High-T heating stage: application for igneous petrogenesis and mantle processes - melt inclusions as key tools -

Transcription

1 High-T heating stage: application for igneous petrogenesis and mantle processes - melt inclusions as key tools - SZABÓ, Csaba Lithosphere Fluid Research Lab (LRG), Department of Petrology and Geochemistry, Institute of Geography and Earth Sciences, Eötvös University (ELTE), Budapest H-1117 Budapest (HUNGARY) L RG ANNO 1998 ELTE

2 High-T heating stage (Linkam) Stage + controller: Small ceramic furnace with a hole at the bottom, covered by sapphire, to provide transmitted light for observation during experiment. Quartz lid window for observation. Gas valves to purge the sample chamber with inert gas. Water valves hooked up with a sealed circulating water tank to keep the stage at low T during heating experiment. Purpose of use of the stage: - to study melt inclusions, - to obtain (partially) homogenized melt, - to record melting sequence (crystallization, identification of phases).

3 PART-I: Forewords on melts and melt inclusions PART-II: Method of melt inclusion study, instrumental/analytical techniques PART-III: Magma drops in olivine, spinel, clinopyroxene, plagioclase and quartz phenocrysts of volcanic rocks (+apatite, zircon) PART-IV: Silicate melts and melt inclusions as evidences for mantle enrichment, interaction, and immiscibility

4 Mantle melts in general Melt inclusions, in general, are small droplets of any kind of melts enclosed in a host mineral, which were trapped accidentally at lithospheric mantle and crustal temperatures and pressures, and subsequently quenched or partially or totally crystallized. Why do we study melt inclusions trapped under lithosphere conditions and occurring in any kind of rocks? To figure out physical conditions of trapping and compositions of the trapped melt (behind these, evolution, interaction, crystallization/solidification, immiscibility, etc.) Possible melts trapped under lithospheric mantle and crustal conditions regardless of primary or secondary entrapment: Silicate melt (ultramafic, mafic, neutral and acidic rocks) Carbonatite melt (alkali and ultramafic rocks) [Sulfide melt (ultramafic and mafic rocks)] In the lithosperic environments silicate melt inclusions are the most abundant, however carbonatite [and sulfide] melt inclusions also occur and are relevant to the focus of interest.

5 Examples for different melt inclusions bubble glass glass bubble opx Primary silicate melt inclusion in orthopyroxene from spinel lherzolite xenolith in basalt (Hungary) 30 m opx 30 m Primary silicate melt inclusion in orthopyroxene from pyroxenite xenolith in basalt (Hungary) cpx MSS ol pn 10 m cp Interstitial sulfide inclusion in spinel lherzolite xenolith in lamprophyre (Hungary) CO 2 50 m 200 m Primary silicate melt inclusions in spinel from basalt (Albania)

6 PART-I: Forewords on melts and melt inclusions PART-II: Method of melt inclusion study, instrumental/analytical techniques PART-III: Magma drops in olivine, spinel, clinopyroxene, plagioclase and quartz phenocrysts of volcanic rocks (+apatite, zircon) PART-IV: Silicate melts and melt inclusions as evidences for mantle enrichment, interaction, and immiscibility

7 Method of melt inclusion study Room temperature useful for: silicate carbonatite (sulfide) Petrography (presence of glass, crystallized and/or volatile phases, rations) Image analysis SEM Major element chemical analysis (EPMA) on solid phases mass balance calculation (rough estimation of bulk composition) Spectroscopic methods (e.g., Raman, IR): volatile content of glass, fluid phases, + solid precipitations Low temperature (freezing experiments) useful for: silicate carbonatite Microthermometry on fluid phases of melt inclusions Spectroscopic methods (e.g., Raman) during phase transitions

8 Instrumental techniques High temperature (heating experiments) useful for: silicate carbonatite Homogenization experiments at mantle temperatures Problems: volatile content (usually high-p is necessary for total homogenization) post-entrapment crystallization (amount of wall crystals, sulfide) Heating experiments (homogenization): heat melt quench high-t stages mounted on petrographic microscopes (direct info on changes) furnace technique (no direct observation) Chemical analysis: EMPA (major elements): heated inclusions (solid phases of inclusions) SIMS (trace elements): heated inclusions (solid phases of inclusions) [LA-ICP-MS: unhomogenized inclusions (whole inclusion is ablated)] [LA-MC-ICP-MS: unhomogenized inclusions (especially on sulfides)]

9 High-T heating stage (Linkam) Stage + controller: Small ceramic furnace with a hole at the bottom, covered by sapphire, to provide transmitted light for observation during experiment. Quartz lid window for observation. Gas valves to purge the sample chamber with inert gas. Water valves hooked up with a sealed circulating water tank to keep the stage at low T during heating experiment. Purpose of use of the stage: - to study melt inclusions, - to obtain (partially) homogenized melt, - to record melting sequence (crystallization, identification of phases).

