Progressive Alternating Field Demagnetization Research Articles

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka.Graphical

A record of the lower Mammoth geomagnetic polarity reversal from a marine succession in the Boso Peninsula, central Japan

SUMMARY Palaeomagnetic records from geological archives provide significant information about the nature of geomagnetic polarity reversals; however, there are few detailed palaeomagnetic records of pre-Pleistocene reversals. The lower Mammoth Subchron boundary (late Pliocene) is recorded in a 10-m interval of a marine succession deposited at high accumulation rates (9–66 cm kyr–1) in the Boso Peninsula, central Japan. Here, we report a continuous palaeomagnetic record of the lower, normal to reverse boundary interval of the Mammoth Subchron, including the geomagnetic field direction and relative palaeointensity, with an average temporal resolution of ca. 800 yr. A hybrid method of thermal demagnetization at 200 °C and progressive alternating field demagnetization were used to effectively extract the primary palaeomagnetic component, which is carried by magnetite. The lower Mammoth transition is characterized by palaeomagnetic direction of instability and decay of the relative palaeointensity, and occurred from late Marine Isotope Stage MG3 (3351 ka) to MG2 (3336 ka) or MG1 (3331 ka), spanning 15–20 kyr. Virtual geomagnetic poles (VGPs), calculated from primary palaeomagnetic directions, rapidly rebounded twice from southern latitudes to northern latitudes within the transition. In contrast to the complex lower Mammoth reversal behaviour recorded in the Boso Peninsula succession, records from a lava sequence in O'ahu (Hawai'i) reveal a rebound following a 180° directional change, and those from a marl succession in Sicily (Italy) indicate a single rapid directional change. Diverse geomagnetic field evolution among these three sections is reflected resolution difference among the records likely in combination with an influence of non-axial dipole field.

Archaeomagnetism of four pottery kilns in central Portugal: Implications for secular variation and dating

We report archaeomagnetic results from four pottery kilns in Portugal which are thought to belong to the period of Roman rule (3rd Century BCE-4th Century CE). Very few details have been published to date, so this broad assignment is based on the general archaeological context at each site. Our motivation was to see if a more precise chronology could be established by means of archaeomagnetic dating. Concomitant goals were to compare these results from Portugal to their counterparts in Spain and to expand geographic coverage of the regional geomagnetic secular variation reference curve. Experimentally, all the samples behaved in a very coherent manner during progressive alternating-field demagnetization and yielded high-precision mean archaeomagnetic directions (a95<3°) for each site. The results suggest that two of the kilns, Castelo de Vide and Peniche, were most likely in use during the late 1st/early 2nd centuries CE, whereas the two kilns at Seixal are somewhat younger, dating to the late 2nd to early 4th centuries CE.

Paleomagnetism of Miocene volcanics on Sao Tome: Paleosecular variation at the Equator and a comparison to its latitudinal dependence over the last 5 Myr

AbstractA collection was made in January 2009 of 10 oriented samples from each of 54 sites in lavas on Sao Tome Island (nominal location 0.3ºN, 6.5ºE). Some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation. Overall magnetic properties were excellent (highly stable magnetizations carried by pseudosingle domain magnetite). After principal component analysis of progressive alternating field demagnetization data for the samples, 22 sites had normal polarity magnetizations (D = 0. 6º, I = −8.3º, α95 = 4.3º, κ = 53.1) and 20 had reverse magnetizations (D = 176.0º, I = 4.2º, α95 = 7.3º, κ = 20.8); the directions are within 5° of antiparallel, yielding a positive reversal test. The combined data set of 42 site mean virtual geomagnetic poles converted to common (normal) polarity yields a pole position at 86.0°N, 211.5ºE, A95=3.1º. Ar/Ar and K/Ar dating reveals that these rocks are Miocene in age (∼5–11 Ma), old enough to allow northerly plate motion to help explain the slightly far‐sided pole position. The between‐site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation, Sb, and equaled 11.4° with 95% confidence interval between 9.9° and 13.4°.

