Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Abstract

Abstract Basalts and peridotites from mid-ocean ridges record f O2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and f O2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe 2 O 3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe 2 O 3 ( $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{spl}/\mathrm{melt}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>spl</mml:mi> <mml:mo>/</mml:mo> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ) increases as spinel Fe 2 O 3 concentrations increase, independent of increases in f O2 , and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{opx}/\mathrm{melt}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>opx</mml:mi> <mml:mo>/</mml:mo> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> = 0.63 ± 0.10 and $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{cpx}/\mathrm{melt}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>cpx</mml:mi> <mml:mo>/</mml:mo> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> = 0.78 ± 0.30. MORB Fe 2 O 3 and Na 2 O concentrations are consistent with a modeled MORB source with Fe 2 O 3 = 0.48 ± 0.03 wt% (Fe 3+ /ΣFe = 0.053 ± 0.003) at potential temperatures ( T P ) from 1320 to 1440 °C. The temperature-dependence of the $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{spl}/\mathrm{melt}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>spl</mml:mi> <mml:mo>/</mml:mo> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> function alone allows ~ 40% of the variation in MORB compositions. If we allow $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{opx}/\mathrm{melt}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>opx</mml:mi> <mml:mo>/</mml:mo> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> and $${D}_{\mathrm{Fe}2\mathrm{O}3}^{\mathrm{opx}/\mathrm{melt}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>D</mml:mi> <mml:mrow> <mml:mi>Fe</mml:mi> <mml:mn>2</mml:mn> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>opx</mml:mi> <mml:mo>/</mml:mo> <mml:mi>melt</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> to also vary with temperature by tying them to spinel Fe 2 O 3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe 2 O 3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe 3+ /ΣFe = 0.03 that predict f O2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with f O2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

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