Magnetic properties of partially-oxidized near single-domain magnetite below the Verwey transition temperature
Oxidation of magnetite occurs readily in ambient atmosphere. Before complete transformation to the stable end-member, the partially-oxidized product can have a range of stoichiometry, increased disorder and anisotropy, and enhanced stress due to lattice mismatch, giving rise to changes in magnetic properties. Gaining a full understanding of partially-oxidized near single-domain (SD) magnetite particles is of special interest because it allows for identification of these particles in natural rocks and sediments, and offers a foundation for comprehending the remanent magnetization (RM) they carry. Here we report useful and new characterizations of partially-oxidized near SD magnetite particle assemblages, with an emphasis on RM, DC and AC susceptibility, and hysteresis below the Verwey transition. We report and discuss anticipated and unusual cooling treatment dependent RM results under the framework of applied-field dependent magnetocrystalline anisotropy easy-axis selection associated with the Verwey transition. Results from two types of cooling treatments will be presented: one with varying field strength and the other with restricted temperature window of applied field. Aside from RM, we will show that AC susceptibility below the Verwey transition is also sensitive to the field applied during cooling. Furthermore, the DC susceptibility changes depending on waiting time in zero field at a lower temperature than the Verwey transition temperature. Lastly, we will report on shifts in the irreversible hysteresis component measured at 10 K, as well as the appearances of double maxima in the first-order reversal curve (FORC) diagrams.
Relationships Between Magnetic Properties and Weathering Indices of Basaltic Rocks
Performance of geophysical sensors is often hampered by soils and regolith with significant levels of magnetic susceptibility and viscosity, primarily dependent on the amount and form of ferrimagnetic iron oxide minerals present. In order to develop predictive models for the occurrence of such conditions, it is crucial to understand how the magnetic signal evolves during weathering from fresh rock to soil material. Rock weathering leads to destruction of primary minerals, formation of secondary minerals, and concomitant changes in magnetic properties and major-, minor-, and trace-element geochemistry. Previous work has examined relationships between magnetic properties (e.g., magnetic susceptibility) and single-element proxies for overall sample weathering state. In this contribution we study the relationships between bulk geochemical and magnetic characteristics of weathered basaltic rock, and regolith and soils with basaltic parent material. Four samples collected from a corestone formed by spheroidal weathering on the Kohala Peninsula on the Big Island of Hawaii represent the earliest stages of weathering; a series of samples from regolith to the B-horizon for a soil on Kaho'olawe Island represent later weathering stages. Our analysis includes X-Ray Fluorescence Spectroscopy of whole-sample geochemistry and magnetic measurements for a range of temperatures and frequencies. The extent of chemical weathering is assessed by use of a number of common (but Fe-free) major-element weathering indices. Progressive spheroidal weathering involves centripetal migration of a weathering front from joints and fractures into the interior of the joint blocks. As the weathering front passes through a volume of material, fresh or slightly weathered rock is transformed to a primary-mineral-depleted, clay-rich shell. The exfoliated shells farthest from the corestone were the first weathered; shells successively closer to the corestone were more recently transformed. In the Kohala corestone-shell complex, some chemical-weathering indices vary monotonically with total Fe (as Fe2O3), whereas other chemical-weathering indices vary monotonically with magnetic susceptibility. Thus, some Fe-free major-element chemical-weathering indices seem to scale more systematically with soil magnetic properties than others. For more strongly weathered soils from Kaho'olawe, there is a weak relationship between magnetic properties and total Fe. We hope that through the use of Fe-free major-element weathering indices an improved relationship can be developed. Such an improved correlation would benefit phenomenological understanding of geophysical sensor performance in areas with basaltic substrate.
GP34A-03 INVITED [WITHDRAWN]
Magnetic Diagenesis in the Gas Hydrate System
Natural gas hydrate is a methane-bearing form of ice which occurs in permafrost and continental slope settings. Geochemical processes associated with gas hydrate formation lead to the growth of iron sulphides which have a geophysically measurable magnetic signature. Detailed magnetic investigation and complementary petrological observations were undertaken on unconsolidated sediments from three gas hydrate (GH) settings: permafrost in fluvial-deltaic silts and sands in the Western Canadian Arctic (Japex et al. Mallik 5L-38 in 2002); diamictons and hemipelagics in the Cascadia accretionary wedge west of Vancouver Island (IODP Exp.311 in 2006); and marine sands and hemipelagics from the Bay of Bengal (NGHP Exp.01 in 2007). These magnetic measurements provide stratigraphic profiles which reveal fine scale variations in lithology, magnetic grain size, and paleo-pore fluid geochemistry. The highest magnetic susceptibility values are observed in strata which preserve high initial concentrations of detrital magnetite, such as glacial deposits. The lowest values of magnetic susceptibility are observed where iron has been reduced to paramagnetic pyrite, formed in settings with high methane and sulphate flux such as at methane vents. Enhanced values of magnetic susceptibility characterize the introduction of the ferrimagnetic iron sulphide minerals greigite and smythite. These magnetic minerals are mostly found immediately adjacent to the sedimentary horizons which host the gas hydrate and their textures and compositions indicate rapid disequilibrium crystallization. The observed diagenesis result from the unique physical and geochemical properties of the environment where gas hydrates form: methane is available to fuel microbiological activity and the freezing which accompanied GH crystallization quickly removed pure water, froze the sediments into an impermeable solid and expelled more concentrated brines into the adjacent less permeable strata to the point of inducing fracture formation. Magnetic surveying techniques can help delineate anomalies related to gas hydrate deposits, and magnetic logging of wells and core samples provide information on the original lithology and diagenesis caused by gas hydrate formation.
Magnetite Dissolution and Authigenic Magnetic Iron Sulphide Formation in Gas Hydrate- bearing Marine Sediments: Implications for Paleo- and Environmental Magnetic Studies
We present a mineral magnetic study of gas hydrate-bearing marine sediments drilled at southern Hydrate Ridge during ODP Leg 204 (Cascadia Margin, offshore Oregon). A combination of magnetic techniques and SEM observations has enabled identification and screening for the occurrence of magnetite and magnetic iron sulphides (greigite and pyrrhotite) in the studied sediments. Magnetite is detrital in origin whereas greigite and pyrrhotite formed authigenically as a result of microbial activity at variable depths within the gas hydrate stability zone (GHSZ). Degradation of organic matter in the sulphate-reducing zone (upper 4-10 m of sediments) drives dissolution of magnetite and authigenic growth of greigite. Anaerobic oxidation of methane also drives authigenic formation of greigite in the vicinity of the sulphate-methane transition, whose location varies between 9 m and the surface depending on the flux of methane. Between the sulphate-methane transition and the base of the GHSZ (down to 200 m), greigite and pyrrhotite formed authigenically during deep anaerobic oxidation of methane. These results have important implications for paleo- and environmental magnetic studies. Reductive dissolution of magnetite results in partial to total destruction of the original magnetic signal. On the other hand, authigenic growth of greigite and pyrrhotite at variable depths within the GHSZ results in acquisition of a diagenetic magnetization that is delayed between a few thousand and a few million years with respect to depositional age of the sediment. Further complexity is added to the magnetic signal as sedimentation continues because of greigite and pyrrhotite formation at variable (and perhaps multiple) times in any horizon until it is buried beneath the base of the GHSZ. Combined with results from other studies, our results suggest that greigite- and pyrrhotite-bearing marine sediments are problematical for paleomagnetic and environmental magnetic research that relies on analysis of a syn-depositional signal, unless its primary origin (bacterial or detrital) is demonstrated or unless part of the detrital magnetite signal is preserved.
Monitoring the growth of magnetic grains at 95°C in immature and early mature claystones
With modern equipment, it is possible to detect very low concentration of magnetic grains, typically within the order of part per billions. In addition, it is possible to obtain additional constraints on the size of magnetic minerals, because it is only above a certain volume that magnetic minerals carry a stable magnetization. We propose a technique based on two steps: 1) we monitor at 95°C the acquisition of a remanent magnetization (RM) for several weeks under a magnetic field of 2 mT, 2) We investigate the magnetic mineralogy before and after heating, using mainly the magnetic properties at low-temperature using a MPMS. In the frame work of nuclear waste redeposit program, we applied this technique on two claystones : one immature claystones from the basin of Paris (Bure claystones burial temperature ~40°C) and one early mature illite-rich claystones from the Jura thrust belt (Opalinus claystones, burial temperature ~85°C). We heated claystones from few weeks up to several months at 95°C under semi-confined atmosphere. We monitored consistent RM for both claystones. Typically, RM is one-order magnitude larger in immature claystones compare to RM of early mature claystones at the end of the experiment. RM is by essence a combination of an isothermal remanent magnetization (IRM), a thermo-visquous magnetization (TVRM), and a chemical remanent magnetization (CRM). CRM is carried by newly formed magnetic grains that passed the blocking volume. We demonstrated that IRM is negligible and we removed successfully the TVRM at several stages during heating experiments. We observed that CRM represented a large contribution to RM after few days of experiments. When demagnetizing CRM, we noticed a range of unblocking temperature consistent with the occurrence of magnetite in early mature claystones and magnetite and an iron sulphide in immature claystones. We calculated that the production of magnetite is ~0.1 ppb/day in early mature claystones, while it is ~1 ppb/day in immature claystones. Low-temperature investigation of both claystones, before and after heating experiments, showed contrasting results. Natural claystones displayed Verwey transition at 120K which demonstrated the presence of magnetite. In early mature claystones, we observed a ~35K sharp transition that we relate to the presence of pyrrhotite. When cooling a saturated isothermal remanent magnetization, we found that the ~35K magnetic transition is an induced magnetization, that we called P- transition. P-transition is distinguishable from Néel-transition of siderite (~38K) or rhodocrocite (~32K). An embryonic P-transition is observed in immature claystones. After heating, we found that a P-transition developed in immature claystones while low-temperature magnetic properties remained the same for the early-mature claystones. From SEM and TEM observations, we proposed that magnetite and pyrrhotite formed within the pyrite framboid, where organic matter and water trigger chemical transformation. We monitored therefore for the first time the production of magnetite and pyrrhotite in natural claystones at 95°C which corresponds to ~3 km depth. This discovery has considerable implications. First, we found a magnetic fingerprint within the oil window. Second, we can explain widespread remagnetizations in sedimentary rocks. Third, our results questioned the significance of magnetostratigraphy results in sedimentary section buried at 2m and more.
