[Show abstract][Hide abstract] ABSTRACT: A quantitative procedure for the determination of hydrogen in silicates was developed for incorporation in the general procedure used with the Cameca IMS 4f ion microprobe for light and trace elements. The procedure employs a static 16O– primary beam and the ‘energy filtering’ technique. Moreover, it allows the analysis of samples placed on separate mounts with respect to standards and, in particular, of mineral grains directly in thin sections of rock. Factors influencing the hydrogen background were studied in order to ascertain the appropriate analytical conditions and to develop a specific procedure for the determination of low hydrogen contents. A low and reproducible background was obtained with simple heating and de-gassing of the sample. The background value was equivalent to 0.007–0.015% H2O and its fluctuation over a 1 week working session indicated that the detection limit was ≈0.015% H2O. The results obtained confirm that the use of high-energy ions reduces the influence of the matrix composition on the ionization of hydrogen (relative to that of Si); effects related to the structure of the sample appear to be negligible. Residual matrix composition effects are related, to a first approximation, to the Si content of the sample. As such effects could not be completely removed, two separate ion yields were used for samples with SiO2 contents in the range 27–51% and for rhyolites (≈72% SiO2). The reproducibility of the measurements was typically ≈3% and the accuracy for the former group of samples was 15% for H2O contents >0.1% and 30% for H2O contents of ≈0.05%.
[Show abstract][Hide abstract] ABSTRACT: An updated REE measurement of Gabbro GOG-1 was carried out by means of secondary ion mass spectrometry (SIMS). The availability of high amount of rock powder allowed the sample to be well ground and homogenized before analysis. Fused glass pellets were employed for SIMS investigations. REE data were compared with new ICP-AES and NAA analyses. REE results from all employed analytical techniques show good agreement and are consistent with in-situ SIMS measurements of REE in major mineral phases forming GOG-1. Extensive homogeneity tests seem to confirm the possibility to using this rock as a reference material for earth sciences.
Full-text · Article · May 2007 · Geostandards and Geoanalytical Research
[Show abstract][Hide abstract] ABSTRACT: Rare earth elements were determined by secondary ion mass spectrometry for 13 silicate rock samples, ranging from ultramafic-mafic to acidic compositions, 2 feldspar and 1 biotite separates. As a whole, the investigated samples are characterized by matrices and rare earth elements spectra covering most geological applications. The present data are compared with reference values. The advantage of using secondary ion mass spectrometry as a fundamental tool for trace element detection in bulk samples in the few ppm-ppb region is demonstrated.
Full-text · Article · May 2007 · Geostandards and Geoanalytical Research
[Show abstract][Hide abstract] ABSTRACT: Quadrupole ICP–MS is currently the method of choice for laser-ablation applications, since sector-field instruments are generally considered too slow for the acquisition of rapidly varying signals. However, the new generation of double-focusing sector-field mass spectrometers is capable of rapidly scanning the mass spectrum both magnetically and electrostatically, and the combination of these two scan modes approaches the performance of quadrupoles for most practical applications. We present the instrumental configuration and operating conditions of a LA–ICP–MS system, assembled by coupling a 266 nm laser source to a double-focusing magnetic-sector mass spectrometer ("Element", Finnigan MAT). The capability of the adopted configuration to furnish high-quality in situ trace-element analysis of minerals and U–Pb geochronological data is described. Concentrations of thirty geochemically relevant elements (LILE, HFSE, REE, actinides) were determined on geological samples with good precision and accuracy (<10%), evaluated using two USGS basaltic glass reference materials, BCR–2 and BIR–1. The high sensitivity and low background allowed extremely low limits of detection (down to the ppb level) for the heaviest elements, and between 10 and 100 ppb for the lighter masses. The first results on U:Pb ratio determinations are also very positive. By adopting an external matrix-matched standard for corrections of mass bias and laser-induced fractionation of elements, precision and accuracy in age determinations to better than ~2% are attained on the zircon standard 91500.
