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1.
Large grains, of the order of tens of microns in size, of synthetic titanomagnetite have been oxidized to produce an intergrowth of phases, essentially similar to that produced by the natural process of deuteric oxidation. The scale of the intergrowth is at the limit of the optical range but the electron microscope reveals the characteristic lamella microstructure. The magnetic hysteresis properties of the most highly oxidized material, having probably about 10% of residual spinel phase, are typical of dispersed monodomain magnetite rods. The investigation therefore supports the model in which it is proposed that titanomagnetite grains in a slowly cooled basalt may carry a highly stable component of natural remanent magnetization.  相似文献   

2.
We report opaque mineralogical observations and magnetic properties of primary titanomagnetites in Tertiary submarine gabbros from DSDP, Legs 30 and 37 and in a late Archean, continental granitic pluton, the Shelley Lake granite. The titanomagnetites and silicates in all the submarine gabbros have been deuterically oxidized. There is no indication of subsequent low-temperature oxidation, although serpentization of olivines is pervasive in the deeper Leg 37 units. The Leg 30 samples, from a single thick sill, contain abundant coarse (≈100 μm) titanomagnetites with fully developed ilmenite exsolution lamellae. Curie temperatures are 515–550°C; there are no low Curie temperatures that would indicate surviving unoxidized titanomagnetite. The unserpentinized Leg 37 gabbros contain scarce opaques with pure magnetite Curie points that are barely resolvable microscopically; most occur as inclusions in pyroxene. In the Shelley Lake granite, on the other hand, many samples exhibit bimodal blocking-temperature spectra, with blocking temperature peaks at 250–300°C and 550–575°C. The low-blocking-temperature phase is unidentified. No pyrrhotite was seen in thin section. Optically homogeneous grains coexist with fully exsolved neighbours, but the electron microprobe indicates no titanium. The lamellae appear to be haematite, not ilmenite, and the primary composition of the opaques is pure magnetite. The oxidation state of the opaques is very inhomogeneous, even on a fine scale.  相似文献   

3.
Synthetic single crystals of titanomagnetite of nominal composition Fe2.4Ti0.6O4 have been oxidized at 1275°C in controlled gas atmospheres, producing multiphase intergrowths to simulate the natural process of deuteric oxidation. The evolution of the intergrowths was monitored using the conventional techniques of petrology: optical and electron microscopy and X-ray and electron microprobe analyses. In addition, the measured magnetic properties — particularly the temperature-dependence of hysteresis properties — provided further information about the composition and concentrations of magnetic phases, and their domain state, as oxidation proceeded. The evolution of a trellis pattern of ilmenite lamellae, characteristic of the “exsolution” stages of deuteric oxidation, was observed in the oxidized crystals. The interlamellar spinel region consisted of two iron-enriched titanomagnetites, one thought to occur along the lamellar boundaries. The magnetic hardness of both phases was found to be greater than the original homogeneous multidomain titanomagnetite crystals, although neither phase achieved monodomain characteristics, and the stability of thermoremanence (TRM) remained quite low (median destructive fields (MDF) of the order of a few thousend A m?1). The lamellae made little contribution to the total remanence. A sharp rise in magnetic hardness, observed during the post-exsolution stages of oxidation, was due to the presence of fine-grain monodomain magnetite, thought to be distributed within the haemoilmenite matrix, but probably not having been formed by lamellar subdivision. The crystals could now carry an intense and stable TRM with MDFs of many tens of thousands of A m?1.  相似文献   

