Natural Remanent Magnetization Values Research Articles

Elemental and isotopic response of different carbon components to anaerobic oxidation of methane: A case study of marine sediments in the Shenhu region, northern South China Sea

• Evidences show that the SMTZs are located at 300–360 and 440–480 cmbsf in core SH-CL52. • Under high methane flux and short methane-seep duration conditions, carbon isotope of TOC can indicate the location of SMTZs. • In the SMTZ, the positive excursion of TOC isotope is possibly due to the relatively high methanogenic rate. • TOC isotope has the potential to be a novel proxy of SMTZ. In the cold seep region, the carbonate rocks/nodules help to reveal the source of methane and gas-hydrate decomposition. Conventionally, authigenic carbonate rocks/nodules are great proxy of AOM, but it only forms under an intermediate methane flow rate. It is thus vital to explore another carbon component proxy of AOM. In this paper, we investigate different carbon component features from a 5.2 m-long sediment core (SH-CL52) in the Shenhu region. We constrain the location of SMTZ based on the natural remanent magnetization (NRM) characteristic, pyrite content, TS/TOC and sulfur isotope, after that we discuss carbon component’s isotopic response to AOM. Pyrite increases in 300–360 and 440–480 cmbsf, negative (NRM AF60mT - NRM AF80mT )/NRM values occurrence in 300–360 and 440–480 cmbsf, bulk-sediment δ 34 S values have positive excursion in 300–360 and 440–480 cmbsf and pyrite δ 34 S positive excursions appear in 300–360 and 440–480 cmbsf. All these reflect that at least two methane release events (MREs) are identified in 300–360 and 440–480 cmbsf in the core SH-CL52, respectively. In these zones, δ 13 C TC , δ 13 C TIC , δ 13 C FC and δ 13 C FGC variety is not discernable, but δ 13 C TOC has positive excursions which are most likely interpreted to the relatively higher methanogenesis rate in SMTZs. Our results suggest that δ 13 C TOC represents a potential proxy of SMTZ under high methane flux and shortduration condition.

Obsidians of Pantelleria (Strait of Sicily): A Petrographic, Geochemical and Magnetic Study of Known and New Geological Sources

Abstract This paper provides new petrochemical and paleomagnetic data from obsidian sub-sources on the island of Pantelleria, exploited since the Neolithic. Data has been obtained from 14 obsidian samples from 4 locations: Fossa della Pernice (2 sites), Salto la Vecchia and Balata dei Turchi. Here, we aim to better characterize these obsidians using a cross-disciplinary and multi-analytical approach, to further understand their archaeological significance. Major element analyses (EMP) have enabled two compositional super-groups to be distinguished: (i) Fossa della Pernice, less peralkaline and (ii) Balata dei Turchi–Salto la Vecchia, distinctly more peralkaline and having almost identical chemical patterns. Trace element analyses (LA-ICP-MS) corroborate major element groupings, with the Balata dei Turchi–Salto la Vecchia super-group being further characterized by a pronounced negative europium anomaly. Glass H2O contents (FT-IR) reveal an overlap among all the sub-sources (H2O = 0.1–0.3 wt. %). Magnetic methods have refined the petrochemical groupings, permitting further distinction between Balata dei Turchi–Salto La Vecchia and the Fossa della Pernice super-groups. The occurrence of sub-microscopic (< 1 μm) ferromagnetic minerals results in different magnetic susceptibility and Natural Remanent Magnetization values and allows the best distinction among the products from the chosen sites. When compared with obsidian tools excavated from Bronze-age settlements on the island of Ustica (230 km NE of Pantelleria), 12% are distinctly peralkaline, indicating their provenance to be from the Balata dei Turchi sub-source.

