Experiments on phosphate-silicate liquid immiscibility with potential links to iron oxide apatite and nelsonite deposits
Experiments on phosphate-silicate liquid immiscibility with potential links to iron oxide apatite and nelsonite deposits
Authors
Lledo, Haroldo L.
Naslund, H. Richard
Jenkins, David M.
Naslund, H. Richard
Jenkins, David M.
Authors
Date
Datos de publicaciĆ³n:
10.1007/s00410-020-01751-8
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Abstract
The formation of phosphorus-iron oxide (P-Fe) immiscible melts and their possible connection to the genesis of Kiruna-type and Nelsonite deposits was experimentally investigated by adding phosphoric acid (H3PO4), water, and sulfur, to andesite at 100-450 MPa, 500-900 degrees C, at the NiNiO and magnetite-hematite fO(2) buffers using internally heated gas vessels. The addition of up to 8.02 wt% of H3PO4 to the andesite causes crystallization of apatite. At higher concentrations of H3PO4 whitlockite crystallizes, and at concentrations above similar to 11.4% H3PO4 (at 800 degrees C, 385 MPa) an immiscible P-Fe melt forms. Adding sulfur at low fO(2) (NiNiO) causes an additional immiscible Fe-S melt to form. Increasing the fO(2) to the hematite-magnetite buffer causes the sulfur-rich melt to shift in composition to a Ca-S-O melt, and the coexisting P-Fe melt to incorporate large amounts of SO4. Immiscible P-Fe melts can form at temperatures above 1100 degrees C down to 600 degrees C (at 400 MPa). Mass balance calculations show that some experimentally produced P-Fe rich immiscible liquids may result in mineral assemblages similar to those found at some Kiruna-type deposits, such as actinolite-rich dikes, and apatite-rich veins. Depending on the geological conditions and the composition the fractionation of a P-Fe melt may result in the formation of nelsonites at high pressures, high temperatures, and low fO(2) or Kiruna-type deposits at lower temperatures and higher fO(2).