Details
Original language | English |
---|---|
Article number | 120765 |
Journal | Chemical geology |
Volume | 594 |
Early online date | 11 Feb 2022 |
Publication status | Published - 5 Apr 2022 |
Abstract
In this study we present the compositional changes of clinopyroxene (cpx), plagioclase (plg), spinel (sp), and glass experimentally solidified from an Icelandic MORB melt. The starting material was cooled at Patm and fO2 of air, in the thermal range of cooling (ΔTc) between 1300 °C (superliquidus) to 800 °C (solidus) with rates (ΔT/Δt) of 1, 7, 60, 180, 1800, and 9000 °C/h. The run products obtained at 1, 7 and 60 °C/h are holocrystalline, whilst between 60 and 180 °C/h plg disappears, and texture of cpx + sp. shifts from faceted to dendritic. As cooling rate increases, we observe that Fe2O3 decreases and Al2O3 increases in sp. and Al2O3 + Fe2O3 increase and CaO + MgO decrease in cpx. These measured variations mirror changes induced by cooling rate in cation (atoms per formula unit, a.p.f.u.) and molecular abundances of these two crystalline phases. Plg composition shows clear linear trends versus cooling rate. The chemistry of sp., cpx and, to a minor extent, plg solidified from this basaltic liquid is thus strictly related to the cooling rate condition and is similar to those observed in previous investigations on alkaline and evolved basaltic systems. In particular, cpx is the only mineral phase profusely present at all the cooling rates, showing the greatest chemical variations in terms of oxides, cations, and components. The intra-crystalline glass (≤ 50 μm from crystal rims) obtained at 180–1800 °C/h shows compositional variations related to the surrounding crystal growth, evidencing strong supersaturation phenomena (such as dendritic texture) due to the establishment of a diffusion-controlled growth regime. Chemical attributes of mineral phases are also quantitatively related with the maximum (Gmax) and average (GCSD) growth rates of sp., cpx, and plg. When compared with the starting melt composition, the chemistry of cpx suggests the attainment of near-equilibrium crystallization conditions at cooling rate ≤ 60 °C/h, whereas disequilibrium effects are found at cooling rate > 60 °C/h. In contrast, plg is in disequilibrium with the initial melt chemistry in all experiments. By using thermometric models, the calculated crystallization of plg takes place at temperatures much lower than those of cpx, when the crystal content is high and the diffusion of cations in the melt is slow due to the higher (residual) melt viscosity. Under such conditions and due to the effect of cooling, the system cannot return to homogeneous concentrations and, consequently, plg more effectively records the disequilibrium partitioning of cations between the growing crystal surface. The data-set reported here captures the entire (superliquidus to solidus) and intrinsic (heterogeneous site-free silicate liquid) solidification behavior from an actual MORB melt from very rapid to extremely sluggish cooling rate. Finally, all analytical relationships found in this work enable careful reconstruction of the solidification conditions of MORB melts, providing novel geo-speedometers for them at high fO2.
Keywords
- Cooling rate, Geospeedometer, Mineral chemistry, MORB, Solidification kinetics
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geology
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Chemical geology, Vol. 594, 120765, 05.04.2022.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Crystal-chemical variations of spinel, clinopyroxene, and plagioclase in MORB basaltic melt induced by continuous cooling
AU - Giuliani, Letizia
AU - Vetere, Francesco
AU - Iezzi, Gianluca
AU - Nazzari, Manuela
AU - Mollo, Silvio
AU - Behrens, Harald
AU - Scarlato, Piergiorgio
AU - Ventura, Guido
N1 - Funding Information: The authors would like to thank the editor and the reviewers for their handling. This study was funded by the “ Fondi Ateneo of the University G. D'Annunzio ”, PRIN ( 2009PZ47NA_003 ) project “ Experimental Determination of the Glass-Forming Ability (GFA), Nucleation and Crystallization of Natural Silicate Melts ” and PRIN ( 2017J277S9_003 ) project “ Time Scales of Solidification in Magmas: Application to Volcanic Eruptions, Silicate Melts, Glasses, Glass-Ceramics ” awarded to G. Iezzi. Most of this study was conducted during the Ph.D. of L. Giuliani. Alexander von Humbold foundation senior research grant to F. Vetere is also acknowledged.
