Geology topics

Alexander T. Bearden

Publications and source records attributed to Alexander T. Bearden.

4 recordsLinked to original sources

USGS2026

Petrological, textural, and volatile constraints on the architecture of Mauna Loa’s magmatic system over 170 years (1852–2022)

Mauna Loa’s reawakening in 2022 highlighted the limited availability of petrological constraints characterizing the geometry and dynamics of pre-eruptive storage at this volcano. In particular, there has been relatively little prior work constraining pre-eruptive volatile budgets, mineral zoning and textures. To address this, we examine olivine- and orthopyroxene-hosted melt inclusions (MIs), their mineral hosts, and matrix glasses from seven Mauna Loa eruptions spanning 170 years (1852–2022). We present the first extensive MI dataset from Mauna Loa with direct Raman measurements of vapor bubble CO 2 . Olivine- and orthopyroxene-hosted MIs contain 0.3–0.5 wt % H 2 O and 100–600 μg/g CO 2 . MI saturation pressures reveal that the dominant reservoir is located at ~2–5 km depth, consistent with fluid inclusion barometry and geophysical constraints. This depth has remained consistent over the past 170 years, although some eruptions appear to have tapped a narrower range of depths than others. Even the high-Mg# orthopyroxene phenocryst-hosted MIs record saturation pressures of <150 MPa (<6 km depth), which is inconsistent with a deep sub-Moho origin proposed by previous studies that rely on thermodynamic modeling. Our Raman measurements also provide an opportunity to assess the accuracy of different methods to reconstruct the amount of CO 2 in vapor bubbles, with applications to MI datasets published without direct Raman measurements. In general, models that partition CO 2 between the melt and bubble using the bubble volume prior to pre-eruptive quenching (e.g. Melt Inclusion Modification Corrections, MIMiC) systematically underestimate CO 2 contents by a median of ~100–200 μg/g (depending on the model and K D choice) and thus underestimate storage depths by ~0.5–1.5 km. While these methods are useful for first-order estimates of bubble CO 2 in published datasets, new studies should measure bubble CO 2 by Raman spectroscopy or perform MI rehomogenization. The zoning and textures of the crystals that host the MIs provide additional constraints on the plumbing system to supplement MI barometry. Orthopyroxene crystals exhibit varied zoning patterns, including oscillatory zoning that records repeated fluctuations in magmatic conditions, and sector zoning indicating rapid, kinetically controlled crystal growth. While faster Fe-Mg diffusion in olivine results in a simpler zoning record, complexly zoned olivines with higher-Fo ‘shoulders’ from 1852, 1868, and 1950 also record the injection of more primitive melts at shallow levels. Electron backscatter diffraction reveals that touching crystals in orthopyroxene glomerocrysts exhibit only very weakly systematic crystallographic orientation relationships, perhaps due to rapid aggregation in a dynamic system. Overall, our data are consistent with a large, shallow (~2–5 km) pre-eruptive reservoir repeatedly recharged by primitive magma injections that homogenized prior to eruption due to their small volume relative to the overall size of this reservoir. This chemical complexity is effectively recorded in mineral and MI compositions (orthopyroxene zoning in particular) but is obscured in glass and whole-rock analyses.

Hawaii↗

USGS2025

Fluid inclusion constraints on the geometry of the magmatic plumbing system beneath Mauna Loa – Part I: Lavas and tephras

There are few petrological constraints on magma storage depths at Mauna Loa, Hawai‘i. Yet understanding the geometry of the magmatic plumbing system is critical for interpreting geophysical signals of unrest at this very high-threat volcano. We address this gap by examining CO₂-rich fluid inclusions (FI) in lava and tephra from seven eruptions (8870 ± 56 14C yr BP, 1852, 1855, 1868, 1949, 1950, and 1984), supplemented with published data from 2022. Raman spectroscopy was used to determine FI densities, from which entrapment pressures were calculated using a CO₂-H₂O equation of state. Most FI record pressures of ~ 0.25–1.25 kbar (~ 2–5 km depth below the summit), consistent with geophysical estimates from the past 40 years. In summit eruptions, FI hosted in more evolved olivine and orthopyroxene clots (Fo and Mg# < 84) record slightly shallower pressures than those in more primitive olivines (Fo > 84) from rift zone eruptions, suggesting a crystal-poor evolved cap near the top of the reservoir (~ 2 km). The similarity in storage depths across all eight eruptions indicates that Mauna Loa’s magmas have tapped a quasi-stable reservoir over the past two centuries, and possibly over 10 kyr. Electron backscatter diffraction reveals deformations to the crystal lattice in Fo82-83 olivines, likely due to deformation during storage in mush piles. The intensity of deformation is comparable to that seen at Kīlauea, implying that mush pile stress may be decoupled from edifice size or longevity. Finally, SO₂ contents in FI increase from ~ 2 mol% at 2 kbar to ~ 15 mol% at 0.5 kbar, suggesting sulphur degassing begins far deeper than the 0.2–0.3 kbar commonly assumed for Hawaiian systems. This validates the newest generation of S degassing models (e.g., Sulfur_X), and explains precursory SO2 emissions in the ~3 hours prior to the onset of the 2022 eruption (Esse et al. 2025).

