Gaining acuity on crystal terminology in volcanic rocks

Zellmer, Georg F.

doi:10.1007/s00445-021-01505-9

Cited by 50 publications

(15 citation statements)

References 36 publications

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“…Olivine crystals were not found in equilibrium (K D Fe-Mg : 0.27 ± 0.03), which in turn suggests that these crystals correspond to antecrysts or xenocrysts (c.f. Jerram and Martin, 2008;Zellmer, 2021). For clinopyroxene (K D Fe-Mg : 0.28 ± 0.08) and orthopyroxene (K D Fe-Mg : 0.29 ± 0.06) in equilibrium, the geothermobarometers applied were equations 32d and 32c (clinopyroxene) and 28a and 29b (orthopyroxene) of Putirka (2008).…”

Section: Geothermobarometrymentioning

confidence: 99%

“…However, the basement rocks in the area do not host this mineral phase and, therefore, an antecryst origin for the olivine seems more plausible (c.f. Jerram & Martin, 2008;Zellmer, 2021). Furthermore, by comparing the variation of incompatible element ratios with respect to SiO 2 (Figures 10A,B) for products of the SMVF and the basement rocks of the Cajamarca Complex (the major possible source of crustal contamination), it can be assessed that any contribution of the Cajamarca Complex is negligible.…”

Section: Crustal Assimilationmentioning

confidence: 99%

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The Northernmost Volcanoes in South America (Colombia, 5–6°N): The Potentially Active Samaná Monogenetic Volcanic Field

2022

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The northernmost volcanism in South America (5–6°N) is defined by the presence of several monogenetic volcanic edifices in Colombia, which have been grouped within the Samaná monogenetic volcanic field. Few volcanoes have been studied so far, but they are recognized as a cluster of volcanoes of intermediate-to-acid composition, formed by both explosive and effusive eruptions. This study aims to 1) characterize four more monogenetic volcanic edifices as part of the Samaná field, 2) highlight the potentially active volcanism in an area previously defined as non-volcanogenic, and 3) give insights into the magmatic evolution of the scarcely studied evolved monogenetic volcanism linked to subduction zones worldwide. To achieve these aims, this study uses petrography, mineral chemistry, whole-rock geochemistry, geochronological analyses, and geothermobarometric calculations. The analyses indicate that the field is formed by at least seven volcanoes with similar composition and that it is long-lived and potentially active. Mineralogically, the erupted products host plagioclase (An26–74) and amphibole (magnesio-hastingsite, tschermakite, and occasionally mangesio-hornblende) as the most abundant phases, although orthopyroxene (enstatite; Wo2–3, En70–76, Fs21–28) and clinopyroxene (diopside and augite; Wo44–45, En41–42, Fs13–15, and Wo42–44, En46–47, Fs10–11) also appear. Less abundant phases such as olivine (Fo81–88), biotite (magnesiobiotite), quartz, and Fe–Ti oxides (Usp4–89 Mag96–11, and Ilm61–92 Hem39–8) were also recognized. Chemically, the volcanoes are of andesitic-to-dacitic composition with calc-alkaline affinity and show similar behavior of LILE, HFSE, and REE, which is typical for magmatism in subduction environments. Ages yield a range between 1.32 ± 0.06 Ma (K/Ar) and 16,919 ± 220 years (14C). The results also indicate that the volcanoes share a common magmatic source that fed the individual eruptions and that the magma differentiation is mainly controlled by processes of fractional crystallization, although evidence of magma recharge processes or magma mixing and assimilation as a minor process are also recognized. Geothermobarometric calculations suggest that the different mineral phases are crystallized between 1,194 and 687 °C and a pressure between 0.88 and 0.19 GPa. This indicates that the aforementioned processes occurred not only at the main magmatic reservoir (∼33–21 km depth) but also at different stagnation zones at shallower levels of the crust (∼7–5 km). Taking this into account, it is shown that the magma evolution of this monogenetic field is more complex than individual batches of magma reaching the surface uninterrupted, as is normally described for monogenetic volcanic fields of more mafic compositions in other tectonic settings.

