Oxygen Isotope Composition of Magnetite Deposits at El. Laco, Chile: Evidence of Formation from Isotopically Heavy Fluids
Amy Larson Rhodes, Naomi Oreskes, 1999. "Oxygen Isotope Composition of Magnetite Deposits at El. Laco, Chile: Evidence of Formation from Isotopically Heavy Fluids", Geology and Ore Deposits of the Central Andes, Brian J. Skinner
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Oxygen isotope analyses of iron oxide at El Laco, Chile, were conducted in order to test whether magnetite formed either by a combination of magmatic and hydrothermal metasomatic processes or by a single hydrothermal-metasomatic process alone. If magnetite formed from two distinct sources—one magmatic, the other hydrothermal—then a contrast in oxygen isotope compositions between magnetite that crystallized in a magma and magnetite that precipitated from a hydrothermal fluid should be expected. In fact, δ18O values in magnetite at El Laco show no significant variability between textural types (mean = 4.1 ± 0.49‰). Perhaps more important, δ18O in wall-rock andesite shows a distinct increase in oxygen isotope values—from 7.2 to 24.2 per mil—with increasing degrees of hydrothermal alteration. This observation strongly suggests that oxygen exchange occurred with an isotopically heavy fluid that was distinct from what might have been generated from a magma. Diopside separated from altered andesite, apatite separated from magnetite, and quartz separated from hydrothermal magnetite-quartz (± apatite) veins all have heavy δ18O values (7.1–8.9‰, 7.8–8.0‰, and 7.1–27.9‰, respectively) relative to values typical of igneous rocks. The quartz values are among the heaviest reported in the literature and are exceedingly variable both between and within individual samples. In contrast to the values for magnetite, apatite, diopside, and quartz, δ18O values of hematite-bearing iron oxide samples have much lower δ18O values, down to a minimum of −8.9 per mil, and the δ18O value decreases as the hematite content increases.
These results strongly support the theory that the bulk of the magnetite at El Laco formed by metasomatic replacement and did not form by direct consolidation from a magma. The hydrothermal fluids that reacted to form magnetite were isotopically heavy in oxygen composition. These 180-rich fluids were also responsible for hydrothermal alteration of wall-rock andesite and the formation of quartz veins. Heated, closed-basin water that experienced significant evaporation, or deep-seated fluids (possibly magmatic) that interacted with buried evaporite deposits, may be the source for these isotopically heavy hydrothermal fluids. The extremely high δ18O values and isotopic variability of the quartz suggests that quartz veins formed as the hydrothermal fluids boiled, perhaps losing large quantities of volatile components. The much lower δ18O values of hematite-rich samples suggest that isotopically lighter meteoric fluids reacted with, and oxidized portions of, the magnetite deposits at temperatures ranging from approximately 65° to 150°C in the near-surface or surficial environment. However, these fluids were volumetrically minor.
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Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner