Transport and storage of chlorine in the upper mantle

Chlorine is the dominant anion in virtually all geological fluids and can play a fundamental role in magmatic and postmagmatic processes including metal transport and deposition, (trace) element partitioning between solids and fluids/melts and partial melting of rocks. In spite of the importance of Cl, little is known about (1) the crust-mantle transfer mechanisms of Cl in subduction zones, (2) the high P-T stability fields of Cl-rich phases such as chlorapatite and the Cl-storage capacities of potential Cl-hosts in the Earth`s mantle, (3) the crystal chemistry of Cl under high P-T conditions and (4) the influence of Cl on the melting behaviour of common rock types such as basalts. In order to address some of these questions we propose a high P-T experimental study in the range 1-15 GPa and 700-1800C based on multi-anvil and piston cylinder experiments in combination with electron microprobe-, ion microprobe analysis, laser Raman spectroscopy and crystal structure analysis. The project will focus (1) on the stability, phase relations and crystal chemistry of Cl-rich apatite and trioctahedral mica, (2) on the Cl-storage capacities of these phases under high P and T and (3) on the partial melting behaviour of MORBs under fluid-present and fluid-absent conditions in the presence of Cl. The results will provide crucial information (1) on the crust-mantle transfer mechanisms of Cl and the Cl storage capacity of potentially important Cl-hosts in the mantle, (2) on the crystal chemistry of Cl in silicates and phosphates under upper mantle P-T conditions, and (3) on the influence of Cl on phase relations and partial melting of MORBs with implications for Cl-transport as solid and/or fluid inclusions in subduction zones and the P-T conditions that allow slab melting and breakdown of hydrous phases. Together, these results will help to devise more accurate models for the global Cl-cycle, for the fluid budget in subduction zones and the generation of certain subduction zone magmas parental to adakites or tonalite-trondhjemite-granodiorite series rocks.

Chlorine is the dominant anion in virtually all geological fluids and therefore it is of fundamental importance to magmatic and post-magmatic processes even though it usually only occurs in minor amounts in most magmas and igneous rocks. Chlorine is preferentially partitioned into fluid or vapour relative to silicate melts at low to medium pressures and acts as a highly effective complexing agent for many metal cations. Thus, chlorine facilitates the transport of ore-forming constituents in mineralizing magmatic-hydrothermal systems and plays a central role in the formation of large platinum group element and base metal deposits. Apatite Ca5 (PO4 ) 3 (OH, F, Cl) is one of the most important carriers of phosphorus, chlorine and water in the Earth`s upper mantle. Thus, in order to study possible pathways for the transport and storage of chlorine and phosphorus under P-T conditions of the Earth`s upper mantle and transition zone, experiments were conducted in a P-T range 3-15 GPa and 850-1450C to study the stability and phase relations of Cl- and OH-apatite in basaltic and peridotitic bulk compositions and to determine the Cl-distribution between apatite and coexisting phlogopite, the latter being a further important halogen and water carrier in the upper mantle. The presence in or absence of apatite from a given mantle volume is of great importance to the halogen-, phosphorus-, and trace element composition of partial melts generated therein. The stability of apatite is determined by P, T, the bulk phosphorus content and the ability of coexisting phases to incorporate phosphorus. The upper pressure stability limit of apatite in typical basaltic high pressure rocks such as eclogites was located between 7 and 8.5 GPa equivalent to a depth of approximately 210-250 km at 950C which is at least 3 GPa below that of pure apatite. With increasing P and T apatite incorporates up to 3 wt% MgO, which indicates that this phase can not only host trace elements with large ionic radii such as Ba, Sr, U or Th but also elements with much smaller radii in the range of Mg. At P > 8-8.5 GPa, apatite breaks down to form Ca3 (PO4 ) 2 . This breakdown results in a release of all water, halogens and elements substituted for Mg from apatite and terminates its role as water and halogen carrier in the upper mantle. The incompatible character of Cl in apatite is retained in its entire stability field, making it extremely difficult if not impossible to stabilize pure Cl-apatite in the presence of even trace amounts of water. This indicates that apatite with significant Cl-contents can only be stable under fluid-absent conditions or in fluid-bearing environments were apatite coexists with small amounts of saline fluid. In the absence of a melt phase, garnet can accomodate up to 1 wt% P2 O5 in a peridotitic bulk composition. This indicates that under appropriate P and T, the phosphorus solubility in garnet is high enough to store the entire primitive mantle budged of around 100 ppm. Experiments exploring the distribution of Cl between apatite and mica under high P and T have shown that Cl is always strongly partitioned into apatite. At temperatures exceeding the melting temperature of the mantle, the silicate melt phase is the major carrier of both chlorine and phosphorus because of the strongly incompatible character of these elements.