Dear Mr. Segun,

there is no other choice than making use of garnet s.s.s. found besides omphacite in eclogite samples. In case of eclogite-amphibolites using hornblende is feasible but you have to keep in mind that this lithology already attests to a retrograde process and it is not identical with the formation of the eclogites, proper. Pure rutile does not offer any possibility to accommodate tetravalent U. There is only one mineral reaction (Pronto reaction) which leads to brannerite (U,Ca) (Ti,Fe)2O6 .This mineral has , however, no meaning for this environment of metamorphism which you are going to study. Brannerite forms from TiO2 and UO2 in the course of ilmenite alteration according to the succeeding reaction: Brannerite formed according to the so-called Pronto reaction between 200 and 300°C from epigenetic alteration of ilmenite  2 FeTiO3 + 4 H2S + UO2 = UTi2O6 + FeS2 +H2O + 2H2 (Ramdohr 1975). This U titanate occurs in a wide range of environments, from paleoplacers such as Witwatersrand/South Africa through Variscan vein-type deposits to Cenozoic Fe deposits (Hüttenberg). I have to note that even this U compound is often a hard nut to crack in case of dating, because of the presence of  another sort of alteration products (U "leucoxene"). This is the only mineral aggregate where you can expect U oxide together with TiO2. We made some attempts but the results were not very much promising with a high error bar.

Best regards

H.G.Dill

Apart from the inherent solubility of U in certain minerals, the other important parameters are the amount of U in the whole rock (typically low in basic rocks), and the presence/absence of other minerals that can accommodate the element of interest. I found, for instance, that apatite from an U-ore had a low U content, likely because other minerals formed that had a far greater affinity for U. Similarly, a colleague told me that phlogopite from a calcio-carbonatite was excellent for Ar-Ar dating, because it contained no calcium whatsoever, as all the calcium had partitioned in coexisting diopside.

Literature data show a wide range of certain

trace elements, in particular Al and Fe, in rutile from crustal and mantle

eclogites, which most likely reflects the presence of corundum and

ilmenite lamellae (Sobolev and Yefimova, 2000).

Most naturally occurring rutile corresponds to the general formula

TiO2, with titanium occurring as Ti4+. Note that titanium also exists in

different states of oxidation: besides Ti4+, there are also Ti3+, Ti2+ and

Ti0 (MacChesney and Muan, 1959). In rutile, there are commonly several

possible substitutions for titanium such as hexavalent (W6+ and U6+),

pentavalent (Nb5+, Sb5+ and Ta5+), tetravalent (Zr4+, Mo4+, Sn4+, Hf4+

and U4+), trivalent (Al3+, Sc3+, V3+, Cr3+, Fe3+ and Y3+) and divalent

(Fe2+, and to a lesser degree Mg2+, Mn2+ and Zn2+) cations (Graham

and Morris, 1973; Brenan et al., 1994; Hassan, 1994; Fett, 1995;Murad et

al., 1995; Smith and Perseil, 1997; Rice et al., 1998; Zack et al., 2002;

Bromiley and Hilairet, 2005; Scott, 2005; Carruzzo et al., 2006)

Rutile is the dominant carrier of HFSE (e.g. Foley et al., 2000; Kalfoun

et al., 2002; Zack et al., 2002). For example, in eclogites 1 modal-% of

rutile can carry more than 90% of the whole-rock content for Ti, Nb, Sb,

Ta and W and considerable amounts (5–45% of the whole-rock content)

of V, Cr, Mo and Sn (Rudnick et al., 2000; Zack et al., 2002).

Rutile typically contains between 3 and 130 ppm of uranium

(Mezger et al., 1989) Fig. 21), but higher concentrations also

occur, making Pb–Pb and U–Pb dating possible (e.g. Mezger et al.,

1989, 1991; Möller et al., 2000; Vry and Baker, 2006). Radiometric

dating of rutile is based on the occurrences of unstable (radioactive)

uranium isotopes 238U and 235U incorporated into the rutile crystal

structure, which decay over several stages (decay chain) into stable

daughter lead isotopes 206Pb and 207Pb, respectively (Jaffey et al.,

1971; By measuring the parent/daughter ratios and known decay constants, it is possible to calculate the age when rutile cooled

below its “blocking” or closure temperature. The latter is around

600 °C for rutile grains larger than about 0.2 mm in diameter

(Cherniak, 2000; Vry and Baker, 2006), which is much higher than

previously thought (370–500 °C: Mezger et al., 1989