The role of sediment melting in Earth's mantle remains controversial, as direct observation of melt generation in the mantle is not possible. Geochemical fingerprints provide indirect evidence for subduction-delivery of sediment to the mantle, however sediment abundance in mantle-derived melt is generally low (0-2%), and difficult to detect. Here we 1 provide evidence for bulk melting of subducted sediment in the mantle through isotopic analysis of granite sampled from an exhumed mantle section. Peraluminous granite dikes that intrude peridotite in the Oman-United Arab Emirates ophiolite have U-Pb ages of 99.8±3.3 Ma that predate obduction at ca. 85 to 90 Ma. The dikes have unusually high oxygen isotope (δ 18 O) values for whole rock (14-23‰) and quartz (20-22‰), and yield the highest δ 18 O zircon values known (14-28‰; values relative to Vienna standard mean ocean water). The extremely high oxygen isotope ratios uniquely identify the melt source as high δ 18 O marine sediment (pelitic and/or siliciceous mud), as no other source could produce granite with such anomalously high δ 18 O. Formation of high δ 18 O sediment-derived (S-type) granite within peridotite requires delivery of sediment to the mantle by subduction, where it melted and intruded the overlying mantle wedge. The granite suite described here contains the most evolved oxygen isotope ratios reported for igneous rocks, yet intruded mantle peridotite below the Mohorovičić seismic discontinuity, the most primitive oxygen isotope reservoir in the silicate Earth. Identifying the presence and quantifying the extent of sediment melting within the mantle has important implications for understanding subduction recycling of crust and mantle heterogeneity over time.

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1-x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ 33 S, δ 34 S, and δ 36 S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ 34 S and Δ 33 S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ 33 S values +0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the 2 magmatic assemblage. The three sulfide phases show slightly variable δ 34 S values (δ 34 Schalcopyrite = 2.9 ± 0.3‰, δ 34 Spentlandite = 3.1 ± 0.2‰, and δ 34 Spyrrhotite = 3.9 ± 0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic-and alteration-related sulfides, which is not resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ 33 S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales. HIGHLIGHTS  Characterisation of four sulfide standards for multiple sulfur isotope analysis: pyrite, chalcopyrite, pyrrhotite, and pentlandite for distribution  Analysis of orientation effect in pentlandite  Natural sulfur isotope fractionation between pentlandite and pyrrhotite  Case study multiple sulfur isotope analysis of three sulfide phases within world-class Long-Victor komatiite-hosted nickel-sulfide deposit KEYWORDS Multiple sulfur isotopes, SIMS, in situ, sulfide minerals, ore genesis 1.

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