Morphology and maturation of melt inclusions in quartz phenocrysts from the Badlands rhyolite lava flow, southwestern Idaho

Morphologies of primary rhyolitic melt inclusions in three successive populations of quartz phenocrysts in a single volcanic unit illustrate the change of inclusion shape after trapping. The 14 Ma Badlands lava flow on the Owyhee Plateau of southwestern Idaho contains two magmas with three distinct quartz populations differing in size and morphology. In tiny quartz crystals that nucleated shortly before the eruption, large melt inclusions retain their initial, irregular shapes, whereas the smallest inclusions show mature negative crystal shapes. Inclusions in a population of larger quartz crystals that nucleated earlier all show either negative crystal shapes or faceted shapes transitional to negative crystals. All inclusions in a third population of large corroded quartz crystals have mature negative crystal shapes regardless of inclusion size. In silicic magmatic systems, irregularly shaped inclusions in quartz may imply trapping just before eruption; analyses of such inclusions should have the greatest likelihood of revealing the magmatic volatile conditions driving the eruption. Electron microprobe analyses show that maturation had no effect on the composition of the trapped melt. After eruption, however, slow cooling led to coarse crystallization and loss of silica from the melt; when these inclusions were revitrified in the laboratory, they did not regain all lost silica and did not become completely homogeneous. Revitrification of inclusions that cooled more quickly showed no such loss of silica to the host. Thus, given appropriate cooling conditions, even very old (Precambrian?) silicic melt inclusions may be suitable for microbeam analysis after any necessary revitrification in the laboratory. INTRODUCTION causing depressions or hollows that are covered by subsequent growth (Wilkins 1979). This is in contrast to secMicrobeam analytical techniques have dramatically inondary inclusions, which are trapped, in either fractures creased the usefulness of igneous silicate melt inclusions (see Roedder 1984) or dissolved reentrants (Donaldson for determining the original volatile contents, composiand Henderson 1988), after the crystal has formed. tions, and temperatures of erupted magmas. To exploit Primary fluid and melt inclusions are thought to be fully the information recorded in melt inclusions, we need trapped only very rarely with negative crystal shapes to understand the inclusions' probable morphology and (Roedder 1984). Sisson et al. (1993) observed irregular composition at the time of their trapping and how these primary fluid inclusions forming in laboratory experimay have changed after entrapment. Such information ments, but the temperatures and conditions involved in has the potential to provide better constraints on magtrapping of primary melt inclusions have so far precluded matic evolution and crystallization processes, triggering direct observation of their formation. In the laboratory, of eruptions, and eruption dynamics. synthetic, secondary inclusions of aqueous fluid, formed Most of what we know about the processes of trapping along fractures in quartz, are commonly observed to neck of fluid and melt inclusions has been gained from microdown into many smaller, isolated inclusions with more scopic study of natural inclusions and more directly by regular shapes by local dissolution and reprecipitation of laboratory observations of crystal growth and fluid-incluquartz (Shelton and Orville 1980; Roedder 1984). Likesion formation in relatively low-temperature aqueous wise, the regular shapes of most primary melt inclusions systems. Primary inclusions, which are trapped during are thought to result from posttrapping evolution or matgrowth of the surrounding host crystal (see discussion in uration of their morphology to minimize surface energy Roedder 1984), form because of a variety of conditions, (Chaigneau et al. 1980; Beddoe-Stephens et al. 1983; including defects in crystal growth, such as kinking of a Roedder 1984). This maturation often yields inclusions new growth layer over a planar crystal face (Sisson et al. with the spherical (lowest surface area per volume) or 1993), and localized temporary stagnation of growth negative crystal shapes (lowest surface energy) most often 0003-004X/96/0102-0158$05.00 158 MANLEY: MELT-INCLUSION MORPHOLOGY AND MATURATION 159 seen in igneous phenocrysts (Roedder 1984). Maturation occurs only if temperature conditions remain suitable; for example, Clocchiatti (1975) reported that a rounded melt inclusion in quartz developed a negative crystal shape after it was held at 700-800 °C for six weeks. Experimental studies at 900°C and 1 GPa (Laporte and Provost 1994) indicate that the equilibrium shape (no net growth, no net dissolution; see Wortis 1988) of a quartz crystal in a silicate liquid is a bipyramid with pristine, flat faces but with smooth apices and interfacial edges. This is also the morphology of most melt inclusions with negative crystal shapes in igneous quartz phenocrysts, i.e., smooth, lacking sharp angles and terminations. It is generally assumed that maturation involves only the redistribution of material of the host crystal, with no net loss or gain of chemical components from the inclusion fluid as the shape of the inclusion changes. This is most easily shown for synthetic, secondary inclusions of aqueous fluid, where the composition of the fluid is very different from that of the host quartz. In contrast, rhyolitic melt inclusions have high Si02 contents in common with their quartz and feldspar hosts, and more mafic melts share more elements with their pyroxene and olivine hosts. The initial shapes of igneous melt inclusions, and how these then change with maturation, are nicely illustrated by rhyolitic melt inclusions in successively nucleated populations of quartz phenocrysts in the Badlands lava flow of southwestern Idaho. GEOLOGICAL CONTEXT OF SAMPLES The Badlands lava flow is a rhyolitic unit with a total known volume of about 15 km3 (Manley 1994, 1995). It is associated with other large-volume rhyolite lavas and ignimbrites erupted between 14 and 9 Ma (Bonnichsen 1982a, 1982b; Ekren et al. 1984; Bonnichsen and Kauffman 1987) when the Yellowstone hotspot was located beneath what is now the southwestern comer of Idaho, also known as the Owyhee Plateau. The main events during the Badlands eruption, which tapped two distinct types of magma, can be determined from field relations in the unit's well-exposed vent area (Fig. 1; Manley 1994, 1995). A dike at least 5-10 km long (in plan view) propagated to the surface, and the first surficial activity involved minor extrusion of aphyric lava at the dike termination and sub-Plinian explosive eruptions from the main eruptive fissures, which built up -50 m thick ridges of nonwelded tephra representing mixed aphyric and phenocryst-rich magma. The narrow dike termination probably quickly became inactive because of rapid cooling (e.g., Delaney and Pollard 1981). When explosive activity ceased, lava effusion began. Near the dike termination, aphyric and phenocryst-rich magmas mingled during ascent and emplacement; the majority of the multilobed 15 km3 Badlands lava is composed of the phenocryst-rich material. The predominant Badlands magma, a phenocryst-rich (30 vol%) rhyolite (-75 wt% Si02, Table 1), had an anhydrous mineral assemblage of quartz + sanidine + plaKEV: UthoJogy