An official website of the United States government
Puyehue-Cordon Caulle (PCC) is an active volcanic complex located in the SVZ of the Andes that has had three major historic rhyodacitic eruptions with the most recent event in 2011-12. We provide petrologic and geochemical evidence that PCC is underlain by a crystal mush using recently identified basaltic mafic enclaves that highlights the involvement of distinct mafic magma components during the 2011-12 eruption. We suggest the mafic enclaves represent remnants of the crystal-rich mush that get entrained during eruption of the crystal-poor rhyodacite melt lens cap. This architecture requires the basaltic mush to produce rhyodacite through efficient fractionation. The dominant population of enclaves are equigranular, crystal-rich (45-55%), vesiculated (10-20%), and display interlocking grains between phases. Vesicles have complex shapes filling the irregular interlocking textures, while phenocrysts show stepwise normal zoning (uniform plagioclase cores, ~An90, overgrown with weakly zoned rims, ~An60). A second porphyritic population may represent mafic recharge into the system that bypasses the mush unperturbed. The porphyritic enclaves have spherical vesicles and tightly bound primitive mineral compositions (Fo80-86 vs Fo70-86 in the equigranular enclaves). Published geothermobarometry from the 2011-12 rhyodacite suggests shallow magma storage (5-7 km, 100-140 MPa, 895°C), which we compare against newly determined mineral-mineral trace-element partitioning based thermometry. Our thermometry indicates the equigranular enclaves were stored at ~900-1000°C at the time of eruption suggesting both a compositionally and thermally zoned magma system. We combine this temperature information with trace element data and mass balance calculations from various minerals phases and melt to substantiate our previous hypothesis that the basaltic enclaves can produce rhyodacite given their crystallinity. These estimates may support a spatially connected basaltic crystal-mush underlying a rhyodacite melt lens cap further proving highly efficient rhyolite formation at PCC. PCC’s enclaves present one of the largest compositional gaps on record globally. We compare them to other enclave-bearing systems and how PCC is an important end-member to understand enclaves as well as rhyolite formation.
more »
« less
The Survival of Mafic Magmatic Enclaves and the Timing of Magma Recharge
Abstract Many intermediate to felsic intrusive and extrusive rocks contain mafic magmatic enclaves that are evidence for magma recharge and mixing. Whether enclaves represent records of prolonged mixing or syn‐eruptive recharge depends on their preservation potential in their intermediate to felsic host magmas. We present a model for enclave consumption where an initial stage of diffusive equilibration loosens the crystal framework in the enclave followed by advective erosion and disaggregation of the loose crystal layer. Using experimental data to constrain the propagation rate of the loosening front leads to enclave “erosion” rates of 10−5–10−8 cm/s for subvolcanic magma systems. These rates suggest that under some circumstances, enclave records are restricted to syn‐eruptive processes, while in most cases, enclave populations represent the recharge history over centuries to millennia. On these timescales, mafic magmatic enclaves may be unique recorders that can be compared to societal and written records of volcano activity.
more »
« less
- PAR ID:
- 10386836
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 14
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The 2018 LERZ eruption of Kilauea featured a wide range of eruptive styles. In particular, Fissure 17 (F17) displayed activity ranging from Hawaiian fountaining in the eastern part of the fissure to Strombolian explosions in the western part. Lava erupted from F17-West was highly viscous and contained magmatic enclaves. Magmatic enclaves have previously been observed in many other volcanic systems (e.g. Vulcano Island, IT and Sete Cidades Volcano, PT), where they have been attributed to injection of mafic magma into an evolved magma chamber, resulting in viscous fingering, quenching, and break-off into fragments. The F17 enclaves differ from previous studies in that the chemical compositions of the enclave and host magmas are very similar, and that the enclaves have a limited spatial distribution and lack signs of viscous behavior and quenching, pointing to a different formation mechanism than inferred for other volcanic systems. In order to test a different formation hypothesis, we conducted fractal analysis of the size distribution of 84 individual enclaves from F17-West lavas. Our results, including a fractal dimension of fragmentation Df of 2.59, indicate that the F17 enclaves likely formed by brittle fragmentation. Since the enclave and host magmas were at temperatures far above the glass transition during the magma hybridization, high strain rates have to be invoked to explain the brittle fragmentation. This may have caused the enclave magma to transition into solid-state behavior, allowing it to break off into fragments that were subsequently picked up by the host magma and carried to the free surface. The enclaves from F17-West therefore offer a unique insight into the diversity of processes that characterizes the shallow parts of volcanic systems, as well as the importance of strain rates in modulating the rheological behavior of magmas.more » « less
-
