Titanomagnetite Grains Research Articles

Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon

The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and 14C AMS stratigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo-single domain titanomagnetite grains, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon. In addition, in core MD77-180, we observe a correlation between magnetic grain size and a chemical index of alteration. This suggests that these magnetic grain-size changes are related to chemical weathering driven by summer monsoon rainfall. A comparison of the GISP2 ice core isotopic record and the magnetic grain-size record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard–Oeschger cycles and Heinrich events), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent. We suggest that rapid cold events of the North Atlantic (Heinrich events) during the last glacial stages are characterized by a weaker summer monsoon rainfall over the Himalaya via an atmospheric teleconnection.

Paleomagnetic and rock magnetic evidence for inverse zoning in the Parguaza batholith (southwestern Venezuela) and its implications about tectonics of the Guyana shield

We report paleomagnetic and rock magnetic data from the rapakivi granites of the Parguaza batholith (Guyana Precambrian Shield, southwestern Venezuela). These results suggest that the pluton is inversely zoned with respect to the cooling ages. In order to explain such an age pattern, tentative structural settings are proposed placing the Parguaza intrusions in a plate tectonic context. Six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith. Thermomagnetic curves, X-ray diffraction and fluorescence, hysteresis loops, thermal and alternating field (AF) intensity plots, transmission (TEM) and scanning (SEM) electron microscope analyses and Königsberger ratios (Qn) values were used to identify the different magnetic mineralogies and their distribution of grain sizes. Magnetite, titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations (NRMs) in these rocks. Magnetic granulometry indicators such as Königsberger ratios (Qn values) suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect. The paleomagnetic results reveal the existence of two primary thermoremanent and/or thermochemical magnetizations (TRMsTCRMs) for sites CSP-3, PI-2 and PI-4 (Decl. = 328°, Incl. = −21°, k = 15, α95 = 11.4°) and site CSP-2, PI-1 and PI-3 (Decl. = 16°, Incl. = 87°, k = 10, α95 = 13°), respectively. There is also a poorly defined G3R2 magnetization (Decl. = 284°, Incl. = −86°, k = 6, α95 = 27°) found in sites CSP-2, PI-1 and PI-3. The overlap of coercivities and the unblocking temperatures spectra, probably resulting from the coexistence of primary single-domain magnetic mineralogies with secondary exsolutions of single-domain-like Ti-poor (Fe-rich) regions (almost pure magnetite) in multidomain titanomagnetite grains, in most cases precludes the complete resolution of hybrid G1 (N + R) or G3 (N + R). The relative ages for these components were determined using a map of RbSr model age ‘chrontours’. G1 and G3 are the older and the younger TRMsTCRMs, respectively, and were acquired at two discrete moments of the batholith's geological history. The final map resulting from integrating the paleomagnetic and RbSr data, shows an age pattern for this batholith, and the rest of the intrusions that belong to the Parguaza Igneous Complex, that could be explained either as the effect of inverse cooling of a single intrusive body, the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of NE-trending horsts and grabens cutting through the pluton. Because of its feasibility and agreement with the most recent theories about the tectonic evolution of the Guyana Shield, we favor the latter hypothesis.

Alteration and dissolution of fine-grained magnetite and its effects on magnetization of the ocean floor

Scanning and transmission electron microscopy (STEM) of ocean-floor pillow basalts with ages between 0 and 70 Ma reveals progressive alteration of submicrometer titanomagnetite to phases such as goethite and clays. In contrast, larger titanomagnetite grains (> 1 μm) oxidize to titanomaghemite without apparent change in crystal morphology. Rock magnetic experiments are consistent with a selective removal of the submicrometer grains as the basalts age, and show good correlations between the ranges of grain sizes observed by STEM and those indicated by hysteresis properties. Remanence contributions from the larger (pseudo-single domain and multi-domain) titanium-iron oxides are inferred to decrease only slightly as the ocean floor becomes older, whereas the overall remanence decays with age as the substantial contribution from stable, single-domain titanomagnetite grains diminishes greatly due to their alteration to other phases. Although more work on many more samples is required to verify our conclusions, the current data imply that this alteration is one of the reasons that amplitudes of marine magnetic anomalies diminish with age over time scales of tens of millions of years.

