Rapid, precise and accurate Os isotope ratio measurements of nanogram to sub-nanogram amounts using multiple Faraday collectors and amplifiers equipped with 10~(12) Ω resistors by N-TIMS

摘要

We present a new approach to obtain rapid and precise (0.1 % or better) Os isotopic compositions for small (nanogram to sub-nanogram) amounts of Os extracted from geological samples using static collection with Faraday cups and amplifiers equipped with 10~(12) Ω resistors, by negative thermal ionization mass spectrometry (N-TIMS/Triton Plus). The results show that the measurement repeatability of Os isotopic ratios (~(190)Os/~(188)Os, ~(187)Os/~(188)Os and ~(186)Os/~(188)Os) changes as a function of signal intensity that varied from 0.005 to 0.05 V for ~(192)OsO_3~? (here and throughout the text, given intensities are equivalent signals that would be obtained using a 1011 Ω standard amplifier; 0.001 V (1 mV) is equivalent to ~62500 cps). At ~(192)OsO_3~? ion beams greater than 0.02 V with 50 ratios measured over a 10 min acquisition time, the repeatability on ~(187)Os/~(188)Os, as well as ~(186)Os/~(188)Os and ~(190)Os/~(188)Os, is better than that obtained by the conventional peak-hopping electron multiplier (SEM) at ~(192)OsO_3~? = ~200,000 cps with 500-100 ratios measured over a 30-60 min acquisition time. At ~(192)OsO_3~? ion beams of ~0.04 V or above, the ~(187)Os/~(188)Os and ~(186)Os/~(188)Os data for loads of 1 and 0.1 ng Os referencematerials can be measured with a repeatability of b0.1% (2σ) and deviate by b0.1% fromthe accepted values. Similar results can be achieved for Os load sizes in geological samples as lowas ~ 0.025 ng. In addition, duplicate Faraday measurements of six spiked peridotitic samples (total Os loaded = 0.3-1.2 ng) yield results within uncertainty of data obtained by peak-hopping SEM, including both ~(187)Os/~(188)Os and Os concentration. At much higher ~(192)OsO_3~? ion beams (~0.5-1 V) with 300 ratios taken (total acquisition time circa one hour), the Faraday measurements of 1 to 3 ng Os loads of reference material solutions can generate high-precision ~(186)Os/~(188)Os data with a repeatability of 30-50 ppmand amean intermediate precision of 10-30 ppm. Collectively, our tests demonstrate that the use of Faraday cups equipped with 10~(12) Ω amplifiers in a static collection mode by N-TIMS can rapidly produce precise and accurate (within 0.1 %) Os isotopic data (both ~(187)Os/~(188)Os and ~(186)Os/~(188)Os) for small amounts of Os (as low as 0.025 ng) extracted from geological samples. The optimum~(192)OsO_3~? signal is 0.04 V or above to achieve the best results.While the smaller signal intensities used in concert with 10~(12)Ω amplifiers necessarily yield lower precision measurements than the best achievable with larger signals on 10~(11)Ω amplifiers, the repeatability we have achieved on reference materials can expand the application of the ~(190)Pt-~(186)Os decay systemto geological samples containing significantly less Os than those normallymeasured on 10~(11)Ω amplifiers, if isotopic variations in excess of 50 ppm are present.