Experimental calibration of a new oxybarometer for silicic magmas based on vanadium partitioning between magnetite and silicate melt
| Autor: | Róbert Arató, Andreas Audétat |
|---|---|
| Rok vydání: | 2017 |
| Předmět: |
010504 meteorology & atmospheric sciences
Analytical chemistry Vanadium chemistry.chemical_element Mineralogy Silicic engineering.material 010502 geochemistry & geophysics 01 natural sciences Silicate Partition coefficient chemistry.chemical_compound chemistry Geochemistry and Petrology Mineral redox buffer engineering Saturation (magnetic) Ilmenite Geology 0105 earth and related environmental sciences Magnetite |
| Zdroj: | Geochimica et Cosmochimica Acta. 209:284-295 |
| ISSN: | 0016-7037 |
| DOI: | 10.1016/j.gca.2017.04.020 |
| Popis: | Partition coefficients of vanadium between magnetite and rhyolitic silicate melt, D V mgt/melt , were experimentally determined as a function of oxygen fugacity (0.7–4.0 log units above the fayalite-magnetite-quartz buffer), temperature (800–1000 °C), melt alumina saturation index (ASI = 0.74–1.14), magnetite composition (0.2–14 wt% TiO 2 ) and pressure (1–5 kbar; at H 2 O saturation). Experiments were performed by equilibrating small (≤20 µm), V-free magnetite grains in V-doped silicate melts (∼100 ppm V) and then analyzing both phases by LA-ICP-MS. Attainment of equilibrium was demonstrated by several reversal experiments. The results suggest that D V mgt/melt depends strongly on f O 2 , increasing by 1.5–1.7 log units from the MnO-Mn 3 O 4 buffer to the Ni-NiO buffer, and to lesser (but still considerable) extents on melt alumina saturation index (ASI; increasing by 0.3–0.7 log units over 0.4 ASI units) and temperature (increasing by 0.3–0.7 log units over a 200 °C interval at a fixed f O 2 buffer). Magnetite composition and melt water content seem to have negligible effects. The data were fitted by the following linear regression equation: log D V mgt / melt = 0.3726 ∗ 10 , 000 T + 2.0465 ∗ ASI - 0.4773 ∗ Δ FMQ - 2.1214 , in which temperature is given in K, ASI refers to molar Al 2 O 3 /(CaO + Na 2 O + K 2 O) and ΔFMQ refers to the deviation of f O 2 (in log units) from the fayalite-magnetite-quartz buffer. This equation reproduces all of our data within 0.3 log units, and 89% of them within 0.15 log units. The main advantages of this new oxybarometer over classical magnetite–ilmenite oxybarometry are (1) that it can be applied to rocks that do not contain ilmenite, and (2) that it is easier to apply to slowly-cooled rocks such as granites. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|
Full Text from ScienceDirect