AGU 2022 Transcript

Note: I used Hawaiian language accent marks for the first appearance of a word, and then used anglicized names due to screen reader issues (tested on NVDA).

Thank you for tuning in to my talk today about Lō‘ihi (Loihi) Seamount, which is the youngest volcano of the Hawaiian Island chain. My name is Thi Truong and I’m presenting research that I have done with my collaborators, David Graham, Mike Garcia and Peter Michael.

To study Loihi, we must start with Hawai‘i (Hawaii). If we orient ourselves on the south coast of Hawaii and move offshore about thirty kilometers, we will see an underwater volcano at about a kilometer below the sea surface. This is Loihi Seamount. It is a young volcano with two rift zones, which developed early in its history.

Loihi has erupted a diversity of lava types despite its short life. For these reasons, Loihi has transformed our understanding of the earliest stage of Hawaiian volcanism, called the pre-shield stage. In addition, the presence of tholeiite basalt flows indicate that a shield transition has already occurred or is ongoing.

Shown here is a summary of the methodological approach to collect basaltic glasses for geochemical analysis. Shown on the right is a 3D scan showing the interior of a glass chip, where vesicles, or vapor bubbles are visible. These types of bubbles are composed mostly of C-O-2, and they are likely to host the majority of Helium, as well.

Early noble studies at Loihi have established that Loihi has among the highest helium isotope values at a Hawaiian volcano. This helium isotope value is believed to arise from the primordial mantle input, that is, a deep mantle source which has obtained chemical characteristics of an early Earth. At the same time, significant variation in helium isotopes results in values as low as 20 R-A, producing over 10 R-A difference of helium isotope values.

Because Loihi is close to the hypothesized melting region of the Hawaiian plume, its variation sheds light not only on the lesser understood pre-shield stage, but on the present day distribution of chemical heterogeneities sampled by the Hawaiian plume.

Some explanations for the lower helium isotope values at Loihi include intrinsic variation within the plume, radiogenic ingrowth of uranium (U) and thorium (Th), as well as atmospheric and seawater contamination. The addition of recycled components such as subduction crustal materials may also account for the signature. Integrating Helium, carbon and water measurements of quenched basaltic glass can be used to explore this hypothesis.

However, most previous studies at Loihi have focused on the shallow summit region where the retention of volatiles from the mantle is a concern. An expanded study of Loihi’s deeply erupted glasses is needed.

The objective of this study is to contribute a comprehensive dataset of coupled volatile measurements from over 40 basaltic glasses collected from Loihi’s deep rift zone. Basaltic glasses were chosen because they represent erupted melt compositions. Collection depths from 1300-5000 meters beneath sea level means that samples were erupted under relatively high confining pressure. In contrast to the shallow summit, where gas loss is expected to be more extensive, high pressure from the overlying water column in the deep rift should promote the retention of magmatic volatiles.

The majority of samples from Loihi with published Helium isotope data do not have values determined by both crushing and melting. In this study, Helium, and C-O-2 contents were measured from both the melt and vesicle components to compute total contents, which were used to evaluate relative behavior and partitioning. Water and trace element compositions were measured on the melt components. This dataset provides a means to evaluate the causes of volatile and helium isotope variations at Loihi Seamount.

For the rest of this talk, I will be presenting results to address the following questions: Is there systematic variation in helium isotopes at Loihi? Are CO-2 and water contents heavily modified by degassing? Does elevated CO2/3He reflect the addition of recycled materials? And do trace element signatures support the addition of recycled materials?

Local variation at a single volcano will be useful to apply to the broader scale, say, of the Hawaiian Islands or the global high 3He/4He reservoir in the deep mantle.

For the rest of this talk, I will be using the same color coding to indicate composition within the sample suite. These designations are based on the normalized major element composition of each glass relative to its position along the MacDonald-Katsura line and based on alkalinity index. Within the South Rift zone of Loihi, alkali basalts are shown in yellow, transitional basalts are shown in cyan or light blue, and tholeiites are shown in red. In the North Rift zone, three samples from the same dredge are tholeiites. These are shown in orange.