Specific features of the mineral composition and petromagnetic properties of rocks from the Minami-Khiosi submarine volcano (Mariana island arc)

Complex studies of the mineral composition and petromagnetic properties of the rocks which compose an edifice of the Minami–Khiosi submarine volcano located in the Mariana island arc are carried out for the first time. The Minami–Khiosi Volcano is a part of the Khiosi volcanic complex within the alkaline province of the Idzu–Bonin and Mariana island arcs. All of the rocks analyzed are enriched in K2O (1.34–3.30%), Ba (370–806 ppm), and Sr (204–748 ppm). The basalt has a porhyric texture and contains mosTy olivine phenocrysts as individual crystals and growths with a size up to 2 cm; the groundmass is finecrystalline. The samples studied contain at least three Fe-bearing oxide minerals. These are predominant magnetite and less abundant ilmenite and Fe hydroxides. It is established that the samples studied are magnetically isotropic and have high values of natural remanent magnetization and Konigsberger ratio. Similarly to the other island-arc Late Cenozoic submarine volcanoes in the western part of the Pacific Ocean, the samples studied are strongly differentiated by the value of natural remanent magnetization and magnetic susceptibility. The low-coercivity magnetic minerals (titanomagnetite and magnetite) of the pseudo-single-domain structure, as well as high-coercivity minerals (hematite) are the main carriers of magnetization. The high values of natural remanent magnetization are explained by the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility result from the high concentration of ferromagnetic grains.

Petromagnetism of ores and host rocks in the Southern Omolon ore district (Northeast Russia)

The petromagnetic properties of rocks from the Verkhnii Omolon, Magnetitovyi, Innyaga, and Alekseeskii areas of the Southern Omolon iron-ore district located in the southern part of the Omolon Massif (Northeast Russia) are investigated. The maximum values of natural remanent magnetization (NRM) and magnetic susceptibility (k) of 22.8–1196 A/m and 1046 × 10–3 SI on average, respectively, are typical of ferruginous quartzite from the Innyaga site. The amphibolites exhibit high values of these magnetic parameters: Jn is approximately 28.6 A/m and k is 45 × 10–3 SI. The magnetic properties (Jn and k) of the plagiogneisses and granite gneisses are highly heterogeneous and variable. The migmatites, rhyolites, and dacites are submagnetic. Magnetic minerals are largely represented by multidomain and pseudosingle-domain magnetites and their oxidation products such as maghemite, hematite, and iron hydroxides. It is revealed that the amphibolites are characterized by normal polarity of the remanent magnetization, while the ferruginous quartzites demonstrate normal and reverse polarities. These rocks may serve as sources of magnetic bipolar anomalies.

Petromagnetic and microprobe studies of rocks in the Sofu underwater volcanic cluster, Izu-Bonin island arc, Pacific Ocean

A multidisciplinary study of samples that were dredged during cruises of the R/V Vulkanolog was continued in order to interpret results from a survey of the anomalous magnetic field on the Pacific seafloor. We acquired data on the main ferromagnetic carrier and the nature of magnetization. The naturally occurring magnetic parameters and petromagnetic properties of the rocks that make up the edifices of the Sofu underwater volcanic cluster at the Izu-Bonin island arc have been studied. We provide the results of thermomagnetic, petrologic, and microprobe studies of the dredged samples. It is shown that the main magnetic mineral carriers of natural remanent magnetization are unaltered titanomagnetite and oxidized titanomagnetite. The differentiation of rocks over values of natural remanent magnetization was due to different crystallization conditions for titanomagnetite, while that over magnetic susceptibility was controlled by the magnetic grain size. The structural, mineralogic, and petrographic features of the samples show that they were taken from rocks that were discharged during different phases of eruptions.

Modification of the magnetic mineralogy in basalts due to fluid-rock interactions in a high-temperature geothermal system (Krafla, Iceland)

SUMMARY Active high-temperature (>150 °C) geothermal areas like the Krafla caldera, NE-Iceland, often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity. The main alteration processes in such an environment are low-temperature oxidation (<350 °C, maghemitization) and fluid–rock interactions. We investigated the rock magnetic properties [natural remanent magnetization (NRM) magnetic susceptibility and their temperature and field variation] and the mineralogy, using X-ray diffraction, microscopic methods and electron microprobe analyses, of two drill cores (KH1 and KH3) from the rim of the Krafla caldera. The drill cores have distinctly lower NRM values (average <3 A m–1) compared to younger surface basalts (average 20 A m–1) along with a large variation in magnetic susceptibility (1.3 × 10−7– 4.9 × 10−5 m3 kg–1). The secondary mineral assemblage (sulphides, sphene, rutile and chlorite) indicates an alteration within the chlorite–smectite zone for both cores without depth zoning. Optical miscroscopy in combination with the Bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite: (1) titanomaghemite with intermediate titanium concentration and probably high vacancy concentration, and (2) titanomaghemite with low titanium concentration and low vacancy concentration. The mineral assemblages, textures and magnetic properties deduced from the mentioned magnetic measurements indicate two-stage transformation mechanism: (1) Dissolution of titanium at low pH under oxidizing conditions. The ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene, and Fe2 + migrates out of the spinel lattice forming titanomaghemite. (2) Formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions. The latter mechanism produces ghost textures (all titanomaghemite is transformed and only their former grain shapes are preserved), with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved. This mechanism could explain the significant magnetization loss, which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like Iceland or Hawaii. The production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of Krafla.

