Magnetic field intensity study of the 1960 Kilauea lava flow, Hawaii, using the microwave palaeointensity technique

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1 Geophys. J. Int. (2) 142, agnetic field intensity study of the 196 Kilauea lava flow, Hawaii, using the microwave palaeointensity technique imi J. Hill and John Shaw Geomagnetism L aboratory, Oliver L odge L aboratory, Department of Earth Sciences, University of L iverpool, Oxford Street, L iverpool, L69 7ZE, UK. mimi@liv.ac.uk Accepted 2 arch 1. Received 2 February 17; in original form 1999 October 13 1 INTRODUCTION When a lava is studied for palaeomagnetic purposes it is assumed that, when the lava cooled, it acquired a natural remanent magnetization (NR) with an orientation and intensity that accurately reflects the geomagnetic field at that time. In addition, it is assumed that the NR has survived, at least in part, to the time that the palaeomagnetic experiments are carried out. The validity of these assumptions is obviously of fundamental importance for all studies of the ancient geomagnetic field using lava. By studying historic lava where the behaviour of the geomagnetic field is known at the time of extrusion these assumptions can be investigated. The high ferromagnetic content of lava can affect the magnetic field that it records. The local magnetic field can be refracted as it enters the cooling lava (Stacey & Banerjee 1974) so that the magnetic field recorded will be this refracted one. The magnetic terrain effect (Baag et al. 1995) occurs when lava flows over a strongly magnetized pre-existing flow: this distorts the magnetic field so once again the magnetic field in the vicinity of the cooling lava is not that of the true geomagnetic field. Palaeomagnetic results from a fresh lava flow are expected to show good internal consistency. If this is not the case then there are a number of possible causes: errors in sampling, errors in the analysis, rapid variation of the geomagnetic field SUARY It is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known. In this study, two vertical sections, 16 m apart, have been sampled from the approximately 1 m thick 196 Kilauea lava flow, Hawaii. Variations are seen in the rock-magnetic and palaeomagnetic properties between and within the two sections, indicating that there are small-scale lateral and vertical variations in the lava flow. The two sections showed different responses to microwave palaeointensity analysis. Section H61 generally gave ideal linear behaviour on plots of natural remanent magnetization (NR) lost against microwave-induced thermoremanent magnetization (T R) gained, whilst the majority of samples from H62 showed anomalous two-slope behaviour. When all plots were interpreted by taking the best-fitting line through all points, the flow mean intensity for H61 was 31.6±3.6 mt and that for H62 was 37.1±6.4 mt, compared with the expected intensity of 36 mt. Additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava, and whether it is best to always interpret NR lost/t R gained plots by taking the line of best fit regardless of shape. Key words: Hawaii, lava, microwaves, palaeointensity. during cooling, or factors that influence the recording mechanism. The remanence held in lava is generally assumed to be a thermoremanence (TR). However, in a study of cooling Hawaiian lava lakes, Grommé et al. (1969) found discrete zones in which the NR was a combination of a TR and a high-temperature chemical remanent magnetization (CR). The high-temperature CR was held by magnetic minerals that had formed by subsolidus reactions at temperatures below their final Curie temperature. The lava contained titaniumpoor titanomagnetite with, in general, microscopically visible ilmenite lamellae, and abundant haemoilmenite. Grommé et al. suggest that the haemoilmenite may be necessary for the high- temperature CR component formation. Palaeointensity determinations require that the NR is a pure TR, so if this is not the case there is no certainty that the results obtained are accurate representations of the past geomagnetic field. The thickness of a lava flow affects its cooling history, with the middle of a flow cooling more slowly than the edges. This cooling history determines the extent of oxidation the magnetic minerals will undergo, and the crystal shape, size and properties. The thinner the flow, the quicker it will cool, resulting in smaller expected variations in magnetic mineralogy and thus smaller variations in palaeomagnetic results. Grommé et al. (1969), however, suggested that thinner, faster-cooled flows might be more susceptible to producing a high-temperature Downloaded from by guest on 19 October RAS 487

