A detailed palaeointensity and inclination record from drill core SOH1 on Hawaii
Introduction
Drill cores provide a unique opportunity to study long continuous sequences of lava flows and gain valuable information on the magnetic field. The advantage of using lavas over the more readily available sedimentary cores is that the acquisition process in lavas is well understood and absolute palaeointensities can be recovered, unlike sediments where only relative intensities can be obtained. However, sediments give a continuous record of the field, compared to lavas which are less continuous because of sporadic eruption rates. Dating of lava sequences rely on linear interpolations between radiogenic tie points, whereas sediments can be dated more precisely by using techniques, such as astronomically calibrated δ . Drilling in lava is also much more costly than in sediments and cores are thus relatively rare.
In Hawaii we are fortunate in having the opportunity to study a number of lava cores, both from the Hawaii Scientific Drilling Project (HSDP) and the Scientific Observation Hole (SOH) projects. The SOH project was drilled to investigate the potential for geothermal energy production on Kilauea. Three holes SOH1, SOH2 and SOH4 were drilled along the Kilauea East Rift Zone (KERZ) near to the rift axis. Temperatures in SOH1 were found to be much too low for geothermal energy production, providing us with an excellent opportunity for studying the magnetic field. The proximity of SOH1 to other lava cores also allows us to compare the SOH1 data with palaeomagnetic studies on SOH4 (Laj et al., 2002) and the HSDP1 pilot hole (Laj and Kissel, 1999).
SOH1 comprises 351 volcanic units with a total length of 1685 m. This study focuses on the upper 241 flows (0–787 m), which consists of subaerially erupted lavas and a few intrusives. Palaeointensity and inclination records were obtained from the core, but due to rotation of the core during drilling, declination information was lost.
Results already obtained from SOH4 and HSDP1 provide detailed records of palaeointensity and inclination for the last 400 kyr over hundreds of flows. SOH1 covers a much shorter time span with a comparable number of flows and, therefore, provides a very detailed record of recent palaeosecular variation on Hawaii.
Section snippets
Stratigraphy and sampling
SOH1 is located on Kilauea volcano, Hawaii, at latitude 19°29′N and longitude 154°54′W, about 2 km north of the axis of the KERZ and close to Lava Trees State Park. The locations of the SOH and HSDP cores are shown in Fig. 1.
All of the 351 units (1685 m) sampled by SOH1 are from Kilauea, they have relatively low alkali (Na2O+K2O) content compared to silica (SiO2), which places them in the tholeiite field, as defined by Macdonald and Katsura (1964). The stratigraphy used for SOH1 is based on an
Chronology
Tholeiitic Kilauean basalts have a low potassium content; typically around 0.5 wt.%. Also SOH1 lavas are very young compared to the half life of . Therefore, very little radiogenic is produced, making radiogenic dating of young SOH1 basalt very difficult. Unfortunately no organic material was found in the core which could be used for radiocarbon dating.
Dating of SOH1 was first attempted using the K–Ar technique, which gave reliable ages for the submarine section, but was less successful
Thermomagnetic properties
The suitability of samples for the time consuming Thellier–Thellier experiments was determined by using a horizontal Curie balance to measure the saturation magnetisation (Js) over a heating/cooling cycle with a maximum temperature of ca. 700 °C. Small amounts of powdered sample were heated by a water-cooled furnace, while a gentle stream of argon was passed over the sample to minimise oxidation.
The evolution of Js with temperature gave important information on how the sample would behave during
Results
In total, 360 samples from 162 flows were studied using the Thellier–Thellier technique (Teanby, 2001). Table 3 reports results from 354 samples (155 flows) from the subaerial section (dataset is available in the electronic archive at, http://www.elsevier.com/locate/pepi). The six samples (from six flows) from the submarine section all gave poor results and are not reported. The results from 24 samples studied using AF demagnetisation are given in Table 4.
Inclination was found by plotting the
Discussion
In Section 3, an age model for the SOH1 record based on radiogenic Ar/Ar and K–Ar dating was discussed along with a number of problems associated with this model. Therefore, features that are independent of the exact age are discussed first, followed by the development of an improved age model based on correlation with previous Hawaiian palaeomagnetic datasets. The new age model is then used to determine secular variation on Hawaii and to compare SOH1 with global records.
Conclusions
A Thellier–Thellier study of 360 samples from lava core SOH1 on Kilauea, Hawaii, yielded around 200 new palaeointensity and inclination determinations. The SOH1 secular variation record contains three geomagnetic excursions, which display reductions in field intensities of around 50%. This agrees with previous studies, although intensities are on the high side of previous observations. This may indicate the presence of strong non-dipole components during excursions A, B and C. Field intensity
Acknowledgements
The authors would like to thank Steve Quane for the use of his detailed stratigraphic log for SOH1 and also for helping during the second sampling session in Honolulu; Mike Garcia for allowing us access to the core and for help in selecting samples for Ar/Ar analyses; Donald Thomas for permission to sample SOH1; Catherine Kissel and Alain Mazaud for their valuable assistance during the Thellier–Thellier experiments; Brian Davidson and Jim Imlach for help in obtaining the Ar/Ar dates at SURRC,
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