Some of the stuff we are doing with melt inclusions and crystals:

 

Melt inclusions are bits of melt trapped inside crystals.  They are valuable magma aliquots -- in most cases they are the only eruptive products that have escaped degassing.  Ion microprobe analyses of the now glassy inclusions provide concentration data on several volatile species (H2O, CO2, S, F, Cl).  The fidelity of the melt inclusion record, especially for water, depends criticallly on the crystal cooling rate, size and diffusivity.  Much of our recent work is aimed at studying volatile diffusion in crystals and glass to get at timescales of magma ascent and cooling.  And we continue to mine melt inclusions in well-quenched tephra to obtain first-order information on the volatile contents of magmas.

Fidelity of Melt Inclusions, Magma Ascent Rate,

Water Diffusion and Water in Arc Magmas

Volatile loss from melt inclusions in pyroclasts of differing sizes.

Alex Lloyd, Terry Plank, Philipp Ruprecht, Erik Hauri and Bill Rose

Contributions to Mineralogy and Petrology, (2013) DOI 10.1007/s00410-012-0800-2


We have investigated the loss of H2O from olivine-hosted melt inclusions (MIs) by designing an experiment using tephra samples that cooled at different rates owing to their different sizes: ash, lapilli, and bomb samples that were deposited on the same day (10/17/74) of the sub-Plinian eruption of Volca´n de Fuego in Guatemala. Ion microprobe, laser ablation-ICPMS, and electron probe analyses show that MIs from ash and lapilli record the highest H2O contents, up to 4.4 wt%. On the other hand, MIs from bombs indicate up to 30 % lower H2O contents (loss of *1 wt% H2O) and 10 % post-entrapment crystallization

of olivine. This evidence is consistent with the longer cooling time available for a bomb-sized clast, up to 10 min for a 3–4-cm radius bomb, assuming conductive cooling and the fastest H diffusivities measured in olivine (D*10-9 to 10-10 m2/s). On the other hand, several lines of evidence point to some water loss prior to eruption, during magma ascent and degassing in the conduit. Thus, results point to both slower post-eruptive cooling and

slower magma ascent affecting MIs from bombs, leading to H2O loss over the timescale of minutes to hours. The important implication of this study is that a significant

portion of the published data on H2O concentrations in olivine-hosted MIs may reflect unrecognized H2O loss via diffusion. This work highlights the importance of reporting clast and MI sizes in order to assess diffusive effects and the potential benefit of using water loss as a chronometer of magma ascent.



Diffusion of water in diopside from Kunlun Mts. by FTIR

Elizabeth Ferriss, Terry Plank and David Walker

2012 AGU Fall Meeting


Diffusion of hydrogen and other water-related species in clinopyroxenes has potential for use quantifying the ascent rate of magma near the surface of the Earth and has implications for our understanding of water in the mantle. Previous work on diopside has yielded diffusivities that range over four orders of magnitude (e.g., very fast diffusion in Woods et al. 2000 versus slow diffusion in Sundvall et al. 2011), and the relationship between diffusivity and mineral chemistry is still poorly understood. The diffusivity of water has been investigated in natural low-Fe (Mg number: 98.3) diopside from the Kunlun Mts., China supplied by the curator of the American Museum of Natural History (AMNH) mineral collection. Polished, small (typically 1.5-2 mm × 1.5-2 mm × 1.5-2 mm) blocks were oriented and heated at 1 atm in a vertical gas-mixing furnace. The samples were held in gold wire, and the oxygen fugacity was buffered at the quartz-fayalite-magnetite buffer (QFM) using a mixture of CO and CO2. Fourier transform infrared spectroscopy (FTIR, at AMNH) was used to measure the water content based on absorption peaks between wavenumbers 3000 and 4000 cm-1. The initial water content (33 ppm) was estimated using polarized spectra obtained in three orthogonal directions and the calibration of Bell et al. (1995). Integrated peak areas were obtained from FTIR spectra through the edges of the uncut, heated block using a linear baseline from wavenumbers 3500-3700 cm-1 and 3200-3700 cm-1 for spectra taken with the polarized infrared beam traveling parallel to [001] and [010], respectively. Diffusion profiles were obtained in each direction by normalizing these peak areas to the initial peak area measured prior to heating in each direction. The interpretation of these profiles and determination of diffusivities is complicated by the interdependence of the profiles and the fact that each FTIR measurement represents an average value of the water contents through the sample. Diffusivity parallel to [010] appears to be slower than parallel to [100]* and [001], in broad agreement with previous observations of anisotropy. Results obtained from this approach will be discussed and verified using profiles measured on a thin plate cut from the center of the sample. Woods et al. (2000) Am. Mineral. 85(3-4). Sundvall et al. (2009) Eur. J. Mineral. 21. Bell et al. (1995) Am. Mineral. 80 (5-6).

Why do Mafic Arc Magmas Contain

~ 4 wt% Water on Average?


