Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Hydrogen diffusion and storage mechanisms in diopside
Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
2008 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
Vätets diffusion och lagringsmekanismer i diopsid (Swedish)
Abstract [en]

Hydrogen is a widespread trace element in many nominally anhydrous minerals (minerals without water or hydroxyl ions in their structural formula) from the Earth's crust and mantle. The hydrogen is normally incorporated in the form of hydroxyl ions and can be regarded as structurally bound water. The most important minerals of the upper mantle: olivine, orthopyroxene, clinopyroxene and garnet, all contain small but significant amounts of hydrogen. This means that the upper mantle has the capacity to store the equivalent of several world oceans. To know how much water there is in the Earth's interior is important knowledge, as small differences in mantle water content influences models of mantle dynamics. The mantle plays an important role in the hydrological cycle as water in oceanic crust and sediments is subducted at converging plate boundaries and again released through volcanic eruptions during millions of years. Detailed knowledge of the amount of water that is retained within the mantle for longer periods of time (~109 years) is still lacking.

Investigating the water content of the mantle is a task shared between the fields of mineralogy, petrology, geophysics, and theoretical physics (i.e. atomistic models).

My approach as a mineralogist has been to investigate in detail the mechanisms that are responsible for water incorporation in nominally anhydrous upper mantle minerals, with a special emphasis on the pyroxenes as they can carry substantial amounts of water, up to 1300 ppm H2O. The fundamental questions here are how much of the original xenolith water is lost during transport to the surface and if the spectroscopic features measured in the minerals are representative for mantle conditions.

The redox reaction: Fe2+ + OH- ↔ Fe3+ + O2- + ½H2, which is relatively fast, is thought to be the dominant hydrogen exchange reaction in many minerals (Ingrin & Skogby, 2000). The reaction is fast enough to suggest that water in nominally anhydrous minerals equilibrates with the transporting magma and related fluids during ascent to the surface. Nevertheless, several studies show systematic variations in water content with geological environment (Bell & Rossman, 1992; Peslier et al., 2002), implying a complex relationship between host mineral, mantle source region, magma type and eruption style.

 

This thesis is focused on the dehydration-hydration mechanisms in diopside, the most common variety of clinopyroxene in the upper mantle. The approach has been to study the kinetics and temperature dependence of the reactions controlling hydrogen diffusion in synthetic Fe-poor diopside.

Other reactions are likely to be obscured by the iron redox reaction if measured in natural mantle diopside containing several wt% FeO. Therefore, synthetic diopside with very low amounts (0.7 wt% FeO) of iron had to be used in order to measure the influence and co-dependence of the iron redox reaction with other possible reactions.

The experiments were carried out by stepwise heating of the samples in both air and hydrogen. After each step, OH-absorbance was measured using Fourier Transform Infrared spectroscopy, and the relative amounts of ferric and ferrous iron was monitored by Mössbauer spectroscopy. When comparing the amounts of ferric iron and hydrogen (in atoms per formula unit), there is considerable deviation from the ideal 1:1 relationship expected from the iron redox reaction. The dehydration process is reversible to a great extent and re-hydration continues even after all iron is reduced.

The results of this study show that other reactions apart from the iron redox reaction are active, and that they are significantly slower. If these slower reactions are active in mantle diopsides, there is a possibility that they may preserve signatures from the mantle source region.

 

Abstract [sv]

Väte är ett vanligt förekommande spårämne i många nominellt ohydrerade mineral (mineral utan vatten eller hydroxyljoner i strukturformeln) i jordskorpan och manteln. Vätet är vanligen bundet som hydroxyljoner och kan betraktas som strukturellt bundet vatten. Den övre mantelns huvudmineral: olivin, ortopyroxen, klinopyroxen och granat, innehåller små men betydelsefulla mängder väte. Detta medför att övre manteln har en vattenlagringskapacitet som motsvarar flera världshavs volym. Att veta hur mycket vatten som finns i jordens inre är viktig kunskap eftersom små skillnader i mantelns vattenhalt påverkar manteldynamiska modeller. Manteln spelar en viktig roll i vattnets kretslopp eftersom vatten i oceanisk jordskorpa och överlagrande sediment förs ned vid subduktionszoner och frigörs åter vid vulkaniska processer under loppet av miljontals år. Detaljerad kunskap om den mängd vatten som lagras i manteln under ännu längre tidsperioder (~109 år) saknas ännu.

Utforskandet av mantelns innehåll av vatten är en uppgift som delas av mineraloger, petrologer, geofysiker och teoretiska fysiker (dvs. atomistisk modellering). Min infallsvinkel som mineralog har varit att närmare undersöka de mekanismer som styr väteutbytet i nominellt ohydrerade mineral från övre manteln, med särskilt fokus på pyroxengruppens mineral då dessa kan innehålla betydande mängder vatten, upp till 1300 ppm H2O. Viktiga frågor är hur mycket av den ursprungliga vattenmängden som bevaras i xenoliter på väg upp till ytan och om de spektroskopiska signaturer som uppmätts i dessa mineral är representativa för förhållanden som råder i manteln. Redoxreaktionen: Fe2+ + OH- ↔ Fe3+ + O2- + ½ H2, som är relativt snabb, tros vara den dominerande väteutbytesreaktionen i många mineral (Ingrin & Skogby, 2000). Reaktionen är tillräckligt snabb för att vattenhalten i nominellt ohydrerade mineral ska komma i jämvikt med den transporterande magman och relaterade fluider vid transport till ytan. Trots detta visar flera studier på systematiska variationer i vattenhalt med geologisk miljö (Bell & Rossman, 1992; Peslier et al., 2002) vilket antyder ett komplext förhållande mellan värdmineral, källområde i övre manteln, magmatyp, och eruptionssätt. 