Paleomagnetic records and mineral-magnetic properties of deep-sea sediments in the NW pacific: Paleoenvironmental implication

The paleomagnetic records and mineral-magnetic properties of unconsolidated core sediment from the east Mariana Basin of the western Pacific have been analyzed to trace the time-dependent variations in sedimentary environments. Progressive alternating field demagnetization effectively extracts a stable remanent magnetization showing both normal and reverse polarities. Comparison of successive polarity changes, recorded in the sediment core, with reference magnetic polarity time-scale, reveals that the recovered sediment column was deposited since the late Pliocene. From the sediment age model, calculated sedimentation rate during the late Pliocene was 9.8 times higher than that during the Pleistocene. Considering the oceanic environments and geologic setting in the study area, the anomalous high sediment flux during the late Pliocene was probably caused by enhanced current flows, such as North Equatorial Current, associated with atmospheric circulation as well as by debris flows from adjacent sea mounts. In addition, the systematic variation of mineral-magnetic properties indicates periodical fluxes of coarse and magnetically stable particles, on the fine-grained dominant sedimentary environments. Such influxes, however, would not be related to syn-volcanic activities, because the summits of seamounts were totally blanketed by biogenic Pliocene-Pleistocene sediments. It is, hence, reasonable to interpret that paleomagnetic and mineral-magnetic data probably reflect drastic paleoenvironmental changes at the boundary between the Pliocene and Pleistocene, where strong current and atmospheric circulations decreased.

Holocene palaeomagnetic secular variation recorded in multiple lake sediment cores from eastern Finland

SUMMARY Holocene palaeomagnetic secular variation (PSV) in inclination and declination recorded in the sediment remanent magnetization of two small lakes, Lake Lehmilampi (63°37′N, 29°06′E) and Lake Kortejarvi (63°37′N, 28°56′E) in eastern Finland is presented. As an outcome of systematic coring, eight cores, 300–753 cm in length, were investigated. All samples (Lehmilampi n= 1320, Kortejarvi n= 943) were subjected to palaeo- and mineral magnetic analyses. The directions of the characteristic remanent magnetization (ChRM) were obtained from progressive alternating field demagnetization of the natural remanent magnetization (NRM) followed by principle component analysis. The younger sections of the sediment columns in the studied lakes are annually laminated, providing detailed chronologies for dating PSV features back to 5100 cal. BP. The underlying older sections of the cores were dated by palaeomagnetic pattern matching in respect to varve-dated Lake Nautajarvi PSV data (Ojala & Saarinen 2002), thus yielding a composite age model covering nearly the whole Holocene epoch. Average sedimentation rates ranging from ∼0.68 to 0.74 mm yr−1 enabled recording of changes in the geomagnetic field at decadal resolution. The carriers of remanence are dominantly magnetite of stable single-domain to pseudo-single-domain grain size, accompanied by magnetic minerals of harder coercivity. The sediments from both lakes exhibit strong and stable single-component magnetizations nearly throughout the whole cores. The sediment magnetization lock-in delay is estimated to range between 80 and 100 yr. PSV data were transformed into time-series and subsequently stacked to comprise North Karelian stack, and Fisher statistics were used to calculate mean directions together with the 95 per cent confidence level (α95). A comparison of declination and inclination features of the North Karelian stack with previously published data expresses remarkable similarity, therefore confirming the similar source behind the changes in the NRM directional records. The high quality of the PSV data extracted from Lehmilampi and Kortejarvi, in terms of dating as well as amplitude reconstruction, have a high potential to improve existing and future geomagnetic field models.

新潟県北蒲原地域の胎内川における鮮新統鍬江層最上部の古地磁気層序

The magnetostratigraphy for the uppermost part of the Pliocene Kuwae Formation was established along the Tainai River (Natsui Section) in the Niigata Prefecture, Japan. Characteristic components were identified from fourteen sites out of 60 collected sites through the progressive thermal demagnetization (PThD) test, but no stable component was recognized for all pilot specimens through the progressive alternating field demagnetization (PAFD) test. After the tilt-correction for inclined sites, the normally and overlying reversely magnetized sequence was recognized. Combining with biostratigraphic-age constraints, the normal and following reversed polarity sequence can be correlated to the uppermost part of the Gauss (C2An.1n) and the lowest Matuyama (C2r.2r) Chrons. Thus the Matuyama/Gauss boundary (2.581Ma) is located at the uppermost part of the Natsui Section, which gives an upper limit for the underlying marine sequence of the Pliocene Kuwae Formation.

