[1] Abundances of major and trace elements in whole rocks and minerals in lherzolites and harzburgites from the northern Oman ophiolite are used to understand the mantle processes creating compositional variation in oceanic lithospheric mantle. Detailed mapping shows that lherzolites occur near the base of a mantle section in the northern Fizh block. Geochemical analyses identify two types of basal lherzolite. The first type (Type I lherzolite) displays porphyroclastic microstructure and occurs sporadically in the basal mylonite zone. Whole rock and clinopyroxene are highly depleted in incompatible elements such as Na, Ti, Zr, and rare earth elements (REE). The chondrite-normalized patterns of Type I lherzolites show steep slopes from heavy REE (HREE) to light REE (LREE) that are ascribed to melt extraction, up to 12-18%, from a source containing a small amount of garnet. The chondrite-normalized patterns have slight enrichment in LREE relative to the patterns expected for residues of partial melting thereby indicating reaction with a LREEenriched melt or fluid at a low melt/rock ratio. The second type (Type II lherzolite) shows mylonitic microstructure and only occurs at the contact between the mantle section and the metamorphic sole. Abundances of incompatible elements in whole rocks and clinopyroxenes are greater than those of Type I lherzolites, and clinopyroxenes in Type II lherzolites have high Na 2 O contents (>1 wt.%). To a first approximation, the high Na content of clinopyroxenes and whole rocks and the LREE-depleted, chondritenormalized whole rock REE patterns are consistent with Type II lherzolite being in equilibrium with a midocean ridge basalt (MORB)-type melt at relatively high pressure (>2 GPa). However, the flatness of chondrite-normalized patterns for middle and heavy REE are inconsistent with residual garnet peridotite. The characteristics of Type II lherzolites are better explained by a mixing process in which residual peridotite was refertilized by addition of a LREE-depleted melt. The large compositional gradient near the basal thrust in the northern Fizh block may have recorded a transient state in which the degree of partial melting was progressively decreased as a result of reducing mantle temperature and upwelling rate. This scenario is consistent with the inferred failing ridge associated with a transform zone in the western side of the northern Fizh block proposed by Nicolas et al. [2000]. In the detachment stage of the Oman ophiolite, a small amount of ascending melt may have crystallized near the basal part of mantle section thereby forming Type II lherzolites. Basal lherzolites and their spatial chemical variations in the northern Fizh block may provide a key for understanding the processes of ridge segmentation and detachment at fast spreading ridges.

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SummaryThis workshop report describes plans for scientific drilling in the Samail ophiolite in Oman in the context of past, current, and future research. Long-standing plans to study formation and evolution of the Samail crust and upper mantle, involving igneous and metamorphic processes at an oceanic spreading center, have been augmented by recent interest in ongoing, low temperature processes. These include alteration and weathering, and the associated sub-surface biosphere supported by chemical potential energy due to disequilibrium between mantle peridotite and water near the surface. This interest is motivated in part by the possibility of geological carbon capture and storage via engineered, accelerated mineral carbonation in Oman. Our International Continental Drilling Program (ICDP)proposal led to the Workshop on Scientific Drilling in the Samail Ophiolite in Palisades, New York, on 13-17 September 2012. There were seventy-seven attendees from eleven countries, including twenty early career scientists.After keynote presentations on overarching science themes, participants in working groups and plenary sessions outlined a ~U.S.$2 million drilling plan that practically addresses testable hypotheses and areas of frontier discovery in the following areas.• understanding the subsurface biosphere • characterizing the rates and mechanisms of ongoing mineral hydration and carbonation • characterizing chemical and physical processes of mass transfer across a subduction zone • evaluating well-posed hypotheses on hydrothermal circulation, cooling, and emplacement mechanisms of igneous rocks in the lower crust • investigating key problems in the dynamics of mantle flow and melt transport beneath oceanic spreading ridgesThis report places these goals in the context of complementary research via ocean drilling and ongoing studies of active processes at oceanic spreading centers, subduction zones, and peridotite-hosted hydrothermal systems. We end with an outline of the synergy between Oman drilling and the specific drilling proposed in the Integrated Ocean Drilling Program (IODP) proposal "Mohole to Mantle Project (M2M)", IODP Proposal 805-MDP. Workshop Proceedings and ResultsKeynote speakers outlined hypotheses and areas of frontier scientific exploration to be addressed via drilling, including• the nature of mantle upwelling, • the chemical and physical mechanisms of mantle melt transport, • the processes of lower crustal accretion and cooling, • the frequency and magnitude of microseismicity during weathering, • the rate and location of ongoing alteration, and • the composition, density, and spatial distribution of subsurface microbial communities.Additional keynote talks covered state-of-the-art geological logging of drillcore, geophysical logging in boreholes, and data management.Breakout groups considered overarching science themes, then designed idealized projects to address these themes, and finally considered practical constraints. We agreed to focus on studies relevant to global processes. There was a consen...