Palaeomagnetic study of the Eastern Kamchatka Cretaceous-Palaeocene island arc: new evidence concerning palaeosubduction zone absolute motion

An important geodynamical problem is testing for the possibility of absolute displacement of subduction zones. To consider the above problem, the most favourable geological situations are those where (1) arc-related continuous volcanism existed long enough to store significant palaeolatitude changes occurring during the accumulation of volcanic sequences and (2) a significant meridional component of the studied block displacement occurred during volcanic activity. The Upper Cretaceous and Lower Palaeocene volcanic island-arc complexes of Eastern Kamchatka, which formed on the Pacific ocean during rapid northward drift, satisfy the above conditions. Palaeomagnetic and petromagnetic investigations of the Upper Cretaceous-Lower Palaeocene volcanics of the Kumroch Ridge at 12 localities (183 hand samples) were carried out to determine the movements of the subduction zone during volcanic activity. The major magnetic mineral in all rocks is low-Ti titanomagnetite, which is typical for island-arc basalt. Most of the titanomagnetite grains from basalts are homogenous, and their Curie points are 450°–500°C; a small fraction of the magnetic grains have Tc=560°–580°C. Most of the titanomagnetite grains in tuffs have been oxidized to magnetite. NRM stepwise thermal demagnetization was carried out, together with principal component analysis. As a result of fold, baked contact, conglomerate and ‘long particles’ tests, the high temperature, close to the primary, ChRM component was found. Mean palaeomagnetic directions for four stratigraphic intervals from the base to the top of the sequence are: (1) D=32°, I=49°, α95=3.8°; (2) D=218°, I= −56°, α95=5.6°; (3) D=251°, I= −66°, α95=2.7°; (4) D=230°, I= −69°, α95=7.8°. The mean palaeolatitude changed from 30° to 53° during formation of the volcanic sequence. The drifting rate was 15–20 cm yr−1, which is close to the independent estimation of the Kula plate rate for the same time. According to palaeodeclinations, the area investigated was rotated clockwise 30°–70° and rocks were partly remagnetized.

Single-domain and superparamagnetic titanomagnetite with variable Ti content in young ocean-floor basalts: No evidence for rapid alteration

High resolution electron microscope studies have been carried out on ‘zero-age’ (New Flow) basalts from the Juan de Fuca Ridge and on young (< 20 ka) basalts from the axis of the East Pacific Rise at 12°N. Such data lead to characterization of the magnetic minerals, especially those of smaller grain size, which have been hypothesized by Kent and Gee to have undergone grain size-dependent alteration. In addition to larger titanomagnetite grains, abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix. These grains, likely to be single-domain (SD) or superparamagnetic, are associated with apatite, uncommon pyrrhotite and residual glass. The submicrometer titanomagnetite grains have a wide compositional range (0 < x < 0.8), where x is the fraction of ulvöspinel component, whereas the larger, multi-domain (MD)-sized titanomagnetite grains have a narrow composition range of approximately x = 0.6. This variability in Ti content provides a ready explanation for the thermal rock magnetic properties observed by Kent and Gee and eliminates the need to invoke extremely rapid (< 20 ka) alteration of these young basalts.

How to include magnetostriction in micromagnetic models of titanomagnetite grains

Numerical models of the micromagnetic structure of titanomagnetite particles require the consideration of magnetostriction. A new technique for calculating magnetostrictive self energies in finite element models is introduced. The method is based on the continuum theory of defects [Kröner, 1958]. By solving the biharmonic equation for the second order stress function tensor in an infinite elastically homogeneous medium the treatment of the magnetostrictive energy becomes similar to the treatment of demagnetizing energy. Fast Fourier Transformation can therefore be applied to the calculation of both, demagnetizing and magnetostrictive energies.