2 488. J. Hill and J. Shaw CR for the particular magnetic mineralogy of the Hawaiian lava lakes they studied. For thin fast-cooled flows, the cooling KILAUEA LAVA FLOW time will be less than the time of geomagnetic fluctuations, so Hawaii is an ideal location for the study of historic flows as this possibility can be eliminated as a cause of within-flow the magnetic field at the time of extrusion is known from palaeomagnetic variations. observatory data, enabling experiments to be conducted whilst Two recent papers (Rolph 1997 and Böhnel et al. 1997) have already knowing the answer. The 196 Kilauea lava flow from presented results of detailed studies through recent lava flows the Big Island was chosen for this study. The eruption occurred and have included intensity analyses. The Rolph (1997) study on Kilauea s east rift zone. The lava erupted from reservoir(s) was of two lava flows less than 3 m thick that were extruded that were also the source for the 1955 lava. These recharged in in 1169 and 1971 from t Etna, Sicily, and the Böhnel et al with relatively low-magnesia magma originating beneath (1997) study was of a 6.6 m Holocene exican lava flow. Kilauea s summit, and again in 196 but this time with a more They both found significant variations in the palaeomagnetic magnesian magma (Wright & Heltz 1996). The 196 eruption results through the flows. The 1169 Etna flow exhibited palaeo- was the first eruption at Kilauea recognized as having produced magnetic results that correlated with rock-magnetic parameters; hybrid lavas. The lava contains phenocrysts of olivine, however, the palaeomagnetic results from the 1971 Etna flow augite and plagioclase. Rare microphenocryst-sized crystals of and the exican flow did not seem to correlate with any hypersthene and ilmenite have also been observed (Wright & differences in magnetic mineralogy. Rolph (1997) suggested Heltz 1996). The 196 eruption lasted for 38 days and covered that the reason for the palaeomagnetic variations in the Etna an area of 1 km2, with the estimated volume of lava extruded flows might be that the NR contains a combination of a being.125 km3 (Richter et al. 197). high-temperature CR and a TR in different proportions The flow is exposed on the eastern tip of the island, and for the two lava flows. Both authors found that the variations samples were taken along the road between Kumukahi lighthouse in intensity were not random but that there were distinct on the coast towards Kaniahiku Village. The 1979 trends in the data. This has serious implications, as flows are aeromagnetic survey of the area (Flanigan et al. 1986) shows usually sampled where there is good exposure, with little no major magnetic anomalies over the sampling area. At this concern for the situation of the sampling site within the flow. location, the flow is around 1 m thick, with approximately To investigate the nature of the field intensity recorded by.5 m of scoriaceous rubble on top. There have been no lava, this study has sought to find as simple a scenario as subsequent flows in the area, so the top of the flow is exposed possible. This has been achieved by studying a historic, thin to the air. flow and by using the microwave palaeointensity technique This flow has been studied previously by a number of (Hill & Shaw 1999). By using high-frequency microwaves authors (Tables 1 and 2), but we believe that this is the first instead of conventional heat, the magnetic minerals are targeted extensive investigation through the whole thickness of the directly. The bulk sample only experiences heat when the flow. Two studies (Castro & Brown 1988 and Hagstrum & subsequent decay of energy from the demagnetization process Champion 1994) have investigated the directions recorded by converts to heat and diffuses through the bulk sample (Walton this lava. Castro & Brown (1987) had previously studied two et al. 1993). The amount of alteration that occurs during the other historic (195 and 1972) Hawaiian flows and found experiment will therefore be reduced. Results from the microwave anomalously shallow inclinations, but for the 196 flow they technique can be compared with those from conventional found that the palaeomagnetic directions were as expected techniques. If all the techniques give equivalent results, there from extrapolation of the observatory data on Oahu. The is more confidence that the features seen are due to the actual Hagstrum & Champion (1994) results were from a much larger field recorded by the lava as opposed to artefacts of the study of recent Hawaiian lava. They too found that, within method/analysis. The Rolph (1997) study used the Shaw (1974) the error limits, the evaluated directions corresponded to those palaeointensity technique with the data analysis adaptations expected. of Rolph & Shaw (1985), whereas the Böhnel et al. (1997) As can be seen from Table 2, a variety of different versions study used the Thellier method as modified by Coe (1967). of the Thellier method and the Shaw method have been used In this study, in addition to the microwave intensity analysis, for palaeointensity analysis, and the results range from a full suite of rock-magnetic experiments and directional to 53.5 mt, with the expected intensity being 36 mt. Tanaka & analyses has been carried out. Kono (1991) showed that for some samples two slopes were Table 1. Directional data for 196 flow. Reference Samples D I N/N a 95 British Geological Survey 196 Field, Honolulu Observatory Castro & Brown (1987) Extrapolation from Observatory data Tanaka & Kono (1991) DGRF Hagstrum & Champion (1994) 2L /8 3. Hagstrum & Champion (1994) B / Castro & Brown (1988) Downloaded from by guest on 19 October 218 Declination D, and inclination I are values for the 196 magnetic field recorded at the Honolulu Observatory (row 1), the expected values for the Island of Hawaii from 195 to 1975 extrapolated from the observatory data (row 2), values from the 1965 DGRF (row 3), and three published directions obtained from the 196 Kilauea lava flow (rows 4 to 6), where N is the number of samples studied, N is the number of samples that yielded a directional result and a 95 is the flow mean error. 2 RAS, GJI 142,