Electric currents along earthquake faults and the magnetization of pseudotachylite veins

Pseudotachylites occur in the form of thin glassy veins quenched from frictional melts along the fault planes of major earthquakes. They contain finely grained magnetite and often exhibit a high natural remanent magnetization (NRM). High NRM values imply strong local electric currents. These currents must persist for some time, while the pseudotachylite veins cool through the Curie temperature of magnetite around 580 °C. There is no generally accepted theory explaining how such powerful, persistent currents may be generated along the fault plane. Data presented here suggest the activation of electronic charge carriers, which are present in igneous rocks in a dormant, inactive form. These charge carriers can be “awakened” by the application of stress. They are electrons and defect electrons, also known as positive holes or p-holes for short. While p-holes are capable of spreading out of the stressed rock volume into adjacent p-type conductive unstressed rocks, electrons require a connection to the hot, n-type conductive lower crust. However, as long as the (downward) electron flow is not connected, the circuit is not closed. Hence, with the outflow of p-holes impeded, no current can be sustained. This situation is comparable to that of a charged battery where one pole remains unconnected. The friction melt that forms coseismically during rupture, provides a conductive path downward, which closes the circuit. This allows a current to flow along the fault plane. Extrapolating from laboratory data, every km 3 of stressed igneous rocks adjacent to the fault plane can deliver 10 3–10 5 A. Hence, the current along the fault plane will not be limited by the number of charge carriers but more likely by the (electronic) conductivity of the cooling pseudotachylite vein. The sheet current will produce a magnetic field, whose vectors will lie in the fault plane and perpendicular to the flow direction.

Comment on a paper “The origin of high magnetic remanence in fault pseudotachylites: Theoretical considerations and implication for coseismic electrical currents” by E.C. Ferré, M.S. Zechmeister, J.W. Geissman, N. Mathana Sekaran, and K. Kocak

Pseudotachylites occur in the form of thin glassy veins quenched from frictional melts along the fault planes of major earthquakes. They contain finely grained magnetite and often exhibit a high natural remanent magnetization (NRM). High NRM values imply strong local electric currents. These currents must persist for some time, while the pseudotachylite veins cool through the Curie temperature of magnetite around 580 °C. There is no generally accepted theory explaining how such powerful, persistent currents may be generated along the fault plane. Data presented here suggest the activation of electronic charge carriers, which are present in igneous rocks in a dormant, inactive form. These charge carriers can be “awakened” by the application of stress. They are electrons and defect electrons, also known as positive holes or p-holes for short. While p-holes are capable of spreading out of the stressed rock volume into adjacent p-type conductive unstressed rocks, electrons require a connection to the hot, n-type conductive lower crust. However, as long as the (downward) electron flow is not connected, the circuit is not closed. Hence, with the outflow of p-holes impeded, no current can be sustained. This situation is comparable to that of a charged battery where one pole remains unconnected. The friction melt that forms coseismically during rupture, provides a conductive path downward, which closes the circuit. This allows a current to flow along the fault plane. Extrapolating from laboratory data, every km3 of stressed igneous rocks adjacent to the fault plane can deliver 103–105 A. Hence, the current along the fault plane will not be limited by the number of charge carriers but more likely by the (electronic) conductivity of the cooling pseudotachylite vein. The sheet current will produce a magnetic field, whose vectors will lie in the fault plane and perpendicular to the flow direction.

