Magma chambers versus mush zones: constraining the architecture of sub-volcanic plumbing systems from microstructural analysis of crystalline enclaves

Abstract

There are clear microstructural differences between mafic plutonic rocks that formed in a dynamic liquid-rich environment, in which crystals can be moved and re-arranged by magmatic currents, and those in which crystal nucleation and growth are essentially in situ and static. Crystalline enclaves, derived from deep crustal mushy zones and erupted in many volcanic settings, afford a unique opportunity to use the understanding of microstructural development, established from the study of intrusive plutons, to place constraints on the architecture of sub-volcanic systems. Here, we review the relevant microstructural literature, before applying these techniques to interrogate the crystallization environments of enclaves from the Kameni Islands of Santorini and Rábida Volcano in the Galápagos. Crystals in samples of deep-sourced material from both case studies preserve evidence of at least some time spent in a liquid-rich environment. The Kameni enclaves appear to record an early stage of crystallization during which crystals were free to move, with the bulk of crystallization occurring in a static, mushy environment. By contrast, the Rábida enclaves were sourced from an environment in which hydrodynamic sorting and re-arrangement by magmatic currents were common, consistent with a liquid-rich magma chamber. While presently active volcanoes are thought to be underlain by extensive regions rich in crystal mush, these examples preserve robust evidence for the presence of liquid-rich magma chambers in the geological record. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Keywords: crystal mush; enclave; magma chamber; microstructure.

Figure 1.

QEMSCAN maps of Ca distribution in cumulates from the Layered Series of the Skaergaard Intrusion, East Greenland. (a) The leucocratic portion of a modal layer from Trough G in Upper Zone. Plagioclase is teal (with darker colours showing more sodic compositions), interstitial phases are quartz (black) and clinopyroxene (grey). Note the euhedral shape of the plagioclase, with most grain boundaries formed by the juxtaposition of these planar growth faces. Although some boundaries show evidence of indentation, the smoothness of these boundaries and the localized presence of late-stage albite on them, together with the absence of a preferred orientation of the late-stage albitic overgrowths on the plagioclase, demonstrates that these indentations are not a consequence of dissolution–reprecipitation in response to an applied stress but are most likely a consequence of irregular, late-stage growth of grains accumulated on the magma chamber floor. The scale bar is 1 mm long. (b) A troctolitic cumulate from Lower Zone. Plagioclase is dark teal, with dark relatively albitic rims. Olivine is grey and augite is bright blue. Low-Ca pyroxene (inverted pigeonite) is a mottled grey. The cores of the plagioclase grains formed close to the magma–mush interface, while the constant composition's relatively sodic rims formed within the mush. The scale bar is 2 mm long.

Figure 2.

(a) Glass-rich portion of the upper crust of the Kilauea Iki lava lake, quenched during drilling. Plagioclase (labelled plag), augite (labelled cpx) and ilmenite (opaque) form a framework with interstitial brown glass (labelled glass). The junctions between adjacent grains are formed by the meeting of planar growth faces (examples are arrowed) with no evidence for minimization of interfacial energies. Plane polarized light. Scale bar is 1 mm long. (b) Plagioclase-rich glassy enclave from Brandur, Iceland, photographed with sensitive tint plate under crossed polars. The glass-filled pores have rounded cuspate margins, with low, equilibrium values of the melt–plagioclase–plagioclase dihedral angle established at pore corners (examples are arrowed). Scale bar is 0.5 mm long. (c) Olivine-rich glassy enclave from Mauna Loa, Hawaii. The rounded olivine grains display dihedral angles close to textural equilibrium at pore corners. Plane polarized light. Scale bar is 0.5 mm long. (d) Troctolitic cumulate from the Eastern Layered Intrusion of the Rum Igneous Complex, Inner Hebrides. Three-grain junctions involving only plagioclase are close to textural equilibrium, with a well-developed granular microstructure in plagioclase-only regions. Note the absence of any evidence of planar plagioclase growth faces, in contrast to figure 1a. The olivine grain on the right has a low dihedral angle where it forms a three-grain junction with two plagioclase grains (arrowed), demonstrating an absence of textural equilibration of this poly-phase junction. Crossed polars. Scale bar is 0.5 mm long.

Figure 3.

