CHLORINE CONTENT IN BIOTITE OF THE TORRES DEL PAINE AND WESTERN ADAMELLO CONTACT AUREOLES: A PROGRADE OR RETROGRADE SIGNAL?

Chloride is the dominant anion in geothermal fluids. Cl concentrations will determine the solubility of major rock-forming elements. Since the mobility of elements strongly depends on their concentration, chlorine concentration in the fluid influences mineral textures and mineral equilibria. The OH^--Cl^- exchange between biotite and fluid, together with fluid-speciation calculations can be used to estimate chlorine compositions¹ of fluids and element mobility.

We have studied Cl contents in biotites using electron microprobe (the detection limit is about 40ppm) and investigated two very different contact aureoles: the shallow crustal Torres del Paine (TP) aureole (3 km; 750 bar) in Patagonia and the mid-crustal Western Adamello (WA) aureole (10 km; 2.5 kbar²) in the Italian Alps. Both aureoles show only very minor retrograde alteration.

The TP granites intruded into Cretaceous turbidites. The Cl content in biotite is nearly constant at about 150ppm across the entire contact aureole. Only some samples close to the contact show higher concentrations of 1500-2000ppm. Note, that in these samples, retrograde biotites are Cl-poor (100-200ppm).

The WA tonalite intruded into a pre-Permian basement with a Permotriassic cover. Cl in biotite is quite constant at 200ppm, and biotite inclusions – found in andalusite or garnet - have the same Cl-concentration as matrix biotites.

These results are surprising and suggest that the Cl concentration of the fluid increase with metamorphic grade due to increasing association of Cl. This contradicts Rayleigh fractionation models, which predict that Cl is quickly depleted during dehydration due to the its preference for the fluid phase. Yet, the similarity of Cl in matrix and in inclusions rules out the possibility of retrograde fixation of the Cl-content. It could be that biotite sequesters Cl upon crystallization, and diffusion (subsequently) is too slow to establish Cl-biotite-fluid equilibrium on the scale of a biotite grain. The predicted zoning requires better analytics. Initial test with the SIMS suggests that we can measure Cl, F and OH with the required higher analytical precision.

¹Zhu and Sverjensky (1991) GCA 55, 1837-1858.

²Floess and Baumgartner (2012) Terra Nova 25, 144-150