TY - JOUR

T1 - Formation and dispersal of pyroclasts on the Moon

T2 - Indicators of lunar magma volatile contents

AU - Morgan, Cerith

AU - Wilson, L.

AU - Head, J.W.

N1 - This is the author’s version of a work that was accepted for publication in Journal of Volcanology and Geothermal Research. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Volcanology and Geothermal Research, 413, 2021 DOI: 10.1016/j.jvolgeores.2021.107217

PY - 2021/5/31

Y1 - 2021/5/31

N2 - We use new estimates of the total content and speciation of volatiles released during the ascent and eruption of lunar mare basalt magma to model the generation and behavior of gas bubbles, the disruption of magma at shallow depth by bubble expansion, and the acceleration and dispersal of the resulting pyroclasts. Lunar eruptions in near-vacuum differ significantly from those on bodies with an atmosphere: 1) exposure to near-zero external pressure maximizes volatile release to form gas bubbles; 2) the infinite potential expansion of the gas bubbles both ensures and maximizes magma fragmentation into pyroclastic liquid droplets with sizes linked to the bubble size distribution; 3) the speeds to which gas and entrained pyroclasts can be accelerated by gas expansion are also maximized. Generation of CO gas bubbles at much greater depths and pressures than bubbles of other volatiles produces bimodal (~120 and 650 μm) total pyroclast size distributions. In the near-vacuum, gas expands to pressures so low that gas-particle interactions enter the Knudsen regime, resulting counter-intuitively in the median grainsize in pyroclastic deposits first increasing, then decreasing, and finally increasing again with increasing distance from the vent, instead of decreasing monotonically as when an atmosphere is present. These complex gas-particle interactions cause clast size distributions to vary in a complex way with distance from the vent and the maximum thickness of the deposit to occur at about 75% of the maximum pyroclast range. Lunar eruptions typically evolve through four stages, which significantly influence gas release patterns. Most volatiles are released during the second, hawaiian-style eruption stage. However, elevated gas concentration can occur both in the short first stage (due to gas accumulation in the dike tip during ascent from the mantle) and in the third and fourth stages (due to reduced volume flux, increased time for gas bubble formation, growth, rise and coalescence, and strombolian activity replacing the hawaiian eruption style). Such gas concentration mechanisms can increase pyroclast ranges by a factor of ~5, but result in very much thinner deposits than if no concentration occurs. Maximum pyroclast range scales essentially linearly with total mass fraction of released volatiles; thus determination of the deposit radius around specific vents can provide data on lunar magma volatile contents. If the volatile inventory of the Apollo 17 orange glass bead picritic magma (~3400 ppm maximum) is typical, maximum ranges of the majority of pyroclasts would have been ~20 km. Such eruptions could explain 79% of the currently recognized pyroclastic deposits on the Moon. A few larger deposits and vents, such as the Aristarchus Plateau Dark Mantle and Cobra Head, suggest higher magma volatile contents. Numerous lunar vents show little evidence of associated pyroclastic deposits. Together, these observations suggest a wide range of volatile contents in lunar basaltic magma mantle source regions.

AB - We use new estimates of the total content and speciation of volatiles released during the ascent and eruption of lunar mare basalt magma to model the generation and behavior of gas bubbles, the disruption of magma at shallow depth by bubble expansion, and the acceleration and dispersal of the resulting pyroclasts. Lunar eruptions in near-vacuum differ significantly from those on bodies with an atmosphere: 1) exposure to near-zero external pressure maximizes volatile release to form gas bubbles; 2) the infinite potential expansion of the gas bubbles both ensures and maximizes magma fragmentation into pyroclastic liquid droplets with sizes linked to the bubble size distribution; 3) the speeds to which gas and entrained pyroclasts can be accelerated by gas expansion are also maximized. Generation of CO gas bubbles at much greater depths and pressures than bubbles of other volatiles produces bimodal (~120 and 650 μm) total pyroclast size distributions. In the near-vacuum, gas expands to pressures so low that gas-particle interactions enter the Knudsen regime, resulting counter-intuitively in the median grainsize in pyroclastic deposits first increasing, then decreasing, and finally increasing again with increasing distance from the vent, instead of decreasing monotonically as when an atmosphere is present. These complex gas-particle interactions cause clast size distributions to vary in a complex way with distance from the vent and the maximum thickness of the deposit to occur at about 75% of the maximum pyroclast range. Lunar eruptions typically evolve through four stages, which significantly influence gas release patterns. Most volatiles are released during the second, hawaiian-style eruption stage. However, elevated gas concentration can occur both in the short first stage (due to gas accumulation in the dike tip during ascent from the mantle) and in the third and fourth stages (due to reduced volume flux, increased time for gas bubble formation, growth, rise and coalescence, and strombolian activity replacing the hawaiian eruption style). Such gas concentration mechanisms can increase pyroclast ranges by a factor of ~5, but result in very much thinner deposits than if no concentration occurs. Maximum pyroclast range scales essentially linearly with total mass fraction of released volatiles; thus determination of the deposit radius around specific vents can provide data on lunar magma volatile contents. If the volatile inventory of the Apollo 17 orange glass bead picritic magma (~3400 ppm maximum) is typical, maximum ranges of the majority of pyroclasts would have been ~20 km. Such eruptions could explain 79% of the currently recognized pyroclastic deposits on the Moon. A few larger deposits and vents, such as the Aristarchus Plateau Dark Mantle and Cobra Head, suggest higher magma volatile contents. Numerous lunar vents show little evidence of associated pyroclastic deposits. Together, these observations suggest a wide range of volatile contents in lunar basaltic magma mantle source regions.

KW - Dark halo

KW - Dark mantle

KW - Explosive eruption

KW - Grain size

KW - Moon

KW - Pyroclast dispersal

KW - Volatiles

KW - Deposits

KW - Expansion

KW - Gases

KW - Size distribution

KW - Gas bubble

KW - Grainsize

KW - Pyroclastic deposits

KW - Size-distribution

KW - Volatile

KW - Volatiles content

KW - Bubbles (in fluids)

U2 - 10.1016/j.jvolgeores.2021.107217

DO - 10.1016/j.jvolgeores.2021.107217

M3 - Journal article

VL - 413

JO - Journal of Volcanology and Geothermal Research

JF - Journal of Volcanology and Geothermal Research

SN - 0377-0273

M1 - 107217

ER -