Paleomagnetic data are the unique source of observations to understand the geomagnetic transitions and hence the geodynamo processes involved in field generation. However, because reversals and excursions occur over a few thousands years at most, it is a real challenge to acquire detailed paleomagnetic information about the reversing and the transitional field. In addition, due to the fact that there is a real need to obtain reversal records from long volcanic sequences in order to have a better understanding of the generation of the field, we propose to study in detail, a long volcanic sequence (about 320 meters) of lavas from the island of Lanai with the main objective of investigating the directional behavior (i.e. Declination and Inclination) as well as the geomagnetic field strength recorded before during and after several successive polarity transitions registered by the Lanai section. We have already identified a volcanic section on the island of Lanai (Hawaii) covering about 400,000 years of geomagnetic field changes that includes the possibility of studying the detailed records of the successive Gilsa, Gardar, Bjorn and Cobb Mountain Subchron and excursions, since the base of the section is 1.6 +/-0.08 Ma old according to Ar/Ar radiometric age determinations. We also propose to do additional high precision Ar/Ar radiometric dating of the reversal boundaries and the expected excursions of the Lanai lava sequences.
It is clear that proper analysis of these observations requires documenting the total field vector and therefore conducting an exhaustive study of the directional changes and absolute paleointensity of the paleo-field recorded by the sequence of lava flows of the Lanai Volcano is proposed here. The objectives of this two year proposed study are primarily related to the knowledge of the field variations during periods like reversals and excursions but also related to the critical time intervals that precede and follow reversals that so far have only been documented from a few sedimentary sequences with conflicting observations, particularly for the interval between 1.6 to ~1.2 Million years that is still not very well defined in terms of the above mentioned reversals and excursions of the geomagnetic field. Reversal studies cannot be separated from the surrounding polarity intervals and most records published so far were not long enough to provide complete information. The Lanai sequence offers the first exceptional opportunity to study field changes over a time period that includes several polarity intervals and thus documents the paleofield behavior over various time constants. This is a basic condition to extract and bring up significant constraints for the theoretical and numerical modeling of the geodynamo. The principal investigator and three undergraduate student(s) will carry out the entire scientific research project proposed here. The involvement of undergraduate students will be a priority in this project, from fieldwork through lab work, analysis and publication. The project will help support a US scientist of an underrepresented ethnic group (the PI). Data will be disseminated through publication the web and community maintained paleomagnetic databases. This project will also provide the stimulus to maintain the paleomagnetics and petrofabrics operation at UH which is used by a diverse group of students and researchers from all over the world.
Intellectual Merit Previous published work on Lanai indicated that the volcano was formed mainly during the Matuyama Chron (Herrero-Bervera et al., 2000). In order to constrain further the timing of the active phases of the Lanai volcano, we conducted a paleomagnetic and rock magnetic study involving a ~500-m vertical thick sequence of lava flows that were erupted between 0.76+/-0.66 Ma and 1.6+/-0.09 Ma according to previous K/Ar and 40Ar/39Ar dating (Leonhardt et al., 2009). Low-field susceptibility versus temperature (k-T) and SIRM experiments performed on a dozen flows indicate that magnetite dominates the remanent magnetization (575°C). In a few cases, a low-temperature mineral phase (300-400°C) could reflect the presence of titanomagnetite with low Ti content, but the presence of maghemite or pyrrhotite cannot be completely excluded. Additional investigations are in progress on this matter. All specimens were step-wise demagnetized by alternating fields from 5 to 100 mT. Companion specimens from the same samples were demagnetized at 15 temperature steps. The demagnetization diagrams obtained with each technique showed a stable direction of remanence. In all cases, the characteristic (ChRM) component was clearly defined from at least seven successive directions isolated during step-wise demagnetization. The succession of the mean directions calculated for each of the 24 lava flows studied revealed the existence of at least one polarity interval. Based on our own radiometric dates, they were assigned to the Gilsa, "excursion" (1.606+/-0.063 Ma). Thus, the present results, along with the radiometric ages of the lavas, indicate that the tholeiitic flows that formed the Lanai volcano were erupted over a short time period, and only during the Matuyama Chron (0.780-2.58 Ma). No eruptions have occurred during the Brunhes Chron (0.78 Ma) as previously indicated from K-Ar data on lavas in the Maunalei Gulch. The excursional VGPs from the onset of the Gilsa excursion recorded on Lanai are situated near the vicinity of the west coast of South America and lingering to the west part off Australia. We have also obtained absolute paleointensity results both from the microwave and Thellier-Coe protocols indicating that the lowest absolute paleointensity values are 3.53-4.69 micro-Tesla and as high as 21.73-26.05 micro-Teslas. These transitional/excursional directions are well correlated to other Subchrons such as the Cobb Subchron, the Punaru, the Kamikatsura excursions, the Halawa Crytochrom and the Matuyama/Brunhes precursor suggesting a dipolar dominance during the onset of these excursions indicating perhaps an influence of lower mantle heterogeneities. Broader Impacts: The principal (E.H-B) investigator, the collaborator (Dr. Brian Jicha), the laboratory assistant and the undergraduate student conducted the 40Ar/39Ar radiometric determination, field and laboratory work to obtain the full vector analyses (i.e. directional and absolute paleointensity determinations) of the Lana’i lava flows in order to understand the volcanic evolution of the island and the short-term behavior of the geomagnetic field recorded by the lavas at ~1.606+/-0.063 Ma that correspond to the Gilsa polarity episode. The involvement of undergraduate student(s) has been the priority in this project, from fieldwork through lab work, analysis and publication. The project has helped out to support a US scientist of an underrepresented ethnic group (the PI). Data will be disseminated through publication the web and community maintained paleomagnetic databases. This project has also provided the stimulus to maintain the paleomagnetics and petrofabrics operation at UH which is used by a diverse group of students and researchers from all over the world.
