Details
Originalsprache | Englisch |
---|---|
Seiten (von - bis) | 2284-2294 |
Seitenumfang | 11 |
Fachzeitschrift | American Mineralogist |
Jahrgang | 102 |
Ausgabenummer | 11 |
Publikationsstatus | Veröffentlicht - 1 Nov. 2017 |
Abstract
One of the various problems faced in experimental petrology is the fact that most experimental products obtained by crystallization experiments are too small, making their accurate identification by electron microprobe and laser ablation analyses very difficult. This problem is magnified when a highly polymerized starting material is used for experiments at low temperature (e.g., 700-800 °C). In this study, we present the results of crystallization experiments performed using a rhyolitic starting glass in which we test the potential of temperature cycling and pre-hydrated starting material to increase crystal size and discuss the effect of those variables on the attainment of chemical equilibrium. Experiments were performed at different temperatures (725 to 815 °C) and pressures (1 and 2 kbar), under water-saturated conditions (aH2O = 1; with aH2O being the water activity). During the experiments, temperature was either constant or cycled to ±15 °C around the target temperature during the first half of the runs. We used either a pre-hydrated (7 wt% H2O) rhyolitic glass or a dry rhyolitic glass to which 7 wt% H2O was added during capsule preparation. Our results differ between 1 and 2 kbar experiments. At 1 kbar, plagioclase and orthopyroxene were the main crystalline phases affected and temperature cycling (±15 °C) did not increase the crystal size of these phases. In contrast, if only the nature of the starting material is considered (dry glass vs. pre-hydrated), the use of a pre-hydrated starting material successfully increased the overall crystal size and decreased the crystal number density. At 2 kbar, plagioclase and amphibole were the main phases and the largest crystals were also obtained when pre-hydrated starting material was used. Contrary to experiments at 1 kbar, temperature cycling also increased the overall crystal size. The different effects of temperature cycling at 1 and 2 kbar are attributed (1) to the different cation diffusivities at 1 and 2 kbar caused by different melt water concentrations and (2) the negligible effect of temperature cycling at 1 kbar (±15 °C) is explained by little dissolution of phases, so that small crystals were already too large to be completely consumed by the dissolution process in the high temperature interval. The results demonstrate that temperature oscillation (depending on the amplitude) and the nature of the starting material (pre-hydrated vs. dry glass + water) are two parameters that can contribute to increase crystal sizes in experiments with rhyolitic melts. However, we also observed that the use of a pre-hydrated starting material increased the occurrence of zoned plagioclase crystals, which may indicate that chemical equilibrium was not perfectly reached.
ASJC Scopus Sachgebiete
- Erdkunde und Planetologie (insg.)
- Geophysik
- Erdkunde und Planetologie (insg.)
- Geochemie und Petrologie
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in: American Mineralogist, Jahrgang 102, Nr. 11, 01.11.2017, S. 2284-2294.
Publikation: Beitrag in Fachzeitschrift › Artikel › Forschung › Peer-Review
}
TY - JOUR
T1 - Crystallization experiments in rhyolitic systems
T2 - The effect of temperature cycling and starting material on crystal size distribution
AU - Da Silva, Marize Muniz
AU - Holtz, Francois
AU - Namur, Olivier
N1 - Funding Information: We thank the head of the workshop at the Institute for Mineralogy at Leibniz University Hannover, Ulrich Kroll, for technical support and Julian Feige for sample preparation. Further thanks go to Eric Wolff and Chao Zhang for analytical support and to Adriana Currin for language revision. We also thank the editor Charles Lesher, as well as the reviewers Julia Hammer and Ryan D. Mills for their comments and suggestions that helped improve the manuscript. This work was funded by the DAAD and CNPq (fellowship to the first author) and the Deutsche Forschungsgemeinschaft (DFG; project HO1337/31 in the frame of the ICDP program). Olivier Namur acknowledges support from an Emmy Noether program from the DFG. Publisher Copyright: © 2017 Walter de Gruyter GmbH. All rights reserved. Copyright: Copyright 2018 Elsevier B.V., All rights reserved.
