Mineralogy and Origin of Exsolution In Ti-Rich Magnetite From Different Magmatic Fe-Ti Oxide-Bearing Intrusions

Abstract

Titanium-rich magnetite from different magmatic Fe-Ti oxide-bearing intrusions consists of host magnetite (nearly pure Fe 3 O 4 ) and different types of exsolution. The exsolution of Ti-rich magnetite is indicative of the physical and chemical conditions at which the host intrusions formed. However, the mechanisms to form these types of exsolution have been debated. To examine the formation mechanisms of the different types of exsolution, multiple microanalytical techniques, such as electron microprobe analysis, micro Raman spectroscopy, and micro X-ray diffraction, were used to examine the chemical composition, morphology, and crystalline structure of exsolved minerals in Ti-rich magnetite from different Fe-Ti oxidebearing intrusions in China. The characterization results indicate that pleonaste and ilmenite are the two dominant exsolution phases in the Ti-rich magnetite, whereas the ulvöspinel phase is barely found. The ilmenite exsolution may have different origins: (1) the cloth-texture lamellae primarily exsolved as ulvöspinel, which was oxidized to ilmenite later at ,600 8C; (2) most of the trellis-and sandwich-type lamellae formed by ''oxy-exsolution'' at .600 8C; (3) some ilmenite lamellae exsolved directly from cation-deficient solid solution due to the substitution of Ti 4þ þ A 2Fe 2þ ; (4) sub-solidus re-equilibration of coexisting Fe-Ti oxides can also lead to ilmenite exsolution in Ti-rich magnetite and magnetite exsolution in ilmenite.

Magnetite exsolution in ilmenite from the Fe-Ti oxide gabbro in the Xinjie intrusion (SW China) and sources of unusually strong remnant magnetization

“…2e), thus different-sized magnetite lamellae may have similar TiO 2 content. This is supported by our previous micro-XRD results showing that the exsolved magnetite has a cell parameter of a 0 ≈ 8.394 Å (Tan et al 2015b), identical to that of pure magnetite (Wechsler et al 1984). The cell parameter of Fe 3-x Ti x O 4 (x = 0~1) is supposed to have a positive correlation with TiO 2 content and can be calculated using a 0 = 8.3957 + 0.0688x + 0.1957x 2 -0.1247x 3 (x = 0~1, r 2 = 0.9995) (Wechsler et al 1984).…”

“…The cell parameter of Fe 3-x Ti x O 4 (x = 0~1) is supposed to have a positive correlation with TiO 2 content and can be calculated using a 0 = 8.3957 + 0.0688x + 0.1957x 2 -0.1247x 3 (x = 0~1, r 2 = 0.9995) (Wechsler et al 1984). If the ultrafine magnetite lamellae have ~5.30 wt% TiO 2 in the lattice (Table 1), the cell parameter would approach to 8.410 Å, higher than the value we obtained by XRD results (Tan et al 2015b). We thus consider that ~1 wt% TiO 2 is actually from the ultrafine magnetite lamellae in host ilmenite.…”

Subsolidus Evolution of the Magnetite-Spinel-UlvöSpinel Solid Solutions in the Kovdor Phoscorite-Carbonatite Complex, NW Russia

“…Lamellae oriented along the {111} planes of magnetite are predominant and originate from reaction (2), while their orientation along the {100} planes of magnetite occurs much rarer as a probable result of consecutive reactions (1) and (3) [39,68]. The cores of magnetite crystals contain comparatively thick ilmenite lamellae, while outer zones are often free of inclusions or include the thinnest lamellae of "ilmenite".…”

“…As for divalent cation relations, the ilmenite compositions range widely from ilmenite-poor to geikielite-and pyrophanite-poor, and correspond to average formula of Mg-rich ilmenite: ( [40]. Fractionation of Mg and Mn between coexisting titanomagnetite and ilmenite (Figure 11b) has resulted in significant losses of Mg in host magnetite in comparison with fresh volcanic rocks [47] due to spinel exsolution following ilmenite oxy-exsolution [39,40].…”

“…Under oxidizing conditions, titanomagnetite commonly undergoes oxy-exsolution that produces ilmenite-geikielite-pyrophanite lamellae instead of ulvöspinel-qandilite [8,38] Mg-Ti-rich magnetite Magnetite Geikielite (2) Besides, exsolution ulvöspinel becomes unstable with decreasing temperature and can be oxy-exsolved at a later stage into ilmenite and magnetite [38,39] (3)…”