3 icrowave intensities of the 196 Hawaiian lava flow 489 Table 2. Intensity data for 196 flow. Paper H a (mt) SD notes Honolulu Observatory Tanaka & Kono (1991) 36.2 DRGF 1965 Tanaka et al. (1995) 36 4 Coe analogue High temperature 44 3 Thellier method KTT analogue 36 7 Kono and Ueno perp analogue 47 7 Tanaka & Kono (1991) 41.7 average if reject 53.5 Coe version Thellier ± Tsunakawa & Shaw (1994) Shaw method rejected from double heating test Abokodair (1977) 47. in air Thellier method 42.3 in vacuum Field intensity, H a in mt from the Honolulu Observatory for 196, DGRF 1965, and determined from the 196 lava flow, where SD is the associated standard deviation. (1) Numerous olivine phenocrysts, many cracked and containing inclusions of titanomagnetite of various sizes. (2) In ground mass numerous fine (1 mm) and submicroscopic titano- magnetites too small to be able to see any details of oxidation. (3) Small titanomagnetite crystals grown around non-magnetic grains and along cracks in olivine. (4) Predominant magnetic mineral in large population size elongate R1 haemoilmenite (1 2 mm by5mm). Sometimes containing glass inclusions. (5) any large (25 2 mm) euhedral opaques grey with a lighter corona titanomagnetite with a g-chromite corona? Some cracked, all homogenous. (6) any skeletal and cruciform titanomagnetites. present on a plot of NR lost versus TR gained. The partial thermoremanent magnetization (ptr) checks commonly used to check for alteration during the experiment failed for the second shallower slope, so the first slope was taken to give the correct intensity estimate. These estimates were high; for one sample 53.5 mt, which is almost 5 per cent greater than the expected field intensity of 36 mt. The other authors also found a range of intensities with a tendency for excessively high values. In this study, two vertical sections were taken a distance 16 m apart. The flow is 1 m high in section 1 (H61) and 75 cm high in section 2 (H62). Standard palaeomagnetic (2.5 cm diameter) cores were taken at 5 cm intervals with a petrol-powered drill and orientated with a sun compass. The lava is vesicular, with little visible sign of weathering, and numerous olivine crystals are present (up to 5 mm in size). 3 AGNETIC INERALOGY 3.1 Reflected-light microscopy Reflected-light microscopy allows direct observation of the magnetic minerals present. Polished surfaces of six samples were examined using reflected light with a Nikon microscope with video attachment at Liverpool John oores University. A 5 air objective was used in addition to, 2, 4 and Table 3. Summary of reflected-light microscopy observations. General observations applying to all samples Some large euhedral grains show cracks that may be due to low-temperature oxidation. The fine titanomagnetites are too small to determine whether any oxidation is present. 3.2 Rock magnetism 1 oil immersion objectives. The key observations are By using a agnetic easurements Variable Field Translation described in Table 3. Balance ( VFTB), hysteresis properties, isothermal remanent There is little visible variation between the six samples, all magnetization (IR) acquisitions, and Curie curves were of which show a high concentration of magnetic minerals, obtained for all samples. Susceptibility experiments from the most abundant being haemoilmenite. There are two other liquid nitrogen to room temperature were also carried out. magnetic mineral populations: large titanomagnetites, and a Representative results are shown in Figs 1 to 3, and Table 4 ground mass containing fine to submicroscopic titanomagnetites. lists all rock-magnetic properties for both the sections. Fig. 4 There is little visible evidence of oxidation, and the majority shows the variation through the two sections of a number of of the oxides can be classified as C1 and R1 (Haggerty 1991). the rock-magnetic parameters. Downloaded from by guest on 19 October RAS, GJI 142,

4 49. J. Hill and J. Shaw Normalised agnetisation H Temperature C Normalised agnetisation H Temperature C Normalised agnetisation H Temperature C Figure 1. Representative Curie curves for H61 and H62. Low temperature susceptibility 6 5 K/K Temperature C agnetisation Am 2 /kg Normalised agnetisation agnetisation 1-3 Am 2 /kg IR Acquistion Field mt H Temperature C Hysteresis Loop Field mt Figure 2. Representative low-temperature susceptibility, hysteresis loop and IR acquisition plots for the two sections. Downloaded from by guest on 19 October 218 The Curie curves show some variation in shape through the two sections (Fig. 1), but all contain a predominant phase with a Curie temperature of about 52 C, characteristic of a lowtitanium titanomagnetite. The majority of the Curie curves in section H61 are linear, showing a monotonic decrease in magnetization indicating a distribution of Curie temperatures. This suggests that cooling was too rapid to have allowed a uniform equilibrium composition of oxide grains to be achieved 2 RAS, GJI 142,

