COG3 Seminar: Marc-Antoine Longpré (Queens College CUNY)
A “Crystal Clock” Case for Multi-Stage Ascent of Mantle Xenolith-Bearing Magma
Estimates of magma ascent rates are significant for understanding the precursors and styles of volcanic eruptions, the chemical evolution of magmas, and — in the case of magmas carrying mantle-derived xenoliths — the composition of the mantle. Most studies of ascent rates of mantle xenolith-bearing magmas assume uninterrupted mantle to surface transport, are based on a Stokes’ law approach, and yield ascent velocities in the range from 0.2 to 0.5 m/s [1,2]. This implies xenolith transport from depths of up to 200 km in less than 12 days, and typically much less. However, some mantle xenoliths feature reaction zones at the melt–xenolith interface that appear to require xenolith residence times in the host magma of years to decades, suggesting prolonged multi-stage ascent [3,4]. Moreover, mantle xenoliths often display sub-rounded to rounded surfaces presumably owing to mechanical abrasion during transport, which could have removed any pre-existing reaction zones and would be expected to produce abundant olivine xenocrysts in the host magma .
Two questions then come to mind: (1) Could multi-stage ascent of mantle xenolith-bearing magma be a common process? (2) Do these magmas contain xenolith-derived xenocrysts, and, if so, can the xenocrysts provide additional constraints on ascent rates?
Here I adopt a “crystal clock” (diffusion chronometry) approach to tackle these questions, focusing on the xenolith-bearing 1730–1736 Timanfaya eruption on Lanzarote, Canary Islands. While the occurrence of 20–30 cm, sub-angular to rounded spinel harzburgite xenoliths at Timanfaya would imply minimum magma ascent rates on the order of 0.2 m/s  from their source at 20–90 km depth , olivine macrocrysts found within basaltic lapilli from throughout the eruptive sequence tell a rather different story. Backscattered electron imaging and high-precision electron probe micro-analysis reveal that, in addition to typical polyhedral phenocrysts (Fo86–81)*, the Timanfaya magma contained abundant olivine macrocrysts characterized high-Fo anhedral cores, dendritic overgrowth rims, and pronounced normal zoning (Fo91–81), which are interpreted to represent mantle xenocrysts that have acted as nuclei for rapid olivine growth from the carrier liquid. The residence time of these xenocrysts in the magma, and thus minimum timescales of mantle to surface magma transport, can be calculated via kinetic modeling of elemental diffusion within the core–rim compositional gradients. I find relatively long transport times ranging from 20–1690 days, with a median of 320 days, translating into sluggish magma ascent rates of 0.0003–0.04 m/s, i.e., 1–3 orders of magnitude lower than estimates based on xenolith sizes.
I propose a multi-stage ascent model that may solve this “xenolith–xenocryst conundrum”: Xenocrysts are derived from mechanical disaggregation of the xenoliths in an early stage of rapid magma ascent, but both xenocrysts and xenoliths ascended to the surface in two or more steps, experiencing prolonged storage in a (crustal?) storage system prior to eruption. Reaction zones that would have formed at the melt–xenolith interface were likely destroyed upon final turbulent magma ascent. These results highlight a novel approach to interrogate magma transport rates at basaltic volcanoes, call for re-evaluation of the classic view on ascent rates of mantle xenolith-bearing magmas, and have important implications for understanding their crystal cargo.
 O’Reilly and Griffin (2010);  Spera (1984), CMP;  Klügel (1998), CMP;  Shaw et al. (2006), CMP;  Brett et al. (2015), EPSL;  Neumann et al. (1995), Lithos *Forsterite content of olivine, Fo = 100 x molar Mg/(Mg+Fe)
About the Speaker
Marc-Antoine Longpré's research interests lie where volcanology and igneous petrology mingle and mix. In the field, he examines and samples tephra and lava sequences to reconstruct the eruptive histories of volcanoes. In the lab, he analyzes the chemical composition of volcanic rocks and their constituents (volcanic glass, minerals and their inclusions) to decipher the dynamics of magma reservoir and eruption processes, such as magma differentiation, mixing and degassing, and the environmental consequences of volcanic eruptions. Topics include:
• Triggering mechanisms of explosive volcanic eruptions
• Architecture of magma plumbing systems
• Volatile element budget of magmas recorded by melt inclusions
• Sulfur in magmas
• Climatic and environmental impacts of volcanic eruptions
• Timescales of magmatic processes
• Geochronology, eruptive history and hazards assessment at individual volcanoes
• Giant landslides and their effects on ocean island volcanoes
About this Seminar
The Chemical Oceanography, Geology, Geochemistry, and Geobiology Seminar [COG3] is a student-run seminar series. Topics include chemical oceanography, geology, geochemistry, and geobiology. The seminars take place on Fridays from 10-11am in Building E25, Room 119, unless otherwise noted (term-time only).