The effect of low‐temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites: A case for acquisition of chemical remanent magnetization

Abstract

The effect of hydrothermal alteration on the thermoremanent magnetization (TRM) of synthetic titanomagnetite (TM40: Fe2.6Ti0.404) has been studied, to simulate the alteration that occurs in the oceanic crust. Pseudo-single-domain titanomagnetite grains, similar in size to those often found in oceanic basalts, were dispersed in a permeable but rigid glass matrix. This resulted in a TRM in the sample which was subsequently oxidized in acidic solutions while a magnetic field (0.1 mT) was applied perpendicular to the TRM direction. The experiments were conducted in a non-magnetic stainless steel pressure vessel at 150øC in solutions of acidity varying from pH=2 to pH=7. In addition to being time and temperature dependent, the acquired chemical remanent magnetization (CRM) was also found to be very pH dependent. The degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions. Long-term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating. It was found that during the alteration of TM40 to titanomaghemite the CRM is along the a•nbie. nt field direction from the onset, and not partly or wholly along the TRM direction as has been found in previous air oxidation experiments. This has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples. 19,54519,546 KELSO ET AL.: CRM IN T1TANOMAGNETITE DUE TO HYDROTttERMAL ALTERATION (i.e., the CRM direction) being perpendicular to the original TRM of the sample. They found that CRM and TRM were parallel up to z = 0.7, where z, the oxidation parameter, is the proportion of ferrous iron converted to ferric iron, which varies from 0 to 1.

Determination of lattice parameter, oxidation state, and composition of individual titanomagnetite/titanomaghemite grains by transmission electron microscopy

“…are incompatible with maghemitization by addition of oxygen, but consistent with the process of loss of iron. Although the iron-removal maghemitization process has been experimentally demonstrated [Worm and Banerjee, 1984;Kelso et al, 1991], the laboratory synthesis of titanomaghemite used in the studies on the relationship between a and z is likely to occur by processes different from those for natural samples and is generally thought to involve oxidation mechanisms other than those of diffusion of Fe within an invariant matrix of closest-packed oxygen atoms. In particular, the changes in x during maghemitization on the ocean floor typically occur by the Fe -loss, as observed in this study.…”

“…maghemitization process has also been experimentally verified in aqueous low-temperature oxidation of synthetic titanomagnetite [Worm and Banerjee, 1984;Kelso et al, 1991]. Studies on naturally occurring samples are very limited [e.g., Xu et al, 1996], and with rare exception parameters have been determined from rock chips or magnetic extracts, thereby averaging the contributions from grains of different origins and properties.…”

Ophiolite Tectonics, Rock Magnetism and Palaeomagnetism, Cyprus

“…We may not be surprised to see so few reversals since the Troodos ophiolite was mostly emplaced during one or more long intervals of normal magnetization. However, appreciating that the remineralization processes are so long and complex, some reversals could still be expected, locked in as late secondary CRMs during the oxidation of titanomagnetite (e.g., Gee et al 1993;Kelso et al 1991;Wang et al 2006). The hope of reconciling paleomagnetic groundstudies with ocean-scale MAGSAT studies is naturally futile.…”

“…Approximate orientation and palaeolatitude. Both this paper and Borradaile and Lucas (2003) use the modern shoreline of Cyprus as a reference with the modern oceans (Christie et al 1986;Gee et al 1993;Kelso et al 1991;Xu et al 1997;Zhou et al 2000).…”

Correlation among the Changes with Alteration in the Mineralogical, Chemical, and Magnetic Properties of Upper Ocean Crust, Hole 896A

“…Additional evidence for different oxidation mechanisms of bulk-rock and Fe-Ti oxides can be seen in the coexistence of sulfides and (oxidized) titanomaghemites in Samples 138-896A-11R-3, 89-91 cm (Piece 15), and 16R-3, 84-86 cm (Piece 4). The timing of the maghemitization can be confined by considering the facts that titanomagnetites acquire secondary components during the oxidation process (Kelso et al, 1991) and that no other than reverse component magnetizations (except for steep drilling-induced magnetization) have been found for Hole 896A (Allerton et al, this volume; Worm et al, this volume).…”