Kerr microscopy of small synthetic Ti‐rich titanomagnetite grains

Magnetic domain structure observations have been carried out on small synthetic Ti‐rich titanomagnetite (TM) grains (ulvöspinel content 50%–70%) employing the magneto‐optical Kerr effect (MOKE). On randomly oriented grains simple domain structures with planar domains or complex domain structures with curved domains and reverse spike domains have been observed. In addition to the description of the domain pattern, a qualitative determination of the magnetization direction within the domains is presented. Planes with a magnetization parallel to the surface show broad domains with nearly straight domain walls while planes with a magnetization normal to the surface show narrow domains and more complex patterns. With the help of the observed domain patterns on the randomly oriented grains and the knowledge about the magnetization direction within the domains a three‐dimensional (3‐D) domain structure model for Ti‐rich TM is proposed. These Ti‐rich titanomagnetites have only one easy direction of the magnetization and show a real unaxial character, although the magnetic mineral is cubic.By analysing the domain structure of more than a hundred TM particles in a range of size between 1.7 and 20 microns the transition from the single‐domain (SD) to the multidomain (MD) state is determined. The fit of the data in this study gives a critical SD size of ≈ 1.1 µm for Ti‐rich TM.

Residual stress and domain structure

Calculations show that as the residual stress increases, the number of magnetic domains in titanomagnetite at first increases and subsequently decreases, and large residual stress causes laminar domain structures in Ti‐rich titanomagnetite, as is often observed. In addition, large residual stress significantly decreases the number of domains, as is also observed. One anticipates that large residual stress also exists in most pure magnetites, but theory and observations are inadequate to confirm this expectation. We conclude that the presence of a large macrostress and possible systematic errors in the observational estimates of the number of domains observed explains the current discrepancy between domain observations and theory for titanomagnetite grains.

Composite magnetic fabric in weakly deformed black shales

The anisotropy of magnetic susceptibility (AMS) of weakly deformed sediments of the subalpine Terres Noires (French Alps) is due to phyllosilicates (matrix contribution) and detrital titanomagnetites (ferromagnetic contribution) of variable grain size. Various rock magnetic and anisotropy techniques have been carried out to understand the origin of the scattering of maximum and minimum susceptibility axes. We demonstrate the major contribution of the matrix, both to susceptibility and AMS anisotropy, even when mean matrix susceptibility is weak. This suggests that phyllosilicates are better oriented within the foliation plane and that intrinsic anisotropy of the detrital titanomagnetite grains is probably weaker than expected. Also, the separation of the two magnetic subfabrics of three selected sites by coupling together low-field AMS, HFA (high-field AMS) and AAS (anisotropy of anhysteretic remanence) demonstrates the existence of a competition between two magnetic lineations and an interchange of the intermediate and minimum AMS axes as a result of fine titanomagnetite. This is the first evidence of the so-called “intermediate fabric” as predicted by theory.

Magnetic Properties and Rotational Hysteresis of A Basalt With Homogeneous Ti-Rich Titanomagnetite Grains 10-20 μm In Diameter

SUMMARY Magnetic hysteresis properties and rotational hysteresis data are reported for basalt samples with homogeneous Ti-rich titanomagnetite grains 10–20 μm in diameter between ∼ 100K and the Curie temperature of ∼ 400K. From the remanence behaviour it is concluded that at low temperatures a pseudo-single domain state exists. Above ambient temperature this character appears to remain up to the Curie point. Magnetic domain-wall observations with the colloid Bitter technique at room temperature reveal wavy patterns that form as a consequence of stress effects. Rotational hysteresis losses occur at high magnetic fields where only reversible magnetization processes are inferred from hysteresis data. This result is interpreted in terms of centres or of internal surfaces with high magnetic anisotropy. the sources are either a very low amount of segregated anti-ferromagnetic phases, leading to exchange anisotropy, or Fe2+ in the environment of lattice defects carrying an angular orbital momentum due to the disturbed crystal field; in both cases, the mechanism leading to high field rotational hysteresis losses may be the same. the assumed high anisotropy Fe2+ and the neighbouring grain regions in pure titanomagnetite particles may be of importance in the formation of a thermo-remanence since they may act like single domain particles.