5 icrowave intensities of the 196 Hawaiian lava flow 491 Table 4. Rock-magnetic properties. H61 sample x RS nrm / H H H /H B j /j t 1 t 2 s rs rs s c cr cr c 1/2 h c c c depth 1 8 m3 1 5 Am2 1 3 Am2 1 3 Am2 mt mt mt C C through kg 1 kg 1 kg 1 kg 1 flow cm average sd per cent H61 sample x RS nrm / H H H /H B j /j t 1 t 2 s rs rs s c cr cr c 1/2 h c c c depth 1 8 m3 1 5 Am2 1 3 Am2 1 3 Am2 mt mt mt C C through kg 1 kg 1 kg 1 kg 1 flow cm average sd per cent Susceptibility, hysteresis and thermomagnetic parameters. x, room temperature susceptibility; RS, ratio of susceptibility at liquid nitrogen temperature to that at room temperature; nrm, natural remanent magnetization intensity; s, saturation magnetization; rs, saturation remanence; H c, coercivity; H cr, coercivity of remanence; B 1/2, the field at which half of the saturation isothermal remanent magnetization is aquired; j h /j c, ratio of room temperature magnetization before and after heating; t c, Curie temperature. The average value per section is given with the associated standard deviation (sd) and percentage error. Downloaded from by guest on 19 October 218 of the section. Samples from 35 to 45 cm depth have two discernible Curie temperatures. The low-temperature com- ponent has a Curie temperature of 29 C, indicative of a more titanium-rich titanomagnetite phase. at any stage of cooling (Grommé et al. 1969). The curves become less linear towards the bottom of this section. In section H62, linear Curie curves are seen for the middle of the section with less linear behaviour at the top and the bottom 2 RAS, GJI 142,

6 492. J. Hill and J. Shaw.6.5 S D.4 P S D H61 H62 rs/ s D Hcr/ Hc Figure 3. Hysteresis properties of the two sections plotted on a Day plot (Day et al. 1977), with the regions for bulk single-domain (SD), pseudo-single-domain (PSD) and multidomain (D) behaviour highlighted. depth through H61 cm! 1-8 m 3 /kg ! 1-8 m 3 /kg 2 4 s 1-3 Am 2 /kg s 1-3 Am 2 /kg H61 rs / s H62 rs / s h cr mt h cr mt B 1/ B 1/ Downloaded from by guest on 19 October 218 depth through H62 cm Figure 4. Variation through the two sections of various rock-magnetic parameters (symbols as in Table 4). 2 RAS, GJI 142,