Helium isotope values are within the range of previous studies. If we focus in on the results of the South Rift, we see that mean helium three helium four values for tholeiites and transitional basalt are different. Specifically, tholeiites are higher than transitional basalts. They are different despite the fact that they often have overlapping values.

This study does establish that the diversity of rock types can be related to the diversity of helium isotope variation. It is also notable that the North Rift tholeiites in this study are higher in Helium-three (3He) compared to the South Rift tholeiites.

Moving on to carbon and water results, we see C-O-2 measurements vary greatly among Loihi deep rift samples. Comparing C-O-2 to a trace element like Barium indicates that samples have degassed by 95 to 99%. That is, the least degassed samples have lost at least 95% of their original carbon inventory. Loihi samples are highly degassed, despite erupting at great depths. However, water-potassium (H-2-O over K-2-O) ratios suggest that dehydration has not occurred. Multi-species element modeling with carbon and water indicate that the least degassed samples correspond to about 2.3 km equilibration depth.

To evaluate the extent of vapor-melt partitioning at Loihi, we examined the CO2 and helium concentrations of the vesicles and the melt. All of Loihi’s samples plot outside the equilibrium field as indicated by Henry’s Law. Vesicle-melt partitioning suggests that helium diffuses faster into bubbles than C-O-2.

Another way to evaluate disequilibrium is to look at the C-O-2 over Helium-3 ratio. This ratio is said to be consistent across mid ocean ridge and ocean Island basalt sources. Deviation from canonical mantle ratios indicate potential recycling processes. In our Loihi suite, we do see some elevated C-O-2 over Helium-3. This might indicate recycled input due to elevated C-O-2. However, our examination of vesicle-melt partitioning indicates that non-equilibrium or kinetic degassing is likely to have occurred. Combining our results from vesicle-melt partitioning and the C-O-2 over Helium-3 ratio suggests the preferential loss of bubbles has led to the loss of 3He. This bubble loss will then drive the CO2/3He ratio upwards. Hence, the C-O-2 over Helium-3 ratio may not reflect the source, or recycled components. Instead, it reflects disequilibrium degassing processes.

While integrated Helium-Carbon results may not support recycled components in the Loihi source, some trace elements are indicators of possible recycled material. For example, we see slight positive anomalies in Niobium, and Tantalum. Loihi is notable for its elevated Niobium anomalies, both in previous studies and this current study. Elevated Niobium has been attributed to recycled crustal materials, for example, eclogite in the mantle.

In addition, Niobium over Uranium ratios, which are typically consistent across O-I-B and MORB are actually elevated at Loihi. This indicates the possible enrichment from recycled materials.

In conclusion, the study yielded a number of results related to volatiles at Loihi. Helium systematically varies between rock types within the South Rift. Furthermore, the North Rift tholeiite samples have among the highest 3He/4He measured for this whole dataset. This suggests some spatial variability, at least in helium isotope ratios between the North and the South Rift.

Another major result is that C contents vary widely at Loihi, and this is due to variable degassing of at least 95% of the least degassed sample. Despite the loss of carbon, water loss has not occurred. Integrated helium and carbon results indicate that preferential loss of helium may occur by bubble loss and non equilibrium, or, kinetic degassing.

In addition, trace element signatures may support the existence of recycled materials. Trace element results indicate that high-3He/4He samples also have positive Niobium anomalies. This supports the existence of recycled materials with primordial material. The trace element signature of Niobium over Uranium at Loihi is elevated compared to global ocean Island and MORB sources. This suggests the addition of enriched or altered material.

Overall, the study sought to incorporate volatile measurements and trace elements of a diverse set of lavas from Loihi Seamount to constrain primordial and recycled components in the mantle source. I hope this talk has convinced you that local variation of a single volcano can be used to understand broader scale processes happening in the global mantle.

Thank you for watching my talk. I also want to thank my collaborators, and I want to thank the session conveners: Val Finlayson, James Dottin, and Jabrane Labidi, for inviting me to give this talk. Thank you to everyone who made this work possible, and I look forward to seeing you in the live session on Tuesday, December 14th.