Evidence for textural and alteration changes in basaltic lava flows using variations in rock magnetic properties (ODP Leg 183)

The different effects of cooling and alteration on magnetic properties, in single thick flows from subaerial and submarine eruptions, cored and logged during Ocean Drilling Program (ODP) Leg 183 at Sites 1137 and 1140 (Kerguelen Plateau) are examined. Downhole logging data from both sites is supplemented by petrology and geochemistry of 32 samples from three subaerial lava flows at Site 1137 and two flows units at Site 1140, covering transects from fresh to highly altered basalts. Changes in magnetic properties have previously been observed in several ODP drill holes, which penetrate basaltic basement. In subaerial basalts, a typical trend is that of high magnetic susceptibility and natural remanent magnetization (NRM) values in the altered flow top, and lower values in the less-altered massive flow interior. In contrast, submarine lava flows display the opposite behavior in their magnetic properties. Altered pillow rims have lower susceptibility and NRM values than the fresh pillow interiors. It is concluded that rate of cooling and degree of alteration are the main factors influencing the magnetization and, hence, the distribution of iron oxides. The effects of low-temperature alteration are most noticeable in the distribution of more mobile elements, such as K. Consequently, the spectral gamma ray (SGR) log, which in basaltic basement is largely controlled by K concentration, is an excellent proxy to the downhole identification of alteration. The strong positive correlation observed for the subaerial basalts between the downhole total magnetic field (F tot) and SGR, suggest a potential link with alteration in the drilled sections. The alteration of the submarine basalts is not as pronounced and therefore no correlation is evident.

Insights into Permian Pedogenesis and Climate: Magnetic Stratigraphy of Four Stacked Paleosols, Lower Permian (Wolfcampian) Roca Shale, Central Kansas: ABSTRACT

Abstract This study focuses on the rock magnetic character of four stacked paleosols (P1 at base, P4 at top) in the Lower Permian (Wolfcampian, Council Grove Group) Roca Shale exposed near Manhattan, Kansas. Magnetic characterization of Quaternary soils has proven a valuable tool for evaluating climatic conditions and pedogenic processes, yet no systematic study of pre-Quaternary soils has been undertaken. The Roca paleosols are protosols, vertisols, calcic vertisols, and calcisols. Cross-cutting relationships and superposition principles suggest that macro- and micro-climatic conditions evolved as soil formation progressed. In particular: 1) in P2, clasts of the lower (vertic) part are entrained in the upper (calcic, leached) part suggesting a shift from subhumid to semiarid; and 2) in P3 and P4, carbonate nodules have argillic coats that are at times overlain by (hematitic) ferrans, suggesting climate transitions from semiarid to subhumid back to semiarid. Rock magnetic data provide further insight into pedogenesis and climate. A preserved magnetic signature is suggested by: l) distinct magnetic susceptability, natural remanent magnetization (NRM), anhysteric remanent magnetization (ARM) and isothermal remanent magnetization (IRM) in B, Bt, Bk, E, and R horizons; and 2) carbonate nodules and adjacent subangular blocky structure which preserve the same susceptability trends. Overall, susceptability ranges from <2.0 to 11.6 x 10{-8}m{3}/kg, but within P2, P3, and P4, there is a general upwards decrease. PI contains upwards increasing susceptability. These changes correspond to a shift in S-ratios (IRM[0.3T]/SIRM) ratios from ~0.9 to ~0.4, reflecting a possible change from magnetite to hematite upwards through the paleosol profile. NRM values range from almost 1 x 10{-8}`to 5 x 10{-5} Am{2}/kg. NRM/susceptability and SIRM/susceptability ratios (proxies for ferrimagnetic vs. paramagnetic contributions to susceptability) decrease from PI to P4 and the position of the highest ratio within each paleosol profile is lowered from the B (P1) to top Bt (P2) to base Bt (P3, P4) horizons. Rock magnetic data and field observations suggest that weathering intensity increased through time from the development of P1 to P3 (P3 represents the most leached and oxidized paleosol profile), but decreased during development of P4. The oxidizing conditions prevalent in upper soil horizons led to increased lessivage in these intervals and subsequent concentration of clays and iron oxides in lower, less oxidizing horizons. Gleyed intervals are characterized by (SD?) magnetite. The results of this study show that coupled field observation and rock magnetic characteristics of pre-Quaternary paleosols potentially will yield significant climatic and pedogenic information.

