Trace element partitioning between orthopyroxene and anhydrous silicate melt on the lherzolite solidus from 1.1 to 3.2 GPa and 1,230 to 1,535°C in the model system Na2O–CaO–MgO–Al2O3–SiO2

Dirk Frei, Axel Liebscher, Gerhard Franz, Bernd Wunder, Stephan Klemme, Jon Blundy

Publikation: Bidrag til tidsskriftArtikelForskningpeer review

55 Citationer (Scopus)

Abstrakt

We determined experimentally the Nernst distribution coefficient Diopx-melt = ciorthopyroxene/cimelt between orthopyroxene and anhydrous silicate melt for trace elements i in the system Na2O-CaO-MgO-Al2O3-SiO2 (NCMAS) along the dry model lherzolite solidus from 1.1 GPa/1,230 °C up to 3.2 GPa/1,535 °C in a piston cylinder apparatus. Major and trace element composition of melt and orthopyroxene were determined with a combination of electron microprobe and ion probe analyses. We provide partitioning data for trace elements Li, Be, B, K, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Sm, Nd, Yb, Lu, Hf, Ta, Pb, U, and Th. The melts were chosen to be boninitic at 1.1 and 2.0 GPa, picritic at 2.3 GPa and komatiitic at 2.7 and 3.2 GPa. Orthopyroxene is Tschermakitic with 8 mol% Mg-Tschermaks MgAl[AlSiO6] at 1.1 GPa while at higher pressure it has 18-20 mol%. The rare earth elements show a continuous, significant increase in compatibility with decreasing ionic radius from DLaopx-melt ∼ 0.0008 to DLuopx-melt ∼ 0.15. For the high-field-strength elements compatibility increases from D Thopx-melt ∼ 0.001 through DNbopx-melt ∼ 0.0015, DUopx-melt ∼ 0.002, DTaopx-melt ∼ 0.005, DZropx-melt ∼ 0.02 and DHfopx-melt ∼ 0.04 to D Tiopx-melt ∼ 0.14. From mathematical and graphical fits we determined best-fit values for D0M1, D0M2, r0M1, r0M2, E0M1, and E0M2 for the two different M sites in orthopyroxene according to the lattice strain model and calculated the intracrystalline distribution between M1 and M2. Our data indicate extreme intracrystalline fractionation for most elements in orthopyroxene; for the divalent cations DiM2-M1 varies by three orders of magnitude between DCoM2-M1 = 0.00098-0.00919 and DBaM2-M1 = 2.3-28. Trivalent cations Al and Cr almost exclusively substitute on M1 while the other trivalent cations substitute on M2; DLaM2-M1 reaches extreme values between 6.5 × 107 and 1.4 × 1016. Tetravalent cations Ti, Hf, and Zr almost exclusively substitute on M1 while U and Th exclusively substitute on M2. Our new comprehensive data set can be used for polybaric-polythermal melting models along the Earth's mantle solidus.

OriginalsprogEngelsk
Sider (fra-til)473-490
Antal sider18
TidsskriftContributions to Mineralogy and Petrology
Vol/bind157
Udgave nummer4
DOI
StatusUdgivet - 2009

Programområde

  • Programområde 4: Mineralske råstoffer

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