DOI: 10.1016/j.epsl.2019.115927
论文题名: Anomalous structure of MgCO3 liquid and the buoyancy of carbonatite melts
作者: Hurt S.M. ; Wolf A.S.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2020
卷: 531 语种: 英语
中文关键词: alkaline earth carbonates
; carbonate liquid structure
; carbonatite density
; MgCO3 melt
英文关键词: Alkaline earth metals
; Buoyancy
; Calcite
; Calcium carbonate
; Carbonation
; Iron compounds
; Liquids
; Magnesite
; Molecular dynamics
; Oxygen
; Positive ions
; Specific heat
; Strontium compounds
; Topology
; Volume measurement
; Alkaline earth
; Bulk compositions
; Carbonatites
; Density crossover
; Empirical potentials
; Liquid structures
; Metal cation order
; Structure of liquids
; Barium compounds
; buoyancy
; carbonate
; carbonatite
; high pressure
; igneous geochemistry
; magnesium
; mantle structure
; melt
; melting
; phase equilibrium
; thermodynamics
英文摘要: MgCO3 is one of the most important components of mantle-derived carbonatite melts, and yet also one of the most difficult to study experimentally. Attempts to constrain its thermodynamic properties are hampered by decarbonation, which occurs at only ∼500 °C, far below its metastable 1 bar melting temperature. Molecular dynamic simulations, however, can predict the thermodynamic properties of the MgCO3 liquid component in spite of experimental challenges. Using the recently developed empirical potential model for high-pressure alkaline-earth carbonate liquids (Hurt and Wolf, 2018), we simulate melts in the MgCO3-CaCO3-SrCO3-BaCO3 system from 773 to 2373 K up to 20 GPa. At 1 bar, MgCO3 liquid assumes a novel topology characterized by a 4-fold coordination of the metal cation (Mg) with both the carbonate molecule and oxygen ion; this is distinct from the other alkaline-earth carbonate liquids in which the metal cation is in ∼6- and ∼8-fold coordination with carbonate and oxygen. With increasing pressure, MgCO3 liquid structure becomes progressively more like that of (Ca, Sr, Ba)CO3 liquids with Mg2+ approaching 6-fold coordination with carbonate groups. The novel network topology of MgCO3 liquid results in a melt that is significantly more buoyant and compressible than other alkaline-earth carbonate liquids. Simulations of mixed MgCO3-bearing melts show that metal cation coordination with O and C is independent of bulk composition. Mixed simulation also reveal that molar volume, compressibility, enthalpy and heat capacity do not mix ideally with (Ca, Sr, Ba)CO3 liquids at 1 bar, a consequence of preferential metal-cation ordering in MgCO3-bearing mixtures. As pressure increases, however, mixing progressively approaches ideality with respect to molar volume, becoming nearly ideal by 12 GPa. The model is further applied to mantle-derived primary carbonatite melts with compositions, temperatures and pressures determined by published phase equilibrium experiments. The voluminous structure of liquid MgCO3 results in a buoyant melt that inhibits a density crossover with the surrounding mantle. Assuming FeCO3 liquid also adopts the same anomalous high-volume structure as MgCO3, we predict that even the most Fe-rich ferrocarbonatites would remain buoyant and be barred from sinking or stagnating in the mantle. © 2019 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/165043
Appears in Collections: 气候变化与战略
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作者单位: Department of Earth and Environmental Sciences, University of Michigan, 1100 N. University Ave., Room 2534, Ann Arbor, MI 48109, United States; Division of Math and Science, National Park College, 101 College Dr., Hot Springs National ParkAR 71913, United States
Recommended Citation:
Hurt S.M.,Wolf A.S.. Anomalous structure of MgCO3 liquid and the buoyancy of carbonatite melts[J]. Earth and Planetary Science Letters,2020-01-01,531