Transdomain thermoremanent magnetization

Transdomain thermoremanent magnetization (TRM) is produced when thermally activated transitions between different domain structures become blocked during cooling. This paper investigates transdomain remanence in the pseudo‐single‐domain size range (&lt;1 μm) in magnetite. We use a one‐dimensional micromagnetic model of single‐domain (SD), two‐domain (2D) and three‐domain (3D) structures to determine the energies of these states, the energy barriers between them, blocking temperatures for SD ↔ 2D and 2D ↔ 3D transitions, and relaxation times for the resulting TRMs. Energy barriers are very high at 20°C, from 2000kT to &gt;6000kT for SD ↔ 2D transitions. Transdomain viscous remanent magnetization (VRM) will not occur, even on geological timescales, by a one‐dimensional excitation such as edge nucleation of a domain wall. Transdomain blocking temperatures, at which energy barriers fall to 25kT–60kT, are ≥553°C for SD ↔ 2D and ≥574°C for 2D ↔ 3D transitions. There are two separate blocking temperatures, e.g. TB1 for SD ↔ 2D and TB2 for 2D ↔ SD transitions. Usually, only the higher of the two has practical significance because the favored (lower energy) state is already 100% populated at this temperature. Our theory is the first to make quantitative predictions of transition paths, relaxation times, and blocking temperatures for transdomain TRM. It is also quite robust. Relaxing the one‐dimensional constraint and introducing crystal defects would make it easier to nucleate domains, but energy barriers and blocking temperatures would not be reduced greatly. Our principal conclusion, that only the lowest energy state at blocking is significantly populated, is a fundamental consequence of the Boltzmann statistics of equilibrium states and is unaffected by the details of transitions between states. Grain interactions may be responsible for the multiplicity of states observed in large titanomagnetite grains following replicate TRM experiments.

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.

An unusual titanomagnetite replacing spinel

n a recent investigation of the Fe-Ti-oxides in the Koster dyke swarm, SW Sweden, one of the dykes from sector I (41197) was omitted because the cubic Fe-Ti-oxides in this dyke were found to deviate from those in all the other dykes. The Fe-Ti-oxides in 41197 consist partly of ilmenite as free grains, and partly of titanomagnetite with sandwich as well as trellis oxyexsolution lamellae of ilmenite. Some of the titanomagnetite grains have a core of grey spine!, and there are also grains consisting mainly of spine!, but with the spine! clearly being replaced by titanomagnetite. Electron microprobe analyses of these Fe-Ti-oxides have revealed that the ilmenite as free grains has a composition quite similar to that in the other Koster dykes from sector I. Apart from a small content of Cr, the ilmenite lamellae in the titanomagne­tite grains also show good agreement with the other Koster dykes from sector I, but there are small but significant differences between the ilmenite lamellae from grain to grain within the sample. The titanomagnetite groundmass also shows small but significant differences be­tween different grains. The composition of the titanomagnetite varies from ferrofer­rites to ferrochromferrites. The grey spine! varies from ferrospinel over ferrochromspinel to ferroferrichrom­spinel. Except for the spine! grain with only incipient replacement by titanomagne­tite, the spinels have a rim which is richer in Mg, Al and Zn and poorer in Cr and Fe+++ than the core. Temperature and fO2 of coexisting pairs of ilmenite and titanomagnetite varies from 991°C, fO2 10-12-9, to 1104°C, fO2 10-11-2.

Volcanic ash particles as carriers of remanent magnetization in deep-sea sediments from the Kerguelen Plateau