Records of the Cobb Mountain Subchron from the Bermuda Rise (ODP LEG 172)

Duplicate records of the Cobb Mountain Subchron were obtained from two holes (Holes 1063B at 33°41.204′N, 57°36′W and 1063C at 33°41.181′N, 57°36′W) drilled in a sediment drift at Ocean Drilling Program (ODP) Site 1063 on the northeast Bermuda Rise. We sub-sampled the cores from both holes using U-channels (up to 1.5 m long samples with 2×2 cm cross-section) focusing on continuous sampling through the late Matuyama interval containing the Cobb Mountain Subchron. Results of standard progressive alternating field demagnetization experiments reveal that these sediments generally have a stable characteristic component after removal of a low-coercivity drilling overprint. Additional discrete samples (∼7 cm 3 oriented cubes) used in progressive thermal demagnetization studies display similar behavior. By taking duplicate sets of U-channels, we were able to identify and remove several measurement artifacts, such as edge effects that affect measurements made near the ends of the U-channel samples. The upper normal-to-reverse polarity transition was not obtained because of a coring gap or an interval of coring deformation. After removing measurement artifacts, the records from the two holes were stacked to obtain a composite record. Virtual geomagnetic poles (VGPs) computed from the composite record follow a path through the central Pacific during the lower polarity transition, very similar to VGP paths from Cobb Mountain Subchron records from other sites around the world. Furthermore, the VGPs cluster near southern Africa early in the reversal process and then group in a nearly antipodal patch in the north Pacific late in the reversal. Taken together, these records indicate that the transitional field remained nearly dipolar through the polarity reversal, or at least during the beginning and end of the reversal when the field was dominated by an axial dipole tilted about 20–40° to the spin axis. If a tilted dipole is an inherent part of the initiation and ending of a reversal, it may help explain why many sediment recorders, which are thought to smooth the field, record antipodal VGP paths through many reversals.

A sedimentary paleomagnetic record of the Matuyama chron from the Western Antarctic margin (ODP Site 1101)

A high-resolution paleomagnetic record for part of the Matuyama chron (0.7–2.1 Ma) is reported for Ocean Drilling Program Site 1101 (Leg 178), off the Antarctic Peninsula Pacific margin. A rock-magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization (NRM) is carried by pseudo-single domain magnetite. Progressive alternating field demagnetization of 83 m of U-channels provided a polarity stratigraphy down to the Olduvai subchron. Two geomagnetic events preceding the Jaramillo subchron were identified, including the Cobb Mountain polarity interval. The bulk magnetic parameters vary by more than a factor of 20 over the entire time interval, but by less than a factor of 6 over the 0.7–1.1 Ma interval. This latter interval was selected for paleointensity determinations, which were carried out by normalizing the NRM by the anhysteretic remanent magnetization (ARM). Direct comparison of the Site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record. A more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0.95–1.1 Ma interval. This test yielded no outlier for the period considered, confirming the geomagnetic character of the records. We have constructed a low-resolution stack revealing some of the characteristic paleointensity features of the Jaramillo subchron.

Magnetic detection of heated soils at paleolithic sites in Japan

We present a methodological approach to detect heated soil on ancient sites, using magnetic measurements. The method is based on changes in magnetic signals of soil by heating. The following three types of soil were used for testing the method: silty soil (SS), weathered volcanic ash (WVA, = loam) and fairly fresh volcanic ash (VA) called Odori tephra. On heating above 250–600°C, the magnetic susceptibility and remanent magnetization intensity increased for the SS and WVA samples, reflecting chemical alteration of magnetic minerals (from goethites to magnetites through hematites). The VA sample showed no susceptibility change suggesting the absence of goethites within it. On heating below 250°C, only the intensities of all the samples increased. This is possibly due to acquisition of thermal remanent magnetization. The largest change of the magnetic signals was identified for the SS sample and the smallest one was seen for the VA sample. Therefore, the in situ susceptibility measurement, which is the nondestructive and indirect method, seems to be effective to detect heated soil for sites of aqueous deposits as the SS. On the other hand, for sites of aeolian deposits as the WVA (loam) and VA, the intensity measurement of collected soils seems to be the most reliable method to detect evidence of heating. The degree of the magnetic stability (coercivity) against progressive alternating-field demagnetization was also an important parameter, indicating whether the investigated soils were heated or unheated. © 1999 John Wiley & Sons, Inc.