7 icrowave intensities of the 196 Hawaiian lava flow 493 In all cases the cooling curves lie above the heating curve The majority of samples were demagnetized by a power of at low temperatures, but on cooling many start out below the 1 W, and all samples were demagnetized by approximately heating curve and then cross it at about 35 C. An indication 15 W. of the alteration that occurs due to heating is given by the The directional results through the flow, using thermal difference in magnetization at 1 C. This is less than 25 per demagnetization, are illustrated in Fig. 6. Data gaps are due cent in all cases, with the average difference 15 per cent for to limited sample availability or unoriented samples. Scatter H61 and 1 per cent for H62. is seen in the directions from both sections, with a maximum The low-temperature susceptibility results (Fig. 2) are all range of 14 and 19 variation in H61 and 22 and 12 in characterized by an extremely high ratio (up to 5) of susceptibility H62 for the declination and inclination, respectively. There at liquid nitrogen temperature to that at room temperature. are a few obvious outliers (such as the inclination value for This is due to the haemoilmenite, which from the low- the top of H61), and these could possibly be due to temperature susceptibility plots has a Curie temperature of orientation error in the field, as the cores, being highly vesicular, approximately 15 C. The haemoilmenite signal will swamp often broke during drilling. out any other signal, such as that from multidomain (D) The mean directions for the two sections, including all data magnetite, that may be present. points, are shown in Table 5. These results can be compared The samples all fall in the pseudo-single-domain (PSD) with the previously published flow mean directions and obserrange of the Day plot (Day et al. 1977), which is a typical vatory data in Table 1. The results from this study encompass result for lava (Fig. 3). From the reflected-light microscopy, the expected directions apart from the inclination for H61, large multidomain grains are present as well as fine grains and which has a mean value that is 7 too shallow. Even when the presumably submicroscopic single-domain (SD) grains, so the outliers at and 65 cm depth are rejected the inclination is average domain state for the bulk sample is PSD. Samples still shallow (31.7±2. ), as all the individual inclination from H61 cluster together with those from the top half of values are less than the expected value. This is contrary to H62. Samples from the bottom half of H62 are more the findings of other authors (Table 1). Castro & Brown scattered and trend towards larger grain sizes. (1987, 1988) found shallow inclinations for the 195 and 1972 During IR acquisition saturation was essentially achieved Hawaiian flows, but found that the 196 flow mean directions by 3 mt (Fig. 2), indicating the predominance of ferrimagnetic correctly represented the geomagnetic field. There were no minerals. B values for H61 are indicative of PSD obvious visible differences between H61 and H62 to 1/2 to SD grains. The B values for H62 indicate a range of account for this difference. 1/2 bulk grain sizes from large D up to PSD to SD. The samples with larger grain sizes indicated by B generally correspond 1/2 to those indicated by the Day plot. 5 INTENSITY ANALYSIS To summarize, there is little variation in the rock-magnetic parameters for section H61, but far greater variation is seen 5.1 ethod in section H62 as illustrated in Fig. 4. All samples contain Two 5 mm diameter core samples taken from each 2.5 cm core low-titanium titanomagnetite, with samples from the middle were subjected to microwave palaeointensity analysis. The of H62 also containing a significant proportion of a higher- technique used, described in detail in Hill & Shaw (1999), is titanium titanomagnetite phase. The grain size of H62 a variant of the stepwise Thellier technique and is similar to generally increases towards the bottom of the section. In that of Kono & Ueno (1977). A microwave thermoremanence addition to titanomagnetite, all samples contain haemoilmenite (T R) is induced perpendicular to the direction of the with a Curie temperature of about 15 C. The mineralogy natural remanence (NR), which has the advantage that only of these samples is similar to those from the cooling Hawaiian one microwave application is required for each power step lava lakes studied by Grommé et al. (1969). However, no microscopically visible signs of oxidation of the titanomagnetite were found in this study. 4 DIRECTIONAL STUDY Stepwise demagnetization of orientated samples was carried out using a thermal demagnetizer, and, for comparative purposes, sister samples were demagnetized with the microwave system. Orientation marks were transferred from the 2.5 cm cores to the small-size samples (5 mm 3 mm cores) that are used with the microwave system. The method of orienting the samples in the microwave system is currently under development, but for this study was done by eye. Considering the errors involved, the agreement of the two techniques was extremely good. Selected representative Zijderveld and intensity plots for both thermal and microwave demagnetization are shown in Fig. 5 for the two sections. Both techniques yield a stable primary remanence with, in some cases, an additional small component of viscous magnetization. Increasing microwave power steps (applied for 1 s) are analogous to increasing temperature steps. (temperature step equivalent). This eliminates the need for accurate reproducibility of the microwave power absorbed by the samples. It should be stressed that it is the power absorbed by the sample that is the critical parameter, not the output power from the microwave source. The amount of power that the sample absorbs is dependent on the resonant microwave cavity characteristics, and these are not fixed as the cavity is part of the microwave circuit and acts as an amplifier. The cavity characteristics are highly sensitive to such factors as sample position and temperature. Table 5. Flow mean directional results from this study. Samples D I N/N a 95 H / H / Declination, D and inclination, I are the values obtained using thermal demagnetization, where N is the number of samples studied, N is the number of samples that yielded a directional result and a is the flow 95 mean error. 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8 494. J. Hill and J. Shaw SAPLE HW1H627A EU E UP H62-7 SAPLE HW1H627EZ E U EUP THERAL ICROWAVE N S N Temperature C Power Watts S Horizontal SAPLE HW1H6113A Vertical N FIELDS Horizontal SAPLE Vertical HW1H616A N FIELDS Horizontal V ti l THERAL THERAL W DOWN EU W DOWN EU WDOWN W H Temperature C S H Temperature C S Horizontal WDOWN SAPLE Vertical HW1H6113ZD E U FIELDS N ICROWAVE Horizontal SAPLE HW1H616EZ Vertical N ICROWAVE FIELDS Horizontal V i l WDOWN E U WWDOWN Power Watts Power Watts Figure 5. Zijderveld and intensity plots for representative samples from both sections using thermal and microwave demagnetization. Open symbols are for the horizontal component of magnetization and closed symbols for the vertical component of magnetization. The y-axis on the intensity plots is the normalized magnetization. The vector sum of the NR and T R components can be decomposed so that NR lost can be plotted against T R gained. Ideally, the data points result in a straight line, the gradient of which is equal to the ratio of the natural to laboratory magnetic fields. In all cases the laboratory field applied was 36±4 mt, giving an ideal result of a straight line with a gradient of one. This method is only applicable to magnetically isotropic samples, which is generally the case for lava. The anisotropy of magnetic susceptibility (AS) has been measured for all the samples using a inisep system. AS levels were found to be less than 2.5 per cent for all samples, and for the majority of samples to be less than 1 per cent (where AS percentage is defined as (k /k 1) 1, where k and k are the maximum and minimum axes of the susceptibility ellipsoid). As an additional check on the method, experiments were repeated for a selection of samples using a method described in Shaw et al. (1996, 1999) in which the laboratory field is applied in the direction of the NR. Comparable results were obtained for both methods. These tests indicate that the anisotropy is palaeomagnetically insignificant and that these samples are suitable for intensity analysis using a perpendicularly applied field. 5.2 Results The intensity results for section H61 are shown in Table 6(a) along with the associated Coe quality parameters (Coe et al. 1978). The gap factor ( g) describes how the points are distributed along the best-fitting line ( g=1 if the points are evenly distributed and g= if they are concentrated at the extremes), and the fraction factor ( f ) is the percentage fraction of the NR used in the determination. These are both used, along with the gradient of the best-fitting line and its associated error, to give a quality factor (q), where q> and for highquality results will have a value of several tens. High q-value S S Downloaded from by guest on 19 October RAS, GJI 142,

