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| Titel |
Melting Phase Relations and ”Stishovite Paradox” in Lower-Mantle System MgO - FeO - SiO2 at 24 GPa |
| VerfasserIn |
Yuriy Litvin, Anna Spivak, Leonid Dubrovinsky |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094423
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| Publikation (Nr.) |
 EGU/EGU2014-9831.pdf |
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| Zusammenfassung |
| Stishovite is missed in model composition of the ultrabasic lower mantle (Akaogi, 2007;
Stixrude, Lithgow-Bertelloni, 2007). It is due to the fact that mineralogy of the lower mantle
is estimated by experimental study of phase relation of the pyrolite composition up to 50
GPa. It was found that ultrabasic assemblage magnesiowustite+Mg-perovskite+Ca-perovskite
is stable at PT-conditions of the lower mantle. However, stishovite is a representative
phase in basic assemblage stishovite+Ca-perovskite+Mg-perovskite+Al-bearing
resulted in similar experiments with basaltic compositions. But in this case stishovite
should be subducted into the lower mantle. Meanwhile, paradoxal intergrowths of
stishovite with magnesiowustite, indicatory mineral of the ultrabasic lower mantle, were
found out as inclusions in “super-deep” diamonds (Kaminsky, 2011, for review).
Physicochemical reasons for in situ formation of stishovite and assemblage of stishovite
and magnesiowustite (“stishovite paradox”) at the primitive lower mantle were
earlier discussed (Litvin et al, 2014). The discussion was based on preliminary
data for melting phase relations of the lower mantle system MgO – FeO – SiO2 –
Ca-perovskite.
The goal of this work is experimental investigation of phase relations on the
ternary MgO – FeO – SiO2 join of the lower mantle system MgO – FeO – SiO2 –
CaO at pressure of 24 GPa. The sections (MgO)70(FeO)30-(SiO2)70(FeO)30 and
(MgO)30(FeO)70-(SiO2)30(FeO)70of the ternary join were studied and melting phase
diagrams for them constructed.
Melting relations of the MgO – FeO – SiO2 join are characterized by formation of
invariant peritectic point (Mg,Fe)-perovskite+(Mg,Fe)O+stishovite+L(liquid) and
monovariant cotectic curve (Mg,Fe)O+stishovite+ L at compositions richer in FeO. Thus,
peritectic reaction of (Mg,Fe)-perovskite and Fe-richer liquid is responsible for
magnesiowustite (Mg,Fe)O + stishovite SiO2paragenesis. Origin of model primary ultrabasic
magma is under control of magnesiowustite+Mg-perovskite+L invariant eutectics. By
scenario of fractional crystallization, a figurative point of the magma composition is moving
along the monovariant curve magnesiowustite+Mg-perovskite+L towards the invariant
peritectic magnesiowustite+Mg-perovskite+stishovite+L where “stishovite paradox” is first
realized. After disappearance of Mg-perovskite in peritectic reaction a further change of the
basic residual magmas and stable existence of stishovite and magnesiowustite is regulated by
the monovariant curve magnesiowustite+stishovite+L. The role of magnesiowustite as a
“through mineral” for the lower mantle ultrabasic and basic materials and, evidently, for
the “super-deep” diamond parental media is justified by this. Thus, experimental
evidences of existence of assemblage magnesiowustite+stishovite were obtained. The
results described above demonstrate that stishovite is in situ mineral of the lower
mantle.
Support: grant of the President of RF #MK-1386.2013.5, grants of RFBR 12-05-33044,
13-05-00835, 14-05-00537. |
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