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Author(s): Stephen Foley (corresponding author) [1]; Massimo Tiepolo [2]; Riccardo Vannucci [2] It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at [...]

To unravel the rare earth element (REE) redistribution during high-pressure-low-temperature metamorphism, we have analyzed by ion microprobe all the minerals from representative Fe-gabbros from Ligurian metaophiolites (northwestern Italy). Contrary to what is observed for fresh Fe-gabbros, the clinopyroxene contribution to the whole-rock REE inventory of blueschists and eclogitized Fe-gabbros is minor or negligible. In both blueschists and eclogites, REE are redistributed among newly formed minerals. In blueschists, lawsonite is the major REE carrier and concentrates almost all the light REE (LREE) in the rock, whereas titanite plays an important role for middle and heavy REE (MREE, HREE). In eclogites, LREE and HREE mainly reside in accessory allanite and garnet, respectively. The development of blueschist and eclogite facies mineral assemblages in mafic oceanic crust, due to subduction to 35-65 km depths, was not accompanied by release of significant amounts of LREE to the overlying mantle wedge. Owing to the stability of lawsonite and epidote at ultra-high-pressure conditions, subduction of mafic rocks is considered an effective mechanism to bring LREE to great depth within the mantle.

A SIMS procedure for Li, Be, and B quantification in silicates has been developed using the empirical approach of working curves via calibration with standards. Medium- to high-energy secondary ions have been used to reduce matrix effects affecting especially Li/Si ionization and to improve measurement reproducibility with respect to low-energy ion analysis for Li, Be, and B. In the case of Be, there has been evidence for a possible reduction of matrix effects, whereas in the case of B, matrix effects have been evidenced to be rather low at any ion energy, and in such a case, the application of a voltage offset has been useful only to improve measurement reproducibility. Positive ions of the isotopes 7Li, 9Be, 11B, and 30Si (assumed as the matrix reference isotope), having emission kinetic energies ranging from approximately 75 to 125 eV, have been monitored using an ion microprobe Cameca IMS 4F. The calibration curves hold over extended concentration ranges from ppm to percent level by weight for light elements and silica contents ranging from a few percent to more than 80% by weight. Precision and accuracy of the method are generally estimated as better than + or - 20% relative at the ppm level and better than + or - 10% relative for element abundances of tens of ppm for Be and B. As for Li, the whole uncertainty in the proposed SIMS procedure is evaluated at about + or - 20% (28% for lead crystal glasses). These results have important implications for more extensive applications of SIMS as a 'routine' microanalytical and bulk technique for the quantification of Li, Be, and B in silicate minerals and rocks.

Two spinel dunite xenoliths ([Fo.sub.89.8-91.2] in olivine) from La Palma contain minor amounts ([is less than] 1%) of a pale-blue sodalite-group mineral with hauyne/lazurite chemistry. Selected-area electron diffraction (SAED) patterns of this phase indicate a cubic unit cell with dimensions 9.12 [+ or -] 0.02 [Angstrom], and space group P43n. Superstructure spots along three directions are common, implying commensurate or incommensurate modulations along directions. Raman spectra show peaks typical of both lazurite and hauyne. It is concluded that the mineral has a structure intermediate between those of pure lazurite and pure hauyne, and it is here referred to as [hauyne.sub.ss]. The [hauyne.sub.ss] occurs together with strongly nepheline-normative glass in thin veinlets ([is less than] 0.1 mm), in interstitial glass pockets, and as inclusions in olivine porphyroclasts. To our knowledge lazurite or hauyne has not previously been described in mantle rocks. The [hauyne.sub.ss] is strongly depleted in REE and most other highly lithophile elements relative to the coexisting glass, whereas [D.sub.mineral/glass] for Sr is [approximately equals] 1.0, and [D.sub.Eu] higher than the other REE. The [hauyne.sub.ss] crystallized from a melt now present as phonolitic glass, probably in response to rapidly decreasing pressure during transport of the xenoliths to the surface. The coexistence of [hauyne.sub.ss] and FeS-rich sulfide globules in some samples suggests slightly more oxidizing conditions than for samples in which the glass contains sulfide globules alone.

