Determination of pre-eruptive conditions of phonolitic magmas using experimental petrology Second year progress report

1. Determination of pre-eruptive conditions of phonolitic magmas using experimental petrology(Second year progress report) Group of Volcanology, CSIC, Barcelona, Spain

3. Working plan 1) Determination of the pre-eruptive conditions ( T, P, f H2O, fO2) of two selected samples from T-PV complex: MB subplinian eruption deposit (2 ka; Ablay et al., 1995) and Lavas Negras (1.2-0.8? ka; Ablay et al., 1998); and one sample from the El Abrigo deposit: (pre-Teide, caldera-forming eruption, 179ka, Mart� et al., 1994) 2) Constraining the present state of the Teide volcano 3) Comparison between the eruption dynamics of Teide (effusive) and Las Ca�adas (explosive). 4) Comparison between Teide and Vesuvius phonolites

4. Experiments planning El Abrigo pumice (pre- Teide phonolite)

5. Experiments planning Monta�a Blanca obsidian (Teide phonolite)

6. RESULTS EA Experimental mineral phases: sanidine/anorthoclase+biotite+magnetite+clinopyroxene+sphene+apatite+nepheline

8. Preliminary results

9. El Abrigo experiments: General observations ? ?O2 ? increase stability of mgn ? H2O ? 50�C increase of the liquidus T of mineral phases, ?px stability, ? tit stability, nepheline is stable at higher P, ? crystallinity at higher T. Presence of magnetite but NOT illmenite Variation on P,T, H2O ? variation on mineral and glass composition, degree of crystallization

10. EA experimental mineral phases

11. EA experimental glasses (sat experiments) SiO2: A T=cte, ?P ? ?[SiO2], Non-T effect Al2O3:A T=cte, ?P ? ?[Al2O3], Non-T effect MgO: A P=cte, ?T ? ?[MgO], ?P ? ?[MgO] CaO: A T=cte, ?P ? ?[CaO];?T ? ?[CaO]; crystal. Titanite. MnO and FeO: non-observed trend; affected by magnetite, biotite and pyroxene crystallization. Na2O: T=cte, ?P ? ?[Na2O]; Non-T effect K2O: Non-observed trend TiO2: A P=cte, ?T ? [TiO2]?

12. EA experimental glasses (non-sat experiments) SiO2: A T=cte, ?P ? ?[SiO2], ?T ? ?[SiO2], Al2O3:not clear influence of T/P MgO: ?T ? ?[MgO], ?P ? ?[MgO] CaO: A T=cte, ?P ? ?[CaO];?T ? ?[CaO]; crystal. Titanite and clpx. MnO and FeO: ?P ? ?[MnO-FeO] Na2O: A T=cte, ?P ? ?[Na2O], Non T effect K2O: Non-observed trend TiO2: A P=cte, ?T ? [TiO2]?

13. Conclusions Comparing previous petrological data with the new experimental results, we can conclude that the pre-eruptive conditions of El Abrigo pumice (pre- Teide phonolite) were: T: 750�C-800�C P:1100 bars ?O2 :NNO-NNO+1. %H2O:3-5%. The presence ha�yne, sodalite and apatite at natural assemblage ? S, Cl i F.

14. Results MB Monta�a Blanca obsidian: NNO+1 Water sat-experiments (top) Non-sat-experiments (bottom)

15. Results MB

16. MB experiments: general observations

17. MB experimental mineral phases Experimental phases:anor+bt+clpx+ill+mgn+tit+ap. Biotites:annite-phlogopite: Ti rich biotites ? T ? ?XMg at NNO+1; ? ?O2,? ?XMg DMg linear relationship with P Feldspars: Ab 56-66.5,Or:30-43.; ? T,?PH2O, ? fO2,? ? Or Or nat duplicated T<800�C,fO2 FQM-PH2O?1kbar Oxides: ? ?O2? ? illmenite (solidus at ?T) - ?Magnetite stability. Mgn: ?T ? ?Mg, on influence of fO2, ? ?O2 ? ?[Fe3+]; ? T ? ?XMg at NNO+1 Clpx: En10-27 Fs22-37 Wo38-43 Ac 7-15(FQM), NNO+1(small crystal size) ? T ? ?XMg at NNO+1 ;? P ? ? Na, ?Fe3+, ? fO2 ? ?XMg

18. MB experimental glasses Glasses: peralkaline phonolites SiO2:?T ? ?SiO2 at all ?O2. MgO: compatible trend at all ?O2 ?crystallisation of Mg bearing phases NaO: incompatible trend at 1Kbar/compatible trend at 2kbar CaO: low content ?clpx-tit crystallisation

19. Conclusions Combining previous petrological data with the new experimental data we can conclude that the Monta�a Blanca magma chamber contained two phonolitic liquids: Pre-eruptive conditions of the uppermost phonolitic layer were : T:755-800�C,P:1500bars, ?O2 :FQM-1, 3-4.5%H2O. while those of the lower layer were: 877�C, water contents<3%.

20. Experimental Problems Under Mike Carroll�s supervision we have detected the following problems: Furnaces: thermocouple calibration, ?O2 Experiments: variation of water activity ? does not allow to compare data between samples; studies of sample crystalinity (constraining pressure and water activity) Microprobe analysis: problems with mineral analysis (stechyometric and standard deviation checking), Na-loss in glass analysis, problems with microprobe standards

21. Problem solutions Furnaces: thermocouple calibration ? gold point, ? correction experiments T; ?O2 ? future calculations. Experiments: variation of water activity ? same water activity for all P; studies of sample crystalinity are in progress. Microprobe analysis: problems with mineral analysis ? (stechyometric and standard deviation checking), Na-loss in glass analysis ? create own standards for analyzing and correct Na-loss, problems with microprobe standards ? find new standards

22. Future work (3rd year) Correction of El Abrigo experimental data. Reversal experiments: checking mineral equilibrium ?Pre-eruptive parameters for EA and MB Experiments for Lavas Negras Experiments for MB (varying water activity) Experiments for Vesuvius

23. The End