9 Table 6(a). Results from microwave palaeointensity analysis for section H61. icrowave intensities of the 196 Hawaiian lava flow 495 H61 sample line of best fit av Fe mt sd initial/high cmpt 2nd/low cmpt average depth high/low through N f g q Fe mt error N f Fe mt N f Fe mt FemT flow (cm) h611da h611db h612da h612db h613ua h613ub h614a h614b h615cb h615cc h616ea h616ch h617ca h617ch h618aa h618ab h619fa h619fb h611ca h611cb h611cc h6111ca h6111cb h6112ea h6112eb h6113a h6113bb h6114da h6114bb h6115da h6115db h6116fa h6116fb h6117da h6117db h6118ea h6118eb h6119fa h6119fb h612da h612db average sd results are obtained for H61, with the two results per 2.5 cm containing only one slope. When the plots were interpreted core giving consistent results for all samples apart from those with the best-fitting straight line, reasonable to good-quality from 45 cm depth. The majority of samples give good straight q-values were still obtained, and the two results per 2.5 cm lines on the NR/T R plots, but a few samples (those from core gave consistent results. The results for section H62 are 45, 7 and one from 65 cm depth) could be interpreted as shown in Table 6(b), and representative NR/T R plots consisting of two slopes (Fig. 7a). The samples that consist of are illustrated in Fig. 7( b). two slopes were analysed with the average best-fitting line Taking only those samples that exhibited single-slope (which still gave high-quality results), in addition to the two NR/T R plots, the variation through the flow is illustrated slopes being analysed separately. in Fig. 8(a). For H61, 34 out of a total of 41 intensity Section H62 exhibits different behaviour, with the majority evaluations were accepted, leaving data gaps only at depths of samples showing two-slope behaviour on the NR/T R of 45 and 7 cm. The mean evaluated field intensity is plots. Only samples from the top of the flow and from depths 31.8±3. mt, which is lower than the expected 36 mt. For between 4 and 55 cm could be interpreted as consistently H62, only 15 out of the 3 intensity estimates were accepted Downloaded from by guest on 19 October RAS, GJI 142,

10 496. J. Hill and J. Shaw Table 6(b). Results from microwave palaeointensity analysis for section H62. H61 sample line of best fit av Fe mt sd initial/high cmpt 2nd/low cmpt average depth high/low through N f g q Fe mt error N f Fe mt N f Fe mt FemT flow (cm) h621fa h621fb h622da h622db h623ea h623eb h624ea h624eb h625ea h625eb h626da h626db h627ea h627eb h628fa h628fd h629cc h629cb h621ca h621cb h6211fb h6211fc h6212fa h6212fc h6213da h6213db h6214ba h6214bc h6215ca h6215cb average sd Intensity results analysed using the line of best fit, and if two slopes are present on the NR/T R plot, analysis of the two slopes separately for (a) H61 and (b) H62. N is the number of data points; f, g and q are the quality factors as defined by Coe et al. (1978); Fe is the evaluated field intensity in mt; error is the level of uncertainty taking the error in the applied laboratory field as 1 per cent plus 1/q; av Fe is the average field intensity in mt at each sampling level, with sd the associated standard deviation. as containing a single-slope NR/T R plot. This section exhibited a greater dispersion of intensity values as shown 5.3 Alteration checks in Fig. 8( b), with the mean, 38.±6. mt, encompassing the Traditionally, ptr checks are used with conventional expected intensity within the error bounds. Thellier-type experiments to monitor alteration during the When the average best-fitting line is taken for all samples experiment. This involves repeating a previous temperature irrespective of whether they could be interpreted as containing step and comparing the two results. In the case of the micro- one or two slopes, the flow mean values do not significantly wave palaeointensity technique it is not as easy to repeat a change from the more limited single-slope data set (Fig. 8c and previous step because, as has already been mentioned, it is Fig. 8d). For H61 the flow mean intensity is 31.6±3.6 mt not the applied power that needs to be accurately reproduced (11 per cent scatter around the mean), and for H62 it is but the power absorbed by the sample. To obtain a ptr 37.1±6.4 mt (17 per cent scatter around the mean). check, two additional steps need to be performed: first a For samples that contain NR/T R plots with two demagnetization step and then the repeat ptr step. It is slopes, the second slope is often interpreted as being due to therefore expected that ptr checks will not necessarily be sample alteration during the experiment. Thus, taking the first good indicators of the alteration that occurs during the micro- slope if two are present, the variation of intensity values wave palaeointensity experiment. ptr checks were, however, through this flow is shown in Fig. 8(e) for H61 and in attempted for six samples, the results of which are shown in Fig. 8(f ) for H62. For H61, intensity values range from Fig. 9. As predicted, the checks in general fail but they are 27. to 42. mt with a mean value of 33.±3.9. For H62 significantly worse for the second slope, where present. the variation of intensity values through the flow range from An alternative way to check for alteration is to monitor 36. to 68.7 mt with a mean value of 43.4±9.7. hysteresis parameters (Haag et al. 1995; Hill & Shaw 1999). Downloaded from by guest on 19 October RAS, GJI 142,

11 icrowave intensities of the 196 Hawaiian lava flow 497 Section depth (cm) Declination H61 Inclination do not show a significant difference from the last microwave step, indicating that no more alteration has occurred. Samples H62-11 and H61-6, however, show a marked decrease in the value of rs / s on heating to 65 C, to hysteresis parameters that are nearer to those of the fresh sample. This is not understood at present. To gain an approximate idea of the temperature that the bulk sample reaches during microwave power application, six samples were removed from the microwave cavity immediately after power application and their temperature measured with a thermocouple. There was a time delay of 12 s between the end of the microwave application and the sample reaching the thermocouple. Results for the six samples, and also for the fire cement used as an adhesive to attach the samples to the sample rods, are shown in Fig. 11. All the lava samples show a similar trend, with the maximum temperature recorded being below 2 C in all cases. Even though there is a 12 s delay before measurement, this is obviously a greatly reduced temperature for the bulk sample to obtain, compared with conventional thermal demagnetization where samples are heated to at least 6 C. Declination Inclination 6 DISCUSSION The two sections in this study, sampled only 16 m apart, show differences in magnetic mineralogy, mean flow directions and different behaviour during microwave palaeointensity analysis, indicating that variations in lava flows are lateral as well as vertical. This study has mainly been concerned with the palaeointensity determinations and these will now be discussed. 25 There is a similarity between these palaeointensity results 3 using the microwave technique and those from conventional 35 H62 techniques; in particular, the two-slope behaviour noted by 4 45 Tanaka & Kono (1991) is also seen here. Heating times 5 during palaeointensity experiments are of the order of an hour for conventional techniques, whereas microwaves are applied for 1 s in this study, yet the intensity results are comparable. It is therefore not necessary, for these samples, to apply a cooling-rate correction (Dodson & cclelland-brown 198). When ptr checks were attempted with the microwave Figure 6. Directional variation throughout the two sections (with associated a ) using thermal demagnetization; the narrow solid line technique, the second shallower slope failed these checks by a 95 is the flow mean direction, and the dashed lines the associated a ; the significant margin, also in agreement with Tanaka & Kono 95 thick solid line is the expected declination/inclination. (1991). Unfortunately, it is not possible to monitor the hysteresis properties of samples whilst they are undergoing microwave intensity analysis. However, a study made of sister samples Six samples were measured on the VFTB to obtain their hysteresis that had their hysteresis properties monitored after cumulative parameters and then subjected to 17 W of microwave power microwave exposure showed that there were detectable changes before repeating the hysteresis measurements. This procedure in the hysteresis properties at high powers. The power at which was repeated for microwave powers of 33, 42, 55, 65, 75, 85, an increase in / value was observed usually corresponded rs s 95, 11 and 125 W. Finally, for comparative purposes, the to the power at which the sample would be nearly or comsame samples were heated up to 65 C (Curie curve measure- pletely demagnetized: thus there is no conclusive evidence that ment on the VFTB) and the hysteresis properties measured alteration occurred during the microwave intensity analysis. once again. The results are shown in Fig. 1, where changes in The unexpected behaviour of samples H61-6 and H62-11 the hysteresis properties can be observed. The most notable when they were heated to 65 C, namely an increase in the change is an increase in the value of /, indicating that average domain state, is contrary to what is expected from rs s the samples are becoming more SD-like, as expected from thermal alteration. thermal alteration. For most samples, this increase of about If the second shallower slope seen on the NR/T R.5 in the value of / occurs at around 1 W. This is plots is an artefact of alteration during the experiment, the rs s generally the highest power used in the intensity experiments, first steeper slope should be taken as the best estimate of as it corresponds to the power at which the samples are nearly the field intensity (as in the previous published studies). When or completely demagnetized (Fig. 5). The hysteresis values for this rule is applied there is little difference in the results from the majority of samples after they have been heated to 65 C H61, as only a few samples showed two-slope behaviour. In Section depth (cm) Downloaded from by guest on 19 October RAS, GJI 142,

12 498. J. Hill and J. Shaw (a) 4 h6119fa 4 h6119fa 3 3 y = -.913x R 2 = h612da h612da h615cb h619fb h6115db y = -.812x R 2 = h615cb y = -1.83x R 2 = h619fb y = -.954x R 2 = h6115db y = -.79x R 2 = h6115db y = -1.24x R 2 =.989 Downloaded from by guest on 19 October y = -.54x R 2 = Figure 7. Representative results from the microwave intensity analysis for (a) H61 and (b) H62. From left to right: all data points, analysis using line of best fit, and, if two slopes are present, the two slopes analysed separately. y-axis: NR lost; x-axis: T R gained in arbitrary units. 2 RAS, GJI 142,