PY - 2017/11/1
Y1 - 2017/11/1
N2 - One of the various problems faced in experimental petrology is the fact that most experimental products obtained by crystallization experiments are too small, making their accurate identification by electron microprobe and laser ablation analyses very difficult. This problem is magnified when a highly polymerized starting material is used for experiments at low temperature (e.g., 700-800 °C). In this study, we present the results of crystallization experiments performed using a rhyolitic starting glass in which we test the potential of temperature cycling and pre-hydrated starting material to increase crystal size and discuss the effect of those variables on the attainment of chemical equilibrium. Experiments were performed at different temperatures (725 to 815 °C) and pressures (1 and 2 kbar), under water-saturated conditions (aH2O = 1; with aH2O being the water activity). During the experiments, temperature was either constant or cycled to ±15 °C around the target temperature during the first half of the runs. We used either a pre-hydrated (7 wt% H2O) rhyolitic glass or a dry rhyolitic glass to which 7 wt% H2O was added during capsule preparation. Our results differ between 1 and 2 kbar experiments. At 1 kbar, plagioclase and orthopyroxene were the main crystalline phases affected and temperature cycling (±15 °C) did not increase the crystal size of these phases. In contrast, if only the nature of the starting material is considered (dry glass vs. pre-hydrated), the use of a pre-hydrated starting material successfully increased the overall crystal size and decreased the crystal number density. At 2 kbar, plagioclase and amphibole were the main phases and the largest crystals were also obtained when pre-hydrated starting material was used. Contrary to experiments at 1 kbar, temperature cycling also increased the overall crystal size. The different effects of temperature cycling at 1 and 2 kbar are attributed (1) to the different cation diffusivities at 1 and 2 kbar caused by different melt water concentrations and (2) the negligible effect of temperature cycling at 1 kbar (±15 °C) is explained by little dissolution of phases, so that small crystals were already too large to be completely consumed by the dissolution process in the high temperature interval. The results demonstrate that temperature oscillation (depending on the amplitude) and the nature of the starting material (pre-hydrated vs. dry glass + water) are two parameters that can contribute to increase crystal sizes in experiments with rhyolitic melts. However, we also observed that the use of a pre-hydrated starting material increased the occurrence of zoned plagioclase crystals, which may indicate that chemical equilibrium was not perfectly reached.
AB - One of the various problems faced in experimental petrology is the fact that most experimental products obtained by crystallization experiments are too small, making their accurate identification by electron microprobe and laser ablation analyses very difficult. This problem is magnified when a highly polymerized starting material is used for experiments at low temperature (e.g., 700-800 °C). In this study, we present the results of crystallization experiments performed using a rhyolitic starting glass in which we test the potential of temperature cycling and pre-hydrated starting material to increase crystal size and discuss the effect of those variables on the attainment of chemical equilibrium. Experiments were performed at different temperatures (725 to 815 °C) and pressures (1 and 2 kbar), under water-saturated conditions (aH2O = 1; with aH2O being the water activity). During the experiments, temperature was either constant or cycled to ±15 °C around the target temperature during the first half of the runs. We used either a pre-hydrated (7 wt% H2O) rhyolitic glass or a dry rhyolitic glass to which 7 wt% H2O was added during capsule preparation. Our results differ between 1 and 2 kbar experiments. At 1 kbar, plagioclase and orthopyroxene were the main crystalline phases affected and temperature cycling (±15 °C) did not increase the crystal size of these phases. In contrast, if only the nature of the starting material is considered (dry glass vs. pre-hydrated), the use of a pre-hydrated starting material successfully increased the overall crystal size and decreased the crystal number density. At 2 kbar, plagioclase and amphibole were the main phases and the largest crystals were also obtained when pre-hydrated starting material was used. Contrary to experiments at 1 kbar, temperature cycling also increased the overall crystal size. The different effects of temperature cycling at 1 and 2 kbar are attributed (1) to the different cation diffusivities at 1 and 2 kbar caused by different melt water concentrations and (2) the negligible effect of temperature cycling at 1 kbar (±15 °C) is explained by little dissolution of phases, so that small crystals were already too large to be completely consumed by the dissolution process in the high temperature interval. The results demonstrate that temperature oscillation (depending on the amplitude) and the nature of the starting material (pre-hydrated vs. dry glass + water) are two parameters that can contribute to increase crystal sizes in experiments with rhyolitic melts. However, we also observed that the use of a pre-hydrated starting material increased the occurrence of zoned plagioclase crystals, which may indicate that chemical equilibrium was not perfectly reached.
KW - Crystal size distribution
KW - Crystallization experiment
KW - Rhyolite
KW - Temperature cycling
UR - http://www.scopus.com/inward/record.url?scp=85048331323&partnerID=8YFLogxK
U2 - 10.2138/am-2017-5981
DO - 10.2138/am-2017-5981
M3 - Article
AN - SCOPUS:85048331323
VL - 102
SP - 2284
EP - 2294
JO - American Mineralogist
JF - American Mineralogist
SN - 0003-004X
IS - 11
ER -