Nickel is a major element in the Earth. Due to its siderophile nature, 93 % of Ni is hosted in the core and the Ni isotope composition of the bulk silicate Earth might inform on the conditions of terrestrial core formation. Whether Earth's mantle is fractionated relative to the chondritic reservoir, and by inference to the core, is a matter of debate that largely arises from the uncertain Ni isotope composition of the mantle. We address this issue through high-precision Ni isotope measurements of fertile-to melt-depleted peridotites and compare these data to chondritic meteorites. Terrestrial peridotites that are free from metasomatic overprint display a limited range in δ 60/58 Ni (deviation of 60 Ni/ 58 Ni relative to NIST SRM 986) and no systematic variation with degree of melt depletion. The latter is consistent with olivine and orthopyroxene buffering the Ni budget and isotope composition of the refractory peridotites. As such, the average Ni isotope composition of these peridotites (δ 60/58 Ni = 0.115 ± 0.011 ‰) provides a robust estimate of the δ 60/58 Ni of the bulk silicate Earth. Peridotites with evidence for melt metasomatism range to heavier Ni isotope compositions where the introduction of clinopyroxene appears to drive an increase in δ 60/58 Ni. This requires a process where melts do not reach isotopic equilibrium with buffering olivine and orthopyroxene, but its exact nature remains obscure. Chondritic meteorites have variability in δ 60/58 Ni due to heterogeneity at the sampling scale. In particular, CI1 chondrites are displaced to isotopically lighter values due to sorption of Ni onto ferrihydrite during parent body alteration. Chondrites less extensively altered than the CI1 chondrites show no systematic differences in δ 60/58 Ni between classes and yield average δ 60/58 Ni = 0.212 ± 0.013 ‰, which is isotopically heavier than our estimate of the bulk silicate Earth. The notable isotopic difference between the bulk silicate Earth and chondrites likely results from the segregation of the terrestrial core. Our observations potentially provide a novel constraint on the conditions of terrestrial core formation but requires further experimental calibration.

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Two major types of mafic granulite layers occur within the Horoman host peridotite (e.g. Nicolas & Jackson, 1982). [In this paper we use 'mafic layers' to describe pyroxenites to peridotite, an 8 km × 10 km × 3 km orogenic lherzolite exposed 'gabbroic' rocks (mafic granulites) whose composition in the high-T and low-P Hidaka metamorphic belt of Hokkaido, and mineral assemblage differs substantially from the Japan. The mineral assemblages and textures of these layers reflect host peridotites.] Hypotheses for the origin of these subsolidus reactions occurring during uplift from the upper mantle layers in orogenic peridotites include: (1) solidification to the crust. Nevertheless, their whole-rock compositions can be used of melt (Niida, 1984;Shiotani & Niida, 1997); (2) to infer the primary mineralogy of these layers, and a genetic crystal cumulates in melt conduits or sills (Loubet & relationship to melts geochemically similar to mid-ocean ridge basalts