Full-text · Article · Apr 2003 · The Canadian Mineralogist
[Show abstract][Hide abstract] ABSTRACT: We present first trace element analyses of the fluid produced during breakdown of antigorite serpentine, a major dehydration reaction occurring at depth within subducting oceanic plates. Microinclusions filled with crystals+ aqueous liquid are disseminated within olivine and orthopyroxene grown at pressures and temperatures beyond the stability field of antigorite. Despite hydrogen loss and significant major element changes that have affected the analyzed inclusions, their trace element composition still reflects characteristics of the subduction fluid released during serpentinite dehydration. The fluid is enriched in incompatible elements indicating either (1) interaction with fluids derived from crustal slab components, or (2) dehydration of altered (serpentinized) oceanic mantle previously enriched in incompatible elements. Several features of the analyzed fluid+mineral inclusions (high Pb/Th, Pb/U and Pb/Ce) are in agreement with available experimental work, as well as with the geochemical signatures of most arc lavas and of several ocean island basalt mantle sources. The trace element patterns of the fluid+mineral inclusions do not display relative enrichment in large ion lithophile elements compared to high field strength elements, thus suggesting that the latter elements may become soluble in natural subduction fluids.
Full-text · Article · Oct 2001 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Selective enrichment or depletion in either Zr and Hf (HFSE4+) or Nb and Ta (HFSE-5+) is a feature commonly observed in many mantle-derived melts and amphiboles occurring as either disseminated minerals in mantle xenoliths and peridotite massifs or in vein assemblages cutting these rocks. The fractionation of Nb from Zr seen in natural mantle amphiboles suggests that their incorporation is governed by different crystal-chemical mechanism. An extensive set of new partitioning experiments between pargasite-kaersutite and melt under upper-mantle conditions shows that HFSE incorporation and fractionation depends on amphibole major-element composition and the presence or absence of dehydrogenation. Multiple regression analysis shows that Amph/LDNb/Zr is strongly dependent on the mg-number of the amphibole as a result of a combination of amphibole and melt structure effects, so that the following generalizations apply: (1) high-mg-number amphiboles crystallized from unmodified mantle melts more easily incorporate Zr relative to Nb leading to an increase of the Nb/Zr ratio in the residual melt; (2) low-mg-number amphiboles, such as those found in veins cutting peridotites, may strongly deplete the residual melt in Nb and cause very low Nb/Zr in residual melts. Implications and applications to mantle environments are discussed.
Full-text · Article · Jan 2001 · Journal of Petrology
[Show abstract][Hide abstract] ABSTRACT: Two spinel dunite xenoliths (Fo89.8–91.2 in olivine) from La Palma contain minor amounts (<1%) of a pale-blue sodalite-group mineral with haüyne/lazurite chemistry.
Selected-area electron diffraction (SAED) patterns of this phase indicate a cubic unit cell with dimensions 9.12 ± 0.02 Å,
and space group P4̅3n. Superstructure spots along three <110> directions are common, implying commensurate or incommensurate modulations along
<110> directions. Raman spectra show peaks typical of both lazurite and haüyne. It is concluded that the mineral has a structure
intermediate between those of pure lazurite and pure haüyne, and it is here referred to as haüyness. The haüyness occurs together with strongly nepheline-normative glass in thin veinlets (<0.1 mm), in interstitial glass pockets, and as
inclusions in olivine porphyroclasts. To our knowledge lazurite or haüyne has not previously been described in mantle rocks.
The haüyness is strongly depleted in REE and most other highly lithophile elements relative to the coexisting glass, whereas Dmineral/glass for Sr is ≈1.0, and DEu higher than the other REE. The haüyness 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 haüyness and FeS-rich sulfide globules in some samples suggests slightly more oxidizing conditions than for samples in which the glass
contains sulfide globules alone.
Full-text · Article · Oct 2000 · American Mineralogist
[Show abstract][Hide abstract] ABSTRACT:  A new set of partitioning data for rare earth elements (REE: La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb), Y, Th, U, and Pb has been obtained for 25 calcic amphiboles (pargasites and kaersutites) crystallized from alkali-basaltic and basanitic bulk rock compositions at pressure P = 1.4 GPa, and temperature T between 950° and 1075°C. The variations of amphibole/liquid partition coefficients and of their ratios relevant to petrogenetic studies are discussed with reference to the major element composition of the amphiboles and of the coexisting melt, and to the crystal chemical mechanisms for trace element incorporation. Our results support the conclusions that REE and actinides are incorporated into the M4 cavity in calcic amphiboles and distributed between the two available sites within that cavity and that Pb is incorporated into the A site. In our sample population, REE patterns are systematically enriched in heavy REE (HREE), as expected from the presence of significant cummingtonite component. No significant fractionation is observed between Th and U. The major factor controlling the amount of trace element incorporation is the SiO2 content of the melt. The major implication of this study is that HREE can become compatible in amphibole in systems with SiO2 content greater than ∼50 wt %, whereas LREE always remain incompatible. We use the new DREEamph/l values to calculate the effects of amphibole crystallization during melt migration in the upper mantle by reactive porous flow as well as fractional crystallization of amphibole during melt migration in veined systems. We show that both processes will lead to residual liquids and solids with extremely variable LaN/YbN ratios.
