Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna

Transcription

1 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna Oliver Beermann Jan Stelling XXVII. (14.) Arbeitstagung DGK, Arbeitskreis Nichtkristalline und Partiellkristalline Strukturen Jena, km

2 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna Introduction the concentration and composition of volatile components in magmatic systems affect magma degassing significantly (magma: in general mixture of silicate melt, minerals and fluid) in contrast to water the concentration of sulfur and chlorine in basaltic magma is quite low (< ~ 5000 ppm). Formation of a fluid phase during magma ascent and pressure release causes an enrichment of these components in the hydrous fluid phase volcanic degassing of sulfur and chlorine has an eminent influence on our atmoshere and it has recently been suggested that supereruptions in Earth s history coupled with massive degassing changed the ecosystem on a global scale the interaction of complex volcanic H-C-O-S-Cl fluids with silicate melts is not well understood 1

3 Mt. Etna H 2 S HCl CO 2 H 2 O SO 2 COS CS2 HF HBr 30 km N 2

4 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna Chlorine and Sulfur solubility and partitioning data of chlorine and sulfur between melt and fluid phase of magmatic systems are important for a better understanding of eruption dynamics of volcanic systems Aiuppa et al. (2004) observed recently a correlation between the S/Cl ratio of the emitted gases and the eruption style of the 2002 eruption of Mt. Etna 1

5 S/Cl ratio vs. volcanic activity of Mt. Etna 2002/03 eruption total SO 2 emission: ~ t global SO 2 emission: ~ t/a modified after Aiuppa et al. (2004) 1

6 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna System Chlorine-H 2 O by now the partition behaviour of the volatile components sulfur and chlorine between hyrous fluids and basaltic melt is unknown even the physicochemical behaviour of simplifed chlorine-h 2 O and sulfur- H 2 O fluid in basaltic magma is not clarified so far thus, in a first step we performed partitioning experiments of the simplified system chlorine-h 2 O-bearing fluid and basaltic melt (fused natural rock sample from Etna volcano) 1

7 Experiments 35 experiments were carried out in one of our internally heated Ar/H 2 pressure vessels (IHPV) experimental conditions were: T = C, P = 200 MPa, 6 h run time, reducing conditions (log ƒo 2 = QFM+1, water saturated) 2

8 Sample preparation capsule material: Au 80 Pd 20 (iron presaturated) charged with rock powder (30-70 mg) + initial 10 wt % fluid phase rock powder: trachybasalt from Mt. Etna (errupt. 2001, Rifugio Sapienza) fluid phase: H 2 O-Cl different ratios Cl source: HCl solutions, PtCl 4, AgCl 2

9 Solubility of Water and Chlorine maximum solubility: Cl ~ 4 wt % H 2 O ~ 5.5 wt % H 2 O solubility increases with increasing Cl conc. in the melt with a maximum at ~ 2.5 wt % Cl within error no temperature dependence between C 3

10 Partitioning of Chlorine between Melt and Fluid increase of Cl conc. in the melt is more pronounced at low Cl# within error no temperature dependence between C Cl# = molar Cl/(Cl+H 2 O) ratio in the fluid coexisting with the silicate melt (calculated by mass balance) 3

11 Involved partitoning of Metal Oxides with increasing Cl#: loss of: Na 2 O, K 2 O, CaO and FeO from melt into the fluid 3

12 Reactions in and between Melt and Fluid up to ~ 2.5 wt % H 2 O in the melt without adding H 2 O chlorine source: MCl + ½ H 2 M + HCl (M = Pt, Ag) (1) reaction in the melt: (2) reaction in the fluid: HCl (Na, K) 2 O + 2 HCl melt HCl + O 2- OH - + Cl - fluid melt fluid (Na, K) 2 O melt 2 (Na, K)Cl + H 2 O fluid 4

13 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna Maximum Chlorine Capacity our experiments: ~ 4 wt % maximum capacity of chlorine in basaltic melt at 200 MPa, C previous studies: ~ 2.9 wt % maximum capacity of chlorine in basaltic melt at 200 MPa, C (Webster et al., 1999) ~ 2.7 wt % maximum capacity of chlorine calculated for our composition and P, T conditions using the model of Webster & De Vivo (2002) 4

14 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna Chlorine affects fluid/melt composition depletion of Na 2 O, K 2 O, CaO and FeO in the melt enrichment of Na 2 O, K 2 O, CaO and FeO in the fluid, forming (Na,K)Cl and (Ca, Fe)Cl 2 with HCl chlorine influences the melt/fluid composition chlorine affects crystallization path of silicate melt! 4

15 Partitioning of chlorine between H 2 O-bearing fluid and basaltic melt of Mt. Etna Conclusion complex behaviour of chlorine in hydreous magmatic systems chlorine has a noticeable influence on both, melt and fluid composition (depletion of Na 2 O, K 2 O, CaO and FeO in the melt, enrichment of Na 2 O, K 2 O, CaO and FeO in the fluid) chlorine has a positive effect on water solubility in basaltic melt water solubility increases from 4.5 to 5.5 wt % at 0.1 to 2.5 wt % chlorine in the melt, respectively (200 MPa, C ) consequently chlorine effects degassing processes significantly thus, compared to a CO 2 -containing magma, a chlorine-rich magma will start to degass at a shallower depth, wich may cause explosive eruption styles near the surface 5

16 Vielen Dank fürs Zuhören!