Oxygen isotope composition of magnetite in iron ores of the Kiruna type in Chile and Sweden

Magnetite-apatite iron ores of the Kiruna type, unaffected by deformation, have structures and textures similar to those of igneous rocks. The best examples are the El Laco deposits in northern Chile which resemble lava flows, pyroclastic deposits and dikes. El Laco magnetites have d18O values between 2.3 and 4.2‰ (V-SMOW). Magnetite from ore with a magmatic texture has a mean of 3.7‰, and the mean for magnetite intergrown with pyroxene in veins is 2.4‰. Oxygen isotope data given here, fluid inclusion results and geological evidence indicate that ore formation took place in a cooling magmatic system. Major orebodies resembling lava flows and near-vent pyroclastic deposits crystallized from magma at ca. 1000°C. Fluids from cooling magma deposited magnetite and pyroxene (±apatite) at ca. 800°C in fissures and open spaces, now present as veins cutting major orebodies. There is little evidence for significant magnetite precipitation during hydrothermal conditions. A large province of magnetite-apatite iron ore in central Chile (the Cretaceous iron belt) and the Kiruna district in northern Sweden also contain primary ore of magmatic appearance. Major deposits in the Chilean iron belt and Kiruna contain magmatic-textured magnetites with the following d18O means: Algarrobo = 2.2‰, Romeral = 1.2‰, Cerro Imán = 1.6‰, and Kiirunavaara = 1.5‰. We consider all oxygen isotope data for unoxidized, magmatic-textured magnetite as representative of the Fe-rich magmas. Magnetites affected by hydrothermal alteration, recrystallization and subaerial oxidation have modified isotope signatures

Immiscible shoshonitic and Fe-P-oxide melts preserved in unconsolidated tephra at El Laco volcano, Chile

The origins of apparently magmatic Fe-P-O deposits like those of the El Laco volcanic complex, Chile, with masses on the order of 1 Gt remain contentious. Previous attention has been focused largely on the high-tonnage massive magnetite bodies that form the economic mineral deposits. Extensive occurrences of unconsolidated granular Fe-P-oxide materials or their apparent metamorphic equivalents have received relatively little attention. Here we report textures and compositions of unconsolidated Fe-P-oxide materials from Laco Sur, El Laco. Unconsolidated tephra at El Laco is dominated by hematite along with subsidiary Fe-phosphates, monazite, and silica. A porous hematite bomb contains menisci of two coexisting materials. One is hydrous shoshonitic glass with perlitic texture, and the other comprises finegrained intergrowths of Fe-P-REE (rare earth element) oxides having the bulk composition of the eutectic in the system FePO4-Fe2O3 with minor S, Cl, and other components. We show by experiment that very similar compositions would have coexisted as immiscible silicate and oxide liquids at 900 °C and 1 GPa in the presence of carbonic vapor, magnetite, and quartz; both will also form anhydrous liquids at 1080 °C and 101 kPa. We infer the former existence of a phosphatic Fe-oxide magma rich in volatiles that underwent explosive degassing and consequent rapid compositional undercooling to produce the observed assemblage of Fe-oxide tephra containing small amounts of Fe-P-REE phosphates and silicate glass

Lower oceanic crust formed at an ultra-slow-spreading ridge; Ocean Drilling Program Hole 735B, Southwest Indian Ridge

Ocean Drilling Program ODP Hole 735B, drilled on Legs 118 and 176, 1508 m of oceanic layer 3 on a transverse ridge adjacent to the Atlantis II Fracture Zone, Southwest Indian Ridge. The cored sequence consists predominantly or olivine gabbro and troctolite and lesser amounts of gabbro, and gabbronorite rich in oxides. The section contains live major blocks of relatively primitive olivine gabbro and troctolite, composed of many smaller igneous bodies. Each Of these composite blocks shows a small upward decrease in Mg# [defined as 100 x Mg/(Mg + Fe 2+)] and contains more fractionated Fe- and Ti-rich gabbros near the top.Small, crosscutting bodies of olivine gabbro and troctolite with diffuse boundaries may represent conduits through crystal mushes for melts migrating upward and feeding individual intrusions. Oxide gabbros and gabbronorites are commonly associated with shear zones of intense deformation, which crosscut the section at all levels, However, oxide-rich rocks decrease in abundance downward and are nearly absent in the lower 500 m of the section. The gabbros and gabbronorites appear to have formed from late-stage, Fe- and Ti-rich, intercumulus melts that were expelled out of fractionating olivine gabbros into the shear zones. The fabrics of the recovered gabbros are consistent with synkinematic cooling and extension of the crustal section in a mid-ocean ridge environment. However, thick intervals of the core have only a weak magmatic foliation. The magmatic foliation is commonly overprinted by a weak, parallel, deformational fabric probably reflecting the transition from a largely magmatic to a largely crystalline state. Deformation in this crustal section decreases markedly downward. Metamorphism and alteration also decrease downward, and much of the core has less than 5% background alteration. Major zones of crystal-plastic (ductile by dislocated creep) deformation in the upper part of the core probably formed under conditions equivalent to granulite-facies conditions when there was little or no melt present. Late-magmatic and hydrothermal fluids produced a variety of plagioclase, amphibole, and diopside veins. Late-stage, low-temperature veins of calcite, smectite, zeolite, prehnite are present in a few intervals. The fact that the cored is unlike ophiolite as defined by the Penrose Conference Participants suggests that no ophiolite representing an ultra-slow-spreading-ridge environment like the Southwest Indian Ridge may be preserved