Constraints on the calculation and use of mineral/melt and two-mineral partition coefficients for Ti (D.sup.Ti) have been derived from current knowledge of the distinct crystal-chemical mechanisms for the incorporation of [Ti.sup.n+] in the amphibole structure as follows: (1) mineral/melt partition coefficients for Ti, and other tetravalent high field-strength elements (HFSE), can be compared only when considering the fraction of [Ti.sup.4+] that enters the same structural site; (2) accurate two-mineral partition coefficients can be obtained only when considering the fraction of [Ti.sup.4+] that is involved in the same crystal-chemical mechanism in the two relevant phases (i.e., [sup.M2][Ti.sup.4+] and [sup.M1][Ti.sup.4+] for amphibole and clinopyroxene, respectively). The complete crystal-chemical characterization of synthetic titanian pargasite and kaersutite and of synthetic richterite (all crystallized under P, T, X, [fo.sub.2] conditions of interest for upper-mantle studies) shows that the site preference of Zr and Hf differs between the two amphibole compositions; these elements are essentially ordered at M2 in pargasite and kaersutite, but preferentially enter M1 in richterite. In the latter case, Ti segregates into the split M1' site with distorted coordination and shorter Ti-O3 distances, whereas Zr and Hf most likely prefer the larger and more regular M 1 site. The observed site preference is strongly controlled by the relative dimensions of the available sites. The crystal-chemical mechanisms that govern the incorporation of octahedral high-charge cations are the local charge balance of [sup.IV]Al (by [R.sup.3,4+] at M2) and of dehydrogenation (by [R.sup.3.4+] at M 1); thus the incorporation of Zr and Hf depends on distinct intensive parameters in the two amphibole compositions. Calculation of partition coefficients and of elastic-site parameters under the assumption that all Ti and other [HFSE.sup.4+] order at the M2 site in amphibole, as is currently done in geochemical studies, is strongly biased. In the presence of significant dehydrogenation, amphibole/melt [D.sub.0] values obtained from modeling based on the elastic-strain theory starting from the more-accurate site populations for Ti may be only 1/4 of those obtained by using the total Ti content, and the derived site parameters E and [r.sub.0] are more consistent with octahedral coordination. This result has important consequences for the prediction of D values under P-T conditions different from those of the experimental work. Applying the above concepts to data from natural assemblages, we obtained a significantly narrower (0.3-2.4 vs. 1.5-8.9) and more reasonable range of variation for amphibole/clinopyroxene [D.sub.Ti]. A relationship between these values for [D.sub.Ti] and pressure is also now apparent.

Partition coefficients (Min/LD) for a series of geochemically relevant elements have been calculated from combined EMP/LA-ICP-HRMS analyses of plagioclase, alkali feldspar, Ti-magnetite, biotite, apatite, and trachytic/trachyphonolitic melt pairs in selected Campi Flegrei rocks. Pl/LD and Kfs/LD values are generally very low for most of the trace-elements but Sr, Ba, and Eu. Kfs/LD for the latter elements record a systematic increase as the melt composition changes from trachyte to trachyphonolite, likely due to increasing structural compliance of the sanidine in the trachyphonolites related to larger Na/K values. Conversely, Kfs/LD values for transitional, highly charged incompatible elements (e.g., LREE) decrease from trachyte to trachyphonolite, possibly in response to the decrease of melt polymerization. Min/LD values for titanomagnetite generally decrease with the increasing melt evolution, the highest values being those measured for Ti, V, and Sc. Ti, Ba, Sc, Rb, Nb, Ta, and V are compatible in biotite in equilibrium with trachytic melt, whereas Cs, Sr, and Pb are incompatible and REE are strongly incompatible, as supported by the extremely low Bt/LDY (0.003-0.008). Partition coefficients for apatite and trachyphonolitic glass pairs are high for Sr, REE (particularly MREE), and Y, large for Th, U, and V, generally low for HFSE, and variable for other LILE. The comparison of measured Min/LD values for Campi Flegrei trachytes/trachyphonolites with other sets of partition coefficients reported in literature for evolved systems suggests that a reliable data set for magma evolution modeling requires: (1) a thorough preliminary selection of natural samples; (2) the adoption of accurate microanalytical techniques; (3) the direct measurement of Min/LD values for each specific melt composition. [ABSTRACT FROM AUTHOR]