Search

The Paleocene lavas from Dianzhong Formation (E1d) in Linzhou basin of southern Lhasa terrane are a key target for paleomagnetic investigations into the timing and paleolatitude of the initial India‐Asia collision. Controversy exists, however, on whether these rocks preserve a primary remanent magnetization. Here we reanalyze previously published thermal demagnetization data and report detailed rock magnetic results and petrographic observations of these rocks. We find that the original magnetic carrier, a magmatic multidomain Ti‐poor titanomagnetite, underwent significant grain size reduction and was variably reacted to single‐domain maghemite and nano‐hematite. Such strong alteration may have resulted from successive hydrothermal events: a first event related to the ∼52 Ma dike intrusions into the E1d that accompanied a massive ignimbrite eruption deposited above the E1d producing heating up to 300°C; and a secondary event related to the 42–27 Ma southward overthrusting of the basin, heating the E1d up to 130–145°C. Unblocking/inversion temperature spectra of the authigenic maghemite and nano‐hematite overlap with those of the titanomagnetite, implying that the primary remanence of the E1d lavas has been contaminated or replaced by thermoviscous and chemical remanent magnetizations. Thus the isolated characteristic remanent magnetization from these rocks, whether slightly or completely altered, cannot be considered primary and should not be used for paleolatitudinal determination. Our study confirms that hydrothermal alteration can seriously jeopardize the remanence carried by titanomagnetite and thus should be tested for paleomagnetic investigations of rock units from tectonically active areas. The study of past magnetic directions preserved in rocks (paleomagnetism) enables to constrain the paleolatitudes of the southern Asian margin where the rocks formed and is thus key to understand the timing and latitude of the initial India‐Asia collision. However, paleomagnetic studies of Paleogene rocks of southern Tibet have returned a surprisingly wide spread of paleolatitude data. One possible explanation is unrecognized remagnetization, a process that may relate to subsequent hydrothermal alteration. Such hydrothermal alteration has recently been documented for Paleocene lavas in the Linzhou basin, paleomagnetic signals from which have previously been interpreted either as primary or secondary (re)magnetizations. Here we analyze previously published and newly acquired paleomagnetic, rock magnetic, and petrographic data of these rocks to identify whether hydrothermal alteration caused remagnetization, and how such events may be fingerprinted. We find that the original magnetic carrier is Ti‐poor titanomagnetite. Hydrothermal alteration, however, led to significant changes in these titanomagnetite crystals and mineral transformations to maghemite and hematite, associated with acquisition of secondary magnetizations with the magnetic behaviors similar to the primary magnetization. This can be misleading for the interpretation of the magnetic directions obtained from these rocks and imply that they are not suitable for paleolatitudinal determination. Hydrothermal alteration intensively destroyed the original multi‐domain titanomagnetite and recorded magnetization in the Dianzhong lavasTitanomagnetite experienced serious grain size reduction and has variably converted to authigenic single‐domain maghemite and nano‐hematitePrimary remanent magnetization has been replaced or contaminated, these lavas should not be used to determine Lhasa terrane paleolatitude Hydrothermal alteration intensively destroyed the original multi‐domain titanomagnetite and recorded magnetization in the Dianzhong lavas Titanomagnetite experienced serious grain size reduction and has variably converted to authigenic single‐domain maghemite and nano‐hematite Primary remanent magnetization has been replaced or contaminated, these lavas should not be used to determine Lhasa terrane paleolatitude

Titanomagnetites in mid‐ocean ridge basalt (MORB) experience variable post crystallization alterations associated with seafloor tectonic and environmental processes. Compared to low‐temperature oxidation, seafloor hydrothermal alteration is thought to be more destructive but its magnetic aftermaths are insufficiently documented. Here we present comprehensive rock magnetic and electron microscopic analyses of fresh and hydrothermally‐altered MORBs dredged from the Longqi and Yuhuang hydrothermal fields, Southwest Indian Ridge. We observe large variations in magnetic properties of fresh MORBs, originated from relative proportions of nano‐scale single‐domain to vortex state and micron‐scale vortex to multi‐domain state dendritic titanomagnetites. Progressive hydrothermal alteration produces secondary magnetite through recrystallization of exsolved and dissolved Fe from primary titanomagnetite. Exsolution is evident by a dual Verwey transition signature and coexisting Ti‐poor titanomagnetites and sphenes in partially chloritized basalts. A schematic model is proposed to explain the variations in magnetomineralogy and magnetic properties with progressive hydrothermal alteration. Intermediate hydrothermal alteration products retain a secondary chemical remanent magnetization (CRM) which is related to the long‐term magnetization variations in oceanic basalts. The established framework allows characterizing MORB hydrothermal alteration and ultimately contributes to resolving the complexity of seafloor magnetism. Seafloor magnetism is essential for understanding geodynamics and resource prospecting. Near the seafloor high‐temperature hydrothermal fields that host “black smokers”, the primary titanomagnetites in oceanic basalts experience complicated alterations that profoundly change seafloor magnetic anomalies. This study investigates the alteration of titanomagnetite through fluid‐rock interactions in such seafloor settings. We find titanomagnetite is disassembled into Fe‐rich and Ti‐rich phases when interacting with fluids, causing a series of magnetic property variations. A schematic model is proposed to explain how high‐temperature fluid‐rock interactions gradually destroy seafloor magnetism. This model improves our understanding of seafloor magnetism from the perspective of hydrothermal alterations, which is important for applying magnetic surveys to prospecting seafloor resources and probing subseafloor structures. Magnetomineralogy and magnetic property of MORB vary with progressive high‐temperature hydrothermal alterationExsolution superimposed on titanomagnetite dissolution, producing a dual Verwey transition feature in partially chloritized basaltsHydrothermal alteration may have contributed to the long‐term magnetization variations of oceanic basalts Magnetomineralogy and magnetic property of MORB vary with progressive high‐temperature hydrothermal alteration Exsolution superimposed on titanomagnetite dissolution, producing a dual Verwey transition feature in partially chloritized basalts Hydrothermal alteration may have contributed to the long‐term magnetization variations of oceanic basalts