10 PART-I: Forewords on melts and melt inclusions PART-II: Method of melt inclusion study, instrumental/analytical techniques PART-III: Magma drops in olivine, spinel, clinopyroxene, plagioclase and quartz phenocrysts of volcanic rocks (+apatite, zircon) PART-IV: Silicate melts and melt inclusions as evidences for mantle enrichment, interaction, and immiscibility

11 Significance of magma drops in ol, sp, cpx, plag, q, zrn, ap Petrography primary or secondary silicate melt inclusions post-entrapment crystallization (not a closed system!) on the wall in the inclusion partially or totally recrystallized silicate melt inclusions reheating to have trapped melt Composition of the trapped (primary? primitive?) melt right composition (should be compared to relevant phase diagram, Frezzotti, 2001) volatile bubble (CO 2, H 2 S, CH 4, N 2 ) glass (H 2 O, Cl, F, S) Provide information on change in composition (fractionation, magma mixing) degassing/partitioning process during crystallization/solidification immiscibility post entrapment crystallization

12 Mafic magma drops in olivine phenocrysts Olivine phenocrysts in basalt (Hungary, Russia, Israel, Korea, etc.) contain silicate melt inclusions (Smi), spinel inclusions (Sp) ±CO 2 fluid inclusions Transmitted light Reflected light Trapping conditions: >1250 o C, >7 kbars Phases of silicate melt inclusions (sequence of crystallization): olivine - on the wall sulfide (sulf) Al-spinel (sp) rhönite clinopyroxene (cpx) apatite (±amphibole) glass (gl, Na- & K-rich ) CO 2 ( rich) bubble

13 Mafic magma drops in spinel phenocrysts Cr-spinel (micro)phenocrysts in alkali basalt (Albania, Korea, etc.) contain silicate melt inclusions Spinel as a host: early crystallizing phase coexisting with olivine composition of the trapped melt oxide no reaction with the enclosed silicate melt but post-entrapment crystallization of Cr-spinel happened Trapping conditions: min o C

14 Mafic magma drops in olivine (and spinel) phenocrysts Major element compositions (TAS diagram) of heated silicate melt inclusions (smi) and host basalts (Hungary) Na 2 O+K 2 O Foidite Trachybasalt Tephriphonolite Tephrite Basanite Trachyandesite Trachydacite Phonolite Andesite Trachyte Dacite SiO 2 glass in unheated smi HTU homogenized smi HTU host rock HTU glass in unheated smi PK homogenized smi PK host rock PK Heating experiment crystallization in smi & fractionation of host basalt Host basalt bulk rock

15 Mafic magma drops in olivine (and spinel) phenocrysts Trace element compositions of heated silicate melt inclusions (smi) and host basalts (Hungary) 10 Sun/McDonough. (1989) Rock/Chondrites heated smi HTU (1250 C) host basalt HTU heated smi PK (1250 C) host basalt PK REEs-Nakamura (1974) heated smi HTU (1250 C) host basalt HTU heated smi PK (1250 C) host basalt PK La Ce Nd Sm Eu Dy Er Yb Rock/OIB Trace element distributions of in smi and host basalt pairs: P (apatite), similarities <-> differences (magma mixing) Ba NbK LaCe Sr P Nd SmEuTiDy Y Yb Heated and exposed silicate melt inclusion in olivine for SIMS

16 PART-I: Forewords on melts and melt inclusions PART-II: Method of melt inclusion study, instrumental/analytical techniques PART-III: Magma drops in olivine, spinel, clinopyroxene, plagioclase and quartz phenocrysts of volcanic rocks (+apatite, zircon) PART-IV: Silicate melts and melt inclusions as evidences for mantle enrichment, interaction, and immiscibility

17 Significance of silicate melts and melt inclusions Provide information on: enrichment of incompatible elements mantle metasomatism (cryptic and modal) mantle/melt interaction crystallization process, (modal metasomatism) formation of melt partial melting at source region depletion in incompatible elements at source region (and enrichment of incompatible elements in melts) melt immiscibility partitioning of elements, crystallization process physical properties of mantle (lattice preferred orientation, elastic feature, anisotropy

18 Silicate glasses in mantle rocks The presence of silicate glasses in mantle rocks always indicate in-situ melting or migration of melts/fluids partial melting (depletion) or metasomatism (enrichment)? Open-system Interstitial glass patches Interstitial silicate melt pockets Mantle minerals may trap and preserve the composition of high-pressure-temperature melts, since the large elastic modulus of their host phase prevents them from low-pressure chemical reequilibration and decompression during ascent/cooling (e.g. Schiano & Bourdon 1999) lucky case Silicate melt inclusion enclosed in mantle minerals Closed -system

19 Silicate melt inclusions in mantle rock - pyroxenite Cpx Opx Opx Qz Smi Incl Cpx Opx 250 m 0.25 cm Quartz (Qz) and CO 2 -bearing silicate melt inclusions (Smi) in pyroxenite xenolith, Hungary. Q Petrography: - Smi primary and secondary - Smi: glass and CO 2 bubble CO 2 Gl 200 m

20 Heating experiments of smi Raman spectroscopy of CO 2 Density= g/cm 3 ~960 C melting temperature Entrapment pressure >1.1 GPa Depth of the present day uppermost mantle

21 Glass composition - major elements:

22 Glass composition - major elements: Opx+qz+cpx+amp(?) fractionation from a hybrid melt formed on peridotite-slab-melt interface

23 Silicate melt inclusion composition (LA-ICP-MS) - trace elements: rutile plagioclase? apatite? garnet Rutile+plagioclase(?)+garnet in the source subducted oceanic crust? Ni-content in SMI ppm; Cr-content in SMI ppm reaction with peridotite

24 Silicate melt inclusions in mantle rocks - peridotite Primary silicate melt inclusions (smi) in clinopyroxene (cpx) and secondary silicate melt inclusions in orthopyroxene (opx) from lherzolite xenolith (Hungary) Smi phases: products of post-entrainment crystallization, glass (gl), mica, fluid bubble

25 The same evolved melt (high volatile content) The same process fractionation (where?)

26 Raman spectroscopy of fluid bubble in silicate melt inclusion Cooling experiment Beside CO 2, H 2 O in peridotite (microthermometry also indicates)

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