Föreliggande studie fokuserar på de mekanismer som styr dehydrering och hydrering i diopsid, den vanligaste varianten av klinopyroxen i övre manteln. Angreppsvinkeln har varit att studera kinetik och temperaturberoende hos de reaktioner som kontrollerar vätediffusion i syntetisk Fe-fattig diopsid.  Andra reaktioner utöver järn-redoxreaktionen torde döljas om de skulle studeras i naturlig diopsid från manteln med flera viktprocent FeO. Därför var det nödvändigt att använda syntetisk diopsid med mycket små mängder järn (motsvarande 0.7 viktprocent FeO) för att kunna mäta redoxreaktionens inflytande och samverkan med andra möjliga reaktioner. Experimenten genomfördes genom stegvis upphettning av prov i både luft och vätgas. Efter varje upphettningssteg mättes OH-absorbansen med Fourier-Transform Infraröd spektroskopi, och de relativa mängderna trevärt och tvåvärt järn uppmättes med Mössbauerspektroskopi. Mängden trevärt järn och väte jämfört i atomer per formelenhet avviker betydligt från det ideala 1:1-förhållande som kan förväntas från järn-redoxreaktionen. Dehydreringsprocessen är i hög grad reversibel, och återhydreringen fortsätter även när allt järn reducerats. Resultaten av denna studie visar att andra reaktioner utöver järn-redoxreaktionen är aktiva, och att de är betydligt långsammare. Om dessa långsamma reaktioner förkommer i naturlig diopsid finns det också en större möjlighet att kristallerna har bevarat vissa ursprungliga egenskaper från källområdet i övre manteln.

Place, publisher, year, edition, pages
2008.
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
URN: urn:nbn:se:su:diva-29035OAI: oai:DiVA.org:su-29035DiVA: diva2:228818
Presentation
(English)
Supervisors
Available from: 2011-08-29 Created: 2009-08-06 Last updated: 2011-08-29Bibliographically approved
List of papers
1. Dehydration-hydration mechanisms in synthetic Fe-poor diopside
Open this publication in new window or tab >>Dehydration-hydration mechanisms in synthetic Fe-poor diopside
2009 (English)In: European journal of mineralogy, ISSN 0935-1221, E-ISSN 1617-4011, Vol. 21, no 1, 17-26 p.Article in journal (Refereed) Published
Abstract [en]

Small amounts (ppm) of OH in nominally anhydrous minerals (NAMs) can have a dramatic effect on the physical properties of the upper mantle. The pyroxenes of the upper mantle have been shown to incorporate substantial numbers of protons forming hydroxyl ions. Enstatite and diopside are the most important endmembers of the pyroxenes in terms of bulk volume in the upper mantle. To further constrain the behavior of hydroxyl ions in clinopyroxene, the dehydration-hydration mechanisms of synthetic 57Fe-doped diopside were investigated. Dehydration was carried out by stepwise heating in air of crystals synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step. Mössbauer spectra were recorded for three of the crystals when there had been a significant decrease in FTIR absorbance intensity. From the Mössbauer spectra we see an increase in the Fe3+ doublet with successive dehydration, although this increase is less than the decrease in OH in terms of atoms per formula unit. This means that the dehydration only partly follows the redox reaction OH + Fe2+ = O2− + Fe3+ + ½H2, and that additional reactions occur. Hydration experiments were conducted on one crystal in the same manner as the dehydrations, with the exception that hydrogen gas was used during heating. Hydration experiments resulted in re-hydration of the sample to 73 % of the original amount of OH.

 

The calculated Arrhenius equation derived from the diffusion rates during dehydration along [010] yields an activation energy (Ea) of −292 ± 50 kJ mol−1, and D0 = 10±1.9 + 2.3 m2 s−1. The result of the rehydration experiment agrees well with the established diffusion law. Diffusion rates determined for synthetic diopside are almost two orders of magnitude slower than for synthetic enstatite with comparable Fe contents. Compared to natural diopside, diffusion rates in these synthetic samples are slower, probably because of the low iron content. Ea is similar to that of dehydration of pure and low-Fe enstatite.

Place, publisher, year, edition, pages
Stuttgart: E. Schweizerbart'sche Verlagsbuchhandlung, 2009
Keyword
diopside, hydrogen incorporation, diffusion, FTIR spectroscopy, Mössbauer spectroscopy
National Category
Geology
Research subject
Mineralogy, Petrology and Geochemistry
Identifiers
urn:nbn:se:su:diva-29033 (URN)10.1127/0935-1221/2009/0021-1880 (DOI)
Available from: 2009-08-06 Created: 2009-08-06 Last updated: 2010-11-08Bibliographically approved

Open Access in DiVA

No full text

By organisation
Department of Geology and Geochemistry
Geology

Search outside of DiVA

GoogleGoogle Scholar

urn-nbn

Altmetric score

urn-nbn
Total: 111 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf