Mark K Reagan
Research
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Mark Reagan © 2008
Current Research Projects
Opportunities are available for motivated graduate students (M.S. and Ph.D.) to be involved in research projects in my lab, particularly those listed below. Please e-mail me for more information.
click here for a description of facilities available for petrological and geochemical research at Iowa.
Rates and timescales of magma degassing and crystallization
We have been funded by NSF to quantify the rates, overall extents, and mechanisms of degassing and crystallization for magmas with differing compositions from different tectonic settings. The ultimate goal is to mitigatef the hazards posed by explosive volcanic eruptions. We do this by measuring (210Pb/226Ra) activity ratios in lavas and contained minerals that have recently erupted in a variety of tectonic settings. The short half-life of 210Pb (T1/2 = 22.6. y) allows us to focus on the crucial period of the final century before an eruption. This ratio is ideal for this study because deficits in 210Pb with respect to 226R in lavas can result from persistent losses of 222Rn as magmas degas before eruption, whereas (210Pb) excesses can result from persistent excesses of 222Rn due to streaming of gasses through magmas. In addition, plagioclase fractionates Pb from Ra, and the (210Pb/226Ra) ratio can be used to determine the proportion of plagioclase mineral growth that occurs shortly before eruption. Our recent observations suggest that lavas erupted from volcanoes associated with subduction have (210Pb/226Ra) values that tend towards equilibrium (1.0; Reagan et al., 2005; Reagan et al., 2006; Reagan et al., in press), whereas ocean island basalts (OIB), tend to have 210Pb deficits. This appears paradoxical, as it is the low-volatile-content OIB lavas that appear to persistently lose 222Rn for years, whereas arc lavas, which lose much more gas, appear to rarely do so for more than a year or two before eruption. The purpose of our ongoing reserarch is to determine whether the root cause of this difference is indeed related to degassing, or to other processes such as fractionation of sulfides. We also have found that a large proportion of mineral growth occurs in both tectonic settings within a few decades of eruption (Reagan et al., 2008; Reagan et al., in press), and that rapid crystal fractionation could be the norm for many volcanoes.
Volcanism and subduction initiation
We have been studying early subduction-related volcanic rocks from the IBM arc collected during recent Shinkai 6500 diving and field work on the forearc islands of Guam, Rota, and Saipan. The goal has benn to develop a comprehensive understanding of the early evolution of the southern IBM arc system. We are producing a standard suite of analyses for all rock types so that sources and melting processes can be compared through time and space. These analyses include major element, trace element, and Sr-Nd-Pb-Hf isotope compositions of whole rocks and pillow-rind glasses, and major, trace, and volatile element concentrations in glass inclusions from olivine and other minerals. The ages of eruption are being constrained by 40Ar/39Ar and U-Pb geochronology. Analyses of “first-arc” volcanic rocks with relatively normal arc compositions from the Mariana forearc islands were published this year (Reagan et al., 2008). The ongoing work is on lavas erupted during the “proto-arc” stage of IBM arc development, which spans the time period of unusual petrogenesis that began at subduction initiation. Most of this work is on igneous rocks collected on the trench slope during Shinkai 6500 diving southeast of Guam in 2006 and 2008. The most abundant rock type collected during these dives were fine-grained gabbroic rocks and quench textured basalts. Based on the work accomplished to date, we hypothesize that these forearc basalts are the first volcanic rocks to erupt after subduction begins. Their genesis could have been largely by decompression melting as the mantle upwelled to fill the space left by the catastrophic initial sinking of the slab. We further hypothesize that continued melting of this now-shallow harzburgitic mantle residue in the presence of a robust flux of water-rich slab-derived fluid resulted in the genesis of boninites that ovelie the basalts. Beginning with the 45-46 Ma rhyolites on Saipan and 41-42 Ma arc tholeiites on Guam, lavas with relatively normal arc signatures started being generated. Thus the changeover from proto-arc mantle upwelling to first-arc mantle counterflow appears to take 4-7 Ma depending on location.
Recent and Active Graduate Students
Mindi Woods (PhD. in progress).
Origin of forearc lavas on the trench slope southeast of Guam.Jennifer Cash (M.S.in progress).
Timescales of degassing and magma differentiation at Augustine volcano.Mathew Wortel (M.S.- 2007).
Magma sources, differentiation processes, and eruption depths for Eocene pillow lavas from Guam.
Recent and Active Undergraduate Student Research Projects
Tawny Bailey (in progress)
Time-scales of degassing at Merapi Volcano, IndoesiaLaura Nuno (in progress)
Digital image analysis of volcanic rock textures from the Mariana Trench slope.Ben Ferriera (2008)
Digital image analysis of volcanic rock textures from the Mariana Trench slope.
Greg Stark (2007)
Time-frame of degassing for magmas erupted in 1980-1986 at Mount St. Helens.Matthew Legg (2006)
Differentiation mechanisms and time-scales for the phonolite erupted in 2004 near Tristan da Cunha.