Paleointensity during periods of rapid reversal: A case study from the Middle Jurassic Shamrock batholith, western Nevada

Paleointensity is one of the least determined parameters in geomagnetism, yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the Earth's core and lower mantle. We test two important hypotheses that affect our understanding of how paleointensity has changed in Earth's past: (1) the Mesozoic Dipole Low (MDL) hypothesis and (2) the inverse relationship between geomagnetic polarity reversal rate and paleointensity. We report paleointensity results determined using both the Coe modified Thellier method and the IZZI protocol on the medium-grained hornblende-biotite granite of the Middle Jurassic Shamrock batholith in the southern Singatse Range, western Nevada (USA). Previous zircon U-Pb dating gives an age of batholith emplacement at ca. 165.8 +/- 0.4 Ma. This age estimate is coincident with a period of unusually high reversal frequency (similar to 10 reversals/Ma) in the Middle Jurassic. Remanence in the Shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase. The laboratory unblocking temperature range for the well-defined thermal remanent magnetization is narrow, between similar to 540 and 575 degrees C, and median destructive fields are between 40 and 50 mT, thus we infer that the age of remanence acquisition is approximated by the U-Pb age estimate for pluton emplacement. Two sets of quality criteria were used to assess the paleointensity results. The first set yielded an 87% success rate while the second set using more stringent criteria resulted in only a 20% success rate. Mean paleointensity values for the Shamrock batholith using the loose and strict quality criteria are 34.0 +/- 6.4 mu T (N = 13) and 33.6 +/- 9.6 mu T (N = 3) respectively, with errors reported at one standard deviation. After correction for both remanence anisotropy and cooling rate, the mean values that pass loose and strict criteria are respectively 17.9 +/- 2.7 mu T and 17.5 +/- 2.6 mu T. The uncorrected and corrected paleointensity values for the strict estimates yield respective mean virtual dipole moments (VDMs) of 62.7 +/- 18 ZAm(2) (Z = 10(21)) and 32.7 +/- 4.9 ZAm(2). The uncorrected value is within error of the VDMs reported for plagioclase grains isolated from ca. 160 Ma basalts- from Ocean Drilling Program Site 801, however these values have an absolute difference ranging from similar to 6 to 20 ZAm(2), while the corrected VDM value is more consistent with the estimated average Jurassic field strength of similar to 29 ZAm(2). Using a recent estimate for the longterm stable field strength of similar to 42 ZAm(2), our corrected values add further credence to both the MDL and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses.