84 research outputs found
Mono Lake or Laschamp geomagnetic event recorded from lava flows in Amsterdam Island (southeastern Indian Ocean)
We report a survey carried out on basalt flows from Amsterdam Island in order
to check the presence of intermediate directions interpreted to belong to a
geomagnetic field excursion within the Brunhes epoch, completing this
paleomagnetic record with paleointensity determinations and radiometric dating.
The directional results corroborate the findings by Watkins and Nougier (1973)
: normal polarity is found for two units and an intermediate direction, with
associated VGPs close to the equator, for the other two units. A notable result
is that these volcanic rocks are well suited for absolute paleointensity
determinations. Fifty percent of the samples yields reliable intensity values
with high quality factors. An original element of this study is that we made
use of the PTRM-tail test of Shcherbakova et al. (2000) to help in the
interpretation of the paleointensity measurements. Doing thus, only the high
temperature intervals, beyond 400 degres C, were retained to obtain the most
reliable estimate of the strength of the ancient magnetic field. The normal
units yield Virtual Dipole Moments (VDM) of 6.2 and 7.7 10e22 Am2 and the
excursional units yield values of 3.7 and 3.4 10e22 Am2. These results are
quite consistent with the other Thellier determinations from Brunhes excursion
records, all characterized by a decrease of the VDM as VGP latitude decreases.
40Ar/39Ar isotopic age determinations provide an estimate of 26+-15 Kyr and
18+-9 Kyr for the transitional lava flows, which could correspond to the Mono
Lake excursion. However, the large error bars associated with these ages do not
exclude the hypothesis that this event is the Laschamp
Magnetic fingerprints of stalagmites
Dating stalagmite usi
ng paleomagnetic methods is still
in its early stage of development.
Questions still remain regarding
the nature and origin of the magnetic carriers and the
reliability of the natural remanent magnetization preserved within the thin carbonat
ed
laminations of stalagmites. Here we apply high
-
resolution rock
-
and paleo
-
magnetic
methods
on two (altered and preserved) stalagmites in order to identify the magnetic
and mineralogical signatures and to
assess
the stability of the remanence
Rock-magnetic properties of topsoils and urban dust from Morelia (>800,000 inhabitants), Mexico: Implications for anthropogenic pollution monitoring in Mexico’s medium size cities
En el presente trabajo, investigamos la correlación
entre algunos parámetros magnéticos y el
nivel de contaminación por metales pesados
en suelos urbanos de la ciudad de Morelia, en
el occidente de México. El estudio magnético
fue llevado a cabo en 98 muestras urbanas
provenientes de diferentes tipos de uso de
suelo. La mayoría de las muestras contienen
minerales ferrimagnéticos como responsables de
la magnetización, pertenecientes probablemente
a las soluciones sólidas de las titanomagnetitas/
titanomaghemitas. Esto es inferido a partir de
las mediciones de susceptibilidad en función
de la temperatura y de los experimentos de
magnetización remanente isotérmica (MRI). Estas
mediciones indican además, que la mayoría de las
muestras se saturan casi completamente antes de
los 300 mT. Adicionalmente, los valores S-200 (S-200
= IRM-200/SIRM, donde IRM-200= magnetización
a campo inverso de 200 mT después de la
saturación magnética) se encuentran entre 0.7
y 1.0, característicos de minerales de baja
coercitividad magnética. Las curvas promedio
de magnetización remanente isotérmica de
saturación (SIRM) pueden ser usadas como un
indicador del nivel de contaminación, ya que estas
curvas muestran diferentes valores de saturación
de acuerdo al nivel de contaminación por metales
pesados: Cu, Ni, Cr y Sr. Estas asociaciones de
(titano)magnetitas con metales pesados fueron
observadas bajo el Microscopio Electrónico de
Barrido, revelando algunos agregados complejos
en lugar de las esférulas detectadas comúnmente.In this work, we investigate the correlation between some magnetic parameters and the level of contamination by heavy metals in urban soils from Morelia city, western Mexico. The magnetic study was carried out on 98 urban soils samples belonging to distinct land uses. Most of analyzed samples contain ferrimagnetic minerals as the responsible for magnetization, most probably corresponding to the titanomagnetites/titanomaghemites solid solutions. This is inferred from the susceptibility vs. temperature
measurements and the isothermal remanent magnetization (IRM) experiments. These measurements also indicate that most of samples are almost completely saturated before 300 mT. Additionally, the S-200 values (S-200 = IRM-200/ SIRM, where IRM-200= Back-field of 200 mT after magnetic saturation) are between 0.7 and 1.0, characteristic of low coercivity magnetic minerals. The averaged saturation isothermal remanent magnetization (SIRM) curves can be used as an indicator of pollution level, as these curves show
