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1. What are the key tectonic processes influencing volcanism on Patmos Island?

2. How is Ar/Ar geochronology conducted?

3. What minerals are common in Patmos trachytes?

4. What is the SiO2 content range in volcanic rock samples?

Accepted Answer

The volcanism on Patmos Island is influenced by several key tectonic processes. The island is located in the Aegean and Western Anatolia region, which experiences widespread and compositionally diverse Cenozoic magmatism. The evolution of the numerous volcanic centers on Patmos is governed by subduction-related, back-arc, and intra-plate magmatic processes. The region has been subjected to compressional and extensional forces, resulting from the subduction of continental platform sediments and roll-back of the subducting slab. This has caused a gradual shift of the trench and related processes to the south, forming the Aegean as an extensional, back-arc basin. In a back arc basin, the overall stress field is extensional, allowing upwelling of magmas originating from deep sources. Additionally, the relics of products of earlier compressional processes survive at depth, adding to the diversification and mingling of the magmatic signatures of volcanic products. Understanding these tectonic processes provides insights into the formation of the Aegean back-arc basin, one of the best studied back-arc basins globally. The island of Patmos, situated between the Cyclades and Menderes core complexes, experienced extreme crustal thinning by low angle normal faulting during the middle Miocene. The volcanism on Patmos was active during the Messinian and early Zanclean, a period of anomalously low water tables throughout the Mediterranean domain, causing decompression in the already thinned crust. The island's volcanic rocks include phonolites, trachytes, phonolites, and rhyolites, with the oldest volcanic rocks being phonolites. New high-resolution 40 Ar/39 Ar ages for the different volcanic units on Patmos and the neighboring island of Chilomodi provide valuable data to refine existing datasets and assess the potential roles of slab roll-back, upwelling of sub-continental lithospheric or asthenospheric melts, and tectonic regime changes in the area at the time. The westward propagation of the Northern Anatolian Fault (NAF) into the Aegean around 5 Ma resulted in changes of extensional tectonics and fault patterns, further influencing volcanism on Patmos Island.

Accepted Answer

Ar/Ar geochronology involves separating minerals, leaching samples, irradiating them, fusing mineral grains, and analyzing released argon isotopes. The process includes using a Jeol SEM to assess potassium contents, hand-picking samples, leaching with diluted HNO3, cleaning with deionized water, irradiating samples in a CLICIT facility, loading samples into copper disks, heating under vacuum, fusing mineral grains with a laser system, cleaning released gas, and analyzing purified Ar isotopes using a ThermoFisher Helix MC mass spectrometer. Gain calibration, system blanks, and interfering isotope production ratios are also considered. The reported full external error includes analytical uncertainty and includes error in J-values, standard age, and decay constants with 2 sigma uncertainty (2s).

Accepted Answer

The most common minerals observed in the trachytes of Patmos are K-feldspar (kfsp), pyroxene (px), biotite (bt), and plagioclase (plag). Some trachytes have big kfsp phenocrysts, such as P4 and P7. Trachyte P7 does not show signs of alteration and contains ~30% quartz (qtz) and ~20% kfsp, as well as clinopyroxene (cpx), biotite (bt), and olivine (ol).

Accepted Answer

The SiO2 content in the selected volcanic rock samples ranges from 51.6 to 80.5 wt.%. This range indicates the varying silica content in the samples, which is a significant factor in determining the rock's classification and properties. Silica content influences the viscosity of magma, the type of volcanic eruption, and the mineral composition of the resulting rock. Higher SiO2 content typically corresponds to more felsic rocks, such as rhyolites, while lower SiO2 content is associated with mafic rocks like basalt. The range of SiO2 content in the samples suggests a diverse composition of volcanic rocks, with some classified as rhyolites, trachytes, phonolites, andesite, basaltic trachyandesite, and trachybasalt.

Accepted Answer

The geochronological framework of volcanism on Patmos and Chilomodi indicates a brief period of 1.4 Ma with at least three distinct intervals of activity. The oldest volcanic activity yielded phonolites at ~6.5 Ma, followed by rhyolitic and trachytic volcanism on Patmos island around ~6.0 Ma, and volcanic activity on Chilomodi island around ~5.8 Ma. Contrary to previous studies, no evidence for ages older than 6.0 Ma in the trachytic rocks was found. Previous K/Ar ages for Patmos were reported by Fytikas et al. (1976) and Wyers (1987), but due to uncertainties and difficulties in the whole rock approach, these ages are not included in the current discussion.

Accepted Answer

Phonolites are commonly found in intra-plate settings, intracontinental rifts, and oceanic island settings. They are derived from silica-undersaturated, mafic magmas. A rare example of phonolites occurrence is in or near a volcanic arc or back-arc setting, such as the phonolites of Patmos. In Western Anatolia, west of the current slab gap, the volcanic centre of Foca is one of the few nearby localities where phonolites are described. Additionally, occurrences of phonolites are found in proximity of Isparta, below the tomographically indicated slab gap, in the area of Afyon and Senirkent graben. Several occurrences of Miocene phonolites are described in the eastern Aegean and western Anatolian volcanic province.

Accepted Answer

The K2O/Na2O ratio in volcanic rocks demonstrates the chronological evolution of their composition. In the case of Patmos, the ratio shows a transition from sodic (phonolites) to potassic (trachytes) and back to almost sodic (Chilomodi rhyolites). This sodic character of the phonolites suggests a possible asthenospheric influence. High K2O+Na2O volcanics are common, but the Chilomodi rhyolite also contains andesitic enclaves with lower K2O+Na2O content, similar to the trachybasalt of Chilomodi and the volcanics from Nisyros and Kolumbo. The Na2O content of the volcanic rocks of Chilomodi island is similar to that of SAVA and Kula. The trend in the youngest volcanics of the study aligns with the more sodic trend observed towards the Quaternary, as seen in the relatively sodic nature of the Kula basalts.

Accepted Answer

The three distinct volcanic intervals on Patmos, as indicated by the new age data, are as follows: 1) Magmatism with an asthenospheric to intraplate signature started at 6.5 Ma with the eruption of sodic phonolites. 2) The next phase of magmatism from 6.0 Ma to 5.6 Ma produced rhyolites and trachytes with a more potassic nature. 3) Trachytic and rhyolitic volcanism on Chilomodi commenced at 5.4 Ma and lasted until 5.2 Ma. These intervals provide insights into the tectonic and magmatic evolution of the region.

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1. What are the key tectonic processes influencing volcanism on Patmos Island?

2. How is Ar/Ar geochronology conducted?

3. What minerals are common in Patmos trachytes?

4. What is the SiO2 content range in volcanic rock samples?

5. What is the geochronological framework of volcanism on Patmos and Chilomodi?

6. Where are phonolites commonly found?

7. What does the K2O/Na2O ratio indicate in volcanic rocks?

8. What are the three distinct volcanic intervals on Patmos?

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