(a) Glassy crystalline enclave from Mauna Loa, Hawaii, with an irregular glass film separating grains of olivine (ol) and augite (cpx). Note the small grain of spinel (labelled ox) sitting in the melt film, demonstrating its primary origin during solidification. Plane polarized light. Scale bar is 0.5 mm long. (b) Plagioclase-rich glassy enclave from Brandur, Iceland, showing lenses of glass (examples are arrowed) on irregular grain boundaries. Crossed polars. Scale bar is 1 mm long. (c) Glassy crystalline enclave from Mauna Loa, Hawaii, with irregular glass pockets (some of which are labelled gl) on boundaries between olivine and orthopyroxene (labelled opx). Note the low dihedral angles at pore corners, denoting approach to super-solidus textural equilibrium. Crossed polars. Scale bar is 0.5 mm long. (d) Plagioclase-rich enclave from Brandur, Iceland, showing a small pocket of glass on a plagioclase–plagioclase grain boundary, now partially infilled by clinopyroxene. Plane polarized light. Scale bar is 0.5 mm long. (e) The chilled margin of the Bracken Bay-Straiton Dyke, SW Scotland, showing a cluster of plagioclase phenocrysts set in a fine-grained groundmass. Note the alignment of the plagioclase and that they are joined along large areas of planar grain boundary parallel to the growth faces. Crossed polars. Scale bar is 0.5 mm long. (f) Gabbro from the Marginal Border Series of the Skaergaard Intrusion, East Greenland, showing highly irregular grain boundaries between adjacent plagioclase grains. Very few of these grain boundaries are parallel to growth faces of the plagioclase. Crossed polars with sensitive tint plate. Scale bar is 1 mm long.

Figure 4.

Cumulates from Middle Zone of the Rustenburg Layered Series of the Bushveld complex, South Africa. (a) Note the abundant tapering and curved deformation twins in the plagioclase and the irregular grain boundaries. Cumulus orthopyroxene grains are also bent although neighbouring interstitial clinopyroxene is not strongly deformed. Crossed polars. Scale bar is 1 mm long. (b) Grain boundaries between strongly deformed, original igneous, plagioclase grains are decorated with undeformed neoblasts that grew as a result of dynamic recrystallization. Crossed polars. Scale bar is 0.5 mm long.

Figure 5.

Sample 458220 from Lower Zone (LZa) of the Skaergaard Layered Series, in which plagioclase is cumulus and augite in interstitial. Note the small range in plagioclase grain sizes and the wide constant composition relatively albitic rims (examples are marked with an asterisk). This sample has only a very weak fabric. Crossed polars. Scale bar is 1 mm long. (a) Sample from the LZa equivalent in the Marginal Border Series. Note the wide range of grain sizes of the plagioclase and the abundant normal zoning (examples are marked with an asterisk). Crossed polars. Scale bar is 1 mm long.

Figure 6.

Troctolitic and gabbroic glass-bearing enclaves entrained in the 1950 andesite flow of the Kameni Islands, Santorini. (a) Clusters of euhedral to subhedral olivine and augite grains, joined by large areas of planar grain boundary consistent with a formation during synneusis. Crossed polars. Scale bar is 1 mm long. (b) Plagioclase grains in this enclave have low aspect ratios, commonly with complex twinning, and no evidence for dislocation creep (the yellow birefringence colour is due to the thickness of this thin section). The arrowed grain boundary is planar, melt-free and joins two plagioclase grains on their (010) faces: this is suggestive of sintering following synneusis. The grain marked with an asterisk is joined by similarly straight, (010) parallel grain boundaries to adjacent grains. Crossed polars. Scale bar is 1 mm long. (c) While many plagioclase grains have low AR, some are more elongate, with evidence of sintering following synneusis (grain marked with an asterisk). Note the planar growth faces where plagioclase is adjacent to large pockets of melt (example is marked by an arrow). Crossed polars. Scale bar is 1 mm long. (d) The grain marked with an asterisk has planar growth facets where it protrudes into a large melt-filled pocket (note the serrated, pale grey margins—these grew rapidly after entrainment). The two arrows show the location of wide melt-filled boundaries bounded by highly irregular plagioclase growth faces. Continued growth of these grains will result in the trapping of residual melt to form impingement lenses. Crossed polars. Scale bar is 1 mm long. (e) The two white arrows on the left denote wide melt films on developing grain boundaries between plagioclase grains, whereas those on the right show the location of highly irregular grain boundaries formed by impingement growth. Crossed polars. Scale bar is 1 mm long. (f) The rounded olivine grains have low dihedral angles where in contact (arrowed) denoting some approach to textural equilibrium at pore corners. There is rather less evidence of textural equilibration at plagioclase–plagioclase junctions, with some rounding caused by diffusion-limited late-stage growth during quenching (an example on the left of the image is arrowed—note the clearly defined smooth inner surface of the overgrowth rim compared with the more irregular outer surface adjacent to the interstitial quenched glass [81]). Plane polarized light. Scale bar is 0.5 mm long.

