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Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation.

ABSTRACT: The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth's continental crust (11-13) compared to basalts (15-16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.

SUBMITTER: Tang M

PROVIDER: S-EPMC6335430 | biostudies-literature | 2019 Jan

REPOSITORIES: biostudies-literature

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Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation.

Tang Ming M Lee Cin-Ty A CA Chen Kang K Erdman Monica M Costin Gelu G Jiang Hehe H

Nature communications 20190116 1

The surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth's continental crust (11-13) compared to basalts (15-16). Here, we show that during arc magma differentiation, the exte ...[more]

PMID: 30651551

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Project description:Oxygen fugacity in metal-bearing systems controls some fundamental aspects of the geochemistry of the early Earth, such as the FeO and siderophile trace element content of the mantle, volatile species that influence atmospheric composition, and conditions for organic compounds synthesis. Redox and metal-silicate equilibria in the early Earth are sensitive to oxygen fugacity (fO(2)), yet are poorly constrained in modeling and experimentation. High pressure and temperature experimentation and modeling in metal-silicate systems usually employs an approximation approach for estimating fO(2) that is based on the ratio of Fe and FeO [called "?IW (ratio)" hereafter]. We present a new approach that utilizes free energy and activity modeling of the equilibrium: Fe + SiO(2) + O(2) = Fe(2)SiO(4) to calculate absolute fO(2) and relative to the iron-wüstite (IW) buffer at pressure and temperature [?IW (P,T)]. This equilibrium is considered across a wide range of pressures and temperatures, including up to the liquidus temperature of peridotite (4,000 K at 50 GPa). Application of ?IW (ratio) to metal-silicate experiments can be three or four orders of magnitude different from ?IW (P,T) values calculated using free energy and activity modeling. We will also use this approach to consider the variation in oxygen fugacity in a magma ocean scenario for various thermal structures for the early Earth: hot liquidus gradient, 100?°C below the liquidus, hot and cool adiabatic gradients, and a cool subsolidus adiabat. The results are used to assess the effect of increasing P and T, changing silicate composition during accretion, and related to current models for accretion and core formation in the Earth. The fO(2) in a deep magma ocean scenario may become lower relative to the IW buffer at hotter and deeper conditions, which could include metal entrainment scenarios. Therefore, fO(2) may evolve from high to low fO(2) during Earth (and other differentiated bodies) accretion. Any modeling of core formation and metal-silicate equilibrium should take these effects into account.

Dataset Information

Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation.

Publications

Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation.

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