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Section: Geothermobarometrymentioning

confidence: 99%

Section: Crustal Assimilationmentioning

confidence: 99%

The Northernmost Volcanoes in South America (Colombia, 5–6°N): The Potentially Active Samaná Monogenetic Volcanic Field

2022

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“…The crystal cargo comprises mineral phases that may be categorised as phenocrysts/autocrysts, antecrysts, or xenocrysts (e.g. Hildreth 2001;Zellmer 2021). Mineral grains may also be zoned, for example having a xenocrystic or antecrystic core and a phenocrystic rim.…”

Section: Geology Geochemistry and Geophysicsmentioning

confidence: 99%

“…Antecrysts are generally defined as crystals either recycled from an earlier phase of magmatism within the same volcanic system, or which initially grew in a different part of the system to that which it finished growing (Hildreth 2001;Charlier et al 2005;Miller et al 2007;Cooper and Reid 2008;Zellmer 2021). Antecrysts often show disequilibrium textures in their core and interior zones owing to changes in their surrounding conditions during growth.…”

Section: Xenocrysts Antecrysts and Autocrysts Vs Phenocrystsmentioning

confidence: 99%

“…Xenocrysts are completely foreign to the magmatic system in which they were erupted, and could originate from a different magmatic system, country rock, or cumulate material (Sollas 1892;Charlier et al 2005;Miller et al 2007). As noted by Zellmer (2021), the term phenocryst is used either to refer to the size fraction of a crystal, or its genesis from entirely within the final erupted melt.…”

Section: Xenocrysts Antecrysts and Autocrysts Vs Phenocrystsmentioning

confidence: 99%

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Geochemistry, Magmatic Processes and Timescales of Recent Rhyolitic Eruptives of the Ōkataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa/New Zealand

