Geomagnetic paleointensity in historical pyroclastic density currents: Testing the effects of emplacement temperature and postemplacement alteration

2019

Fine-grained, Ti-poor titanomagnetite in the~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain-size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier-type paleointensity estimates from the lowermost~7.0 m of the flow. Magnetic hysteresis, frequency-dependent susceptibility, and thermomagnetic data show that sample grain-size distribution is dominated by SP in the lower~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm 2 ) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality-control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal-chemical remanent magnetization if microcrystal growth continued at T < T c and/or the section is affected by post-emplacement vapor-phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction. Plain Language SummaryThe strength-or intensity-of Earth's magnetic field changes over geologic time, and these changes are important in learning about the internal evolution and processes of Earth's core. These "paleointensity" variations are recorded in many Earth materials, including volcanic ash flows, sometimes called "tuffs." Magnetic minerals in these flows acquire a magnetization proportional to Earth's field strength when they cool. The approximately 12 million-year-old Tiva Canyon Tuff in the western United States contains very fine-grained magnetite particles, the size of which increases upwards from the base of the flow. This small size is typically considered ideal for a paleointensity recorder, and the systematic size variation allows us to test effect of grain size on the paleointensity estimation across the lower end of the size spectrum. Samples collected from the lowermost 7 m of the Tiva Canyon Tuff were subjected to laboratory experiments designed to recover paleointensity. We find that the smallest particles perform poorly in the experiments and do not produce consistent results, while the slightly larger particles perform ideally and provide a consistent answer. This tells us that smaller is not always better, and there is a very narrow grain size range that will produce accurate results.

Section: Possible Effects Of (Re)hydration And/or Tcrm On Paleointensitymentioning

confidence: 77%

Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

Abdulghafur

2019

“…St. Helens 1980 deposits are described in Bowles et al . []. Comparison of the models with these data (Figure ) shows some similarities, but also significant differences, as we describe below.…”

Section: Discussionmentioning

confidence: 99%

“…IZZI-style paleointensity results from the Mt. St. Helens 1980 deposits are described in Bowles et al [2015].…”

Section: 1002/2016gc006607mentioning

confidence: 99%

Effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments

Jackson

2016

Self Cite

Titanomagnetite (Fe3‐xTixO4, 0 ≤ x ≤ 1) is a common, naturally occurring magnetic mineral critical to many paleomagnetic studies. Underlying most interpretations is the assumption that, lacking chemical alteration, Curie temperature (Tc) remains constant. However, recent work has demonstrated that Tc of many natural titanomagnetites varies strongly as a function of thermal history, independent of chemical alteration. This is inferred to arise from reordering of cations and/or vacancies in the crystal structure, and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments. Because changes take place at T < Tc, they have the potential to dramatically affect thermal remanence acquisition or demagnetization, impacting interpretation of paleomagnetic results. Here we have modeled the effects of reordering on standard thermal demagnetization and paleointensity experiments. Results suggest that Tc changes during laboratory heating make it impossible to accurately measure the unblocking temperature spectrum without modifying it. Samples with a starting Tc0 less than the closure temperature (Tclose) for the reordering process will develop a high‐temperature “tail” that did not exist prior to heating. Samples with a starting Tc0 > Tclose will have their original Tb spectrum truncated at T ≈ Tclose. Predicted behavior during Thellier‐type paleointensity experiments results in only modest deviations in NRM‐lost or pTRM*‐gained from the nonreordering case. Much larger deviations are predicted for pTRM checks. Compared to paleointensity results from titanomagnetite‐bearing pyroclastic deposits, modeled nonideal behavior occurs in the same temperature intervals, but is much more systematic. Reordering is likely one contributing factor to failure of paleointensity experiments.

“…MSH14-08 and MSH14-01 pumice samples are unoriented, but previous work on MSH14-08 has demonstrated that the single-component magnetization is parallel to the 1980 field (Bowles et al, 2015), and there is no reason to suspect that the same would not be true for MSH14-01. The minimum T dep derived from pumice clasts is consistently higher than both the lithic estimates and the measured temperatures, sometimes by hundreds of degrees.…”

Section: Thermal Demagnetization Datamentioning

confidence: 99%

Assessing New and Old Methods in Paleomagnetic Paleothermometry: A Test Case at Mt. St. Helens, USA

Gerzich

Jackson

2018

Self Cite

Paleomagnetic data can be used to estimate deposit temperatures (Tdep) of pyroclastic density currents (PDCs) by finding the laboratory temperature at which a PDC‐associated thermal remanence is removed. Paleomagnetic paleothermometry assumes that (1) blocking (Tb) and unblocking (Tub) temperatures are equivalent, and (2) the blocking spectrum remains constant through time. The first assumption fails for multidomain (MD) grains, and recent evidence shows that the second is violated in many titanomagnetites, where Tc is a strong function of thermal history. Here we assess the extent to which the standard paleomagnetic method may be biased by a changing Tb spectrum, and we explore a new magnetic technique that instead exploits these changes. Using samples from the 1980 PDCs at Mt. St. Helens, we find that standard methods on oriented lithic clasts provide a Tdep range that overlaps with measured temperatures, but is systematically slightly higher. By contrast, juvenile pumice give Tdep_min estimates that greatly exceed lithic estimates and measured temperatures. We attribute this overestimate to (1) depth‐dependent variations in Tc and Tub resulting from thermally activated crystal‐chemical reordering and (2) MD titanomagnetite where Tub > Tb. Stratigraphic variations in Tc are interpreted in terms of Tdep, giving results mostly consistent with measured temperatures and with the lower end of estimates from lithic clasts. This new method allows us to evaluate temporal and spatial variations in Tdep that would not have been possible using standard paleomagnetic techniques in these lithic‐poor deposits. It also provides information on deposits not accessible by surface temperature probes.