Solving the mystery of the 1960 Hawaiian lava flow: implications for estimating Earth's magnetic field

Studying historic lava flows provides a rare comparison between direct measurements of the Earth's magnetic field and the field information recorded in the rock record. Connecting direct and indirect measurements provides a way to bridge the gap between historic data in the last 50-100 yr to geologic data over kyr to Gyr. The field strength in Hawaii in 1960 was directly measured at the Honolulu observatory to be 36.47 mu T, so our palaeointensity analysis of the 1960 flow is expected to give the same value. Two vertical sections of the 1960 flow (section 1 and section 2) were the focus of a previous microwave palaeointensity study. The microwave experiments were run using the non-standard, perpendicular modified Thellier-type protocol and produced either apparently good quality data that were biased to low values (section 1) or more scattered results averaging close to the expected value (section 2). The cause of the non-ideal behaviour observed in the data from the 1960 flow is a long-standing mystery that it is important to resolve to confirm the reliability of palaeointensity measurements in general, and the microwave demagnetization mechanism in particular. Here, we test the hypothesis that higher quality, unbiased (only random noise) measurements are possible using an improved Thellier-type protocol coupled to an updated microwave system. New palaeointensity experiments were performed primarily using the IZZI protocol (which allows alteration checks during the experiment) adapted for the microwave system. The specimens from section 1 produced more linear Arai plots and gave an estimate of 36.8 +/- 3.4 mu T, whereas those from section 2 gave an estimate of 39.1 +/- 4.6 mu T. Our new experiments demonstrate the microwave system's ability to produce accurate results and efficiently run any Thellier-style experiment. We investigate correcting perpendicular data for undetected alteration and find that using too strict selection criteria can be counterproductive to obtaining accurate and precise microwave palaeointensity results.