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Iron partitioning in natural lower-mantle minerals; toward a chemically heterogeneous lower mantle

Felix V. Kaminsky and Jung-Fu Lin
Iron partitioning in natural lower-mantle minerals; toward a chemically heterogeneous lower mantle
American Mineralogist (April 2017) 102 (4): 824-832

Abstract

The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep lower mantle are iron-rich and differ markedly from a pyrolitic composition. In the lowermost lower mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association [K (sub D) (super Bridg-fPer) = ([Fe/Mg] (super Bridg) )/([Fe/Mg] (super fPer) ) (sub at) in juvenile diamond inclusions is as low as 0.1-0.2. During ascent of the diamonds with their inclusions to the surface, the K (sub D) (super Bridg-fPer) eventually increases to values of 0.4-0.5 and even as high as 0.7.The details of the element partitioning between natural Bridg and fPer in the lower mantle are as follows: iron in Bridg is ferrous Fe (super 2+) in the A site, substituting for Mg (super 2+) ; almost all iron in fPer is ferrous Fe (super 2+) ; the share of ferric Fe (super 3+) iron in fPer is Fe (super 3+) /Sigma Fe = 8-12 at%; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the lower part of the lower mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse K (sub D) (super Bridg-fPer) values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe (super 2+) in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the lower mantle.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 4
Title: Iron partitioning in natural lower-mantle minerals; toward a chemically heterogeneous lower mantle
Affiliation: KM Diamond Exploration, West Vancouver, BC, Canada
Pages: 824-832
Published: 201704
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 59
Accession Number: 2017-041514
Categories: General geochemistrySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
S34°00'00" - N05°15'00", W74°00'00" - W34°00'00"
N07°15'00" - N12°40'00", W15°00'00" - W07°40'00"
S38°00'00" - S26°00'00", E129°00'00" - E141°00'00"
N42°00'00" - N84°00'00", W141°00'00" - W52°00'00"
Secondary Affiliation: University of Texas at Austin, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201723
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