GP43A-01 INVITED
Michel Prévot: More Than Thirty Years Reconnaissance of Thermoremance and Viscosity
Since 1968 Michel Prévot has published more than 30 articles on rock magnetic properties, magnetic minerals and mechanisms how they carry a magnetic remanence. The studied minerals have been various titanomagnetites, titanomaghemites and hemoilmenites from continental as well as from submarine volcanic rocks, but also hematite from sediments or pyrrhotite from metamorphic rocks. All these works deal with natural magnetic minerals and persuade the understanding, how thermoremanence and/or viscous remanence are formed and retained. Contributions to the formation of magnetization from chemical processes as well as self- reversals have been studied. Many of these works have been carried out in the context of paleointensity experiments and how various magnetizations can corrupt Thellier experiments. The scope of this fruitful work and its impact on the scientific community will be reviewed and acknowledged.
GP43A-02
Rock Magnetic Properties and Magnetic Petrology of Hawaiian and Icelandic Basalts: a key for the Understanding of high Crustal Magnetizations on Earth and Earth-like Planets?
The rock magnetic behavior and Fe-Ti oxide petrology of subaerial and submarine basalts have been extensively studied over the last decades because of their importance in understanding Earth's magnetic anomalies. Combined rock magnetic and magneto-mineralogic investigations on different basalt lithologies from scientific drillings on Hawaii and Iceland have shown that multiple processes, related to the geodynamic setting, the emplacement and cooling history and the alteration history affected the texture and composition of the originally homogeneous titanomagnetite and caused significant variation in rock magnetic properties like magnetic susceptibility, Curie temperature, coercivity force or natural remanent magnetization (NRM). Hawaiian basalts from the scientific drillings HSDP-2, SOH-1 and SOH-4 have e.g. NRM intensities between <1 and 13 A/m, which is distinctly lower than those measured on several localities on Iceland. Surface samples from fissure eruptions of the Reykjanes peninsula show 4 - 32 A/m and drill cores from the Stardalur central volcano (subaerial basaltic lava flows) show very high values up to 120 A/m. Although the main controlling factors for these high NRM values seem to be the primary magma composition and cooling history, secondary processes like high-temperature decomposition reactions and hydrothermal alteration can also play a role. During the latter process, titanomagnetite is altered to titanomaghemite and furthermore to (cation-deficient) magnetite. Secondary magnetite can be additionally formed due to hydrothermal activity increasing the total magnetization. Modifications of rock magnetic properties related to these magnetic petrology changes will be discussed for basalts of different strong magnetization in relation to geologic processes.
GP43A-03
Early Cretaceous absolute geomagnetic paleointensities from Arapey Formation (Uruguay)
Absolute paleointensity determinations before and during CNS are still scarce and of variable qualities. In this study, we report new paleointensity data from Arapey Formation (part of the Paraná-Etendeka large igneous province) which have several advantages: (1) they are widely distributed in a large volcanic province and easy to access; (2) they record faithfully the magnetic field that existed at the time of their eruption; (3) Most of them are fresh for isotopic dating and have already yielded reliable K-Ar and Ar-Ar ages. 56 samples from 11 individual flows yielded acceptable paleointensity estimates. The mean paleointensity values per flow are ranging from 47.5 to 21.6 microT and the corresponding Virtual Dipole Moments (VDMs) are ranging from 8.9 to 4.3 (1022 Am2). This correspond to the mean value of 6.8 (1.7) x 1022 Am2, which is only slightly lower with respect to the present geomagnetic axial dipole. Currently available selected paleointensity data from 140 to 70 Ma suggest that geomagnetic field strength frequently fluctuated before and during the Cretaceous Normal Superchron while the magnetic polarity maintained stable.
GP43A-04
The strength of the Geomagnetic Field During the Cretaceous Normal Superchron: Preliminary Results Using the Microwave Palaeointensity Technique
The relationship between the strength of the Earth's magnetic field and its tendency to reverse has been a subject of discussion for many years. A key time period to investigate this is the Cretaceous Normal Superchron (CNS). With a paucity of high quality palaeointensity data for this time period however, deducing any such relationship is difficult. Here we present preliminary results from an ongoing investigation, concentrating on producing a set of palaeointensity data spanning the CNS, utilising the microwave palaeointensity technique. The microwave method can significantly reduce alteration during the experimental phase of the investigation compared to the conventional Thellier method, therefore potentially giving higher success rates. As in all palaeointensity investigations, whatever method, ensuring an original thermal remanence is present is paramount. Rock magnetic analyses, SEM imaging and EBSD analyses have been carried out in order to determine the suitability of samples for palaeointensity work. Microwave palaeointensity results are presented and compared to previously published Thellier results (where present) for whole rock samples from Inner Mongolia (113 Ma), Liaoning Province, China (102 Ma) and Madagascar (88 Ma).