Palaeomagnetic and petrological reconstruction of the Seychelles

Abstract The geology of Mahe-Island and adjoining islets, Seychelles, consists of three types of late Precambrian granitic rock series: (1) a gneissose granodiorite-gneissose tonalite and amphibolite-leucogranodiorite series, (2) a porphyritic granite-pink granite series, and (3) a grey granite-gneissose grey granite series. All these granitic rocks are cut by dykes of doleritic compositions, mostly with a WNW-ESE trend. The gneissose granodiorite is a calcalkaline I-type granite with normative Ab ⪢ Or, the porphyritic granite is a sub-alkaline A-type granite with normative Ab > Or, and the grey granite is an alkaline A-type granite with normative Ab > Or. RbSr radiometric analyses yielded ages of 713±19 Ma for the gneissose granodiorite, 683±16 Ma for the porphyritic granite and 570±5 Ma for the grey granite. Petrological, geological, geochronological and geochemical studies suggest that the late Precambrian granitic rocks of the Seychelles are very similar to, and are well correlated with those of the Arabian-Nubian shield. 408 late Precambrian granite specimens were collected from the Mahe island group, Seychelles. Progressive alternating field demagnetization and thermal demagnetization have been successfully performed to isolate stable remanence with well-grouped direction from natural remanent magnetization of the granites. The stable remanences of the granite fall around a northerly mean direction, of which the declination is 348.5° and the inclination +48.7°. The stable remanences of the Mahe island group granite yield a palaeomagnetic pole at 54.0°N, 38.2°E. From the above data, in conjunction with reported polar wander path and plate rotation poles, it is concluded that the Seychelles were originally located near the eastern end of the Horn of Africa and have drifted to the present position since continental break-up.

Preliminary paleomagnetic results from the Triassic Prida Formation (Lower Star Peak Group), west‐central Nevada, and implications for latitudinal displacement

Paleomagnetic data were obtained from two oppositely dipping carbonate sections of the Triassic Prida Formation, lower Star Peak Group, in the Humboldt Range of west‐central Nevada. Progressive demagnetization of the rocks indicated that the remanent magnetization consists of three or fewer vector components. A characteristic component, with the highest unblocking temperatures, was most effectively isolated by low temperature (200–300°C) thermal followed by progressive alternating field demagnetization. The sections yield in situ mean directions that differ significantly, but that have similar inclinations (Star Canyon: D=311, I=+34, α95=5, k=22, N=38 samples; Fossil Hill: D=281, I=+33, α95=2°, k=2814, N=3 section means, 154 samples). The directions diverge upon simple structural correction, from which we infer that at least one and probably both of the sections were remagnetized, probably in latest Middle to Late Jurassic time. These data cannot be used to indicate significant southward translation of Golconda allochthon rocks, as inferred from magnetizations in underlying volcanic rocks. The translation interpretation may require revision if magnetizations in the volcanic rocks are also secondary.

Paleomagnetism of ash‐flow tuffs: Microanalytical recognition of TRM components

A microanalytical technique, employing a cryogenic magnetometer and progressive alternating field demagnetization, is described and used to investigate the natural remanent magnetization of individual components (groundmass, crystals, and lithic and pumice fragments) in seven ash‐flow tuff lithologies and to determine if subblocking temperature flow‐or compaction‐related movements took place in the units on scales ranging from the volume of the individual microspecimen (1–2 mm in diameter) to that of a standard paleomagnetic core specimen. Closely grouped directions from microspecimens of vitrophyre (α95′s = 4.9° ‐ 12.4° ) and close correspondence of these directions with their site mean directions determined from standard samples suggest that the magnetic components of these units have not been disturbed by movements at low temperatures. Microspecimens from devitrified portions of the cooling units are of significantly lower coercivity than vitrophyre material and yield directions either more scattered about their site mean (α95 greater than 15° ) or uniformly displaced in one direction (up to 35°) away from the mean. Devitrification may be as significant a contributor to the scatter of microspecimen directions as are postblocking temperature randomizing or systematic microtectonic movements. Vitrophyres near the base and extremities of ash‐flow tuff cooling units are probably the most accurate indicators of the paleofield in each cooling unit.

Paleomagnetism of a Paleozoic anorthosite from the Appalachian Piedmont, northern Delaware: possible tectonic implications

Two components of magnetization have been observed in fourty-four samples (five sites) of the anorthosites in the Arden Pluton. One component, with D = 325°, I = −75°, k = 32, α 95 = 13.6°, was isolated in many samples by progressive alternating field demagnetization and in the remainder of the collection by the use of intersecting great circles of remagnetization. The corresponding pole is located at 16°N, 303°E, dp = 22.7°, dm = 24.9°. Assuming the age of the last metamorphism (Taconic, ca. 440 Ma) of the Cambrian Arden Pluton to be the age of the magnetization, this pole deviates significantly from coeval poles thus far obtained from the North American craton. The preferred explanation for this deviation is that the Arden Pluton and the surrounding Piedmont rocks belonged to a different Early Paleozoic plate on the south or east side of the Iapetus Ocean, most likely the African (Gondwana) plate, and that it was transferred to the North American plate during a subsequent continental collision.