different saturation values according to the level of contamination by heavy metals: Cu, Ni, Cr and Sr. These associations of (titano)magnetite with heavy metals were observed by Scanning Electron Microscope revealing some complex aggregates rather than commonly detected spherules.Universidad
Nacional Autónoma de México through the
projects CONACYT 118971 and PAPIIT IN 22311
X-ray magnetic circular dichroïsm provides strong evidence for tetrahedral iron in ferrihydrite
International audience[1] Ferrihydrite is an important iron oxyhydroxide for earth and environmental sciences, biology, and technology. Nevertheless, its mineral structure remains a matter of debate. The stumbling block is whether a significant amount of tetrahedrally coordinated iron is present. Here we present the first X-ray magnetic circular dichroïsm (XMCD) measurements performed on a well characterized synthetic sample of 6-line fer-rihydrite, at both K and L 2,3 energy edges of iron. XMCD results demonstrate unambiguously the presence of tetrahedrally coordinated Fe(III) in the mineral structure, in quantities compatible with the latest extended X-ray absorption fine structure (EXAFS) analyses suggesting a concentration of 20–30%. Moreover , we find an antiferromagnetic coupling between tetrahedral and octahedral sublattices, with the octa-hedral sublattice parallel to the external magnetic field. Components: 5100 words, 3 figures
Depositional setting, provenance and tectonic-volcanic setting of Eocene-Recent deep-sea sediments of the oceanic Izu-Bonin forearc, NW Pacific (IODP Expedition 352)
New biostratigraphical, geochemical, and magnetic evidence is synthesized with IODP Expedition 352 shipboard results to understand the sedimentary and tectono-magmatic development of the Izu–Bonin outer forearc region. The oceanic basement of the Izu–Bonin forearc was created by supra-subduction zone seafloor spreading during early Eocene (c. 50–51 Ma). Seafloor spreading created an irregular seafloor topography on which talus locally accumulated. Oxide-rich sediments accumulated above the igneous basement by mixing of hydrothermal and pelagic sediment. Basaltic volcanism was followed by a hiatus of up to 15 million years as a result of topographic isolation or sediment bypassing. Variably tuffaceous deep-sea sediments were deposited during Oligocene to early Miocene and from mid-Miocene to Pleistocene. The sediments ponded into extensional fault-controlled basins, whereas condensed sediments accumulated on a local basement high. Oligocene nannofossil ooze accumulated together with felsic tuff that was mainly derived from the nearby Izu–Bonin arc. Accumulation of radiolarian-bearing mud, silty clay, and hydrogenous metal oxides beneath the carbonate compensation depth (CCD) characterized the early Miocene, followed by middle Miocene–Pleistocene increased carbonate preservation, deepened CCD and tephra input from both the oceanic Izu–Bonin arc and the continental margin Honshu arc. The Izu–Bonin forearc basement formed in a near-equatorial setting, with late Mesozoic arc remnants to the west. Subduction-initiation magmatism is likely to have taken place near a pre-existing continent–oceanic crust boundary. The Izu–Bonin arc migrated northward and clockwise to collide with Honshu by early Miocene, strongly influencing regional sedimentation
Expedition 391 Preliminary Report : Walvis Ridge Hotspot: drilling Walvis Ridge, Southeast Atlantic Ocean, to test models of ridge hotspot interaction, isotopic zonation, and the hotspot reference frame
Hotspot tracks (quasilinear chains of seamounts, ridges, and other volcanic structures) provide important records of plate motions, as well as mantle geodynamics, magma flux, and mantle source compositions. The Tristan-Gough-Walvis Ridge (TGW) hotspot track, extending from the active volcanic islands of Tristan da Cunha and Gough through a province of guyots and then along Walvis Ridge to the Etendeka flood basalt province, forms one of the most prominent and complex global hotspot tracks. The TGW hotspot track displays a tight linear age progression in which ages increase from the islands to the flood basalts (covering ~135 My). Unlike Pacific tracks, which are simple chains of seamounts that are often compared to chains of pearls, the TGW track is alternately a steep-sided narrow ridge, an oceanic plateau, subparallel linear ridges and chains of seamounts, and areas of what appear to be randomly dispersed seamounts. The track displays isotopic zonation over the last ~70 My. The zonation appears near the middle of the track just before it splits into two to three chains of ridge- and guyot-type seamounts. The older ridge is also overprinted with age-progressive late-stage volcanism, which was emplaced ~30–40 My after the initial eruptions and has a distinct isotopic composition. The plan for Expedition 391 was to drill at six sites, three along Walvis Ridge and three in the seamount (guyot) province, to gather igneous rocks to better understand the formation of track edifices, the temporal and geochemical evolution of the