Understanding the enigma of carbonate remagnetization: researching the causes and consequences for authigenic Fe-oxide production
Paleomagnetic and rock magnetic studies of carbonate units from around the world and across the geologic timescale reveal a ubiquity of remagnetizations. And although the occurrences of these remagnetizations are widely accepted, their mechanism(s) of acquisition, and particularly the relationship between magnetization acquisition and penecontemporaneous regional geologic phenomenon, are not fully understood. What are relatively well understood, are the distinct rock magnetic signatures that distinguish remagnetized from unremagnetized carbonates. Of these signatures, hysteresis properties have been the most universal, with typical remagnetized carbonates having consistently high coercivity ratios with respect to remanence ratios. Such ratios are typically associated with bimodal distributions of vastly different magnetic coercivity phases caused by mixtures of either grain size (i.e., a combination of superparamagnetic (SP) and single-domain (SD) grains), magnetic mineralogy, or particle anisotropy. For remagnetized carbonates, these ratios are widely attributed to abundant volumes of secondary SP grains, and thus by inference to growth of new Fe-oxides during remagnetization. The lingering question is then - what produced such large volumes of authigenic Fe- oxides? Several Fe-oxide producing reactions have been proposed, including: 1) illitization, 2) dedolomitzation, 3) crystallization from exotic fluids, and 4) oxidation of Fe-sulfides. All of these reactions require some interaction between the rock and fluids, whether the fluids are meteoric, basinal, or exotic. Recent work investigating smectite-illite transformations suggest that clay reactions are potentially an important source for remanence-carrying authigenic Fe-oxides in remagnetized rocks. There is still, however, significant uncertainty as to the mechanism for clay transformation and the relevance of other Fe-oxide producing reactions to the remagnetization process. Thus, further work is needed to better understand the remagnetization process and its implications for rock-fluid interactions, maturation and migration of hydrocarbons and ore-generating fluids, and orogenic evolution.
Thermal Transformations of Chernozem-like Soil Samples During Laboratory Heating and Their Implications for Revealing Degree of Soil Maturity
Complicated shapes of thermomagnetic curves representing temperature dependence of magnetic susceptibility k(T) are quite usual in many rock magnetic studies on soils and sediments. They, however, can give also useful information about the kinetics of mineral alteration and provide an additional tool for characterization of the initial material. Experimental data obtained for a collection of Chernozem-like soils from Bulgaria using partial k(T) heating - cooling curves are used to deduce the type, grain size and stability of the pedogenic minerals. Appearance of new magnetite phase as a result of heating, which gives several-fold increase in the room temperature susceptibility values is represented by the ratio Kcool/Kinit and gives valuable information about the origin of the newly created phase on heating. The observed linear relationship between the ratio Kcool/Kinit and the relative amount of oxalate-soluble iron (Feo) suggests that thermal transformation of amorphous Fe-containing phases are responsible for the increase of the signal after cooling. Temperature dependence of the exact position of a kink on the cooling branch of k(T) partial curves is supposed to be due to thermal ordering of lattice defects, substitutions and vacancies in the structure of newly created magnetic phase. Differences in microenvironmental conditions in different genetic soil horizons (soil pH, clay content, organic matter, permeability) are present, while parent material is the same. Variation in the values of the ratio Kcool/Kinit, accompanied by similar shapes of thermomagnetic curves suggest that one and the same mechanism operates in all samples, only the relative amount of organic matter decreases with depth and thus determines the rate of Fe3+ reduction necessary for magnetite formation.
Magnetic Signatures of Several Synthetic Iron Oxides Alteration Pathways
The alteration of naturally occurring iron oxides and oxyhydroxides, under the influence of varying environmental
conditions, including micro-environments and biological activity, is an important research topic because their
connection to past climate variations remains to be elucidated. One approach to this subject matter is to
perform laboratory experiments using synthetic iron oxides (sensus lato) subjected to specific physical and
In our presentation, we will report on alteration experiments performed on synthetic lepidocrocite (γ-FeOOH) and maghemite (γ-Fe2O3) particles. A first approach consists in using the starting materials as electron acceptors in bio-reduction experiments involving the iron- reducing bacteria Shewanella putrefaciens and leading to the formation of magnetite (Fe3O4) particles. In a parallel approach, the starting materials are converted to magnetite by slow heating in CO/CO2 atmosphere. Further alteration can be done by slow heating in air, this time inducing aging and oxidation of the material.
At various stages of our experiments, the samples are characterized using both magnetic (low-temperature, low-field and high-field magnetic measurements, Mossbaüer spectroscopy, etc.) and non-magnetic techniques (XRD, HRTEM, etc.). The various experiments conducted on these samples will allow us to study different pathways of magnetite formation and alteration in the environment, including solid-state conversion, partial or total dissolution/precipitation, and particles aggregation.