Full-text · Article · Aug 2000 · Geochemistry Geophysics Geosystems
[Show abstract][Hide abstract] ABSTRACT: New partition coefficients between liquid and pargasitic/kaersutitic amphiboles (Amph/LDNb,Ta) experimentally determined for Nb and Ta at upper-mantle conditions, combined with single-crystal structure refinement of the synthesised amphiboles, show that Amph/LDNb,Ta are strongly dependent on the structure and composition of both amphibole and melt. The correlation of the Amph/LDNb,Ta with the amphibole oxy-component is explained by the ordering of Nb and Ta at the M1 site, contributing with the fraction of Ti at M1 to locally balance the O3O2−↔O3(OH)− substitution. In our set of dehydrogenated amphiboles, variations in the SiO2 content of the melt from 41.5 to 54.6 correspond to a six-fold increase of the Amph/LDNb,Ta, in which Amph/LDNb varies from 0.14 to 0.71 and Amph/LDTa from 0.11 to 0.54. Partition coefficients for Nb and Ta abruptly increase in Ti-depleted compositions (Amph/LDNb up to 1.63 and Amph/LDTa to 1.00). The ratio of DNb to DTa (Amph/LDNb/Ta) varies from 0.71 to 1.63, and is a function of the M1 site dimension, which in turn depends on its Fe, Mg and Ti contents. The observed variations can be explained by assuming that the ionic radius of Nb is (∼0.01–0.02 Å) larger than that of Ta, contrary to the common assumption that they are both equal to 0.64 Å. We calibrated a simplified model for the prediction of Amph/LDNb/Ta values shown to be negatively related mainly to mg# [Mg/(Mg+Fe)] and to Ti content. High-mg# amphiboles have Amph/LDNb/Ta close to unity, so the low Nb/Ta found in convergent margin volcanics and in the continental crust cannot be explained by the involvement of amphibole in the mantle wedge. Amphibole in the subducting slab may have lower mg# and consequently high Nb/Ta values, and thus may give rise to subchondritic Nb/Ta values in coexisting melts. Nb/La is also negatively correlated with mg#, and strongly increases in Ti-depleted compositions.
Full-text · Article · Mar 2000 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite,
and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the
results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare
earth elements) ordering at M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical
knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within
the M4 cavity (M4 for Ca and Na, M4′ for Fe2+ and Mg), we propose a new model for REE incorporation. LREE order at the M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component,
and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple
crystal-chemical mechanisms for REE3+ incorporation explains why measured and estimated Amph/LDHREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on
the basis of a single curve may appear correct, but the values obtained for r0 are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple
sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy
REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within
the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach
allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive
models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves
constructed from accurately determined Amph/LDREE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not
available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates
the general idea that Ca acts as a “carrier” for REE.
Full-text · Article · Oct 1999 · Contributions to Mineralogy and Petrology
[Show abstract][Hide abstract] ABSTRACT: Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Canary Islands include veined spinel harzburgites
and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a gradual transition in composition
from broad veins to narrow veinlets. Broad veins contain alkali basaltic glass with semi-linear trace element patterns enriched
in strongly incompatible elements. As the veins become narrower, the SiO2-contents in glass increase (46 → 67 wt% SiO2 in harzburgite, 43 → 58 wt% in dunite) and the trace element patterns change gradually to concave patterns depleted in moderately
incompatible elements (e.g. HREE, Zr, Ti) relative to highly incompatible ones. The highest SiO2-contents (ca. 68% SiO2, low Ti-Fe-Mg-Ca-contents) and most extreme concave trace element patterns are exhibited by glass in unveined peridotite
xenoliths. Clinopyroxenes shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-diopside in highly silicic
glass. Estimates indicate that the most silicic glasses represent melts in, or near, equilibrium with their host peridotites.