Elms¹

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This thesis investigates the magmatic processes that operated at Ōkataina volcanic centre prior to and during the rhyolitic events of its most recent eruption sequence, the Rotorua Subgroup (erupted since 25.4 ka), with a specific focus on timescales of human interest. Further context is derived with data from the 52.8 ka Rotoiti (caldera-forming) and Earthquake Flat eruptions, and selected Mangaone Subgroup events that occurred between the 52.8 and 25.4 ka. The regular occurrence of these events provides regular snapshots of the magmatic system over this time period. Whole-rock, glass, and mineral compositional data and timescales modelled from diffusion gradients are compared within geographical and physical volcanological contexts. These data provide a picture of the recent evolution of Ōkataina, shedding light on how quickly the volcano could awaken from its current dormancy to produce a new rhyolitic event, and what that event might be like. Geochemical data from Mangaone Subgroup eruptives show that the post-caldera magmatic system recovered over c. 27 kyr, with increasingly evolved and radiogenic compositions erupted during these events. The post-25.4 ka Rotorua Subgroup shows a more steady evolution, trending towards less radiogenic products towards the present day. Modern Ōkataina comprises three vent clusters, the intra-caldera Mt. Tarawera (formed by the 21.9 ka Ōkareka, 17.5 ka Rerewhakaaitu, 14 ka Waiohau and 1314 AD Kaharoa eruptive episodes) and Haroharo Massif (the 25.2 ka Te Rere, 9.4 ka Rotomā, 7.9 ka Mamaku and 5.5 ka Whakatāne), and the Ōkareka Embayment (the 25.2 ka Te Rere, 15.6 ka Rotorua) on the western edge of the caldera. Most eruptions were geochemically complex, with multiple magmas, some of which were repeated between eruptions. Variations in whole-rock and glass geochemistry are controlled by both the dominant mineral assemblage and external inputs, and relate to the geographical region and timing of when each magma erupted. Each vent region shows evidence for distinct features in its shallower magmatism (i.e. that producing the final-erupted magma compositions), super-imposed on temporal trends controlled by the deeper system. This deeper system beneath Haroharo and Tarawera appears to be more unitary or inter-connected. However, the Ōkareka Embayment is unique in that it appears to involve multiple magmatic systems. Back-scattered electron imaging of orthopyroxenes from the Rotorua, Waiohau and Whakatāne eruptives (and selected plagioclase from the Rotorua), coupled with major-element analyses of the orthopyroxenes, imply that open-system processes are common at Ōkataina, at least in the early stages of eruptible magma body assembly. These processes are reflected in diverse core compositions and complex zonation patterns, narrowing to more uniform rims that are in equilibrium with the compositions of the host melts. Not all cores are in equilibrium with the associated whole-rock compositions, but are in equilibrium with whole-rock compositions from other magmas erupted nearby, further demonstrating open-system crystal exchange and the repeated tapping of some magma sources from one event to another. Melt segregation and magma body assembly takes place over decades up to a century or two, with magma residence times on the order of centuries to millennia, or (if prematurely triggered) as short as months. Priming or triggering events such as heating by basaltic injection (common in the Ōkataina rhyolite eruptions) occur over decades prior to eruption. Quartz-hosted, sealed melt inclusions preserve volatile contents of c. 3-6 wt% H2O and c. 15-125 ppm CO2, with values tending to decrease in younger eruptions, although Waiohau samples have relatively low H2O, and the Rotorua samples have relatively low CO2. Overall, the Ōkataina eruptives have remarkably low CO2 contents compared with international datasets. There are weak correlations between H2O and CO2 contents coexisting in magmas within individual vent regions, but little system-wide coherence due to each region having differing trends (a step-wise drop at Tarawera versus a steady decrease in the young Haroharo eruptions). Scatter in data from sealed melt inclusions, and relatively lower H2O contents in open-ended inclusions (embayments) indicate pre-eruptive volatile losses, driven by crystallisation and/or open-system degassing. The H2O contents of the inner parts of embayments suggest an initial slow decompression at the start of the final magma ascent may also play a minor role. Magma ascent rates are low to moderate (0.1-4.4 m.s-1), with the Ōkareka Embayment eruptives having notably faster ascent rates than those from Tarawera or Haroharo. Both the sealed melt inclusion volatile contents and derived magma ascent rates are similar to those observed or inferred from other silicic eruptions worldwide and are notably comparable to those of super-eruptions, indicating that magma volume and overpressure do not influence ascent rate at eruption onset. Current geophysical observations suggest that a viable magma source currently exists under the southwestern end of Haroharo, and that basaltic diking activity is ongoing in the Ōkataina area. While basaltic eruptions can occur with little warning (e.g. Tarawera, 1886 CE), it is more likely that a future Ōkataina event will be prolonged and rhyolitic. The volcano can rejuvenate into an eruption-ready state within human lifetimes (i.e. decades), and final pre-eruptive warnings from geophysical monitoring could be as short as only a few hours. A new rhyolitic episode could have major impacts, lasting several years and involving both voluminous lavas and widespread fall deposition.

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Shallow magmatic processes revealed by cryptic microantecrysts: a case study from the Taupo Volcanic Zone

Lormand

Zellmer

Sakamoto

et al. 2021

Contrib Mineral Petrol

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Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating during pre-eruptive magma ascent. They indicate a plumbing system periodically intruded by short-lived sub-aphyric dykes that entrain microantecrysts grown under diverse physico-chemical conditions and stored in rapidly cooled, previously intruded dykes. Changes in temperature gradients between the intrusion and the host rock throughout ascent and repeated magma injections lead to fluctuations in cooling rates and generate local heterogeneities illustrated by the microlite textures and rim compositions. Late-stage degassing occurs at water saturation, forming thin calcic microcryst rims through local partitioning effects. This detailed investigation of textures cryptic to conventional imaging shows that a significant proportion of the micrometre-sized crystal cargo of the TVZ is of antecrystic origin and may not be attributed to late-stage nucleation and growth at the onset of volcanic eruptions, as typically presumed.

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Gaining acuity on crystal terminology in volcanic rocks

Cited by 50 publications

References 36 publications

The Northernmost Volcanoes in South America (Colombia, 5–6°N): The Potentially Active Samaná Monogenetic Volcanic Field

The Northernmost Volcanoes in South America (Colombia, 5–6°N): The Potentially Active Samaná Monogenetic Volcanic Field

Geochemistry, Magmatic Processes and Timescales of Recent Rhyolitic Eruptives of the Ōkataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa/New Zealand

Shallow magmatic processes revealed by cryptic microantecrysts: a case study from the Taupo Volcanic Zone

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