4.
Magnetic minerals are mostly identified by a combination of rock magnetic and microscopic techniques and the ferrofluid method in combination with an optical microscope was applied during the last decades. But today, scanning electron microscopy (SEM) is preferred for the observation of mineral phases because its depth of focus and resolving capability at the same magnification. In this study, we report on a method, which allows ferrofluid application under the SEM. We coated a polished basalt sample containing titanomagnetite grains with high demagnetizing fields, with a colloidal suspension containing magnetite particles (ferrofluid) ranging in size between 11 and 20 nm. Due to large gradients of multidomain grains the ferrofluid particles adhered to their surfaces. Other grains of similar composition present (ilmenohematite) but with low demagnetizing fields do not generate large magnetic gradients and therefore do not attract the colloidal particles. Upon evaporation of the ferrofluid and covering the sample with conducting material the magnetic grains with high demagnetizing field are easily identifiable under the scanning electron microscope. The different mineralogy observed by this method is confirmed by temperature dependent variation of magnetic susceptibility, revealing titanomagnetite and ilmenohematite as magnetic carriers in the basaltic samples.  相似文献   

5.
Titanomagnetites of basalts and diorites from an off-axis volcanic episode in the Pacific Ocean (Leg 61, DSDP) were studied for their ulvospinel composition and for changes in the iron-titanium ratio with progressive low-temperature oxidation. Thermomagnetic analysis and microscopy indicate that these Fe/Ti spinels show relatively little low-temperature oxidation, despite their Cretaceous age. Microprobe investigation shows that the Fe/Ti spinels have a wide range of ulvospinel content directly reflecting the range of parent rock chemistries. This is an important demonstration that ulvospinel contents are not constant but vary within a restricted range of compositions, namely at least x =0.5 to x=0.75 (Fe2?xTixO4). Recognition of this fact is important in attempting to elucidate changes in cation ratio as a result of progressive low-temperature oxidation.Examination of samples of different oxidation states for cation ratio changes as a result of low-temperature oxidation revealed lower titanium in the more oxidized samples and Ti enrichment in the less oxidized ones. This is probably a reflection of changing x values with progressive crystallization of individual sills, and thus unrelated to cation ratio changes with oxidation.In more than 100 grains analyzed, no evidence was found for a more oxidized shell on the exteriors of grains, suggesting that the oxidation process did not proceed uniformly with respect to grain geometry.  相似文献   

6.
To understand the oxidation state and process of oxidation of lava domes, we carried out magnetic petrological analyses of lava samples obtained from domes and block-and-ash-flow deposits associated with the 1991–1995 eruption of Unzen volcano, Japan. As a result, we recognize three different types of magnetic petrology, each related to deuteric high-temperature oxidation during initial cooling. Type A oxides are characterized by homogenous titanomagnetite and titanohematite, indicating a low oxidation state and high titanomagnetite concentrations. Type B oxides are weakly exsolved and contain titanohematite laths and rutile lenses, indicating a higher oxidation state. Type C oxides, which represent the highest oxidation state, are completely exsolved and composed of Ti-poor titanomagnetite, titanohematite, rutile, and pseudobrookite, indicating high hematite concentrations. Some grains in Types A and B show indications of reduction, which was related to interaction with volcanic gases subsequent to high-temperature oxidation. In terms of geological occurrence, the oxidation processes probably differed for endogenous and exogenous domes. Endogenous dome lavas are oxidized concentrically and are classified into the three types according to their location within the dome: samples from the surface are strongly oxidized and classified as Type C, while the inner part is unoxidized and classified as Type A. Exogenous dome lavas are unoxidized and assigned to Type A. Some samples show signs of reduction, which may have occurred around fumaroles. We propose that location within the dome and the process of dome growth are the factors that control oxidation.  相似文献   

7.
The variation of the low-field susceptibility of basalts down to liquid-nitrogen temperature always falls into one of three types that depend on the composition and grain size of the titanomagnetite grains present. Group 1 basalts contain predominantly unoxidised, multidomain homogeneous titanomagnetites having x 0.3. Group 2 basalts contain predominantly titanomagnetite grains with many exsolved ilmenite lamallae that subdivide the grains so that they act similarly to single domains. Group 3 basalts contain predominantly multidomain magnetite or magnetite-rich titanomagnetite having x 0.15. After repeated heating to 615°C, the group 1 basalts gradually oxidise above 300°C to produce the characteristics of group 2 basalts, owing to the exsolution of ilmenite. On the other hand, both group 2 and 3 basalts are stable to oxidation until at least 500°C. They are therefore the most useful material for palaeointensity studies.  相似文献   