13 icrowave intensities of the 196 Hawaiian lava flow 499 (b) h622db h622db h622db 2 2 y = -.83x R 2 = y = x R 2 = y = -.757x R 2 = h625eb 2 h625eb 2 h625eb h626da h627ea h6214ba y = -.959x R 2 = h626da y = x R 2 = h627ea y = -1.51x R 2 = h6214ba 1 y = x R 2 =.999 y = -.535x R 2 = h626da y = -1.51x R 2 =.997 y = -.767x R 2 = h627ea y = x R 2 =.998 y = -.732x R 2 = h6214ba y = x Downloaded from by guest on 19 October y = -.946x R 2 = R 2 = y = -.78x R 2 = Figure 7. (Continued.) 2 RAS, GJI 142,

14 5. J. Hill and J. Shaw (a) Depth through flow (cm) Depth through flow (cm) (c) e)(e) Intensity (µ T) Intensity (µ T) H Intensity µ T H H61 (b) Depth through flow (cm) (d) Depth through flow (cm) (f) Intensity (µt) H Intensity (µ T) H62 Downloaded from by guest on 19 October 218 Figure 8. Variation of intensity through the two sections. (a) and (b) samples that exhibit single-slope NR/T R plots only; (c) and (d) using the line-of-best-fit analysis on all samples; (e) and (f ) taking the first slope if two slopes are present. The narrow solid line is the flow mean intensity, with the dashed lines the associated standard deviation; the thick line is the expected intensity. 2 RAS, GJI 142,

15 icrowave intensities of the 196 Hawaiian lava flow NR lost (AU) H62-1 NR lost (AU) H icrowave TR gained (AU) icrowave TR gained (AU) 12 1 H62-3 NR lost (AU) NR lost (AU) icrowave TR gained (AU) icrowave TR gained (AU) H61-6 Figure 9. icrowave intensity results (diamonds) with ptr checks (crosses). NR lost (AU) H icrowave TR gained (AU) H62, however, it leads to a larger scatter in results with From the reflected-light microscopy it is known that there visible trends in the data seen through the section. This scatter are D grains present in these samples. To try to determine is similar to that found by Rolph (1997) and Böhnel et al. if D behaviour is manifesting itself, experiments were carried (1997) from t Etna and exican lava, respectively. A possible out on six samples that had undergone microwave palaeoreason for this scatter given by Rolph (1997) is that the NR ntensity analysis. First, to ensure that the remanence held by is not solely a TR but also contains a high-temperature the samples was purely the laboratory-applied T R, micro- CR as found by Grommé et al. (1969) in cooling Hawaiian waves at a power somewhat greater than the last power in lava lakes. These Hawaiian samples have a similar mineralogy the intensity experiment were applied. The microwave palaeo- to those of Grommé et al. (1969) so this is a plausible intensity experiment was then repeated. If D behaviour was explanation for the behaviour of these samples. occurring, two-slope behaviour should again be present. It It is known (for example Dunlop 1998) that D behaviour was found, however, that ideal linear NR/T R plots were manifests itself in producing curved NR/TR plots similar produced in all cases. This could be because the two-slope to the two-slope behaviour seen in this study. This is due to behaviour is solely a function of the original NR (e.g. the the D tail effect, in which the blocking and unblocking NR is not solely a TR); alternatively, in ensuring that all spectra are not equivalent. When a field is applied at a the original NR had been removed by the application of particular temperature, the TR produced has a remanence high-power microwaves, it could be that the samples altered that persists up to the Curie point. It is still possible to obtain and become more SD-like. intensity estimates if the gradient is taken using only the total Irrespective of the cause for the two-slope behaviour, when NR and total TR points. This assumes that no alteration the average best-fitting line is taken the results show reduced has occurred during the experiment. ptr checks on D scatter and are close to the expected field intensity as shown material are not expected to predict accurately if alteration in Table 6 and Fig. 8. There appears to be no benefit in only during the experiment has occurred. accepting single-slope results, as this does not significantly Downloaded from by guest on 19 October RAS, GJI 142,

16 52. J. Hill and J. Shaw H H62-1 rs / s.45.4 rs / s H cr /H c H cr /H c rs / s rs / s H H cr /H c H cr /H c H61-6 rs / s rs / s H H cr /H c H H cr /H c Figure 1. Hysteresis properties plotted on a Day plot for increasing microwave power exposure: square, fresh sample; diamonds, powers of 33, 42, 55, 65, 75, 85, 95 and 11 W; triangle, maximum microwave power, 125 W; and circle, after heating to 65 C. o C Temperature h616t h625t h617t h623t h621t h6211t fire cement room temp Downloaded from by guest on 19 October 218 icrowave power Watts Figure 11. Plot of the temperature that six samples and fire cement (used as adhesive to attach samples to sample rods) reached after microwave exposure for 1 s measured with a thermocouple. There was a 12-s delay between the end of microwave exposure and the temperature measurement. 2 RAS, GJI 142,