Investigation of magnesium isotope fractionation during basalt differentiation

Implications for a chondritic composition of the terrestrial mantle

Fang Zhen Teng, Meenakshi Wadhwa, Rosalind T. Helz

Research output: Contribution to journalArticle

221 Citations (Scopus)

Abstract

To investigate whether magnesium isotopes are fractionated during basalt differentiation, we have performed high-precision Mg isotopic analyses by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) on a set of well-characterized samples from Kilauea Iki lava lake, Hawaii, USA. Samples from the Kilauea Iki lava lake, produced by closed-system crystal-melt fractionation, range from olivine-rich cumulates to highly differentiated basalts with MgO content ranging from 2.37 to 26.87 wt.%. Our results demonstrate that although these basalts have diverse chemical compositions, mineralogies, crystallization temperatures and degrees of differentiation, their Mg isotopic compositions display no measurable variation within the limits of our external precision (average δ26Mg = - 0.36 ± 0.10 and δ25Mg = - 0.20 ± 0.07; uncertainties are 2SD). This indicates that Mg isotopic fractionation during crystal-melt fractionation at temperatures of ≥ 1055 °C is undetectable at the level of precision of the current investigation. Calculations based on our data suggest that at near-magmatic temperatures the maximum fractionation in the 26Mg/24Mg ratio between olivine and melt is 0.07‰. Two additional oceanic basalts, two continental basalts (BCR-1 and BCR-2), and two primitive carbonaceous chondrites (Allende and Murchison) analyzed in this study have Mg isotopic compositions similar to the Kilauea Iki lava lake samples. In contrast to a recent report [U. Wiechert, A.N. Halliday, Non-chondritic magnesium and the origins of the inner terrestrial planets, Earth and Planetary Science Letters 256 (2007) 360-371], the results presented here suggest that the Bulk Silicate Earth has a chondritic Mg isotopic composition.

Original languageEnglish (US)
Pages (from-to)84-92
Number of pages9
JournalEarth and Planetary Science Letters
Volume261
Issue number1-2
DOIs
StatePublished - Sep 15 2007
Externally publishedYes

Fingerprint

magnesium isotopes
Fractionation
fractionation
Isotopes
basalt
Magnesium
magnesium
Earth mantle
isotope
mantle
lava
lakes
Lakes
Chemical analysis
isotopic composition
melt
olivine
lake
Earth (planet)
crystal

Keywords

  • isotope fractionation
  • Kilauea Iki
  • magma differentiation
  • magnesium isotopes

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics
  • Earth and Planetary Sciences (miscellaneous)

Cite this

Investigation of magnesium isotope fractionation during basalt differentiation : Implications for a chondritic composition of the terrestrial mantle. / Teng, Fang Zhen; Wadhwa, Meenakshi; Helz, Rosalind T.

In: Earth and Planetary Science Letters, Vol. 261, No. 1-2, 15.09.2007, p. 84-92.

Research output: Contribution to journalArticle

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abstract = "To investigate whether magnesium isotopes are fractionated during basalt differentiation, we have performed high-precision Mg isotopic analyses by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) on a set of well-characterized samples from Kilauea Iki lava lake, Hawaii, USA. Samples from the Kilauea Iki lava lake, produced by closed-system crystal-melt fractionation, range from olivine-rich cumulates to highly differentiated basalts with MgO content ranging from 2.37 to 26.87 wt.{\%}. Our results demonstrate that although these basalts have diverse chemical compositions, mineralogies, crystallization temperatures and degrees of differentiation, their Mg isotopic compositions display no measurable variation within the limits of our external precision (average δ26Mg = - 0.36 ± 0.10 and δ25Mg = - 0.20 ± 0.07; uncertainties are 2SD). This indicates that Mg isotopic fractionation during crystal-melt fractionation at temperatures of ≥ 1055 °C is undetectable at the level of precision of the current investigation. Calculations based on our data suggest that at near-magmatic temperatures the maximum fractionation in the 26Mg/24Mg ratio between olivine and melt is 0.07‰. Two additional oceanic basalts, two continental basalts (BCR-1 and BCR-2), and two primitive carbonaceous chondrites (Allende and Murchison) analyzed in this study have Mg isotopic compositions similar to the Kilauea Iki lava lake samples. In contrast to a recent report [U. Wiechert, A.N. Halliday, Non-chondritic magnesium and the origins of the inner terrestrial planets, Earth and Planetary Science Letters 256 (2007) 360-371], the results presented here suggest that the Bulk Silicate Earth has a chondritic Mg isotopic composition.",
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