8.
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.  相似文献   

9.
The paper is devoted to studying the mineral composition and magnetic properties, mainly at the cryogenic temperatures, of the Middle–Late Devonian basalts from North Timan. The magnetic minerals in these basalts are dominated by intermediate-composition titanomagnetites (TM25–TM30) which demonstrate unusual magnetic properties in a wide temperature range. At room temperature, a low coercive force coexists with relatively high Mrs/Ms ratios. At cryogenic temperatures, the dependences of magnetic susceptibility on the temperature and frequency of the applied field are characteristic of this titanomagnetite composition, whereas the remanent saturation magnetization acquired at 2 K is destroyed at significantly lower temperatures compared to the synthetic analogs. The obtained results again highlight the necessity of studying the low-temperature properties of titanomagnetite samples with a controlled composition and grain size.  相似文献   

10.
Oligocene basalt samples, cored from Yarraman Creek in the Liverpool Range, New South Wales, Australia, have been found to contain two remanence components with opposing directions. The self-reversed component has been clearly identified in several samples subjected to detailed thermal demagnetization. Moreover, through AF demagnetization results alone, some unheated specimens display consistent multicomponent behavior.Several flows under investigation were extruded during what appears to have been an excursion of the geomagnetic field, each possessing a magnetization direction far from the full polarity state. The possibility that the self-reversed moments are simply secondary components, acquired in an opposing field direction is, therefore, ruled out.Optical examinations and thermomagnetic curves indicate that the samples contain from nearly unoxidized, to moderately low-temperature oxidized, titanomagnetite. The self-reversed component is most clearly removed during thermal demagnetization between about 230 and 290°C, above the observed Curie point of the titanomagnetite, and is believed to be associated with titanomaghemite. Consistently, the role of the self-reversed moment is seen to be most dramatic in samples showing the highest degree of low-temperature oxidation. Moreover, thermal demagnetization of samples from varying depths below the creek bed provide strong evidence that the self-reversed component in some, if not all, of the samples investigated develops at room temperature. The relevant self-reversal mechanism is currently under investigation.  相似文献   

11.
Systematic magnetic and crystallographic studies were made on 57 titanomaghemite samples produced from sintered titanomagnetites with x values of 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0. Curie temperature and lattice parameter values are often significantly different from results of previous studies (Ozima and Sakamoto, 1971; Readman and O'Reilly, 1972; Nishitani, 1979), particularly for x values of 0.4 and 0.6. Other important results are: (1) Oxidation of titanomagnetite is mainly a function of temperature. Atmosphere and Ti content have little influence. (2) Progressive oxidation of homogeneous titanomaghemites can take place in the presence of a rhombohedral phase. The amount of rhombohedral phase produced during oxidation increases with increasing temperature and (less conclusively) with decreasing Ti content. (3) Saturation magnetization measurements at low temperatures show both P- and Q-type ferrimagnetic behavior in titanomaghemites, and also show the Verwey (?) transition in titanomaghemites with composition x=0.0. (4) The temperature of spinel inversion increases with degree of oxidation. Slightly oxidized samples invert near 300°C; for samples with z > 0.8 the inversion temperature is above 450°C. This last result, although neither expected nor understood, is supported by results of thermomagnetic studies on some oceanic basalts.  相似文献   