hotspot, and the variation in paleolatitudes at which the volcanic edifices formed. After a delay of 18 days to address a shipboard outbreak of the coronavirus disease 2019 (COVID-19) virus, Expedition 391 proceeded to drill at four of the proposed sites: three sites on the eastern Walvis Ridge around Valdivia Bank, an ocean plateau within the ridge, and one site on the lower flank of a guyot in the Center track, a ridge located between the Tristan subtrack (which extends from the end of Walvis Ridge to the island of Tristan da Cunha) and the Gough subtrack (which extends from Walvis Ridge to the island of Gough). One hole was drilled at Site U1575, located on a low portion of the northeastern Walvis Ridge north of Valdivia Bank. At this location, 209.9 m of sediments and 122.4 m of igneous basement were cored. The latter comprised 10 submarine lava units consisting of pillow, lobate, sheet, and massive lava flows, the thickest of which was ~21 m. Most lavas are tholeiitic, but some alkalic basalts were recovered. A portion of the igneous succession consists of low-Ti basalts, which are unusual because they appear in the Etendeka flood basalts but have not been previously found on Walvis Ridge. Two holes were drilled at Site U1576 on the west flank of Valdivia Bank. The first hole was terminated because a bit jammed shortly after penetrating igneous basement. Hole U1576A recovered a remarkable ~380 m thick sedimentary section consisting mostly of chalk covering a nearly complete sequence from Paleocene to Late Cretaceous (Campanian). These sediments display short and long cyclic color changes that imply astronomically forced and longer term paleoenvironmental changes. The igneous basement yielded 11 submarine lava units ranging from pillows to massive flows, which have compositions varying from tholeiitic basalt to basaltic andesite, the first occurrence of this composition recovered from the TGW track. These units are separated by seven sedimentary chalk units that range in thickness from 0.1 to 11.6 m, implying a long-term interplay of sedimentation and lava eruptions. Coring at Site U1577, on the extreme eastern flank of Valdivia Bank, penetrated a 154 m thick sedimentary section, the bottom ~108 m of which is Maastrichtian–Campanian (possibly Santonian) chalk with vitric tephra layers. Igneous basement coring progressed only 39.1 m below the sediment-basalt contact, recovering three massive submarine tholeiite basalt lava flows that are 4.1, 15.5, and >19.1 m thick, respectively. Paleomagnetic data from Sites U1577 and U1576 indicate that their volcanic basements formed just before the end of the Cretaceous Normal Superchron and during Chron 33r, shortly afterward, respectively. Biostratigraphic and paleomagnetic data suggest an east–west age progression across Valdivia Bank, becoming younger westward. Site U1578, located on a Center track guyot, provided a long and varied igneous section. After coring through 184.3 m of pelagic carbonate sediments mainly consisting of Eocene and Paleocene chalk, Hole U1578A cored 302.1 m of igneous basement. Basement lavas are largely pillows but are interspersed with sheet and massive flows. Lava compositions are mostly alkalic basalts with some hawaiite. Several intervals contain abundant olivine, and some of the pillow stacks consist of basalt with remarkably high Ti content. The igneous sequence is interrupted by 10 sedimentary interbeds consisting of chalk and volcaniclastics and ranging in thickness from 0.46 to 10.19 m. Paleomagnetic data display a change in basement magnetic polarity ~100 m above the base of the hole. Combining magnetic stratigraphy with biostratigraphic data, the igneous section is inferred to span >1 My. Abundant glass from pillow lava margins was recovered at Sites U1575, U1576, and U1578. Although the igneous penetration was only two-thirds of the planned amount, drilling during Expedition 391 obtained samples that clearly will lead to a deeper understanding of the evolution of the Tristan-Gough hotspot and its track. Relatively fresh basalts with good recovery will provide ample samples for geochemical, geochronologic, and paleomagnetic studies. Good recovery of Late Cretaceous and early Cenozoic chalk successions provides samples for paleoenvironmental study
Data report: hysteresis properties of igneous rocks from Holes U1439C, U1440B, and U1442A
International audienc
Data report: hysteresis properties of igneous rocks from Holes U1439C, U1440B, and U1442A
International audienc
Data report: hysteresis measurements on basalts from Holes U1346A, U1347A, and U1350A on Shatsky Rise
International audienceI measured hysteresis loops and direct current backfield curves on 11 samples from Hole U1346A, 58 samples from Hole U1347A, and 51 samples from Hole U1350A, which were cored during Integrated Ocean Drilling Program Expedition 324 on Shatsky Rise. The hysteresis parameters of these samples showed a variety of values, mostly ranging from single-domain to pseudosingle-domain behaviors
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