Terry Plank, Katherine Kelley, Mindy Zimmer, Erik Hauri and Paul Wallace


EPSL (2013) Frontiers Article

http://dx.doi.org/10.1016/j.epsl.2012.11.044

Abstract

Over the last fifteen years there has been an explosion in data on the volatile contents of magmas parental to arc volcanoes. This has occurred due to the intense study of melt inclusions, trapped primarily within olivine, as aliquots of magma that have escaped degassing during eruption. The surprising first-order result is the narrow range in H2O concentrations of the least degassed melt inclusions when averaged by volcano (based on 7 arcs for which such data exist for > 5 volcanoes:  Central America, Mexico, Kamchatka, Marianas, Cascades, Tonga and the Aleutians). Nearly all arc volcanoes are sourced with mafic magmas that contain 2-6 wt% H2O. Moreover, the average for each arc varies even less, from 3.2 (for the Cascades) to 4.5 (for the Marianas), with an average for all seven arcs of 3.8 +/- 0.5 wt% H2O.  The narrow range and common average value for H2O is in stark contrast to that for most other subduction tracers, such as Nb or Ba, which vary by orders of magnitude. 


A modulating process, either in the crust or mantle, is likely responsible for the restricted range in the H2O contents of melt inclusions. One possibility is that melt inclusions reflects vapor saturation at the last storage depth prior to eruption. Magmas rise from the mantle with variable H2O contents (> 4 wt%), start degassing at the depth of H2O-saturation, and continue to degas up until the depth at which they stall.  If the stalling depths were ~6 km, not atypical for storage depths beneath volcanoes, magmas would be saturated at ~4 wt% H2O, and melt inclusions, which become sealed during ascent, would thus record ≤ 4 wt% H2O. Another possibility is that the melting process modulates water content in the melt such that magmas rise out of the mantle with ~4 wt% H2O. A strong relationship between the water content of the source (H2Oo) and the degree of melting (F) maintains nearly constant water contents in the melt for a restricted range in mantle temperature. Magmas with 3-4 wt% H2O can be generated at 1230-1280°C and 1.5 GPa for a wide range in F and H2Oo. Crust and mantle controls may dominate in different regions and may be distinguished from coupled trace element or CO2 variations.


 

New Stuff !

Recently Published!

NanoSIMS results from olivine-hosted melt embayments: Modeling ascent rate in explosive basaltic eruptions

Alex Lloyd, Terry Plank, Philipp Ruprecht, Erik Hauri, Helge Gonnermann and Bill Rose

AGU Meeting, 2012


A critical parameter governing the explosivity of volcanic eruptions is the rate at which magma ascends and degases, because this affects bubble nucleation, coalescence, and ultimately fragmentation. Although several methods have been used to determine magma ascent rates, it remains a poorly constrained parameter for most eruptions. One promising method employs diffusion modeling of H2O and CO2 concentration gradients in melt embayments/open melt inclusions [1,2]. Here we utilize the fine spatial resolution of the nanoSIMS to obtain concentration gradients for five volatile species, improving upon previous efforts that were more limited in spatial resolution (FTIR, [1]) and in number of volatile analytes (H2O only by BSE, [2]). Focusing on explosive basaltic eruptions, for which very little is known about ascent rates, we chose ash and lapilli samples from the Oct 1974 sub-plinian eruption of Volcán de Fuego. Glassy, olivine-hosted embayments with evidence of outlet bubbles were analyzed by nanoSIMS at a minimum distance between spots of 15 μm. Major element zonation in the embayments was investigated by EMP, and high resolution BSE images were captured to complement the nanoSIMS spot measurements for H2O (as in [2]). We report analyses for 5 embayments that vary in length from 100 to 350 μm. Low-solubility volatiles (CO2, H2O, S) decrease towards the embayment outlet, consistent with diffusive reequilibration with the more-degassed surrounding melt. High-solubility volatiles (Cl, F) increase towards the outlet, apparently behaving as magmaphile elements. Major elements exhibit constant concentrations along the embayment, except for a 20-50 μm wide zone near the embayment outlet, perhaps representing a boundary layer at the outlet bubble, where concentrations vary consistent with olivine and clinopyroxene microlite growth. BSE grayscale values are thus affected by both H2O diffusion and major element zonation at the embayment outlet, and cannot be used to estimate H2O concentration gradients [2]. Forward modeling of CO2 and H2O profiles takes into account temperature- and composition-dependent diffusivities and a closed-system degassing path for the exterior magma (as observed in melt inclusions from the same sample). Assuming a constant decompression rate from 200 MPa and an initial composition of 600 ppm CO2 and 4.3 wt% H2O at 1030°C, models yield preliminary results with very rapid ascent times (100 s, or 2 MPa/s). A two-stage model, however, allows slower decompression during CO2 exsolution (0.1 MPa/s) and faster ascent when H2O begins to exsolve (1.5 MPa/s), for total ascent times on the order of 10 to 20 minutes. This example highlights the additional constraints that come from measuring multiple diffusing species. [1] Liu et al, JGR, 2007 [2] Humphreys et al, EPSL, 2008.