Hawaii↗

USGS2025

Fluid inclusion constraints on the geometry of the magmatic plumbing system beneath Mauna Loa – Part 2: Xenoliths

Mauna Loa volcano erupts crystal-poor material at its summit and more crystal-rich material on its rift zones. Some of the more olivine-rich lava flows contain xenoliths with diverse mineralogy, including cumulate harzburgites with high-Mg# orthopyroxenes and high-Fo olivines (both > 84). Previous experimental work and thermodynamic modelling has proposed that high-Mg# orthopyroxenes only crystallize from Mauna Loa melts at high pressures (> 6 kbar, > 20 km), leading to suggestions that there is a region of sub-Moho magma storage at Mauna Loa in addition to the geophysically imaged magma reservoir at 2–5 km depth below the summit. We use melt and fluid inclusion barometry combined with thermodynamic models to further investigate this suggestion. Fluid inclusion data from harzburgites and dunitic xenoliths yield storage depths remarkably similar to those found in non-xenolithic crystals from lavas and tephras, with a clear peak at ~ 2–3 km (below the summit). Depths from melt inclusions in these xenoliths overlap with fluid inclusion pressures, ruling out the possibility of fluid inclusion re-equilibration during a period of stalling in a shallower reservoir. We examine five different thermodynamic models and find that the minimum pressure of olivine-orthopyroxene co-saturation varies by ~ 4 kbar (~ 12 km). These models also fail to predict that orthopyroxene is stable in ~ 15–80% of compositionally relevant experimental charges which grew orthopyroxene. Overall, this shows that phase stability modelling is an unreliable method of determining magma storage depth at Mauna Loa. We suggest that model discrepancies reflect a lack of experimental constraints on orthopyroxene stability at > 1200 ℃ and 0.01–5 kbar. Based on the presence of large oikocrystic orthopyroxenes completely enclosing rounded olivine chadacrysts, we suggest that these harzburgitic xenoliths formed through the reaction of intruding melts with olivine mush piles within the Mauna Loa edifice at ~ 3 km depth below the summit, with no need for a deeper storage reservoir. The predominance of pre-eruptive shallow storage means that there is more chance of detecting reservoir destabilization with geophysical monitoring techniques compared to a scenario where melts are supplied from sub-Moho reservoirs.

Hawaii↗

USGS2024

Depths in a day - A new era of rapid-response Raman-based barometry using fluid inclusions

Rapid-response petrological monitoring is a major advance for volcano observatories, allowing them to build and validate models of plumbing systems that supply eruptions in near-real-time. The depth of magma storage has recently been identified as high-priority information for volcanic observatories, yet this information is not currently obtainable via petrological monitoring methods on timescales relevant to eruption response. Fluid inclusion barometry (using micro-thermometry or Raman spectroscopy) is a well-established petrological method to estimate magma storage depths and has been proposed to have potential as a rapid-response monitoring tool, although this has not been formally demonstrated. To address this deficiency, we performed a near-real-time rapid-response simulation for the September 2023 eruption of Kīlauea, Hawaiʻi. We show that Raman-based fluid inclusion barometry can robustly determine reservoir depths within a day of receiving samples — a transformative timescale that has not previously been achieved by petrological methods. Fluid inclusion barometry using micro-thermometric techniques has typically been limited to systems with relatively deep magma storage (>0.4 g/cm 3 or >7 km) where measurements of CO 2 density are easy and accurate because the CO 2 fluid homogenizes into the liquid phase. Improvements of the accuracy of Raman spectroscopy measurements of fluids with low CO 2 density over the past couple of decades has enabled measurements of fluid inclusions from shallower magmatic systems. However, one caveat of examining shallower systems is that the fraction of H 2 O in the fluid may be too high to reliably convert CO 2 density to pressure. To test the global applicability of rapid response fluid inclusion barometry, we compiled a global melt inclusion dataset (>4000 samples) and calculate the fluid composition at the point of vapor saturation (⁠X H 2 O⁠ ). We show that fluid inclusions in crystal-hosts from mafic compositions (<57 wt. % SiO 2 ) — likely representative of magmas recharging many volcanic systems worldwide — trap fluids with X H 2 O low enough to make fluid inclusion barometry useful at many of the world’s most active and hazardous mafic volcanic systems (e.g., Iceland, Hawaiʻi, Galápagos Islands, East African Rift, Réunion, Canary Islands, Azores, Cabo Verde).

Hawaii↗