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| Titel |
Oxygen fugacities determined from iron oxidation state in natural (Mg,Fe)O ferropericlase: new insights into lower mantle diamond formation |
| VerfasserIn |
Micaela Longo, Catherine McCammon, Galina Bulanova, Felix Kaminsky, Ralf Tappert |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043647
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| Zusammenfassung |
| Mineral inclusions in diamonds reflect the chemical composition and mineral assemblages of
the two principal rock types occurring in the deep lithosphere, peridotite and eclogite.
However, in the past two decades, the discovery of rare diamonds containing inclusions such
as former Mg,Si-perovskite and (Mg,Fe)O ferropericlase led to the possibility that diamonds
can form also at greater depths. (Mg,Fe)O ferropericlase is the most commonly found
inclusion in lower mantle diamonds (more than 50% of the occurrences). Since the Fe3+
concentration in (Mg,Fe)O is sensitive to oxygen fugacity also at high pressures
(Frost et al., 2004), the determination of Fe3+/-
Fe in such inclusions provides a
direct method for investigating lower mantle redox conditions during diamond
formation. In the present study we explore whether variations in mantle oxygen
fugacity exist as a function of chemical, physical and geographic parameters, by
studying (Mg,Fe)O inclusions in lower mantle diamonds from a wide range of
localities.
Eighteen (Mg,Fe)O ferropericlase inclusions from lower mantle diamonds selected
worldwide were measured by the flank method using the calibration previously established
for synthetic ferropericlase (Longo et al., in preparation). The Fe3+/-
Fe measured in
(Mg,Fe)O inclusions of the present work (Juina, Brazil, Machado River, Brazil and Orroroo,
Australia) were compared to data already available for other inclusions of larger size
previously measured by Mössbauer spectroscopy (McCammon et al. 1997, 2004). Oxygen
fugacity was estimated for each specimen relative to two reference buffers such as the
Fe-(Mg,Fe)O buffer (reducing conditions) and the Re-ReO2 buffer (oxidizing conditions).
Our results show a dependence on geographical location, and in particular, inclusions from
the African province (Kankan Guinea) seem to record more reducing mantle conditions than
the inclusions measured from the other provinces, which cover a larger range of fO2
conditions.
It is noteworthy that a variation of oxygen fugacity was registered in multiple inclusions
extracted from the same host diamonds. However, because the inclusions were removed
from the host without textural control, information on the direction of any redox
gradient that may have evolved, and possible correlation with diamond growth or
anomalies in the variation of the redox conditions through time, were lost. These
observations combined with the geographical correlation observed among all inclusions
measured in the present work and from previous studies leads to the suggestion of other
mechanisms than subducted slabs being involved in diamond formation. In order
to provide insights on the mechanisms controlling the redox conditions at lower
mantle depths and how such oxygen fugacities may affect the physical and chemical
properties of the lower mantle, new measurements are planned to increase the data
set on ferropericlase inclusions. Moreover, a multi disciplinary study involving
cathodoluminescence studies and isotopic and optical studies is suggested for further
work.
References
Frost et al. (2004) Nature, 428, 409-412.
Longo et al., In preparation
McCammon et al. (2004) Earth and Planetary Science Letters, 222, 423-434.
McCammon et al. (1997) Science, 278, 434-436. |
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