PY - 2022/4/5
Y1 - 2022/4/5
N2 - In this study we present the compositional changes of clinopyroxene (cpx), plagioclase (plg), spinel (sp), and glass experimentally solidified from an Icelandic MORB melt. The starting material was cooled at Patm and fO2 of air, in the thermal range of cooling (ΔTc) between 1300 °C (superliquidus) to 800 °C (solidus) with rates (ΔT/Δt) of 1, 7, 60, 180, 1800, and 9000 °C/h. The run products obtained at 1, 7 and 60 °C/h are holocrystalline, whilst between 60 and 180 °C/h plg disappears, and texture of cpx + sp. shifts from faceted to dendritic. As cooling rate increases, we observe that Fe2O3 decreases and Al2O3 increases in sp. and Al2O3 + Fe2O3 increase and CaO + MgO decrease in cpx. These measured variations mirror changes induced by cooling rate in cation (atoms per formula unit, a.p.f.u.) and molecular abundances of these two crystalline phases. Plg composition shows clear linear trends versus cooling rate. The chemistry of sp., cpx and, to a minor extent, plg solidified from this basaltic liquid is thus strictly related to the cooling rate condition and is similar to those observed in previous investigations on alkaline and evolved basaltic systems. In particular, cpx is the only mineral phase profusely present at all the cooling rates, showing the greatest chemical variations in terms of oxides, cations, and components. The intra-crystalline glass (≤ 50 μm from crystal rims) obtained at 180–1800 °C/h shows compositional variations related to the surrounding crystal growth, evidencing strong supersaturation phenomena (such as dendritic texture) due to the establishment of a diffusion-controlled growth regime. Chemical attributes of mineral phases are also quantitatively related with the maximum (Gmax) and average (GCSD) growth rates of sp., cpx, and plg. When compared with the starting melt composition, the chemistry of cpx suggests the attainment of near-equilibrium crystallization conditions at cooling rate ≤ 60 °C/h, whereas disequilibrium effects are found at cooling rate > 60 °C/h. In contrast, plg is in disequilibrium with the initial melt chemistry in all experiments. By using thermometric models, the calculated crystallization of plg takes place at temperatures much lower than those of cpx, when the crystal content is high and the diffusion of cations in the melt is slow due to the higher (residual) melt viscosity. Under such conditions and due to the effect of cooling, the system cannot return to homogeneous concentrations and, consequently, plg more effectively records the disequilibrium partitioning of cations between the growing crystal surface. The data-set reported here captures the entire (superliquidus to solidus) and intrinsic (heterogeneous site-free silicate liquid) solidification behavior from an actual MORB melt from very rapid to extremely sluggish cooling rate. Finally, all analytical relationships found in this work enable careful reconstruction of the solidification conditions of MORB melts, providing novel geo-speedometers for them at high fO2.
AB - In this study we present the compositional changes of clinopyroxene (cpx), plagioclase (plg), spinel (sp), and glass experimentally solidified from an Icelandic MORB melt. The starting material was cooled at Patm and fO2 of air, in the thermal range of cooling (ΔTc) between 1300 °C (superliquidus) to 800 °C (solidus) with rates (ΔT/Δt) of 1, 7, 60, 180, 1800, and 9000 °C/h. The run products obtained at 1, 7 and 60 °C/h are holocrystalline, whilst between 60 and 180 °C/h plg disappears, and texture of cpx + sp. shifts from faceted to dendritic. As cooling rate increases, we observe that Fe2O3 decreases and Al2O3 increases in sp. and Al2O3 + Fe2O3 increase and CaO + MgO decrease in cpx. These measured variations mirror changes induced by cooling rate in cation (atoms per formula unit, a.p.f.u.) and molecular abundances of these two crystalline phases. Plg composition shows clear linear trends versus cooling rate. The chemistry of sp., cpx and, to a minor extent, plg solidified from this basaltic liquid is thus strictly related to the cooling rate condition and is similar to those observed in previous investigations on alkaline and evolved basaltic systems. In particular, cpx is the only mineral phase profusely present at all the cooling rates, showing the greatest chemical variations in terms of oxides, cations, and components. The intra-crystalline glass (≤ 50 μm from crystal rims) obtained at 180–1800 °C/h shows compositional variations related to the surrounding crystal growth, evidencing strong supersaturation phenomena (such as dendritic texture) due to the establishment of a diffusion-controlled growth regime. Chemical attributes of mineral phases are also quantitatively related with the maximum (Gmax) and average (GCSD) growth rates of sp., cpx, and plg. When compared with the starting melt composition, the chemistry of cpx suggests the attainment of near-equilibrium crystallization conditions at cooling rate ≤ 60 °C/h, whereas disequilibrium effects are found at cooling rate > 60 °C/h. In contrast, plg is in disequilibrium with the initial melt chemistry in all experiments. By using thermometric models, the calculated crystallization of plg takes place at temperatures much lower than those of cpx, when the crystal content is high and the diffusion of cations in the melt is slow due to the higher (residual) melt viscosity. Under such conditions and due to the effect of cooling, the system cannot return to homogeneous concentrations and, consequently, plg more effectively records the disequilibrium partitioning of cations between the growing crystal surface. The data-set reported here captures the entire (superliquidus to solidus) and intrinsic (heterogeneous site-free silicate liquid) solidification behavior from an actual MORB melt from very rapid to extremely sluggish cooling rate. Finally, all analytical relationships found in this work enable careful reconstruction of the solidification conditions of MORB melts, providing novel geo-speedometers for them at high fO2.
KW - Cooling rate
KW - Geospeedometer
KW - Mineral chemistry
KW - MORB
KW - Solidification kinetics
UR - http://www.scopus.com/inward/record.url?scp=85125009898&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2022.120765
DO - 10.1016/j.chemgeo.2022.120765
M3 - Article
AN - SCOPUS:85125009898
VL - 594
JO - Chemical geology
JF - Chemical geology
SN - 0009-2541
M1 - 120765
ER -