The observed trace element changes are compatible with formation of the silicic melts by processes involving infiltration
of basaltic melts into mantle peridotite followed by reactions and crystallization. The Fe-Mg interdiffusion profiles in olivine
porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction processes
have taken place in situ in the upper mantle. The CaO-TiO2-La/Nd relationships of mantle rocks may be used to discriminate between metasomatism caused by carbonatitic and silicic melts.
Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with relatively constant,
low-CaO contents which is compatible with metasomatism of “normal” abyssal peridotite by silicic melts. Peridotite xenoliths
from Tenerife show somewhat higher CaO and TiO2 contents than those from the other islands and may have been affected by basaltic or carbonatitic melts. The observed trace
element signatures of ultramafic xenoliths from La Palma and other Canary Islands may be accounted for by addition of small
amounts (1–7%) of highly silicic melt to unmetasomatized peridotite. Also ultramafic xenoliths from other localities, e.g.
eastern Australia, show CaO-TiO2-La/Nd relationships compatible with metasomatism by silicic melts. These results suggest that silicic melts may represent
important metasomatic agents.
Full-text · Article · Sep 1999 · Contributions to Mineralogy and Petrology
[Show abstract][Hide abstract] ABSTRACT: Partition coefficients for Rb, Sr, Sm, Nd, U, Th, Pb, La, Ce, Ba, Nb, and Li between phlogopite, clinopyroxene and melt were determined in order to characterize the effect of metasomatically grown minerals on the isotopic evolution of the earth's lithospheric mantle. Experiments were performed in a piston–cylinder-apparatus at 15 kbar and 1040–1175°C with leucite lamproite as starting composition. Three phlogopite/glass and one clinopyroxene/glass pairs were analyzed by secondary ion mass spectrometry (SIMS). Partition coefficients (D-values) for phlogopite/melt of Rb, Cs and Ba and Ti are ≥0.5. DCs is significantly lower than DRb (0.50/1.34); in contrast to these two elements, DBa varies between 0.3 and 1.6 in phlogopites of different major element composition. A similar behavior is suggested for the elements Sr (D=0.02–0.06) and Pb (D≈0.02). The partition coefficients measured for REE, Th and U are in the order of 10−4–10−6, those for Zr and Nb are 0.015 and 0.07 respectively, and DLi for phlogopite and melt is 0.2. Partition coefficients for REE and Sr between clinopyroxene and melt are relatively high compared to other studies, presumably because of the high wollastonite component in the clinopyroxenes of this work. The data for clinopyroxene as well as the data for phlogopite were used to calculate crystal lattice-site parameters for the M2-site in clinopyroxene and the X-site in phlogopite. Parameters for clinopyroxene are in good agreement with data from other works. For the phlogopite interlayer site, an optimum ionic radius of 1.71 Å and polyhedral Young's moduli of 552 kbar (1+-cations) and 1087 kbar (2+-cations) were found. Recent data and the results of this study show that phlogopite is able to store significant amounts of Rb, Cs and Ba in the lithospheric mantle, thus causing a radiogenic evolution of Sr. In contrast, the influence of phlogopite on the Nd-isotope evolution is negligible because of the very low partition coefficients for Sm and Nd. The concentrations of Sr, Nd and Sm and the evolution of Nd isotopes in metasomatized lithospheric mantle are controlled by clinopyroxene, which significantly contributes to a less radiogenic evolution of Sr relative to the one caused by phlogopite. This implies that the evolution of Sr isotopes depends on the modal proportion of phlogopite and clinopyroxene.
No preview · Article · May 1999 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: The trace element distribution in three selected olivine-bearing gabbros from the Northern Apennine ophiolites has been determined.
