Radioactive disequilibria from 2D models of melt generation by plumes and ridges

doi:10.1016/0012-821X(94)90160-0

Earth and Planetary Science Letters

Volume 128, Issues 3–4, December 1994, Pages 425-437

https://doi.org/10.1016/0012-821X(94)90160-0 Get rights and content

Abstract

We present a simple method of calculating radiogenic excesses caused by the melting process in 2D models of melting regions. ²³⁸U series disequilibrium in basalt from recent Hawaiian eruptions can be explained using an axisymmetric plume model and a melt parameterisation based on laboratory experiments. The model is also applied to mid-ocean ridges and shows that the disequilibria are controlled by the mean melting rate at the base of the melting region and by the melt fraction present during melting. ( $^{230} Th^{235} U$ ) data constrains the mean melting rate at the onset of melting to less than 8 × 10⁻⁸ yr⁻¹, which is consistent with that expected from the experimental parameterisation. Limited ( $^{231} Pa^{235} U$ ) data give a maximum initial melting rate of 0.4 × 10⁻⁸ yr⁻¹. Melting begins in the garnet peridotite stability field and melt extraction is rapid ( ∼ 2000 yr). Extraction on this time scale is most easily explained if melt transport is by channel flow. The geochemical observations require the melt fraction in the region where U, Th and Ra are extracted from the source to be ∼ 0.1%, in agreement with estimates from a simple fluid dynamical model of melt separation.

References (46)

D. McKenzie
²³⁰Th²³⁸U disequilibrium and the melting processes beneath ridge axes
Earth Planet. Sci. Lett.
(1985)
M. Spiegelman et al.
Consequences of melt transport for uranium series disequilibrium in young lavas
Earth Planet. Sci. Lett.
(1993)
Z. Qin
Disequilibrium partial melting model and its implications for trace element fractionations during mantle melting
Earth Planet. Sci. Lett.
(1992)
H. Iwamori
²³⁸U²³⁰Th²²⁶Ra and ²³⁵U²³¹Pa disequilibria produced by mantle melting with porous and channel flows
Earth Planet. Sci. Lett.
(1994)
P. Beattie
The generation of uranium series disequilibria by partial melting of spinel peridotite — constraints from partitioning studies
Earth Planet. Sci. Lett.
(1993)
T.Z. LaTourrette et al.
Experimental determination of U-partitioning and Th-partitioning between clinopyroxene and natural and synthetic basaltic liquid
Earth Planet. Sci. Lett.
(1992)
M. Spiegelman et al.
Simple 2-D models for melt extraction at mid-ocean ridges and island arcs
Earth Planet. Sci. Lett.
(1987)
D.M. Shaw
Trace element fractionation during anatexis
Geochim. Cosmochim. Acta
(1970)
M. Spiegelman et al.
The requirements for chemical disequilibrium during magma migration
Earth Planet. Sci. Lett.
(1992)
Z. Qin
Dynamics of melt generation beneath mid-ocean ridge axes: Theoretical analysis based on ²³⁸U²³⁰Th²²⁶Ra and ²³⁵U²³¹Pa disequilibria
Geochim. Cosmochim. Acta
(1993)

A.S. Cohen et al.

Melting rates beneath Hawaii: evidence from uranium series isotopes in recent lavas

Earth Planet. Sci. Lett.

(1993)

S.J. Goldstein et al.

²³¹Pa and ²³⁰Th chronology of mid-ocean ridge basalts

Earth Planet. Sci. Lett.

(1993)

D. McKenzie

Some remarks on the movement of small melt fractions

Earth Planet. Sci. Lett.

(1989)

S. Newman et al.

²³⁰Th²³⁸U disequilibrium systematics in oceanic tholeiites from 21°N on the East Pacific Rise

Earth Planet. Sci. Lett.

