Chrome-spinels - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 98

Cr #

a

b c

d

Mg # Mg # Mg # Mg #

Figure 2.5 The graphs of relationship between Cr # % = Cr ⋅ 100/(Cr + Al), % and Mg # % = Mg ⋅ 100/
(Mg + Fe 2+ ) of the accessory chrome-spinels from ultramafic rocks from some ultramafic
massifs. a - Ergak massif (West Sayan, Russia) (data [Krivenko et al ., 2004]). In the square -
average values for the general totality of analysis, b - Idzhimsky massif (West Sayan, Rus-
sia) (data [Sibilev, 1980]): 1-5 - dunites, 6 - harzburgite; 7, 8 - lherzolites; c - Kempirsay
massif (South Urals, Kazakhstan) (data [Makeev, 1992]): 1 - chrome-spinels from dunites
(average of 21 analyses), 2 - alumochrome-spinels and subferrialumochrome-spinels from
harzburgites (average of 19 analyses), 3 - chromepicotites and pikotites from lherzolites
(average of 34 analyses); d - Ray Iz massif (North Urals, Russia) (data [Makeev, 1992]): 1 -
subferrichrome-spinels from dunites (average of 16 analyses), 2 - subferrialumochrome-
spinels from dunites (average of 17 analyses), 3 - ferrialumochrome-spinels from dun-
ites (average of 11 analyses), 4 - subferrialumochrome-spinels from dunites (average of 7
analyses), 5 - ferrichrome-spinels from dunites (average of 4 analyses). D melt , % - degree
of partial melting of mantle ultramafic protolite during the formation of restites (data [Page
et al ., 2008]).

Similar calculations of the D melt values of were performed using the data on
chemical compositions of chrome-spinels from ultramafic rocks of several other
mafic-ultramafic massifs. Thus, it was found that for the ultramafic rocks of Idzhimsky
massif, that is just like Ergak is a part of the Kurtushibinsky ophiolite belt (West Sayan,
Russia), the values of D melt successively reduce from dunite to harzburgite and lherzo-
lite (Figure 2.5, b). The values obtained for D melt from ultramafic rocks of Kempirsay
(Kazakhstan) and Ray Iz massifs (Polar Urals, Russia), are shown in Figure 2.5, c
and d.
Thus, the described calculation method allows us to obtain approximate estimates
of the degree of partial melting of mantle sources during the formation of ultramafic
restites using chromium indicator of the accessory chrome-spinels represented in it. If
we accept the results of these calculations to be close to the real values of D melt , we can
conclude that during the formation of ultramafic restites, that composed fairly large