Pillow basalt fragments from the East Pacific Rise, dredged during the Phoenix expedition, often show discoloured rims due to alteration. A suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their Fe-Ti-oxide mineralogy and rock magnetic properties. These large pillow fragments show relatively unaltered grey interiors, surrounded by darker, concentric halos, which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior. The discoloured zones are characterized by precipitation of abundant secondary minerals, such as Fe3+-rich clays that filled vesicle spaces. Fe-Ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques. The subsamples come from traverses that are parallel to the outer glassy pillow rims, allowing us to study the low-temperature alteration effects and rock magnetic properties without having to take variable grain size into account.Not surprisingly, titanomaghemites in discoloured rims are, in a general sense, oxidized to a higher degree (z typically > 0.55) than those in the relatively unaltered grey interior (z typically < 0.55). However, exceptions are numerous and reveal that oxidation state of the Fe-Ti oxides and visible alteration in the discoloured rims are not directly correlated. Moreover, the alteration front of the discoloured rims does not generally coincide with a pronounced jump in z. The titanomaghemite within the discoloured rims appears to have oxidized relatively quickly, reaching z > 0.6 within 200 000 yr. The difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age, so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim.Our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z. Because average Ti content can vary from sample to sample, Curie temperatures are also inaccurate proxies for z. If one wants to study possible correlations between z and rock magnetic parameters, the best technique is to determine z for each subsample by using transmission electron microscopy (TEM), electron microprobe, Mossbauer or similar techniques. In agreement with many (but not all) previous observations on natural samples, we find that bulk coercivity (H-c), and high-field susceptibility (chi(hf)) increase, whereas low-field susceptibility (X-lf), natural remanent magnetization intensity and saturation magnetization (M,) generally decrease with increasing oxidation state.

The fact that the natural remanent magnetization (NRM) intensity of mid-oceanic-ridge basalt (MORB) samples shows systematic variations as a function of age has long been recognized: maximum as well as average intensities are generally high for very young samples, falling off rather rapidly to less than half the recent values in samples between 10 and 30 Ma, whereupon they slowly rise in the early Tertiary and Cretaceous to values that approach those of the very young samples. NRM intensities measured in this study follow the same trends as those observed in previous publications. In this study, we take a statistical approach and examine whether this pattern can be explained by variations in one or more of all previously proposed mechanisms: chemical composition of the magnetic minerals, abundance of these magnetization carriers, vectorial superposition of parallel or antiparallel components of magnetization, magnetic grain or domain size patterns, low-temperature oxidation to titanomaghemite, or geomagnetic field behavior. We find that the samples do not show any compositional, petrological, rock-magnetic, or paleomagnetic patterns that can explain the trends. Geomagnetic field intensity is the only effect that cannot be directly tested on the same samples, but it shows a similar pattern as our measured NRM intensities. We therefore conclude that the geomagnetic field strength was, on-average, significantly greater during the Cretaceous than during the Oligocene and Miocene.

Hysteresis measurements have been carried out on a suite of ocean-floor basalts with ages ranging from Quaternary to Cretaceous. Approximately linear, yet separate, relationships between coercivity (Bc) and the ratio of saturation remanence/saturation magnetization (Mrs/Ms) are observed for massive doleritic basalts with low-Ti magnetite and for pillow basalts with multi-domain titanomagnetites (with x=0.6). Even when the MORB has undergone low-temperature oxidation resulting in titanomaghemite, the parameters are still distinguishable, although offset from the trend for unoxidized multidomain titanomagnetite. The parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types. This plot provides a previously underutilized and non-destructive method to detect the presence of low-titanium magnetite in igneous rocks, notably MORB.

Mid‐ocean ridge basalt (MORB) samples, varying in age from Recent to Jurassic, were selected for electron microscopic and rock magnetic studies. Our observations indicate that the degree of oxidation of titanomagnetite in MORB increases only gradually with sample age. The titanomagnetite in recent MORB (<20,000 years) shows no sign of alteration (z∼0). Quaternary samples near the ridge (<2 Ma) typically have z values of less than 0.35, indicating a low degree of oxidation, whereas samples with ages of tens of millions of years have z values of up to 0.9. Some older samples show lower z values, but the upper envelope of our observations in a z versus age plot can be represented by the function z = p + q log t, where p ≈ 0.38, q ≈ 0.38, and t is in millions of years (for t>100 ka). Both electron microscopic observations and rock magnetic data support the notion that low‐temperature oxidation of titanomagnetite to titanomaghemite in MORB is a gradual process. Moreover, the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales, including regional oceanic crustal structures, lithological features, grain size, and surrounding matrices. Pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows, owing to higher porosity and permeability of the former. In contrast, interstitial glass can protect fine titanomagnetite grains from alteration. The natural remanent magnetization (NRM) intensity of MORB varies as a function of age, magnetic granulometry, concentration of magnetic materials, and the degree of alteration. Fine‐grained MORB samples typically have higher NRM intensity. The NRM intensity appears to decrease substantially with increasing degree of maghemitization and, hence, with age. The envelope of the smoothed magnetic anomaly amplitudes resembles the change in NRM intensity for MORB samples of the last 30 million years, but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported. Maghemitization of the micrometer‐sized and larger grains is probably only partly responsible, whereas a significant contribution to NRM intensities is inferred from those submicrometer‐sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass.