Magnetic alteration of zero-age oceanic basalt

The youngest sampled submarine lava flow, which erupted June 1993 on the Juan de Fuca Ridge, provides the basis for a tight constraint on the initial or zero-age magnetization state of MORB. Detailed profiles of magnetic hysteresis parameters, Curie temperatures, and unblocking temperatures of NRM with respect to the chilled margin of a pillow fragment show evidence of significant oxidation, which preferentially affected the finest grain-size fraction and principal remanence carrier of the titanomagnetite magnetic mineralogy. The oxidation must have occurred during or immediately after initial cooling, implying that MORB is already appreciably magnetically altered before aging. Nevertheless, successful results of Thellier paleointensity experiments on the basalt sample lend support to the idea that crustal magnetization represented by MORB preserves a record of geomagnetic intensity variations that may be reflected in small-scale magnetic anomalies

Magnetic telechemistry and magmatic segmentation on the Southern East Pacific Rise

Results from axial dredges and a profile inversion of magnetic anomaly data along the axis of the East Pacific Rise (EPR) at 13-23°S provide an estimate of the average degree of fractionation for the extrusive layer at this ultrafast-spreading (∼145 mm/yr full rate) ridge. We find a high correlation (R=0.81) between dredge mean FeO* (total iron as FeO) and natural remanence for 34 axial dredges with multiple samples having coincident geochemical and magnetic data. We attribute this good correlation to detailed sampling spanning the full range of cooling-related magnetization changes within a flow and to the young age (0-6 ka) of these axial samples, which effectively minimizes time-dependent magnetization changes due to geomagnetic intensity or alteration. A composite axial magnetic anomaly profile shows large amplitude (up to 400 nT) fluctuations with wavelengths of 50-200 km, which theoretical considerations suggest can reliably be related to the magnetization directly beneath the ship. For much of the southern EPR, seismic data provide independent limits on the axial thickness (259 ± 55 m) and the pattern of off-axis thickening of the extrusive magnetic source layer. These data also provide evidence for an axial magma lens that effectively eliminates anomaly contributions from deeper magnetic sources. Inversion of the axial magnetic anomaly data utilizing these geophysical constraints yields a magnetization solution which, through use of the regression relating FeO* and natural remanence, may be related to the average degree of differentiation of the extrusive source layer. The magnetic data reveal a pattern of magmatic segmentation that closely parallels the tectonic segmentation of the ridge, suggesting that magma supply may be an important control on the average degree of differentiation of the extrusive layer

Magnetic hysteresis in young mid“ocean ridge basalts: Dominant cubic anisotropy?

Magnetic hysteresis data from young mid“ocean ridge basalts include samples with saturation remanence to saturation magnetization (Mrs/Ms) ratios greater than 0.5, the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy. Under the usual assumption of dominant uniaxial anisotropy, the narrow single domain grain size distribution implied by these high Mrs/Ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains. Dominant cubic anisotropy provides a plausible explanation for the high Mrs/Ms ratios, and if generally valid, requires reinterpretation of granulometric and domain state inferences made from hysteresis data

Variations in layer 2A thickness and the origin of the central anomaly magnetic high

The seismically determined off-axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high typically associated with the ridge crest. The positive magnetization contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales (2-3 km) comparable to that of the proposed Layer 2A thickening. We present magnetic remanence data from basalts dredged on and near the East Pacific Rise axis at 12°N which are compatible with a rapid magnetization reduction (~20 k.y. to decay to 1/e). Together with near bottom magnetic profiles from the ultra-fast-spreading East Pacific Rise at 19.5°S, these data suggest that previous estimates of the time constant of alteration inferred from slow-spreading ridges (0.5 m.y.) may be more than an order of magnitude too high

Calibration of magnetic granulometric trends in oceanic basalts

The validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains. For example, populations with dominantly superparamagnetic (SP) or multidomain (MD) grains both are characterized by low remanence and coercivity, and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain (SD) size range. The natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size, from SP–SD mixtures in submarine basaltic glass to SD–MD mixtures in flow interiors. Based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the Mid-Atlantic Ridge (ages < 1 Ma to 70 Ma), a model of preferential dissolution with time of the finest-grained titanomagnetites has recently been suggested as the major process contributing to long-term temporal changes in remanent intensity of mid-ocean ridge basalts. We evaluated the local and long-term temporal trends in effective magnetic grain size predicted by this model using hysteresis data from a large number of submarine basalt samples which span a range of ages from ∼0 to ∼122 Ma. Specimens were systematically taken along transects perpendicular to the chilled margin of each sample. The large number of data (∼750 loops) and the inferred progressive change in grain size approaching the chilled margin allow recognition of mixing trends between MD and SD grains and between SD and SP grains on a Day-plot. These trends in hysteresis parameters are crucial to resolving the inherent, but frequently overlooked, ambiguity in inferring grain size from hysteresis parameters. We illustrate that two additional rock magnetic tests (warming of a low-temperature isothermal remanence and hysteresis loop shapes) often used to address these ambiguities are inconclusive, requiring some independent knowledge of whether SP or MD grains are likely to be present. Even with a considerably larger data set the substantial intrasample variability in oceanic basalts precludes recognition of any systematic trend in magnetic grain size with age

Paleointensity Estimates From Ignimbrites: The Bishop Tuff Revisited

Volcanic ash flow tuffs (ignimbrites) may contain single domain‐sized (titano) magnetite that should be good for recording geomagnetic field intensity, but due to their complex thermal histories also contain other magnetic grains, which can complicate and obscure paleointensity determination. An initial study of the suitability of the ~767 ka Bishop Tuff for measuring paleointensity found an internally consistent estimate of 43.0 ± 3.2 μT. This initial study also showed a spatial heterogeneity in reliable paleointensity estimates that is possibly associated with vapor‐phase alteration and fumarolic activity, which motivated resampling of the Bishop Tuff to examine spatial changes in magnetic properties. Three new stratigraphic sections of the Bishop Tuff within the Owens River gorge were sampled, and the paleointensity results from the initial study in the same locality were reinterpreted. The mean of all sites is 41.9 ± 11.8 μT; this agrees with the initial study\u27s finding but with substantially greater scatter. Two sections show evidence of vapor‐phase alteration where the presence of titanohematite, likely carrying a thermochemical remanence, produces nonideal behavior. This thermochemical remanence in the upper portion of the section also produces some paleointensity estimates of technically high quality that have significantly higher intensity than the rest of the tuff. Our best estimate for paleointensity, 39.6 ± 9.9 μT, comes from the densely welded ignimbrite that was emplaced above the Curie temperature of magnetite. The low permeability of this unit likely shielded it from vapor‐phase alteration. Our results suggest that care must be taken in interpreting paleointensity data from large tuffs as nonthermal remanence may be present

Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff

Ash flow tuffs, or ignimbrites, typically contain fine-grained magnetite, spanning the superparamagnetic to single-domain size range that should be suitable for estimating geomagnetic field intensity. However, ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history. We examined three stratigraphic sections through the ~0.76 Ma Bishop Tuff, where independent information on postemplacement cooling and alteration is available, as a test of the suitability of ignimbrites for paleointensity studies. Thermomagnetic curves suggest that low-Ti titanomagnetite (Tc = 560°C–580°C) is the dominant phase, with a minor contribution from a higher Tc phase(s). Significant remanence unblocking above 580°C suggests that maghemite and/or (titano)maghemite is an important contributor to the remanence in most samples. We obtained successful paleofield estimates from remanence unblocked between 440°C and 580°C for 46 of 89 specimens (15 sites at two of three total localities). These specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43.0 µT (±3.2), equivalent to a VADM of 7.8 × 1022 Am2. The most densely welded sections of the tuff have emplacement temperatures inferred to be as high as ~660°C, suggesting that the remanence may be primarily thermal in origin, though a contribution from thermochemical remanence cannot be excluded. These results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

Timing of magnetite formation in basaltic glass: Insights from synthetic analogs and relevance for geomagnetic paleointensity analyses

Absolute paleointensity estimates from submarine basaltic glass (SBG) typically are of high technical quality and accurately reflect the ambient field when known. SBG contains fine-grained, low-Ti magnetite, in contrast to the high-Ti magnetite in crystalline basalt, which has lead to uncertainty over the origin of the magnetite and its remanence in SBG. Because a thermal remanence is required for accurate paleointensity estimates, the timing and temperature of magnetite formation is crucial. To assess these factors, we generated a suite of synthetic glasses with variable oxygen fugacity, cooling rate, and FeO* content. Magnetic properties varied most strongly with crystallinity; less crystalline specimens are similar to natural SBG and have weaker magnetization, a greater superparamagnetic contribution, and higher unblocking temperatures than more crystalline specimens. Thellier-type paleointensity results recovered the correct field within 1σ error with 2 (out of 10) exceptions that likely result from an undetected change in the laboratory field. Unblocking and ordering temperature data demonstrate that low-Ti magnetite is a primary phase, formed when the glass initially quenched. Although prolonged heating at high temperatures (during paleointensity experiments) may result in minor alteration at temperatures \u3c 580°C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies

Cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows

Cooling rate effects on the intensity of thermoremanent magnetization (TRM) have been well documented in ceramics. In that case, laboratory cooling is generally more rapid than the initial cooling, leading to an overestimate of the paleofield by 5-10% in Thellier-type paleointensity experiments. The reverse scenario, however, has never been tested. We examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass (SBG) samples from 13 sites at 17°30'-18°30'S on the East Pacific Rise. Absolute cooling rates determined by relaxation geospeedometry at five of these sites range from ~10 to ~330°C min⁻¹ at the glass transition (~650°C). Over the dominant range of remanence blocking temperatures (~200-400°C), the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the Thellier experiment. These results suggest that while the cooling rate effect might introduce some within-site scatter, it should not result in a systematic bias in paleointensity from SBG. Paleointensity estimates from the 15 sites range from ~29 to 59μT, with an average standard error of ~1μT. Comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent (indistinguishable from present-day) and the oldest is at least 500, and probably several thousand, years old. These age estimates are consistent with available radiometric ages and geologic observations

Grain Size-Dependent Alteration and the Magnetization of Oceanic Basalts

Unblocking temperatures of natural remanent magnetization were found to extend well above the dominant Curie points in samples of oceanic basalts from the axis of the East Pacific Rise. This phenomenon is attributed to the natural presence in the basalts of three related magnetic phases: an abundant fine-grained and preferentially oxidized titanomagnetite that carries most of the natural remanent magnetism, a few coarser and less oxidized grains of titanomagnetite that account for most of the high-field magnetic properties, and a small contribution to both the natural remanent magnetism and high-field magnetic properties from magnetite that may be due to the disproportionation of the oxidized titanomagnetite under sea-floor conditions. This model is consistent with evidence from the Central Anomaly magnetic high that the original magnetization acquired by oceanic basalts upon cooling is rapidly altered and accounts for the lack of sensitivity of bulk rock magnetic parameters to the degree of alteration of the remanence carrier in oceanic basalts