GP43A-05 INVITED
Gilbert-Gauss geomagnetic reversal recorded in Pliocene volcanic sequences from Lesser Caucasus: Revisited
We carried out a detailed paleomagnetic, rock-magnetic and Thellier paleointensity study of a ~ 3.6 My Pliocene lava flow succession from southern Georgia. Previous study (Camps et al. 1996, PEPI, vol. 96, pp. 41-59) revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. The transitional field was interpreted as an excursion within chron 2Ar or an upper Cochiti-Gilbert reversal. New paleomagnetic data reported in present study are obtained from nearby lava successions. This allowed the better knowing the nature and morphology of geomagnetic record. In total about 140 standard paleomagnetic cores belonging to 23 consecutive lava flows were obtained during the 2006 sample collection campaign. Rock-magnetic experiments show that the remanence is carried by Ti-poor titanomagnetite in most of cases. The fraction of grains with multidomain magnetic structure does not seem to be important. Characteristic remanent magnetization is successfully determined on all samples. The direct correlation with original (Thoki) sequence and field observations allowed to establish a new magnetic stratigraphy. The lower part of section is characterized by intermediate magnetic polarity followed by thick reversely magnetized lavas. The upper sequence, represented by 11 consecutive flows yielded normal magnetic polarity. The mean paleointensity of the intermediate field is 12.8 (2.7) microT (10 flows). The reverse polarity paleointensity is higher with a mean 27.3 (9.3) microT (22 flows) while normal polarity yielded in average 34.2 (6.8) microT. Considering all available radiometric ages and new paleomagnetic data it may be speculated that Gilbert-Gauss (R-N) reversal is recorded at the upper part of sequence. Lower intermediate polarity flows possibly represent a kind of precursor of this reversal similarly to Matuyama-Brunhes geomagnetic transition.
GP43A-06
Volcanic Record of the Halawa Excursion (ca 2.514+/-0.039 Ma), Koolau Volcano, Oahu Hawaii, USA: Full Vector Analyses
New paleomagnetic measurements (directions and paleointensity determinations), coupled with precise 40Ar/39Ar radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the evolution of Koolau Volcano (one of the volcanoes comprising Oahu Island) and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the buttressed flank of the volcano within Halawa Valley. Prior paleomagnetic and K-Ar investigations of the Koolau (Volcano) Series revealed excursional directions (Site F of Doell and Dalrymple, 1973). The alkaline composition of lava flows, easy access, and close geographical proximity to K-Ar dated lava flows made this newly studied 120 m thick sequence of flows in Halawa valley an excellent candidate for detailed paleomagnetic analysis. At least eight samples collected from each of 28 successive flow-sites were stepwise demagnetized by both alternating field (5mT to 100mT) and thermal (from 28° C to 575-650°C) methods, and the mean directions obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility versus temperature (k-T) analyses were conducted for individual lava flows, and the majority of them show reversible curves. Curie point determinations revealed a temperature close to or equal to 580°C, indicative of almost pure magnetite for most of the flows. Magnetic grain sizes analysis indicated SD-PSD sizes. The mean directions of magnetization of the entire section sampled indicate that about 10 m of the section are characterized by excursional directions (5 lava flows). In addition to the directional analyses we performed absolute paleointensity determinations on the 28 lavas sampled. We used the modified Thellier-Coe double heating method to determine paleointensities. pTRM checks were performed systematically one temperature step down the last pTRM acquisition in order to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50°C between room temperature and 500°C and every 25-30°C. The paleointensity determinations were obtained from the slope of the Arai diagrams. Special care was taken to interpret the Arai diagrams within the same range of temperatures lower than 300°C unless a clear and unique slope would be present. Our paleointensity results indicate a near-zero reduced strength of the field during the excursional period ranging from 5 to 9 micro-Tesla. The corresponding VGPs are located off the southeast part of Africa, close to Madagascar. 40Ar/39Ar incremental heating experiments on groundmass from nine flow-sites located at different stratigraphic levels yielded isochron ages ranging from 2.64+/-0.25 to 2.40+/-0.46 Ma indicating that the excursion may correlate with the C2r.2r-l Cryptochron of Cande and Kent [1995]. This is potentially the first terrestrial record of the ca. 2.514 +/- 0.039 Ma Cryptochron, a finding that will place important constraints on evolution of the entire Koolau shield edifice also.