Magnetic properties of dredged oceanic gabbros and the source of marine magnetic anomalies

Summary. Magnetic property studies (natural remanent magnetization, initial susceptibility, progressive alternating field demagnetization and magnetic mineralogy of selected samples) were completed on 45 samples of gabbro and metagabbro recovered from 14 North Atlantic ocean-floor localities. The samples are medium to coarse-grained gabbro and metagabbro which exhibit subophitic intergranular to hypidiomorphic granular igneous textures. The igneous mineralogy is characterized by abundant plagioclase, varying amounts of clinopyroxene and hornblende, and lesser amounts of magnetite, ilmenite and sphene. Metamorphic minerals (actinolite, chlorite, epidote and fine-grained alteration products) occur in varying amounts as replacement products or vein material. The opaque mineralogy is dominated by magnetite and ilmenite. The magnetite typically exhibits a trellis of exsolution-oxidation ilmenite lamellae that appears to have formed during deuteric alteration. The NRM intensities of the gabbros range over three orders of magnitude and give a geometric mean of 2.8 x 10-4gau~~ and an arithmetic mean of 8.8 x 10-4gauss. The Konigsberger ratio, a measure of the relative importance of remanent to induce magnetization, is greater than unity for the majority of the samples and indicates that remanent magnetization on average dominates the total magnetization of oceanic gabbros in the Earth's magnetic field. The magnetic properties of fresh and metamorphosed gabbros appeared to be similar. The majority of gabbros studied were characterized by median destructive fields greater than 200 Oe. The high stability is attributed largely to the effective subdivision of the magnetite grains by the

Paleomagnetism of the Deccan Trap Basalts in India

The paleomagnetism of the Deccan Trap lavas in the western Ghats were studied in detail. Several stability tests were performed but the most effective one seemed to be progressive alternating field demagnetization. The direction of stable remanence was determined in 28 Mahabaleshwar and 5 Amboli lavas. It was noted that there is a good correlation between the stability of remanence and certain properties of magnetic minerals. Most rocks with evidence of high temperature oxidation are magnetically more stable than others.As there is no significant serial correlation nor systematic large movement in directions of NRM in Mahabaleshwar lavas, the directions of the NRM may well represent the secular variation in the geomagnetic field in Eary Tertiary. The present data show a scatter of directions larger than the Quaternary paleomagnetic results, suggesting a bigger secular variation at that time. A statistical method to determine the upper limit of the time interval of volcanism was applied to the Mahabaleshwar section with the result that their activity had not lasted more than 5m.y. at 5% significance level. The mean pole position for all the lavas in this study shows a good agreement with the hypothesis that India moved northward in the Cenozoic era with great speed.

Paleomagnetic Evidence for a Cretaceous Disturbing Event in the Precambrian Johnny Lyon Granodio-Rite, Cochise County, Arizona

Block samples were collected in an east-west profile across the 1,655 m.y.-old Johnny Lyon granodiorite, Cochise County, Arizona. Progressive alternating field demagnetization shows that the NRM directions are stable to 175 Oe, and remain tightly grouped. Beyond this field the angular changes are large and irregular. The mean directions initially and after 175 Oe are remote from either the axial or the inclined present fields. The corresponding paleomagnetic pole after 175 Oe shows good agreement with poles from Cretaceous rocks from North America. This independent evidence supports the conclusion from isotopic studies (Silver and Deutsch 1963) that the granodiorite was profoundly affected by Cretaceous alteration processes.

Effects of moderate stresses on directions of thermoremanent magnetization

Experiments designed to test the effects of directed stress on the thermoremanent magnetization (TRM) of igneous and metamorphic rocks are described. Basalt and andesite specimens exhibited no anomalous directions of TRM when cooled from 600°C under uniaxial stresses of up to 350 bars. Metamorphic rocks with preferred crystallographic orientations were found to exhibit anomalous directions of induced TRM after such treatment. These anomalous TRM directions were apparently related to the lineation of the specimens rather than applied stress. Residual magnetizations of the metamorphic specimens were found to approach the direction of the magnetizing field upon progressive alternating-field demagnetization.