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Flow Banding in Obsidian: a Record of Evolving Textural Heterogeneity During Magma Deformation

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Flow Banding in Obsidian: a Record of Evolving Textural Heterogeneity During Magma Deformation Earth and Planetary Science Letters 236 (2005) 135–147 www.elsevier.com/locate/epsl Flow banding in obsidian: A record of evolving textural heterogeneity during magma deformation Helge M. Gonnermann*, Michael Manga1 Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767, USA Received 29 October 2004; received in revised form 11 April 2005; accepted 12 April 2005 Available online 20 June 2005 Editor: S. King Abstract We perform a quantitative textural analysis of banding in obsidian from Big Glass Mountain (BGM), Medicine Lake Volcano, California and from Mayor Island (MI), New Zealand. The samples are compositionally homogeneous, with banding defined by variable microlite content (BGM) or vesicularity (MI). The spatial distribution of alternating light and dark bands has a 1/wavenumber power-spectral density, as well as multifractal characteristics. Through the use of simple mathematical models, we demonstrate that formation of the observed banding structure is consistent with a continuous deformational reworking of magma. Our results indicate that changes in crystallinity (vesicularity) were concurrent with magma deformation and are random multiples of the total amount of crystallinity (vesicularity) that was already present at any given time. Banded obsidian from BGM and MI can be found in close spatial proximity to brittle deformational textures. We therefore propose that a key element of banding formation in our samples may be magma brecciation or fragmentation. Fragmentation can result in variable rates of degassing of fragments, which in turn can lead to nonuniform crystal (vesicle) nucleation and growth rates. Repeated fragmentation, welding and subsequent viscous deformation can thus lead to banding formation. D 2005 Elsevier B.V. All rights reserved. PACS: 91.40.Àk; 91.50.Hw; 91.60.Ba; 83.50.Àv; 83.70.Hq; 83.80.Nb; 47.20.Ft; 47.50+d; 47.52.+j; 47.53.+n; 47.54.+r; 47.55.Àt; 47.70Àn Keywords: volcanology; mixing; flow banding; obsidian; fragmentation; fractal 1. Introduction Banding in obsidian samples from Big Glass * Corresponding author. Department of Earth and Planetary Mountain (BGM), Medicine Lake Volcano, California Sciences, Harvard University, 20 Oxford Street, Cambridge, MA and from Mayor Island (MI), New Zealand preserves 02138, USA. Tel.: +1 617 496 6796; fax: +1 617 495 8839. E-mail addresses: [email protected] a record of deformation associated with magma as- (H.M. Gonnermann), [email protected] (M. Manga). cent, eruption, and emplacement. Alternating light and 1 Tel.: +1 510 643 8532; fax: +1 510 643 9980. dark bands ranging in width from tens of microns to 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.04.031 136 H.M. Gonnermann, M. Manga / Earth and Planetary Science Letters 236 (2005) 135–147 decimeters are defined by differences in crystallinity scale-invariant record, whereas multifractal analysis (BGM) or vesicularity (MI). Proposed processes for allows the identification of a multiplicative formational banding formation include viscous magma mixing process. In the context of the two samples analyzed (e.g., [1–4]), repeated autobrecciation and welding of here, this corresponds to a repeated (continuous) change fragments during flow at the surface (e.g., [5,6]), weld- in crystallinity (BGM) or vesicularity (MI) that is a ing and rheomorphism of pyroclastic fragments (e.g., small random multiple of the total amount already [7–11]), vapor-precipitated crystallization in stretched present. Such a process is called multiplicative and, in vesicles [12], or brittle deformation and welding of contrast to cumulative processes, results in binomial fragments during magma ascent in the volcanic conduit frequency distribution of the given attribute. Using a [13–16]. series of hypothesis tests (Section 5), we provide con- Although textural heterogeneity caused by mixing straints on the required dynamics of such a process. of preexisting parent magmas has received consider- Finally, in Section 6 we discuss brittle deformation of able attention, the origin of compositionally homoge- magma as a possible physical mechanism for the for- neous banded lavas, such as BGM or MI samples, mation of banding in BGM and MI obsidian samples. remains relatively unexplored. This is an important area of study because crystal or bubble nucleation and growth are intimately linked to magma degassing [17– 3. Samples 19], which in turn is responsible for the buildup of overpressure required for the transition from effusive Banding in BGM and MI obsidian is defined by to explosive eruptive behavior [20–25]. One impor- alternating planar layers of varying color as a conse- tant unanswered question is whether the deformation- quence of varying micorocrystallinity (BGM) or al process that results in banding is separate from (i.e., microvesicularity (MI). In cross-section this layering subsequent to) the actual development of textural ranges in thickness from microns to decimeters and heterogeneity or if both processes are related in may attain lengths of up to several meters. Often, the some manner. layering can be distorted or folded [26]. 