Natural and Induced Remanent Magnetism in Birds

We measured values for natural remanent magnetization (NRM) and isother- mal-induced remanent magnetization (IRM) in the head and neck for relatively large samples of eight bird species, and smaller samples of 13 additional species. Significant differences were found in mean NRM values among species; values ranged from 3.090 x 10-12 T (tesla) for the Carolina Wren (Thryothorus ludovicianus) to 38.069 x 10-12 T for the Northern Bobwhite (Colinus virginianus). Mean IRM values ranged from 337.6 x 10-12 T in Chimney Swifts (Chaetura pelagica) to 1,889.1 x 10-12 T in European Starlings (Stutnus vulgaris), with intraspecific variation being notably high. For two European Starlings and a Common Grackle (Quiscalus quiscula), about three-fourths of the IRM was located in the head and one-fourth in the neck; heads of two Northern Bobwhites contained an even greater proportion of the IRM. In general, the direction of the ferromagnetic material varied substantially among individuals within species. No significant differences were found in mean-vector directions among species. Linear regressions of NRM and IRM values on the logs of mean body mass indicate that the intensity of magnetism is related to species size. Insectivores, which also were the smallest species, had lower NRM and IRM values than found for omnivores sampled. Characteristic demagnetization-remagnetization curves suggest that most of the magnetic materials are interacting single-domain or pseudosingle-domain grains of magnetite. Three species may contain some superparamagnetic material. No differences were found between migratory and nonmigratory species with respect to the amount of remanent magnetism, or the extent of intraspecific variability in orientation direction of NRM. Received 24 May 1990, accepted 23 September 1991.

Magnetic structure of the upper kilometer of the marine crust at Deep Sea Drilling Project Hole 504B, eastern Pacific Ocean

Deep Sea Drilling Project hole 504B is located in 5.9 m.y. crust in the eastern Pacific Ocean about 200 km south of the Costa Rica Rift. At 1076 m subbasement, it is the deepest penetration of marine crust yet achieved. We present here magnetic data from this hole, especially from the recently cored leg 83 section (which constitutes the lowermost 500 m). These data, when combined with those of other studies, yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available. The basement rocks from hole 504B can be divided into three major magnetic units. The upper units (the top 500 m or so of basement) are essentially similar to other shallow marine basement sections, consisting of a mixture of various types of extrusive basalt. Low‐temperature altered titanomagnetite is the dominant magnetic carrier, and the magnetic properties of these rocks are comparable to other shallow marine basalts. Below the upper units is a 200‐m transition zone, consisting of a mixture of extrusives and dikes. High‐temperature hydrothermal alteration (much of it greenschist facies grade) has caused oxidation‐exsolution of primary titanomagnetite as well as its partial or total replacement by silicates. The dominant magnetic carrier is an Fe‐rich titanomagnetite, magnetically indistinguishable from pure magnetite. The combination of oxidation‐exsolution and silicate replacement results in a very low natural remanent magnetization (NRM). The remaining 300 m are in the upper portion of the sheeted dike complex. Primary titanomagnetite is exsolved, but alteration is less intense than in the transition zone. NRM values are substantially higher than those of the transition zone, due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite (which occurs as a silicate alteration product). The NRM magnitude is sufficiently high that the sheeted dike complex, in this location at least, may make a significant if not substantial contribution to the magnetic anomaly. Overall, the magnetic properties of this hole, especially the lower 500 m, are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling. As hydrothermal temperatures do not appear to have exceeded 400°C, the NRM in the lower two sections is apparently a chemical rather than a thermal remanence. As such, models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust. A better understanding of the nature of postemplacement alteration in the marine crust is required, especially its characteristics at high temperatures, and its effects on magnetic properties.

Distribution of the values of natural remanent magnetization and magnetic susceptibility of some minerals

A statistical treatment is presented of the observed values of natural remanent magnetization and of magnetic susceptibility of natural minerals: magnetite, chromite, ilmenite pyrrhotite, haematite, cassiterite and garnets. It was found that for most minerals the distribution of the natural remanent magnetization as well as the magnetic susceptibility is logarithmically normal at a significance level of p=0.05. The typical values of Jn and x, the limits of the intervals of reliability of these typical values for p=0.05, and the standard deviations of the distribution were determined for the individual minerals. The end values of the sets were tested by two independent tests of extreme deviations at a level of significance of p=0.05. Following statistical deliberations it was proved that the lognormal distribution of the Jn and x values depended on the number of factors affecting these values, independently of the type of distribution of these so-called “disturbing” factors. By generalizing for rocks it was shown that the lognormal and normal types of distribution of Jn and x values are extreme cases as regards the observable types with rocks.