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The southern massifs of the Oman ophiolite were often considered as a model for the structure and composition of crust at fast-spreading ocean ridges. However, the influence of late-stage convergent plate margin magmatism on the evolution of lower crustal and upper mantle rocks in the ophiolite is still poorly understood. Here, we report new constraints from highly siderophile and other chalcophile element abundances and 187Os/188Os on samples from the CM-1 drill core of the Oman Drilling Project. Main results are: (1) Abundances of most elements in this study were not affected by low-temperature alteration. (2) In harzburgites and dunites from the top of the mantle section, chalcophile elements predominantly are controlled by traces of sulfides, precipitated from sulfide-saturated, incompatible element-depleted magmas. (3) The overlying dunite-rich crust-mantle transition zone (CMTZ) is extremely depleted in Pd, Se and Re, indicating that parent magmas were sulfide-undersaturated. (4) The mafic unit on top shows local sulfide saturation. (5) Ru/Ir and Os/Ir in CMTZ dunite are suprachondritic, whereas values in the overlying mafic unit are subchondritic. Hence, rocks of the mafic unit are products of magmas that initially crystallized cumulate olivine and a laurite-ehrlichmannite phase in CMTZ. (6) Suprachondritic initial 187Os/188Os of CMTZ, the mafic unit and other lower crustal gabbros suggest that most of the lower crust was affected by magmas that contained excess radiogenic 187Os, presumably a result of slab-derived inputs during the short-lived subduction zone stage.

The ultra-slow spreading South West Indian Ridge (SWIR) near Marion Rise comprises segments of thin crust and abundant abyssal peridotites exposed near the ridge axis. Zhou and Dick (2013) proposed that buoyant depleted mantle beneath the SWIR may represent Proterozoic melting residues recycled from Gondwana lithosphere, resulting in excess buoyancy at Marion Rise. In order to test this hypothesis, serpentinized harzburgites, dunites and scarce lherzolites were dredged and sampled by remotely operated vehicle along SWIR (36°54.0’ E to 39°13.5’ E) during expedition SO 273 in 2020. The major and trace element compositions of the peridotites (Mg# = 0.90-0.92) are similar to other depleted abyssal peridotites and mostly lack visible clinopyroxene. Here, we present new highly siderophile element (HSE) mass fraction and Os isotopes, that constrain the time of melt extraction, style of mantle melting and melt transport. Low HSE abundances (Ru = 0.07-0.3x primitive mantle) coupled with strongly fractionated HSE patterns (high Ru/Os, Ir/Os, Ru/Re) in studied peridotites exposed at normal faults suggest higher melting degrees than in most abyssal peridotites elsewhere. The HSE patterns are similar to those in harzburgites and dunites from ophiolites, interpreted to reflect sub-arc mantle lithosphere that was depleted by flux melting above subduction zones. Measured 187Os/188Os range between 0.119 and 0.142, possibly reflecting melt depletion at 1.4 Ga and interaction with sulfide-undersaturated melts or fluids with suprachondritic 187Os/188Os. The data provide evidence for locally strong depletion of mantle rocks at Marion Rise and the occurrence of ancient subarc mantle underneath a modern oceanic ridge.

Anorthosite sind Plagioklas-Kumulatgesteine und charakteristische Bestandteile der proterozoischen Erdkruste. Ihre Entstehung wird im Allgemeinen entweder als partielles Schmelzen des oberen Erdmantels oder Aufschmelzung mafischer Unterkruste diskutiert. Die magmatische Entwicklung wird als polybare Kristallisation mit Bildung von Plagioklas-Flotationskumulaten und Platznahme als Kristallbrei interpretiert. Der mesoproterozoische Kunene-Intrusiv-Komplex (Namibia/Angola) ist einer der größten, nicht metamorph überprägten Anorthosit-Komplexe weltweit. Die Gehalte lithophiler und hoch siderophiler Spurenelemente und die Verhältnisse stabiler und radiogener Isotope der Anorthosite des Kunene-Intrusiv-Komplex belegen deutlich die Herkunft der primären Schmelze aus dem verarmten oberen Erdmantel. Die petrographischen und geochemischen Unterschiede zwischen verschiedenen Anorthositgenerationen offenbaren jedoch auch einen erheblichen Einfluss der unteren bis mittleren Erdkruste. Die krustale Kontamination des Stammmagmas fand dabei in einem frühen Stadium der magmatischen Entwicklung statt und ist am deutlichsten in den ältesten Anorthositen ausgeprägt. Die chemische Zonierung der Plagioklase erbringt keine deutlichen Belege für eine polybare Kristallisation und die Platznahme der Anorthosite als Kristallbrei. Eine fraktionierte Kristallisation im Intrusionsniveau erscheint daher wahrscheinlich. Anorthosites are plagioclase cumulate rocks and characteristic features of the Proterozoic earth crust. Their formation is generally discussed in terms of either partial melting of the upper mantle or melting of mafic lower crust. The magmatic evolution is interpreted as polybaric crystallization with formation of plagioclase flotation cumulates and emplacement as crystal mush. The Mesoproterozoic Kunene Intrusive Complex (Namibia/Angola) is one of the largest non metamorphic anorthosite complexes worldwide. Lithophile and highly siderophile trace element contents and stable and radiogenic isotopic compositions of the anorthosites of the Kunene Intrusive Complex clearly prove its origin from the depleted upper mantle. The petrographic and geochemical differences between different anorthosite generations, however, reveal a significant influence from the lower to middle crust. This crustal contamination of the parental magma took place during an early state of the magmatic evolution and mainly affected the oldest anorthosite units. Chemical zoning of plagioclase provide no clear evidence for a polybaric crystallization and emplacement as a crystal mush. Fractional crystallization at the intrusion level therefore seems to be more likely.