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Titel |
CaSiO3-walstromite inclusions in super-deep diamonds |
VerfasserIn |
Chiara Anzolini, Fabrizio Nestola, Sula Milani, Frank E. Brenker |
Konferenz |
EGU General Assembly 2015
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 17 (2015) |
Datensatznummer |
250110337
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Publikation (Nr.) |
EGU/EGU2015-10321.pdf |
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Zusammenfassung |
Diamonds are considered the unique way to trap and convey real fragments of deep material
to the surface of our planet. Over the last thirty years, great strides have been made in
understanding of Earth’s lower mantle, mainly thanks to technological and instrumental
advances; nevertheless, it is only in the last two decades that a whole range of inclusion
parageneses derived from the lower mantle was discovered in diamonds from São Luiz
(Brazil) (Kaminsky, 2008 and references therein), thereby establishing a “window” into the
lower mantle. These so-called super-deep diamonds form at depths greater than lithospheric
diamonds, more precisely between 300 and 800 km depth, and contain mostly ferropericlase,
enstatite (believed to be derived from MgSi-perovskite) and CaSiO3-walstromite (believed to
be derived from CaSiO3-perovskite). Even though CaSiO3 not only adopts the perovskite
structure with increased pressure and temperature, but also it is considered the dominant
Ca-bearing phase in the Earth’s lower mantle (Tamai and Yagi, 1989), at the present day there
are no reliable literature data on the pressure at which CaSiO3 crystallizes within
diamonds.
In order to obtain for the first time a pressure of formation value for CaSiO3-walstromite,
several inclusions still trapped in a diamond coming from Juina (Mato Grosso, Brazil) were
investigated both by in-situ microRaman spectroscopy and in-situ single-crystal X-ray
diffraction. First, we applied “single-inclusion elastic barometry” as improved by Angel et al.
(2014) to determine the pressure of formation of the diamond-inclusion pairs. Starting
from the maximum remnant pressure value ever reported (Joswig et al., 2003) and
adopting the thermoelastic parameters already present in literature (Swamy and
Dubrovinsky, 1997; Liu et al., 2012), we obtained an apparent entrapment pressure of ~7.1
GPa, corresponding to ~250 km, at 1500 K. The presence of fractures around the
inclusions indicates this is a minimum estimate, and it is possible that the entrapment
pressure falls at least into the stability field of Ca2SiO4-larnite + CaSi2O5-titanite. In
support of this hypothesis we secondly compared our Raman spectra with reference
spectra of the same phases obtained from an experimental product of Gasparik et al.
(1994). Our preliminary results indicate in at least one inclusion the coexistence of
CaSiO3-walstromite + Ca2SiO4-larnite, suggesting that CaSiO3-walstromite forms in
sub-lithospheric conditions from the back transformation from CaSiO3-perovskite. Further
investigations are in progress in order to find evidence of CaSi2O5-titanite in these
inclusions.
References
Angel R.J., Alvaro M., Nestola F., Mazzucchelli M.L. (2014) Diamond thermoelastic
properties and implications for determining the pressure of formation of diamond-inclusion
systems. Russ Geol Geophys, in press; Gasparik, T., Wolf, K. and Smith, C.M. (1994) Am
Mineral, 79,1219-1222; Joswig, W., Paulus, E.F., Winkler, B. and Milman, V. (2003) Z
Kristallogr, 218,811-818; Kaminsky, F. (2008) Earth Sc Rev, 110,127-147; Liu, X., Wang, S.,
He, Q., Chen, J., Wang, H., Li, S., Peng, F., Zhang, L. and Fei, Y. (2012) Am Mineral
97,262-267; Swamy, V. and Dubrovinsky, L.S. (1997) Geochim Cosmochim Acta,
61,1181-1191; Tamai, H. and Yagi, T. (1989) Phys Earth Planet Inter, 54,370-377. |
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