Determination of preeruptive conditions of phonolitic magmas using experimental petrology Second yea

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

• Group of Volcanology, CSIC, Barcelona, Spain

Joan Andújar Fabio Teixidó Joan Martí
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General Objetives
Objective 1 To determine the pre-eruptive
conditions (P, T, volatile content) of phonolitic
magmas from Teide and Vesuvius by using
experimental petrology
Objective 2 To constrain mineral-melt
partitioning of various components (especially
volatile species, which largely determine the
explosivity potential of eruptions
Objective 3 To demonstrate the utility of
experimental petrology in volcanic risk assessment
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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, Martí et al.,
  1994)
• 2) Constraining the present state of the Teide
  volcano
• 3) Comparison between the eruption dynamics of
  Teide (effusive) and Las Cañadas (explosive).
• 4) Comparison between Teide and Vesuvius
  phonolites

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Experiments planning

• El Abrigo pumice (pre- Teide phonolite)

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Experiments planning

• Montaña Blanca obsidian (Teide phonolite)

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RESULTS EA

• Experimental mineral phases
• sanidine/anorthoclasebiotitemagnetiteclinopyrox
  enespheneapatitenepheline

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Preliminary results
El Abrigo pumice (pre- Teide phonolite)
water-sat exp.
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Preliminary results
El Abrigo pumice (pre- Teide phonolite) non-sat
exp
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El Abrigo experiments General observations

• ? ?O2 ? increase stability of mgn
• ? H2O ? 50ºC 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

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EA experimental mineral phases

• Experimental phasessan/anorbttitpxap(neph)
• Biotites -phlogopite-annite(XMg vs. Al)
• - XMg Fe/Mg nat are between
  NNO-NNO1??O2 ? ?XMg
• - TinatgtTiexp
• - Pcte linear relationship of XMg
  vs.T ?
• formation T 763ºC (sat) and
  765ºC (non-sat)
• - Tcte linear relationship between
  DMg vs.P ?
• formation P 1270(sat) and 1100 (non-sat)

• Feldspars sanidine-anorthoclase,
  Orexp29.6-55.6, Abexp77.4-45,
  Anexp0.52-5.73
• Ornat(34.5-40.3) is duplicated at 750ºC and
  1000 bars, and NNO,H2O3-5. ?H2O
  experiments ? ? Or

• Oxides titanomagnetites.(no illmenite). ?no nat
  data.
• Titanite very frequent at NNO1 and water
  saturated, low compositional variation with P T
  changes.
• Apatite small crystal size ? analysis errors
• Pyroxene small crystal size at water-sat
  conditions water non-sat Exp. Pyr
  En4.64-14 Wo31.9-64.8 Fs 0.56-42.7
• Nepheline small crystal size ? analysis errors

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EA experimental glasses (sat experiments)

• SiO2 A Tcte, ?P ? ?SiO2, Non-T effect
• Al2O3A Tcte, ?P ? ?Al2O3, Non-T effect
• MgO A Pcte, ?T ? ?MgO, ?P ? ?MgO
• CaO A Tcte, ?P ? ?CaO?T ? ?CaO crystal.
  Titanite.
• MnO and FeO non-observed trend affected by
  magnetite, biotite and pyroxene crystallization.
• Na2O Tcte, ?P ? ?Na2O Non-T effect
• K2O Non-observed trend
• TiO2 A Pcte, ?T ? TiO2?

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EA experimental glasses (non-sat experiments)

• SiO2 A Tcte, ?P ? ?SiO2, ?T ? ?SiO2,
• Al2O3not clear influence of T/P
• MgO ?T ? ?MgO, ?P ? ?MgO
• CaO A Tcte, ?P ? ?CaO?T ? ?CaO crystal.
  Titanite and clpx.
• MnO and FeO ?P ? ?MnO-FeO
• Na2O A Tcte, ?P ? ?Na2O, Non T effect
• K2O Non-observed trend
• TiO2 A Pcte, ?T ? TiO2?

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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 750ºC-800ºC
• P1100 bars
• ?O2 NNO-NNO1.
• H2O3-5.
• The presence haüyne, sodalite and apatite at
  natural assemblage ? S, Cl i F.

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Results MB

• Montaña Blanca obsidian NNO1
• Water sat-experiments (top)
• Non-sat-experiments (bottom)

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Results MB
Montaña Blanca obsidian FMQ Water
sat-experiments (left) Non-sat-experiments
(right)
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MB experiments general observations

• ?H2O (NNO1) ?solidus T of feldspars, biotite
  about 50ºC illmenite ? solidus T about 25ºC and
  reduce its stability
• ?H2O (FMQ) ?solidus T of feldspars about 30ºC
  disappearance of titanite.
• ? ?O2 Increase on illmenite stability to higher
  T low decrease on solidus T of pyroxene, apatite
  and titanite at water saturated conditions
  disappearance of titanite.
• Variation on P,T, H2O ? variation on mineral,
  glass composition, degree of crystallization

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MB experimental mineral phases

• Experimental phasesanorbtclpxillmgntitap.
• Biotitesannite-phlogopite
• Ti rich biotites
• ? T ? ?XMg at NNO1 ? ?O2,? ?XMg
• DMg linear relationship with P
• Feldspars Ab 56-66.5,Or30-43.
• ? T,?PH2O, ? fO2,? ? Or
• Or nat duplicated Tlt800ºC,fO2
  FQM-PH2O?1kbar
• Oxides ? ?O2? ? illmenite (solidus at ?T) -
  ?Magnetite stability.
• Mgn ?T ? ?Mg, on influence of fO2, ? ?O2
  ? ?Fe3 ? T ? ?XMg at NNO1
• Clpx En10-27 Fs22-37 Wo38-43 Ac 7-15(FQM),
  NNO1(small crystal size)
• ? T ? ?XMg at NNO1 ? P ? ? Na,
  ?Fe3, ? fO2 ? ?XMg

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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

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Conclusions

• Combining previous petrological data with the new
  experimental data we can conclude that the
  Montaña Blanca magma chamber contained two
  phonolitic liquids
• Pre-eruptive conditions of the uppermost
  phonolitic layer were
• T755-800ºC,P1500bars, ?O2 FQM-1,
  3-4.5H2O.
• while those of the lower layer were
• 877ºC, water contentslt3.

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Experimental Problems

• Under Mike Carrolls 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

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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

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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

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The End