The late Pennsylvanian glacial rhythmites of the Mafra Formation (Itararé Group) from the Paraná Basin of Southern Brazil, have a complex rock‐magnetic signature. Rock‐magnetic tests imply that both hematite and magnetite in varying grain sizes are responsible for the magnetic remanence. Thermal and alternating field (AF) demagnetization of the natural remanence reveal different behaviors that are attributed to a mixture of remanence‐carrying magnetic minerals and grain sizes, implying a series of magnetic overprints. A great circle analysis of the distribution characteristic directions reported suggests that these rocks may not be entirely immune from magnetic overprints acquired during the Jurassic‐Cretaceous in the context of widespread remagnetizations in South America. The nature of the magnetization recorded by these rocks warrants caution in their use for the evaluation of paleosecular variation models. The Late Pennsylvanian glacial rhymites of the Mafra Formation have a complex rock‐magnetic signatureThese rocks have been overprinted during the Jurassic‐Cretaceous in the context of widespread remagnetizations in South AmericaThe nature of the magnetization recorded by these rocks precludes their use for the evaluation of paleosecular variation models

Several types or grain sizes of ferromagnetic minerals can contribute to a rock's remanence and anisotropy of remanence. Each subpopulation may have a different fabric. Measuring anisotropy of partial anhysteretic remanent magnetization (ApARM) allows one to determine the anisotropy contribution of subpopulations with different coercivity distributions. Separating these contributions to remanence anisotropy can provide information about early versus late stages of deformation in fabric studies and is the basis for improved anisotropy corrections in paleomagnetic studies. Unfortunately, collecting multiple ApARM tensors on each specimen is time‐consuming and not often done. Measuring a smaller number of carefully chosen ApARM tensors and obtaining the remaining tensors of interest by tensor calculation would be more efficient. This can only be done, however, when ApARM tensors are additive. Here we investigate the additivity of ApARM tensors in a range of lithologies, by measuring a total of seven ApARM and anisotropy of anhysteretic remanent magnetization (AARM) tensors for each specimen, and comparing the tensors calculated from a combination of ApARM tensors to the corresponding measured AARM. Differences in principal directions between measured and calculated tensors are often smaller than the confidence angles of the measurements. Mean anhysteretic remanences are additive to within ±5%. The anisotropy degree varies by ±30% (k′) or ±0.15 (P), and the shape parameter Uby ±0.4. These error limits will help to determine whether or not it is necessary to measure each ApARM tensor in future fabric or paleomagnetic studies, or if these tensors can be calculated from a smaller set of measurements. Sets of 7 ApARM and AARM tensors measured for 93 samples to test additivity of ApARMsPrincipal directions are additive within confidence limit of measurementMean pARMs additive to ±5%; error limits for anisotropy degree: ±30% (k′), ±0.15 (P), and shape: ±0.4

Magnetic fabrics are powerful tools in structural geology and tectonic studies, because they provide a fast and efficient measurement of mineral alignment, which helps interpret a rock's (de)formation history. The magnetic fabric of remanence‐carrying minerals provides useful information when these grains record different deformation stages than the bulk minerals in a rock. When rocks contain several subpopulations of remanence‐carrying minerals, each of these potentially displays a distinct fabric. This can lead to complex remanence anisotropies, being a superposition of all subpopulations' individual anisotropies. Characterization of partial remanence anisotropies has been used to investigate changes in fabric with grain size. However, most studies still report one bulk remanence anisotropy tensor per sample, and it remains to be determined how commonly different populations of remanence‐carrying grains reflect different subfabrics. Based on a large sample collection including 93 specimens from different lithologies, we have investigated the coercivity dependence of anisotropy of (partial) anhysteretic remanent magnetization A(p)ARM. We find that the principal directions, degree, and shape of A(p)ARM are generally dependent on the coercivity window used to impart the anhysteretic remanent magnetizations (ARMs). Depending on the carrier minerals and their fabrics, ARM anisotropy can either increase or decrease when the ARMs are applied over larger coercivity windows. Additionally, the coercivity fraction that dominates the ARM anisotropy is not always the coercivity fraction that acquires the strongest mean ARM. This illustrates the complexity of characterizing remanence anisotropy, and highlights the importance of carefully choosing experimental parameters in A(p)ARM determination for both magnetic fabric and anisotropy correction studies. A(p)ARM tensors, principal directions, and anisotropy parameters depend on coercivityFabrics of grain subpopulations can interfere positively or negativelyCarefully choose and report experimental parameters for AARM determination