The crystal chemistry of titanian clinohumite (ticl) is of considerable interest as it is a stable phase in peridotites under high pressure (Scambelluri et al., 1995) and ultra-high pressure (Okai, 1994) conditions as well as in kimberlites (McGetchin et al., 1970). In the Alps titanian clinohumite is a widespread accessory mineral in serpentinized peridotite bodies from greenschist to eclogite facies conditions (Malenco, Saas-Zermatt, Erro-Tobbio).
In the Malenco serpentinite, the breakdown of ticl to olivine+ilmenite+H2O is well documented in the tonalite contact aureole of the Bergell intrusion at p~0.3 GPa, T~520°C (Trommsdorff and Evans, 1980). Ticl-bearing veins occur in several metamorphic generations concordant and discordant to the main Alpine schistosity. They consist of highly variable amounts of ticl, diopside, olivine, magnetite, chlorite and antigorite. Minor phases are perovskite, calcite, ilmenite and apatite.
Major element abundances and some trace elements (Ni, Cr, Co, Sc, V) were analyzed by XRF. REE and other trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Bulk rock analyses of ticl-bearing veins are extremely variable in major and trace elements and highly depend on modal amounts of the phases present. In many veins the modal amount of apatite can be as high as 5 percent. In these veins the P, Y, LREE and MREE are dominated by apatite. REE show a flat pattern with a slight depletion in the LREE. Apatite- and diopside- free veins show low REE contents with an increase in the HREE.
The composition of ticl is rather homogeneous in terms of major element composition (i.e. XMg = 0.87-0.90, XTi= 0.456, XOH= 1). Carefully handpicked ticl mineral separates from several veins from Malenco (purity > 99%) have been analysed by ICP-MS and in situ PIXE. The chondrite normalized REE abundance of ticl is relatively uniform. Ticl has low REE content (0.1 < REE/chondrite < 1) and they show a flat to slightly U-shaped REE pattern with a strong increase in the HREE from Dy to Lu (Dy/Ybn: 0.01-0.24). In a normalized trace element abundance diagram ticl shows positive Zr, Hf and Nb, Ta anomalies. Nb, Ta abundance is unrelated to Zr, Hf and may vary almost two orders of magnitude (from less than 0.5 ppm to more than 25 ppm for Nb). This broad variation may reflect distinct concentrations of Nb and Ta in the parental rock.
The observation that ticl can be an important host for Nb and Ta is important for the storage of these elements in the mantle. Recent breakdown experiments of ticl (Weiss and Ulmer, 1995) has shown that ticl is stable down to at least 6 GPa at 950°C. The stability field of natural antigorite (Ulmer and Trommsdorff, 1995) is considerably smaller than that of ticl, thus indicating that during breakdown of antigorite and concomitant water release, Nb and Ta can still be retained in ticl and may explain the relative Nb, Ta depletion of subduction related volcanic rocks.
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