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Complete sample digestion is a prerequisite for achieving reproducible and accurate analytical results for geological samples. Open-vessel acid digestions successfully dissolve mafic samples, but this method cannot achieve complete dissolution of felsic samples, because of the presence of refractory minerals such as zircon. In this study, an efficient and simplified digestion technique using the solid compound NH(4)HF(2) in a screw-top vial has been developed for multielement analysis of different types of rock samples. NH(4)HF(2) has a higher boiling point (239.5 °C) than conventional acids such as HF, HNO(3) and HCl, which allows for an elevated digestion temperature in open vessels, enabling the decomposition of refractory phases. Similar to HF, HNO(3) and HCl, ultrapure NH(4)HF(2) can be produced using a conventional PFA sub-boiling (heating and cooling) purification system. A digestion time of 2-3 h for 200 mg NH(4)HF(2) in a Savillex Teflon vial at 230 °C is sufficient to digest 50 mg of the felsic rock GSP-2, which is ~6 times faster than using conventional closed-vessel acid digestion at 190 °C (high-pressure PTFE digestion bomb). The price of a Savillex Teflon vial is far less than the price of a high-pressure PTFE digestion bomb (consisting of a PTFE inner vessel and an outer stainless steel pressure jacket). Moreover, the NH(4)HF(2)-open-vessel acid digestion is not hampered by the formation of insoluble fluorides. We have successfully applied the NH(4)HF(2)-open-vessel acid digestion to the digestion of a series of international geological reference materials, including mafic to felsic igneous rocks and shales. This method provides an effective, simple, economical, and comparatively safe dissolution method that combines the advantages of both the open- and closed-vessel digestion methods.

Molybdenum isotopes are fractionated by Earth‐surface processes and may provide a tracer for the recycling of crustal material into the mantle. Here, we examined the Mo isotope composition of arc lavas from Martinique in the Lesser Antilles arc, along with Cretaceous and Cenozoic Deep Sea Drilling Project sediments representing potential sedimentary inputs into the subduction zone. Mo stable isotope composition (defined as δ98Mo in ‰ deviation from the NIST 3134 standard) in lavas older than ∼7 million years (Ma) exhibits a narrow range similar to and slightly higher than MORB, whereas those younger than ∼7 Ma show a much greater range and extend to unusually low δ98Mo values. Sediments from DSDP Leg 78A, Site 543 have uniformly low δ98Mo values whereas Leg 14, Site 144 contains both sediments with isotopically light Mo and Mo‐enriched black shales with isotopically heavy Mo. When coupled with published radiogenic isotope data, Mo isotope systematics of the lavas can be explained through binary mixing between a MORB‐like end‐member and different sedimentary compositions identified in the DSDP cores. The lavas older than ∼7 Ma were influenced by incorporation of isotopically heavy black shales into the mantle wedge. The younger lavas are the product of mixing isotopically light sedimentary material into the mantle wedge. The change in Mo isotope composition of the lavas at ∼7 Ma is interpreted to reflect the removal of the Cretaceous black shale component due to the arrival of younger ocean crust where the age‐equivalent Cretaceous sediments were deposited in shallower oxic waters. Isotopic fractionation of Mo during its removal from the slab is not required to explain the observed systematics in this system.