Carbonate sediments from the Kerguelen Plateau (ODP Leg 120) of Eocene to Pliocene age were investigated with rock magnetic, petrographic and geochemical methods to determine the carriers of remanent magnetization. Magnetic methods showed that the major magnetic minerals were titanomagnetites slightly larger than single domain particles. Submicrometre to micrometre-size grains of titanomagnetite were identified as inclusions in volcanic glass particles or as crystals in lithic clasts. Volcanic fallout ash particles formed the major fraction of the magnetic extract from each sediment sample. Three groups of volcanic ashes were identified: trachytic ashes, basaltic ashes with sideromelane and tachylite shards, and palagonitic ashes. These three groups could be equally well defined based on their magnetic hysteresis properties and alternating field demagnetization curves. The highest coercivities of all samples were found for the tachylite, due to the submicrometre-size titanomagnetite inclusions in the matrix. Trachytic ashes had intermediate magnetic properties between the single-domain-type tachylites and the palagonitic (altered) basaltic ashes with low coercivities. Samples which contained mixtures of these different volcanic ashes could be distinguished from the three types of ashes based on their magnetic characteristics. There was neither evidence of biogenic magnetofossils in the transmission electron micrographs nor did we find magnetic particles derived from continental Antarctica. The presence of dispersed volcanic fallout ashes between visible ash layers suggests continuous explosive volcanic activity on the Kerguelen Plateau in the South Indian Ocean since the early Eocene. The continuous fallout of volcanic ash from explosive volcanism on the Kerguelen Archipelago is the source of the magnetic particles and thus responsible for the magnetostratigraphy of the nannofossil oozes drilled during Leg 120.

Origin of the sedimentary magnetic record at Ocean Drilling Program Sites on the Owen Ridge, western Arabian Sea

The 3.2‐m.y. whole core magnetic susceptibility record obtained during Ocean Drilling Program leg 117 from the Owen Ridge in the western Arabian Sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information. The salient features of this record are that (1) it correlates strongly with variations in the concentration and flux of eolian dust; (2) it is driven strongly at Earth orbital periodicities; and (3) there is a significant change in the spectral character at 2.4 m.y., which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas. In view of its potential paleoclimatic significance, we have examined the origin of the rock magnetic signal from the Owen Ridge in more detail. We find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content, there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis. We ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process. The first occurs at about 1.5 m depth and is characterized by a loss of fine‐grained ferrimagnetic material. The second stepshift in magnetic properties occurs at a depth of 6–8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity (magnetite) and high coercivity (hematite and/or goethite) magnetic mineral components. Scanning electron microscope and energy dispersive Xray analyses show that the bulk of the coarse‐grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the Fe:Ti ratio of these grains across the lower diagenetic front. This suggests that titanomagnetite grains with higher Ti contents may be more resistant to the process of reductive diagenesis, and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front.

Diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments, South Island, New Zealand

Detailed magnetostratigraphic studies of Late Neogene siliciclastic sediments of the Awatere Group, South Island, New Zealand (41°45′S, 174°05′E) have revealed a wide range of palaeomagnetic behaviour. Examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic, X-ray diffraction and electron microprobe analyses. These analyses indicate that the ferrimagnetic iron sulphide minerals, greigite and pyrrhotite, are responsible for a stable and intense magnetic remanence in fine-grained sediments, whereas titanomagnetite is the only remanence-bearing mineral identified in coarser-grained sediments, which are less strongly and less stably magnetised than the fine-grained sediments. Detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation, which occurs commonly in rapidly deposited, anoxic sediments that support active sulphate reduction and H 2S formation. Preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process, probably due to the low permeability of the fine-grained sediments and consumption of available H 2S before full reaction to pyrite occurred. Relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution.

A Detailed Magnetic and Opaque Petrologic Study of A Thick Tholeiite From the Rajmahal Volcanics, India

Summary A 22 m thick Rajmahal tholeiite basalt lava flow shows wide variations in magnetic properties which have been explained by opaque petrologic studies. Microscopic ore evidence clearly demonstrates that the upper one-third of the lava is dominated by Fe-rich titanomagnetites from high temperature (deuteric) oxidation and that the lower two-thirds are dominated by unoxidized Ti-rich titanomagnetite except for a thin low temperature oxidation zone. the natural remanent magnetization (NRM) intensity appears to be related to the composition of magnetic minerals, giving rise to high values for Fe-rich titanomagnetites and lower values for their Ti-rich counterparts. Stable magnetization is confined to two narrow zones in the flow where the titanomagnetite grains are intensively subdivided by lamellar ilmenites or by wide cracks. the initial susceptibility and the concentration of magnetic oxide grains both show little variation, suggesting the dependence of the former on the latter. Although the relation of hysteresis properties and opaque mineralogy is usually complex, the ratio of remanent magnetization to the saturation remanent magnetization, JRS/JS, is chiefly dependent on the effective grain size and the coercive force Hc, tends to be higher for anisotropic grain fabrics resulting from lamellar ilmenite formation in titanomagnetite. the Q-values of the specimens suggest that the remanent magnetization is significant only in the high temperature oxidation zone of the lava.

The magnetic properties of Quaternary and Tertiary sediments in the southern North Sea

AbstractCores from boreholes penetrating late Quaternary, glacial, interglacial and postglacial sediments and the underlying late Cenozoic delta complex of the southern North Sea have been examined for their magnetic properties. A magnetic polarity stratigraphy has been established as an aid to biostratigraphic dating of the sediments; the Kaena‐Gauss and Gauss—Matuyama transitions and the base and top of the Olduvai subchron have been identified. The strength and stability of laboratory‐induced isothermal remanent magnetisation display clear magneto‐petrological variations, which match lithostratigraphic changes in the cores. Principal component analysis has picked out a basin‐wide and palaeoenvironmental consistency in the magnetic data. Large, multi‐domain magnetite grains predominate in the post‐deltaic and fluvio‐deltaic sediments, whereas smaller greigite or titanomagnetite grains are concentrated in the intertidal and marine deltaic facies. Since heavy mineral analysis indicates that most of the deltaic detritus derived from common source areas, the differences in magnetic mineralogy have probably been caused by the sediment transport processes operating within the delta complex.

Magnetic property zonation in a thick lava flow

In this study, grain size and composition‐dependent magnetic properties of titanomagnetite minerals are used as indicators of intraflow structures and magmatic evolution in an extensive and thick (30–60 m) basaltic lava flow. Similar zonation occurs in this flow at three localities separated by tens of kilometers. The magnetic properties subdivide the flow to three zones. The upper layer, representing the top 1/3 of the lava (≤ 20 m), has higher magnetic stability due to smaller and more deuterically oxidized titanomagnetite grains, approaching pure magnetite. The central layer in the underlying 2/3 of the flow (≤ 35 m) has larger, magnetically less stable, and less oxidized grains with relatively uniform magnetic properties. The basal layer, the bottom 1/10 of the flow (≤ 5 m), has near primary, least oxidized titanomagnetites (Ulv68Mag32). The upper intraflow boundary of the magnetic properties appears to coincide with the transition from entablature (above) to colonnade (below), distinguishing between regions of faster and slower cooling. Microprobe data indicate that the intraflow oxidation state (Fe3+/Fe2+) of the initially precipitated primary titanomagnetites increases with falling equilibrium temperature from the flow margins to a maximum near the center, the position of lowest equilibrium temperature. In contrast, Curie temperature measurements indicate that titanomagnetite oxidation increases with height in the flow. Modification of the initially symmetric equilibrium titanomagnetite compositions was caused by subsolidus high‐temperature oxidation possibly due to hydrogen loss produced by dissociation of magmatic water, as well as unknown contributions of circulating air and percolating water from above. The titanomagnetites of the basal layer of the flow remain essentially unaltered.

The microstructure and domain structure of multiphase oxidized titanomagnetites

An estimation of the domain state of 15 natural and synthetic samples containing both homogeneous and multiphase oxidized titanomagnetites was made by means of J rs J s and H cr H c ratios, the Lowrie-Fuller criterion, the thermomagnetic criterion, F criterion and the Preisach diagram. The J rs J s and H cr H c ratios and the Lowrie-Fuller criterion are shown to be not sufficiently informative for a determination of the domain state. In the case where the lamellae thickness became thinner than 0.1 μm, titanomagnetite grains demonstrate multidomain behaviour independent of the size of the interlamellar regions (cells). If the lamellae become thicker than 0.1 μm the domain state depends on the size of the cells. Single-domain behaviour is obtained for a cell size less than 1 × 1 μm; in agreement with the results of others, larger cells have multidomain properties.