GP43A-07 INVITED
The 16.6 Ma Steens Mountain Geomagnetic Polarity Reversal: Additional Complexity From a Composite Record of Five Stratigraphic Sections.
The best known record of the earth's magnetic field behavior during a geomagnetic polarity reversal preserved in volcanic rock is the reverse to normal (R-N) polarity reversal found in the Steens Basalts of SE Oregon. At three locations where reverse to normal sections are found (Steens Mountain, Catlow Peak, and Poker Jim Ridge), four high precision 40Ar/39Ar plateau ages of plagioclase separates from transitionally magnetized rocks were determined. The ages are the same within error and have a weighted mean age of 16.58 ± 0.14 Ma. Errors are two sigma. A more precise constraint on the youngest possible age of the reversal is 16.548 ± 0.050 Ma determined from the normally magnetized Oregon Canyon tuff capping the Catlow Peak section. Comparison of these ages to the new geomagnetic polarity time scale of Gradstein et al. (A Geologic Time Scale 2004, 589 pp., Cambridge University Press, 2004.), after adjustments due to differences in Fish Canyon sanidine (FCs) standard ages (28.02 Ma, this study; 28.24 Ma, Gradstein et al.), shows that the Steens reversal is uniquely identified as the top of the C5Cr chron. The high precision of the ages and the Steens' reversal location in the geomagnetic polarity timescale convincingly demonstrate that these stratigraphically uncorrelated transitional sections were erupted during the same transition and their transitional paths should be combined. The high-quality, detailed benchmark record of this reversal (Mankinen et al., JGR, 90(B), 10.393-10.416, 1985; Prevot et al., Nature, 316, 230-234, 1985) is a composite derived from two sampled sections 2 km apart on Steens Mountain that overlapped significantly, Steens A above and Steens B below. This study showed that the magnetic field during the reversal moved from reverse to normal and then bounced back to transitional before finally returning to normal (a R-T-N-T-N path). The unexamined upper part of the Steens B section was later sampled and revealed an additional bounce of the field during the transition (Camps et al., JGR, 104(B8), 17747- 58, 1999). This increased the reversal's complexity to a R-T-N-T-N-T-N pattern. We have studied a R-N volcanic section at Catlow Peak 70 km SSE of Steens Mountain with 32 flows erupted during the transition. The transitional directions trace a path very close to the Steens A and B reversal path but contain an additional large swing through the reversed field direction, demonstrating an even more complex R-T-N-T-N-T-R-T-N path. We will also report on two R-N sections recently sampled at Poker Jim Ridge 80 km west of Steens Mountain that add new directions to the Steens record. The complex composite Steens reversal path recorded in these high fidelity lavas gives some credence to suggestions of very complex magnetic field behavior during reversals, previously seen only in sediment records where the acquisition of magnetization is less well understood.
GP43A-08
Did Episodes of Very Rapid Field Change Occur During the Steens Mountain Reversal?
We have discovered new evidence relating to the hypothesis of very rapid jumps in direction during the 16.6 Ma reversal recorded by Steens Basalt flows. The benchmark record of the polarity transition is punctuated by three large directional gaps (Mankinen et al., 1985; Prevot et al., 1985). An important question is whether any of the gaps mark impulsive changes in the field, as suggested earlier, or just signify longer than usual hiatuses between successive flows. Within each of the first and second directional gaps a lava flow was found with directions prominently streaked in such a way that it might have resulted from the field changing at an astonishing rate (several degrees/day) as the flow cooled. An alternate explanation could involve some kind of subtle overprinting mechanism that remagnetized samples in the interior of the flow more than nearer the top. An important additional observation is that the directions in the flow within the second gap are streaked along the great circle containing the directions of the underlying and overlying flows, but in the flow within the first gap they are not. Our new results from other contemporaneous sections of Steens Basalt (see abstract by Jarboe et al.) shed new light on this problem. Lava flows at the position of the second directional gap display a whole new swing in field direction that is missing in the Steens benchmark record. Thus, the second gap marks a significant hiatus, not field change during cooling of a single flow, and the streaked within-flow directions probably signify varying degrees of partial remagnetization induced by the overlying flow by a mechanism not yet identified. At the position of the first gap there are also flows with directions new to the Steens benchmark record, but these directions lie near the end of the directional streak, i.e., near the direction recorded by the samples in the flow interior that were the last to cool through their principal blocking temperatures. This finding is most consistent with rapid field change as the flow cooled at the first directional gap.