C2/c pyroxene phenocrysts from three potassic series in the Neogene alkaline volcanics, NE Turkey: their crystal chemistry with petrogenetic significance as an indicator of PT conditions

  • Faruk Aydin
  • Richard M. Thompson
  • Orhan Karsli
  • Hinako Uchida
  • Jason B. Burt
  • Robert T. Downs
Original Paper


Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20–58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe3+-rich Al-diopsides for Groups A and B (AB-cpxs) and Ti- and Fe3+-poor Al-diopsides for Group C (C-cpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 Å), V cell (437.2–440.9 Å3), and 〈beta〉 angle values (106.01°–106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by larger c (5.27–5.30 vs. 5.25–5.28 Å), V T (2.27–2.30 vs. 2.23–2.28 Å3), and V M2 (25.53–25.72 vs. 25.41–25.59 Å3) values and smaller b (8.87–8.88 vs. 8.88–8.91 Å) and V M1 (11.49–11.63 vs. 11.64–11.83 Å3) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of V M2/V M1 (2.20–2.23) due to large V M2 and small V M1 compared to the V M2/V M1 ratios of the C-cpxs (<2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes of different groups support the presence of evolved host rocks with different alkaline character (i.e. silica-undersaturated Groups A–B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6–10.6 kbars), suggesting that the AB-cpxs crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate higher cooling rates. The PT estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a closed plumbing system at a moderate- to low-pressure regime.


Clinopyroxene Crystal-chemistry Low pressure Alkaline Volcanics NE Turkey 



The authors are glad to thank the DAAD for the partial financial support. Many thanks are given to M. Burhan Sadiklar (Karadeniz Teknik University, Trabzon, Turkey) for logistic support. Moreover, they would like to give special thanks to Rainer Altherr and Hans-Peter Meyer (Mineralogisches Institut, Universität Heidelberg, Germany) for the electron microprobe analyses of some clinopyroxene phenocrysts. Thoughtful reviews made by Keith Putirka and anonymous peer-reviewers greatly improved the original manuscript. Editorial managing by Gordon Moore is really appreciated. We would like to thank the National Science Foundation for funding RMT, HU, and JAB through grants No. EAR-0622371 and No. EAR-0609906.


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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Faruk Aydin
    • 1
  • Richard M. Thompson
    • 2
  • Orhan Karsli
    • 3
  • Hinako Uchida
    • 2
  • Jason B. Burt
    • 2
  • Robert T. Downs
    • 2
  1. 1.Department of Geological EngineeringNigde UniversityNigdeTurkey
  2. 2.Department of GeosciencesUniversity of ArizonaTucsonUSA
  3. 3.Department of Geological EngineeringGumushane UniversityGumushaneTurkey

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