Subsolidus evolution and alteration of titanomagnetite in ocean ridge basalts from Deep Sea Drilling Project/Ocean Drilling Program Hole 504B, Leg 83: Implications for the timing of magnetization

摘要

Magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B, Leg 83, were studied by methods of rock magnet:ism and transmission electron microscopy (TEM). TEM observations showed that the magnetic mineral in these basalts is end-member magnetite (TM0) of extremely fine-grain size (30-100 nm) primarily in the range of psuedo-single-domain magnetite, consistent with the rock magnetic properties including hysteresis parameters, Curie temperature, and low-temperature measurements (Verwey transition). Magnetite formed by two different processes: (1) oxidation-"exsolution," true exsolution, and hydrothermal alteration, and (2) oxidation-exsolution, a second stage of oxidation-exsolution, and hydrothermal alteration. The primary titanomagnetite (TM60-70) that crystallized from the melt thus evolved to end-member magnetite coexisting with titanite (sphene), kassite, ulvospinel (TM similar to 87), and ilmenite on a submicroscopic scale. On the basis of the formation mechanisms of the magnetic carrier, the primary titanomagnetite (TM similar to 60) with Curie temperature of similar to 180 degrees C did not acquire thermoremanent magnetization (TRM) in these basalts. Instead, the Ti-bearing magnetite (TM similar to 10-20) that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization (CRM) at similar to 500-400 degrees C during subsolidus cooling. Upon the onset of hydrothermal alteration the recrystallized end-member magnetite acquired a second CRM. The natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of CRMs that were acquired when titanomagnetite altered, in part, to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis. [References: 53]