Petrology/Volcanology

Eruption Timescales

The goal of this research project is to determine how long an individual flood basalt eruption lasts, as this has large implications for CO2 and other volatile fluxes to the atmosphere during these events. To do this, I invented a new method called magnetic geothermometry (MGT). MGT uses a combination of petrology, paleomagnetism, and thermal modeling to constrain the active lifetime of any igneous intrusion.  Long-lived intrusions will transfer significant amounts of heat into the surrounding wall-rock, while short-lived intrusions will transfer much less heat. In the wall rock, magnetite can be paleomagnetically reset and geochemically altered from this heating, and these changes are particularly sensitive to the duration of heating. By measuring the degree of paleomagnetic resetting and geochemical reordering in wall-rock magnetites, I can determine which intrusions are relatively short- or long-lived. I then construct thermal models of the intrusion to quantify how long it must have been active to produce the observed effects in the wall rock. I applied MGT to feeder dikes of the Columbia River flood basalts and determined that these dikes were transporting magma for less than a few years in most cases. This intensely focused high effusion-rate activity implies that global volatile and CO2 emissions reached several orders of magnitude above ‘steady state’ during eruptions. Thus, it is plausible major climate perturbations could arise from the eruption of a single flood basalt flow.

Magnetic Volcano Monitoring

I have recently developed a novel technique for remote sensing of subsurface magma. Since the temperature of magma (≳700℃ ) is higher than the Curie temperatures of magnetic minerals (<700℃), magma has no magnetic signal and has a demagnetizing effect on the host rock around it. This produces magnetic ‘lows’ when measuring the magnetic field at earth’s surface. By continually monitoring a volcanic system, one can track the movement of these magnetic lows and therefore track the movement of subsurface magma. Modeling results from Mt. St. Helens, Kilauea, Axial Seamount, and Bardarbunga volcanoes show that magnetic monitoring can be an effective tool to detect volcanic unrest. This technique allows for extremely low-cost and high-quality monitoring of volcanoes, which is particularly beneficial to low-income countries with high volcanic risk. This technique will also greatly advance our understanding of magma transport and fluid dynamics. Test deployments at Axial Seamount have been successful, and I am working on future deployments at other volcanoes.

Mafic Terranes of Northern Alaska

Much of my work has focused on the geochemistry and tectonics of the Angayucham Large Igneous Province (LIP) in Northern/Interior Alaska. The Angayucham LIP plays a central role in the assembly of Alaska, but its origins, age, and tectonic history are poorly constrained. This sequence of mafic-ultramafic rocks is equivalent in total area to the country of Switzerland, but fewer than ten modern ICP-MS trace element analyses were published from this LIP. I am currently increasing the number of high-quality analyses by an order of magnitude, including detailed petrography, bulk geochemistry, mineral chemistry, and petrogenetic modeling. Based on my results and previous mapping, I found that portions of the LIP are an accreted oceanic arc/forearc, while the majority of the LIP has an N/E-MORB affinity. These portions seem to be genetically unrelated, and therefore the Angayucham LIP should be split into two separate LIPs. These results may help to resolve several outstanding issues regarding the assembly of Alaska and timing of orogenic events in the Arctic.

Baffin Flood Basalt Eruption Tempo

The flood basalts of southeastern Baffin Island (North Atlantic Igneous Province) probably represent the earliest magmas from the Iceland mantle plume. They are famous among isotope geochemists for their high 3He/4He ratios and primitive geochemistry. However, these rocks have not been dated via any modern geochronological technique, and have poorly characterized stratigraphy, physical volcanology, and surface distribution. This project uses 40Ar/39Ar geochronology, mapping, volcanostratigraphy, petrological modeling, and paleomagnetism to establish the age, eruptive tempo, emplacement mechanisms, and paleoclimate impacts of this flood basalt. Based a variety of evidence for rapid eruption, I found that all the exposed basalts likely erupted between 61.7- 62 Ma over a ~2500-year timespan. This suggests that they are responsible for the hyperthermal ‘Latest Danian Event’, a period of rapid global warming. Finally, these results also date the onset of rifting between western Greenland and the rest of the North American craton at this latitude.

40Ar/39Ar Geochronology of Altered Basalts

The Columbia River Basalts have been dated by 40Ar/39Ar geochronology, and more recently by U-Pb Zircon geochronology. These two methods disagree significantly, and the U-Pb results are now widely accepted as ‘correct’. Here I use a combination of petrologic, geochemical, and magnetic data to show clear evidence for three periods of alteration in the flood basalt. CRB samples that were altered at high temperatures during cooling of the lava show anomalously old ages, while samples that were altered at low temperatures via groundwater flow show anomalously young ages. These results will help to resolve discrepancies between 40Ar/39Ar and U-Pb geochronology in basalts globally.

Contact:
biasi@dartmouth.edu

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