12.
The aim of this study was to investigate the effect of meteorite impacts on magnetic properties including magnetic susceptibility and the Verwey transition of Ti-poor titanomagnetite of volcanic rocks from the 3.6 Ma old El’gygytgyn impact structure located in the Okhotsk-Chukotka volcanic belt in north-eastern Russia. The target rocks consist mainly of rhyolite with some andesites, and is a rare example of impact structures within volcanic target rocks on Earth. 27 samples from outside the crater, the crater rim and from the depth interval 316 to 517 m below lake bottom (mblb) of the El’gygytgyn ICDP drilling were studied. A significant decrease of the average specific magnetic susceptibility by around 90% was observed between felsic volcanic rocks from the surface (18.1 × 10-6 m3/kg) and the drill cores from near the crater central uplift (1.9 × 10-6 m3/kg). Ferrimagnetic Fe-Ti oxide assemblages (Verwey transition temperature, TV: -161 to -150°C, Curie temperature, TC: 451 to 581°C), occurring in all studied samples, differ significantly. At the surface titanomaghemite is ubiquitously associated with titanomagnetite. The drill cores lack titanomaghemite, but either show a transformation into titanomagnetite and ilmenite or a strong fragmentation associated with a second TV between -172 and -188°C. Reversible curves of temperature dependence of magnetic susceptibility in the suevite indicate high depositional temperatures of at least 500°C. In the polymict and monomict impact breccia mechanical deformation of titanomagnetite and temperatures of at least 200-350°C related to the shock are suggested from temperature dependent magnetic susceptibility cycling. Lowtemperature oxidation along strongly brecciated grain surfaces in titanomagnetite is suggested to cause the lower TV and we suggest that this phenomenon is related to postimpact hydrothermal activity. The strong magnetic susceptibility decrease at El’gygytgyn is mainly influenced by shock, and post-impact hydrothermalism causes a significant additional depletion. These observations explain why magnetic lows are a ubiquitous phenomenon over impact structures.  相似文献   

13.
A three-dimensional model describing the homogenization of titanomagnetites with magnetite–ulvospinel exsolution structures in the course of thermomagnetic analysis is presented. The implications of the size and shape of the exsolution structures and the initial titanomagnetite composition for the temperature dependence of saturation magnetization during repeated heating is analyzed. It is found that the dimension of the exsolution structures has the strongest effect, whereas the shape and composition only have an effect in the case of the small exsolution structures. A method is suggested for estimating the dimensions of the exsolution structures from the thermomagnetic curve. A close consistency of the results is revealed by comparing the distributions of the dimensions of the exsolution structures estimated by the electron microscopy and by the analysis of the thermomagnetic curve for the same sample.  相似文献   

14.
Summary The changes of the anisotropy of the magnetic susceptibility of alkaline Tertiary basalts under titanomagnetite oxidation are investigated. The oxidation of the titanomagnetites was achieved by the following thermal processing of rock samples: 60 mins at a temperature of 400°C, 300 mins at T=400°C and 60 mins at T=500°C. It is shown that considerable changes of the values of the magnetic susceptibility, as well as of the degree of the preferred orientation of the titanomagnetites and of the ellipsoid of anisotropy in the sample occur as a result of the oxidation of titanomagnetites, contained in the samples, due to the said thermal processing. Moreover, the micro-structure of the grains, created by the oxidation of the titanomagnetites, causes the mostly linearly parallel configuration to change to a planar-parallel configuration.  相似文献   

15.
Summary Experiments of heating-cooling cycles in zero magnetic field were performed in order to study self-reversal of NRM in basaltic rocks from Lower Silesia. Complete self-reversal occurred in one sample containing titanomagnetite withT c of 170°C and a small amount of a phase with higher Curie point. During consecutive heat treatments the phenomenon became less conspicuous. In three samples of higher oxidation level, containing several magnetic phases, only partial self-reversal of NRM occurred. For the most oxidized sample no changes of direction of NRM were observed. We suggest that the investigated phenomenon of self-reversal of NRM is due to a negative magnetic interaction between primary titanomagnetite and products of its oxidation. It seems that complete self-reversal can take place in a restricted state of oxidation.  相似文献   

