Preservation of pristine titanomagnetite in older ocean-floor basalts and its significance for paleointensity studies

Transmission electron microscopy observations and rock magnetic measurement s reveal that alteration of fine- and large-grained iron-titanium oxides ca n occur at different rates. Fine-grained titanomagnetite occurs as a crysta llization product within interstitial glass that originated as an immiscibl e liquid within a fully differentiated melt; in several samples with ages t o 32 Ma it displays very little or no oxidation (z approximate to 0). In co ntrast, samples with ages of 10 Ma or older are observed to also contain hi ghly oxidized (z greater than or equal to 0.66) large-grained titanomaghemi te. These large grains, having originated by direct crystallization from me lt, are associated with pore space. Such pore space can serve as a conduit for fluids that promote alteration, whereas fine grains may have been "armo red" against alteration by the glass matrix in which they are embedded. App arently, alteration of oceanic crust is a heterogeneous process on a micros copic scale. The existence of pristine, fine-grained titanomagnetite in the interstitial glass of older ocean-floor basalts that have undergone signif icant alteration implies that such glassy material is capable of carrying o riginal thermal remanent magnetization and may be suitable for paleointensi ty determinations.