These rocks consist of euhedral plagioclase and olivine, and subhedral to poikilitic clinopyroxene. Fe-Ti-oxides, titanian
pargasite, orthopyroxene and apatite occur as interstitial accessory minerals. Plagioclase, clinopyroxene and accessory minerals
were analysed for rare earth (REE) and selected trace elements by secondary ion mass spectrometry. Both plagioclase and clinopyroxene
are compositionally zoned. The plagioclase rims have slightly lower anorthite component and higher light REE (LREE), Ba and
K than the cores. Likewise, the clinopyroxene rims show a slight Mg and Cr decrease, and a marked increase in Zr, REE and
Y relative to the core. The rims of plagioclase and clinopyroxene, Fe-Ti-oxides, apatite and titanian pargasite most likely
formed through post-cumulus fractional crystallization of interstitial liquid. It is argued that such interstitial liquid
had an exotic component, probably related to the infiltration of highly evolved, slightly LREE enriched liquid in the cumulate
pile. On the basis of mass balance calculations, we show that Fe-Ti-oxides play an important role in the Ti budget of the
whole rock, as does apatite for LREE.
No preview · Article · Jan 1999 · Contributions to Mineralogy and Petrology
[Show abstract][Hide abstract] ABSTRACT: The observation of glass inclusions with alkalic and transitional compositions in single olivine crystals from Aci Castello hyaloclastites indicates that small volumes of alkalic melts preceded tholeiite eruptions during the onset of Mt Etna volcanism. The gradual shift in composition from alkalic to transitional melts is explained by simple partial melting of a compositionally uniform source, with the degree of melting increasing by a factor of 3–4 during this early melting process. Such variation controls both the chemical and mineralogical signatures of emitted lavas. The source shows trace-element and isotopic signatures which may be representative of a HIMU-type plume for Etnean lavas.
Full-text · Article · Nov 1998 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the
shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary
series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements,
with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20 °C)
and similar volatile abundances. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages
and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic
mantle. The younger primitive melt is characterized with respect to the older one by higher K2O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree
of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies
in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination.
Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary
paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions
during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive
mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions
suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively
homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical
configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs.
Full-text · Article · Oct 1998 · Bulletin of Volcanology
[Show abstract][Hide abstract] ABSTRACT: Ion microprobe analyses of clinopyroxenes in equilibrium with glasses (SiO2 from 42 to 69 wt%) formed as the result of AFC-type reactions between infiltrating basaltic melt and peridotite wall-rock in the upper mantle under La Palma (Canary Islands) reveal trace element signatures usually ascribed to carbonatite metasomatism (i.e. strong REE-enrichment and Zr,Ti-depletion). Cpx/glass partition coefficients for REE, Y and Sr progressively increase with increasing SiO2 in response to liquid composition/structure effects, approaching unity for La, Ce and Sr, and exceeding unity for the other REE in the most silicic glasses. Partition coefficients for Zr and Ti remain constant or decrease, probably as the combined effect of melt composition/structure and difficulty of charge-balancing 4+ cations in the crystal. Incorporation of the higher-charged HFSE in the cpx lattice requires complex coupled substitions [see original article for formula]. Our results demonstrate that REE-enrichment and REE/HFSE fractionation of clinopyroxene do not necessarily reflect these characteristics in the metsomatic agent and that cpx/basaltD cannot confidently be used to infer the geochemical nature of equilibrium melts under upper mantle conditions. Moreover, we provide evidence that the strong REE-enrichment and Zr,Ti-depletion of some clinopyroxene from La Palma xenoliths is a feature commonly observed in clinopyroxenes from various mantle occurrences often interpreted as reflecting chemical interaction with metasomatic fluids and melts of different composition (either alkaline or carbonatitic) and provenance (from either the enriched asthenospheric mantle or the subducted slab). Therefore, claims of carbonatite metasomatism as opposed to silicate melt infiltration based on trace element signatures of investigated clinopyroxene should be regarded with caution unless the role of liquid composition/structure and of crystal chemical control is investigated.