Quantifying the buildup of pre-eruptive excess pressure in volcanic systems is crucial for the understanding of eruption-triggering mechanisms. The 2006 eruption of Augustine Volcano, Alaska, provides an excellent opportunity to investigate the response of the crystal cargo within a crystal-rich, intermediate host magma to temperature and pressure perturbations induced by mafic recharge shortly before the eruption. We use compositional changes of plagioclase together with adjoined and concurrently grown orthopyroxene to quantify these changes in magma conditions in response to this recharge event. We find ∼3.9 mol% anorthite increase in plagioclase is best explained by a 7.7 MPa (0.4−37 MPa, 80% uncertainty bounds) increase in magmatic pressure prior to the 2006 eruption. This value is close to the estimated tensile strength of rocks, suggesting that the pressure increase due to mafic recharge was likely sufficient to initiate dike intrusion and possibly trigger the eruption. This approach can be implemented broadly at crystal-rich, intermediate, and hydrous arc volcanoes, can provide independent estimates of pre-eruptive pressure changes to complement geophysical models, and can add new insights into the physical processes that lead to volcanic eruptions.more » « less
-
Abstract Two distinct types of rare crystal-rich mafic enclaves have been identified in the rhyolite lava flow from the 2011–12 Cordón Caulle eruption (Southern Andean Volcanic Zone, SVZ). The majority of mafic enclaves are coarsely crystalline with interlocking olivine-clinopyroxene-plagioclase textures and irregular shaped vesicles filling the crystal framework. These enclaves are interpreted as pieces of crystal-rich magma mush underlying a crystal-poor rhyolitic magma body that has fed recent silicic eruptions at Cordón Caulle. A second type of porphyritic enclaves, with restricted mineral chemistry and spherical vesicles, represents small-volume injections into the rhyolite magma. Both types of enclaves are basaltic end-members (up to 9.3 wt% MgO and 50–53 wt% SiO 2 ) in comparison to enclaves erupted globally. The Cordón Caulle enclaves also have one of the largest compositional gaps on record between the basaltic enclaves and the rhyolite host at 17 wt% SiO 2 . Interstitial melt in the coarsely-crystalline enclaves is compositionally identical to their rhyolitic host, suggesting that the crystal-poor rhyolite magma was derived directly from the underlying basaltic magma mush through efficient melt extraction. We suggest the 2011–12 rhyolitic eruption was generated from a primitive basaltic crystal-rich mush that short-circuited the typical full range of magmatic differentiation in a single step.more » « less
-
Abstract Magmatic systems below volcanoes are often dominated by partially crystalline magma over the long term. Rejuvenation of these systems during eruptive events can impact lava composition and eruption style—sometimes resulting in more violent or explosive activity than would be expected, as was the case at Fissure 17 during Kīlauea’s 2018 eruption. Here, we explore how the crystallinity of unerupted intrusion magmas affect hybrid magma compositions and petrological signatures by constructing phase-equilibria models to evaluate mineral and melt compositions of low-MgO lavas erupted along the East Rift Zone of Kīlauea volcano on 30 to 31 January 1997 (Episode 54, Fissures A-F). We then compare calculated mixing proportions and petrologically derived magma volumes to GPS-based geodetic inversions of ground deformation and intrusion growth in an attempt to reconcile geodetic and petrologically estimated magma volumes. Open-system phase-equilibria thermodynamic models were used to constrain the composition, degree of differentiation, and thermodynamic state of a rift-stored, two pyroxene + plagioclase saturated low-MgO magma body immediately preceding its mixing with high-MgO recharge and degassed drainback (lava lake) magma from Pu‘u‘ō‘ō‘, shortly before fissure activity within Nāpau Crater began on 29 January 1997. Mixing models constructed using the Magma Chamber Simulator reproduce the mineralogy and compositions of Episode 54 lavas within uncertainties and suggest that the identity of the low-MgO magma body may be either variably differentiated remnants of un-erupted magmas intruded into Nāpau Crater in October 1968, or another spatially and compositionally similar magma body. We find that magmas derived from a single, compositionally stratified magma emplaced beneath Nāpau Crater in 1968 can mix with mafic Kīlauea magmas to reproduce average Episode 54 bulk lava, mineralogy and mineral compositions without necessitating the interaction of multiple, low-MgO rift-stored magma bodies to produce Episode 54 lava compositions. Further, by constructing phase equilibria-based mixing models of Episode 54, we can better define the pre-eruptive state of the magmatic system. The resultant mineral assemblages and compositions are consistent with the possibility that the now-fractionated, rift-stored magma body was compositionally stratified and ~ 40% to 50% crystalline at the time of mixing. Finally, we estimate the volume of the low-MgO magma body to be ~7.51 Mm3. Phase-equilibria model results corroborate field and geochemical relationships demonstrating how shallow intrusions at intraplate shield volcanoes can crystallize, evolve, and then be remobilized by new, later batches of mafic magma. Most notably, our MCS models demonstrate that the pre-eruptive conditions of an intrusive body may be recovered by examining mineral compositions within mixed lavas. Discrepancies between the geodetic constraints on volumes of stored rift versus newly intruded (recharge) magma and our best-fit results produced by MCS mixing models (which respectively are mmafic:mlow-MgO ≈ 2 vs. mmafic:mlow-MgO ≈ 0.75) are interpreted to highlight the complex nature of incomplete mixing on more localized scales as reflected in erupted lavas, compared to geodetically constrained volumes that likely reflect large spatial scale contributions to a magmatic system. These dissimilar volume relationships may also help to constrain eruptive versus unerupted volumes in magmatic systems undergoing mixing. By demonstrating the usefulness of MCS in modeling past eruptions, we highlight the potential to use it as a tool to aid in petrologic monitoring of ongoing activity.more » « less