Figure 7.

Photomicrographs of the Rábida enclaves, Galápagos. (a) Plagioclase-rich enclave, with interstitial clinopyroxene and vesicular glass, photographed under plane polarized light. (b) shows the same area photographed under crossed polars. Note the absence of preferred orientation of the euhedral, low AR, plagioclase grains, their uniform grain size, and the frequency with which grain boundaries are formed by the juxtaposition of planar growth faces. Scale bar in both images is 1 mm long. (c) Plagioclase-rich enclave, with abundant small rounded grains of oxide and interstitial vesicular glass, photographed under plane polarized light. (d) shows the same area photographed under crossed polars. The range of grain sizes in this enclave is large, and the larger grains are commonly normally zoned. Note the evidence for complex zoning, with other grains apparently not zoned at all. The scale bar in both images is 1 mm long. (e) A strong preferred orientation of relatively elongate plagioclase. Note the scattering of small equant clinopyroxene grains. Crossed polars. Scale bar is 1 mm long. (f) Euhedral, equant plagioclase grains are separated by irregular but parallel-sided voids, with some transgranular fractures. The grain boundary voids are likely to have been created by decompaction caused by devolatilization of interstitial melt during ascent. Note the interstitial quartz (labelled qtz; examples are arrowed), which is also separated from the adjacent plagioclase by parallel-side voids. Crossed polars. Scale bar is 1 mm long.

Figure 8.

QEMSCAN images highlighting the Ca content of plagioclase in the Rábida enclaves, Galápagos. Bright colours denote high Ca content and the darker teal more albitic compositions: phases other than plagioclase are black in these images. (a) Same enclave as depicted in figure 7c,d. Note the presence of several different zoning patterns, with particular examples of either normal, patchy or oscillatory zoning marked by asterisks. It is not possible to reproduce this range of apparent zoning types by sectioning through a single type. Scale bar is 2 mm long. (b) Same enclave as depicted in figure 7e. The two large plagioclase grains either side of the asterisk are the two larger grains shown in figure 7e. Note the strong preferred orientation. The relatively larger grains in this field of view have strongly calcic cores, in contrast to the majority of the smaller grains which have complex partially resorbed cores attesting to a different growth history. Scale bar is 2 mm long.

Figure 9.

Photomicrographs of the Rábida enclaves, Galápagos. (a) Euhedral plagioclase and rounded grains of Fe–Ti oxides, set in highly vesicular interstitial glass. Plane polarized light. Scale bar is 0.5 mm long. (b) All grain boundaries in this enclave have been opened and are now either voids or locally contain some vesicular glass. Note the abundant interstitial quartz. Crossed polars. Scale bar is 1 mm long. (c) A grain boundary containing vesicular glass separates two plagioclase grains, photographed under plane polarized light. The same area is photographed under crossed polars in (d), in which it is apparent that the grain boundary has been opened by decompaction caused by devolatilization during ascent, as the newly opened fracture also cuts one of the plagioclase grains. Scale bar in both images is 0.5 mm long. (e) Interstitial pockets of vesicular glass are bounded by either planar plagioclase growth faces (with localized limited approach to texturally equilibrated dihedral angles) or form impingement lenses along grain boundaries. Crossed polars with a sensitive tint plate. Scale bar is 0.5 mm long. (f) Irregular grain boundary between two plagioclase grains, containing vesicular glass. Crossed polars with a sensitive tint plate. Scale bar is 0.5 mm long.

Figure 10.

Photomicrographs of the Rábida enclaves, Galápagos. (a) Enclave containing primocrysts of plagioclase, clinopyroxene, Fe–Ti oxides, with abundant apatite. The central part of this image is magnified in (b) Plane polarized light. Scale bar is 1 mm long. (b) Primocrysts of apatite (labelled ap) are concentrated in, or adjacent to, pockets of glass, demonstrating that they crystallized relatively late. Plane polarized light. Scale bar is 0.5 mm long. (c) Plagioclase adjacent to pockets of (oxidized) glass or minerals that arrive relatively late on the liquidus has developed a sieve texture consistent with partial melting of the most evolved (and hence with the lowest temperature of crystallization) regions of the enclave during entrainment and ascent in a relatively hot magma. Plane polarized light. Scale bar is 1 mm long.