16.
Based on X-ray spectrometry and thermomagnetic analysis, the chemical composition is studied and the Curie points are determined for the Early Cenozoic basalts and limburgites of the Northern Tien Shan. The microprobe analysis used in combination with the thermomagnetic analysis unambiguously identified a series of homogeneous primarily magmatic titanomagnetites in the studied samples against the broad variety of grains of titanomagnetite that underwent single-phase heterophase oxidation, which are often undetectable by a microprobe alone. The titanium content or TiO2/FeO ratio in titanomagnetites and, correspondingly, the Curie points reflect the depth of the most recent equilibrium state of the magma, i.e., the depth of the magma sources. According to the dependence of the content of primarily magmatic titanomagnetite on the source depth, the latest equilibrium state of the magmatic melt of the Northern Tien Shan occurred at a depth of 40 ± 5 km. The obtained results agree with the reduction in the seismic velocities by a few percent below the Moho revealed by seismic tomography.  相似文献   

17.
Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ～1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ～375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.  相似文献   

18.
The rock magnetic properties of the samples of dredged rocks composing the submarine volcanic edifices within the Sea-of-Okhotsk slope of the northern part of the Kuril Island Arc are studied. The measurements of the standard rock magnetic parameters, thermomagnetic analysis, petrographical studies, and microprobe investigations have been carried out. The magnetization of the studied rocks is mainly carried by the pseudo-single domain and multidomain titanomagnetite and low-Ti titanomagnetite grains. The high values of the natural remanent magnetization are due to the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility are associated with the high concentration of ferrimagnetic grains. The highest Curie points are observed in the titanomagnetite grains of the igneous rocks composing the edifices of the Smirnov, Edelshtein, and 1.4 submarine volcanoes.  相似文献   

19.
Al- and Mg-doped titanomagnetites were synthesized at 1300°C using the gas-mixing technique. A composition, representative of average natural titanomagnetites in ocean floor basalts, was sought.

The samples were ball-milled in acetone to average grain sizes of 0.5 μm and 5 μm and the material was then oxidised, in air, at temperatures below 300°C. This procedure formed titanomaghemite, a cation-deficient titanomagnetite.

Low-temperature oxidation is described as the diffusion of Fe-ions out of the spinel lattice and the process is observed to be distinctly dependent upon grain size.  相似文献   


20.
Electron microprobe and reflected light microscopic examinations confirm the presence of composite grains of ferrian ilmenite with Xilm = 0.53 and titanomagnetite with Xusp = 0.13 in a dacite with self-reversed TRM. A parallel TRM component associated with titanomagnetite and a reversed component associated with self-reversing ferrian ilmenite are the principal NRM components. A subordinate, parallel component is associated with ferrian ilmenite which is not magnetically coupled to an “χ-phase”. The natural self-reversing properties are mainly a consequence of the dacite's high quenching temperature, calculated at 862–864°C using the Fe—Ti oxide geothermometer, and are most consistent with the self-reversal mechanism proposed by Lawson et al. [9].The conduction of thermal demagnetization and TRM induction tests in air had a much greater effect on the Fe—Ti oxides than did natural cooling, and resulted in significant oxidation with the consequent modification of some magnetic properties and the formation of another reversed TRM component. The subdivision of titanomagnetite grains by oxidation along fractures decreased its effective grain size and caused an apparent increase in its magnetic intensity, in addition to a slight increase in its resistance to alternating field demagnetization. The χ-phase associated with the reversed NRM component, with 0.42 > Xilm 0.31, became Fe-enriched during the earlier stages of heat treatment. It is suggested that after heating at 600°C for two hours or more, this χ-phase exsolves as titanohematite with Xilm < 0.33. The ferrian ilmenite host is consequently enriched in Ti, and another χ-phase much closer in composition to the host generates a reversed TRM component with Tb < 200°C.  相似文献   

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