3.1. BGM obsidian 2. Approach BGM is a 900-year-old rhyolite-to-dacite obsidian The main goal of our analysis is to assess whether flow [27–30]. It is the youngest part of the Medicine banding in the BGM and MI obsidian samples pre- serves a record from which we can gain insight into the general nature of the underlying formational pro- cess, and hence about magma ascent and emplace- ment processes inaccessible to direct observation. We use spectral and multifractal analyses of banding to test if the formation of textural heterogeneity occurred prior to deformation, or if both processes are concur- rent. We focus on obsidian because the optical trans- parency allows measurements of structures to be made over a broad range of scales. Moreover, the obsidian samples under consideration bear evidence of a ge- netic relation between brittle magma deformation and banding formation. A petrographic description of both samples is provided in Section 3. In Section 4 we describe the results from power- spectral and multifractal analyses of each sample. Fig. 1. Photograph of banded obsidian sample from Big Glass Powerspectral analysis permits the identification of a Mountain, Medicine Lake Volcano, California. H.M. Gonnermann, M. Manga / Earth and Planetary Science Letters 236 (2005) 135–147 137 Fig. 2. Two different views of the same sample of obsidian from Big Glass Mountain. The sample is a rectangular slab and (a) is the side view perpendicular to (b). The sample shows a continuous gradation from angular, fused fragments (right third), to deformed fragments (middle), to well-developed banding (left third), consistent with brittle deformation, welding of fragments and subsequent viscous deformation. Lake composite shield volcano [27] and provides an cating that brittle deformation, welding of fragments example of the effusive emplacement of a cubic kilo- and subsequent deformation may have resulted in at meter-sized volume of silicic melt. Effusion was pre- least some of the observed banding. ceded by an explosive phase that resulted in the deposition of tephra over an area of approximately 3.2. MI obsidian 320 km2 [31]. BGM therefore provides an excellent example of a sizable silicic eruption with a transition MI is the visible portion of a 700-m-high and 15- from explosive to effusive behavior. The youngest km-wide Quaternary shield volcano [32]. Despite its eruptive sequence includes obsidian that is ubiqui- uniform peralkaline magma composition, MI’s history tously banded (Fig. 1), with some samples bearing includes virtually the full range of known eruption evidence for a genetic relation between fragmentation styles over a wide range of eruption sizes [33,34], and banding (Fig. 2). including fire-fountaining and spatter-fed lava flows, Banding is predominantly defined by varying con- such as the 8-ka obsidian flow from which our sample centrations of pyroxene microlites (Fig. 3), with a was collected [7,10,34]. Banding in the MI obsidian higher microlite content resulting in a darker color. sample under consideration is predominantly defined At times, dark bands also contain increased concen- trations of oxides (e.g., the darkest band in Fig. 3). However, overall there appears to be no compositional difference or difference in water content (J. Castro, personal communication) between individual bands of BGM obsidian. Some samples show a continuous gradation from angular, fused fragments (right third of Fig. 2), to deformed fragments (middle of Fig. 2), to well-developed banding (left third of Fig. 2), indi- Fig. 3. Photomicrograph of obsidian from Big Glass Mountain. Banding is generally defined by varying microlite content in a Fig. 4. Polished sample of Mayor Island banded obsidian (provided glassy matrix. Some dark bands also contain oxides. by K. V. Cashman, University of Oregon). 138 H.M. Gonnermann, M. Manga / Earth and Planetary Science Letters 236 (2005) 135–147 multifractal or not. Unlike for example Perugini et al. [4], where bands are defined by compositional variations, we do not obtain a precise calibration of color index to microcrystallinity or microvesicularity of our samples. It should be noted that, our trans- formation from natural samples to measures does not bias the outcome of our analysis, as will be Fig. 5. Photomicrograph of obsidian from Mayor Island. Banding is shown in Section 5. defined by varying vesicularity. by varying bubble content (Figs. 4 and 5), with higher 4. Analysis of banding vesicularity corresponding to lighter bands. The MI sample provides a sample of banded obsidian with 4.1. Power spectrum evidence of a clastogenic origin and close spatial association with magma brecciation [7]. The wavenumber power spectrum, S(k), is one of the most often used tools for studying complex 3.3. Digitized banding spatial patterns, because of its compact quantitative description of the presence of many spatial scales Our subsequent analysis is based on photomicro- [35].
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