(1983)

S. Newman et al.

Comparison of ²³⁰Th²³⁸U disequilibrium systematics in lavas from three hotspot regions: Hawaii, Prince Edward and Samoa

Geochim. Cosmochim. Acta

(1984)

S.J. Goldstein et al.

Th and U isotopic systematics of basalts from the Juan de Fuca and Gorda ridges by mass spectrometry

Earth Planet. Sci. Lett.

(1989)

M. Condomines et al.

²³⁰Th²³⁸U disequilibria in historical lavas from Iceland

Earth Planet. Sci. Lett.

(1981)

S. Krishnaswami et al.

The behaviour of ²³²Th and the ²³⁸U decay chain nuclides during magma formation and volcanism

Geochim. Cosmochim. Acta

(1984)

K.H. Rubin et al.

Dating of neovolcanic MORB using ( $^{226} Ra^{230} Th$ ) disequilibrium

Earth Planet. Sci. Lett.

(1990)

I.M. Reinitz et al.

The behaviour of the uranium decay chain series nuclides and thorium during the flank eruptions of Kilauea (Hawaii) between 1983 and 1985

Geochim. Cosmochim. Acta

(1991)

A.M. Volpe et al.

²²⁶Ra²³⁰Th disequilibrium in axial and off-axis mid-ocean ridge basalts

Geochim. Cosmochim. Acta

(1993)

R.W. Williams et al.

Effects of partial melting on the uranium decay series

Geochim. Cosmochim. Acta

(1989)

P. Beattie

Uranium-Thorium disequilibria and partitioning on melting of garnet peridotite

Nature

(1993)

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For example, most geochemical models for U-series disequilibria and REE depletion are 1D (e.g., McKenzie, 1985; Spiegelman and Elliott, 1993; Iwamori, 1993; Lundstrom, 2000; Jull et al., 2002). Where 2D geochemical models are used, the physical aspect of the model is highly simplified (e.g., constant porosity or diffuse porous flow, Richardson and McKenzie, 1994; Spiegelman, 1996; Behn and Grove, 2015). Most 2D geodynamic models for mid-ocean ridges focus on porosity distribution, solid and melt flow (e.g. Phipps Morgan, 1987; Spiegelman and McKenzie, 1987; Keller et al., 2017).
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Until more robust compositional constraints are available for calculating the U and Th partition coefficients of clinopyroxene based on mineral composition, however, we consider this approach the best available option. The dynamic melting calculations are based on the equations of McKenzie (1985) that were later modified by Richardson and McKenzie (1994) to include non-modal melting and variable partition coefficients (their equations 10 and 11). We numerically integrated these equations using the MATLAB software and performed calculations for constant solid upwelling rates and melt porosity values along a one-dimensional melt column.
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U-series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid-ocean ridges. Here we present the first ²³⁸U–²³⁰Th–²²⁶Ra isotope data for a suite of 83 basalts sampled between 5°S and 11°S along the South Mid-Atlantic Ridge. This section of the ridge can be divided into 5 segments (A0–A4) and the depths to the ridge axis span much of the global range, varying from 1429 to 4514 m. Previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts. Accordingly, this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials. ²²⁶Ra–²³⁰Th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their ²³⁰Th values do not require any age correction. The U–Th isotope data span a significant range from secular equilibrium up to 32% ²³⁰Th excess, also similar to the global range, and vary from segment to segment. However, the (²³⁰Th/²³⁸U) ratios are not negatively correlated with axial depth and the samples with the largest ²³⁰Th excesses come from the deepest ridge segment (A1). Two sub-parallel and positively sloped arrays (for segments A0–2 and A3 and A4) between (²³⁰Th/²³⁸U) and Th/U ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies. Despite abundant evidence for source heterogeneity, there is no simple correlation between (²³⁰Th/²³⁸U) and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions >350 kyr prior to partial melting to produce the basalts. In our preferred model, the two (²³⁰Th/²³⁸U) versus Th/U arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous peridotite source regions.
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