No preview · Article · May 1998 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Melt impregnation and peridotite-melt interaction are ubiquitous phenomena in the oceanic-type lithospheric mantle. Nevertheless, the nature of the infiltrating melts is still poorly understood. We performed detailed textural and chemical investigations (by means of electron and ion microprobe) on: (1) impregnated plagioclase-bearing ophiolitic peridotites from the Internal Ligurides (Northern Apennine, Italy) and Mt. Maggiore (Corsica), and (2) olivine cumulates (consisting of 85 vol% olivine plus interstitial plagioclase and rare poikilitic clinopyroxene) from magmatic pods intruded within the Mt. Maggiore peridotites. Field evidence indicates that such cumulates crystallized from the melts, which impregnated the surrounding peridotites. Melt impregnation in the peridotites is verified by the occurrence of peculiar microstructures: (1) plagioclase blebs and/or veins confined along grain boundaries or crosscutting mantle minerals; (2) partial dissolution of mantle clinopyroxene and replacement by orthopyroxene and plagioclase aggregates, which indicate disequilibrium between melts and host peridotites. Interaction with melts also produces chemical modifications in mantle clinopyroxenes, i.e., Ti, M(middle)- to H(heavy)-REEs (and, to a lesser extent, Zr, Y, and Sc) enrichment coupled to depletion in Al. Minerals crystallized from the melts have depleted Geochemical signature: plagioclases are highly Anorthitic (An88An94 in the peridotites; An81An85 in the cumulates), and show extremely low Sr (<26 ppm) and LREE () concentrations. Interstitial clinopyroxenes in the cumulates are characterized by high Mg values (90.6–91.7): their REE spectra show significant LREE depletion (), high M- to H-REE concentrations (15–30 × C1) and pronounced negative EuN anomalies. Geochemical modeling indicates that the impregnating liquids probably consisted of unmixed depleted melt increments produced by 6–7% fractional melting. The results of this study therefore suggest that the impregnating melts originated at deeper mantle levels and presumably represented the last melt increments of a fractional melting process. There is thus a higher probability that they will remain incorporated in the upper mantle. They subsequently ascended, partly crystallized as cumulate pods, and interacted with the studied peridotites dissolving mantle clinopyroxene and precipitating orthopyroxene. Such a process has been increasingly invoked in studies of melt/rock reaction zones from ophiolitic peridotites.
No preview · Article · Jul 1997 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Systematic investigations (electron- and ion-microprobe, and X-ray structure-refinement) of pyroxenes and amphiboles close to a cm-wide hornblendite vein have been carried out on a composite peridotite sample from the Lherz massif with the aim of constraining the processes of melt infiltration in the subcontinental mantle.Vein amphiboles have light rare earth element (LREE)-enriched patterns with the maximum at Nd and Sm, and positive Ba, Nb, Sr, and Ti anomalies. The liquid flowing in the vein had an alkaline geochemical signature (i.e., large-ion lithophile element (LILE)-enriched without significant high field strength element (HFSE) anomalies). Modal metasomatism, represented by crystallization of amphibole and consumption of clinopyroxene, occurred in a 2-cm thick layer of wall-rock. Newly formed amphiboles, i.e., those in the vein and in the modally metasomatised layer, have a lower Al disorder than the pre-existing disseminated amphiboles, which formed during an older metasomatic event.Cryptic metasomatism is recorded beyond the modally altered layer in the cores of clinopyroxenes and in amphiboles. Moving away from the vein towards the peridotite, amphiboles and pyroxenes display systematic compositional variations, and approach equilibrium with the wall-rock within a few cm. Porphyroclastic clinopyroxenes record chromatographic separation of elements: less incompatible elements reach equilibrium with the wall-rock lherzolite at shorter distances than the more incompatible ones. The inverse correlation between the depth of the metasomatic effect and partition coefficients in porphyroclastic clinopyroxenes indicates that different concentration fronts developed for different elements during liquid/rock interaction, suggesting that liquid percolation along grain boundaries was responsible for clinopyroxene metasomatism.As in the clinopyroxene, abundances of incompatible elements in the amphibole decrease in the peridotite with distance from the vein. However, a less marked separation of elements is recorded by the amphiboles. Elements that are markedly more compatible in amphibole than in other minerals of a lherzolite, such as K, Ba, and Rb, have abrupt gradients which are confined to the modally-metasomatised region. The lack of metasomatic enrichment in these elements in the farther, disseminated amphiboles, indicates that amphibole crystallization was synchronous with melt infiltration and acted as a buffer for such elements.Micro-analytical investigations indicate that the geochemical effects associated with small-scale liquid infiltration can be successfully described by models which take into account both the chromatographic fractionation of elements and the influence of mineral-assemblage variations.