Iron-titanium oxides and oxygen fugacities in volcanic rocks

Abstract

It is shown that in silicate liquids the ferric‐ferrous equilibrium is controlled by temperature, oxygen fugacity, and the composition of the liquid, particularly its alkali content. Thus, if the iron‐titanium oxide minerals that precipitate from a silicate liquid reflect the ferric‐ferrous equilibrium, the oxygen geobarometer of Buddington and Lindsley will have to be calibrated, especially for such volcanics as phonolites and pantellerites, which are rich in alkali. Estimates of oxygen fugacities for average basalts show that they probably crystallize through a more restricted range of oxygen fugacity than acid volcanics. Alkali gabbros with pseudobrookite, for which analytical data is given, have crystallized at relatively high oxygen fugacities, as have the volcanics (and kimberlites) that contain perovskite and an oxide phase. The calculated oxygen fugacities of sphene‐bearing acid rocks approximate the values derived from the coexisting oxide equilibration data and indicate higher values than pyroxeneor olivine‐bearing acid volcanics. The data on oxygen fugacity and crystallization temperature for basaltic liquids have been used to predict the composition of their coexisting oxide minerals; accordingly, basalts with Curie temperatures significantly in excess of approximately 200°C are considered to have been subsequently oxidized. Unless the temperature at which this later oxidation occurs is known, thermoremanent magnetization (TRM) cannot be demonstrated to exist in basalts. TRM can only be mineralogically demonstrated in the basaltic rocks that contain an orthorhombic oxide phase or its breakdown products.

A Comparison of Three Methods of Determining Geomagnetic Paleointensities

“…All three have JS-T curves that suggest that their oxide minerals have undergone varying degrees of high-temperature oxidation and disproportionation during cooling after eruption (LARSON et al, 1969;GROMME et al, 1969). Several Workers (CARMICHAEL and NICHOLLS, 1967;GROMME et al, 1969;DUNLOP, 1969) have wondered whether such processes may occur below the Curie temperature in some rocks, thereby contaminating the NRM with chemical remanent magnetization and rendering the rocks unsuitable for paleointensity studies. In addition, all three flows appear to have reversed interaction fields between high-and low-blocking temperature TRM that range in magnitude up to 35% of the external field that caused the remanence.…”

Partitioning of Ni, Co and V between spinel-structured oxides and silicate melts: Importance of spinel composition

“…t4.37 Carmichael, 1967) and spinel compositions were cal-174 culated to three cations for use in plotting (e.g., X Ti 175 refers to the amount of Ti calculated around three 176 cations). All analyses are reported in Tables 3-7.…”

Litoquímica e química mineral da Formação Quarenta Ilhas no Distrito Mineiro de Pitinga (AM)

“…Essas feições indicam que o resfriamento relativamente lento das rochas intermediárias, aliado a mais alta fO 2 , deve ter favorecido reações de oxidação/exsolução de ilmenita em magnetita (Buddington & Lindsley 1964). A titanita está frequentemente associada à magnetita e ilmenita, o que sugere oxidação em estágios subsolidus (Carmichael & Nicholls 1967).…”