Discordia, augmented by high precision Hf isotope data obtained on the same spot, reveal invaluable information that permit unique insight to geological processes not available by other means. We demonstrate that our technique is useful for provenance studies of small, complex detrital zircons in sedimentary and high‐grade metamorphic rocks, in relation to crustal growth and evolution.