Research ArticleGEOPHYSICS

Climbing the crustal ladder: Magma storage-depth evolution during a volcanic flare-up

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Science Advances  10 Oct 2018:
Vol. 4, no. 10, eaap7567
DOI: 10.1126/sciadv.aap7567
  • Fig. 1 Schematic showing the temporal evolution and map of the central TVZ highlighting the units of interest in this study.

    Key deposits are shown on both panels, with eruptive volumes indicated in parentheses on the left (see also table S1). In the left panel, the vertical position of each bar represents the best estimate for deposit age, while the horizontal position of bars depicts whether deposits are part of the western or eastern central TVZ, or both. Color of units in the map matches the colors on the diagram on the left.

  • Fig. 2 Crystallization pressures for magmas of the TVZ volcanic flare-up and the late-Whakamaru group.

    (A) Each data point represents a single pressure determination based on rhyolite-MELTS calculations for a given matrix glass composition. Error bars are 25 MPa and represent our best estimate of the uncertainty associated with each determination (27). Different colors correspond to different eruptive units. Eruptive units are organized according to age (increasing to the right), but individual samples are ordered according to crystallization pressure to facilitate visualization. (B) Each data point represents the average of all determinations for the corresponding unit (or subunit, in the case of Pokai); error bars are the SD associated with that average. Units are organized according to age, like in (A); the ordering of Pokai subunits is arbitrary, with shallower subunits shown to the left for ease of visualization. (C) Bar diagrams of crystallization pressure for each unit. Diagrams are organized in order of increasing age, but age is not to scale. The vertical (pressure) axis is the same for all diagrams. The horizontal (number of analyses) scale is the same for all diagrams, except for Mamaku rocks, as indicated. Typical crystallization pressures for TVZ magmas derived from calculations by Bégué et al. (29) are indicated by the yellow band. The data reveal a time progression for the storage depths of magmas feeding flare-up eruptions, particularly for the hot-dry-reducing magmas in the western portion of the central TVZ: Storage starts at relatively deep levels for Chimpanzee magmas and gradually becomes shallower until the co-eruption of Mamaku and Ohakuri magmas.

  • Fig. 3 Trace-element plots showing the compositional evolution of flare-up magmas in comparison with late-Whakamaru magmas.

    Whakamaru glass has very distinct Ba contents, probably a consequence of equilibration with sanidine. Flare-up magmas form a distinct array, with glass from older units (e.g., Chimpanzee) being less evolved [i.e., lower light rare earth element (LREE) and higher Sr] than glass from younger units (e.g., Ohakuri), paralleling the time progression observed in crystallization pressures. ppm, parts per million.

  • Fig. 4 Example of CL and Ti zoning in quartz, including calculation of growth time and rate from detailed CL images.

    CL image and Ti map shown in (A) demonstrate the correlation between CL intensity and Ti contents in quartz when there are large changes in CL; it can be seen that CL images capture the zoning in more detail, justifying the use of CL images for crystallization time and growth rate determination. Two areas were selected for detailed CL imaging, as indicated. (B) Detailed CL images with corresponding profiles used for calculations of crystallization time and growth rate. Each observed profile (red curve) is the average of 11 parallel profiles obtained from the image (shown in red in the corresponding image). The best-fit error function is shown in gray, from which the characteristic diffusion length (L) is extracted, which allows calculation of crystallization times (t). Calculation of growth rate (G) requires obtaining the distance between the boundary and the edge of the crystal from the larger CL image. See text for more details.

  • Fig. 5 Calculated crystallization times and growth rates for quartz from the TVZ volcanic flare-up and the late-Whakamaru group deposits.

    (A) Each data point represents an individual determination from a crystal from the corresponding unit. Units are organized according to age (increasing to the right), but individual data points for each unit are placed in order of increasing time to facilitate visualization. (B) Histogram showing distribution of crystallization times for each unit. (C) Histogram showing distribution of growth rates for each unit. The range of times is almost invariably within 1 and 100 years for all units, and growth rates are the same for all units.

  • Fig. 6 Results of rhyolite-MELTS simulations showing the enthalpy lost over the course of crystallization for magmas from the TVZ volcanic flare-up and the late-Whakamaru group deposits.

    Each curve corresponds to the enthalpy loss from liquidus (crystal-free) conditions to 15 wt % crystals. Plagioclase is a liquidus phase in all cases. The data points in each curve represent the crystal content and enthalpy at which the system saturates in quartz (qtz) for that composition. Pressures used are the average for each unit (Fig. 2B). Water contents are set to yield fluid saturation at the liquidus, and fO2 is set to the Ni-NiO buffer. Starting compositions and pressure used are given in table S2. Pumice from the TVZ flare-up typically contains <10 wt % crystals, suggesting that measured quartz growth times (Fig. 5) in general correspond to at least 25% of the total crystallization times (under the limiting assumption that magmas were initially crystal free). For Whakamaru, crystal contents can be as high as 25 wt % crystals. This suggests that crystallization times based on quartz crystallization times are typically within a factor of 4, and no more than within a factor of 10, of the total crystallization time.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/10/eaap7567/DC1

    Table S1. Characteristics of eruptive deposits from the central TVZ analyzed in this study.

    Table S2. Magma compositions used for rhyolite-MELTS modeling of energy change associated with crystallization.

    Data file S1. Includes supplementary tables S3 to S5.

  • Supplementary Materials

    The PDF file includes:

    • Table S1. Characteristics of eruptive deposits from the central TVZ analyzed in this study.
    • Table S2. Magma compositions used for rhyolite-MELTS modeling of energy change associated with crystallization.
    • Legend for data file S1

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    Other Supplementary Material for this manuscript includes the following:

    • Data file S1 (Microsoft Excel format). Includes supplementary tables S3 to S5.

    Files in this Data Supplement:

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