A preliminary investigation into the feasibility of laser ablation U–Pb isotope ratio measurement via all-faraday cup detection with 10¹¹- and 10¹³-Ω amplifiers on the Neoma multicollector-inductively coupled plasma-mass spectrometer

Rationale: Signal detection for uranium–lead (U–Pb) dating of zircon is typically performed via ion counters. Here, we develop a preliminary understanding of the strengths and limitations of faraday-cup-based detection. Methods: A suite of zircon reference materials and the NIST-610 glass were sampled using laser ablation followed by U–Pb isotope ratio measurement on a Neoma multicollector-inductively coupled plasma-mass spectrometer. Results: We were able to produce geologically accurate ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U, and ²⁰⁷Pb/²³⁵U ratios for the NIST-610 glass and the zircon standards, with ages ranging from ~2.5 Ga to ~337 Ma (TanBrown A, Oracle, 91550, Mud Tank, Temora, and Plešovice). Two of the younger zircon standards examined (94-35, ~55.6 Ma, and Fish Canyon, 28.6 Ma) yielded accurate ²⁰⁶Pb/²³⁸U but not ²⁰⁷Pb/²³⁵U or ²⁰⁷Pb/²⁰⁶Pb ratios, whereas the youngest zircon standard (Penglai, ~4.4 Ma) failed for all three ratios of interest. The accuracy and precision of the all-faraday method are directly tied to signal intensity, with reliable data capable of being produced even when both isotopes in a ratio have signals below ~0.001 V (equivalent to ~62 500 cps on an ion counter). Conclusion: The all-faraday cup multicollection method provides sufficient sensitivity to obtain geologically meaningful U–Pb data, with possible advantages being that laser pit depth-dependent changes in the observed interelemental fractionation behavior may be easier to correct using a static collector configuration compared to when the ion beam is swept across a single detector while also removing the need for an interdetector-type calibration. Further work is needed to refine the all-faraday cup method (e.g., application of background subtraction and common Pb corrections, outlier removal, and interelement as well as down-hole fractionation corrections), but our initial results demonstrate that the faraday detector method has sufficient sensitivity to warrant further study.