Elisabetta Rampone

Chemistry and origin of trapped melts in ophioiitic peridotites

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 (An88An94 in the peridotites; An81An85 in the cumulates), and show extremely low Sr (<26 ppm) and LREE (< 1 × Cl; CeNSMN = 0.13−0.50) concentrations. Interstitial clinopyroxenes in the cumulates are characterized by high Mg values (90.6–91.7): their REE spectra show significant LREE depletion (CeNYbN = 0.027−0.039), 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.

Subsolidus reactions monitored by trace element partitioning: the spinel- to plagioclase-facies transition in mantle peridotites

Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.

The ophiolite-oceanic lithosphere analogue: New insights from the Northern Apennines (Italy)

In this paper, we discuss the results of recent petrologic and isotopic studies on the Northern Apennine ophiolites, which are remnants of the oceanic lithosphere of the Jurassic Ligurian Tethys Ocean. In the Northern Apennines, ophiolites crop out in two distinct geologic units, the External and Internal Ligurides, which have been related, respectively, to pericontinental and intraoceanic settings of the Ligurian Sea sector of the Tethys. In the External Ligurides, subcontinental-lithosphere peridotites (presumably Proterozoic) are associated with Hercynian continental crust. Both mantle and crustal rocks display a composite subsolidus retrograde evolution, related to their uplift toward the seafloor, where they are associated with Jurassic mid-oceanic-ridge basalts (MORBs). The Internal Liguride ophiolites comprise the spatial association of depleted peridotites (of Permian age) and younger (mostly Jurassic) MORB-type magmatism, not linked to the peridotites by a genetic melt and residua relationship. Structural and petrologic features recorded by the Northern Apennine ophiolites thus indicate that the Jurassic Ligurian Sea sector of the Tethys was mostly underlain by older (Proterozoic and Permian), subcontinental lithospheric mantle and younger, unrelated, MORB seafloor. This peculiar lithologic association cannot be produced at mid-ocean ridges of mature oceans; rather, it most likely originated after the breakup of the continental crust in response to passive extension of the continental lithosphere. The exposure of subcontinental mantle peridotites on the ocean floor is a unique feature testifying to the early stages of inception of an oceanic basin by means of passive lithospheric extension. Modern oceanic analogues of the Ligurian Sea sector of the Tethys represented by the Northern Apennine ophiolites are currently found in embryonic oceans (e.g., the Red Sea) and at the transition zone between continental and oceanic lithosphere of mature oceanic basins (e.g., the Galicia margin, western Spain).

Origin of LREE-depleted amphiboles in the subcontinental mantle

Ion-microprobe analyses of interstitial kaersutite and Ti-pargasite grains from orogenic peridotites and lherzolite xenoliths reveal that LREE-depleted amphiboles are common in the subcontinental mantle samples. Incompatibility diagrams for the investigated amphiboles show that REEs almost parallel those of coexisting clinopyroxenes, whereas Sr, Zr, and Ti show variable anomalies (i.e., Sr/Sr∗ and Ti/Ti∗ > 1 and Zr/Zr∗ < 1). In the chondrite-normalized incompatibility diagrams, Sr lies almost a factor of two above Ce and Nd and is usually depleted relative to HREEs. Average amphibole/clinopyroxene partition coefficients for spinel-bearing assemblages range from 1.4–1.8 for LREEs and from 1.8–2.2 for HREEs. Corresponding D values for Zr, Sr, and Ti are about 1, 3, and 5, respectively. Present data apparently contrast with the conventional wisdom that the presence of amphibole in mantle rocks is related to the introduction of melt or fluids enriched in incompatible elements. In the absence of experimental evidence that aqueous fluids in equilibrium with deep mantle are LREE-, Sr-depleted, we propose either a diffusive redistribution (near solidus or at subsolidus) or a chromatographic process to account for the formation of depleted amphibole from LREE-, Sr-enriched fluids. The crystallization of LREE-, Sr-depleted kaersutite and Ti-pargasite has important geodynamic implications, since it refers, at least for some peridotite massifs (i.e., Zabargad, External Ligurides, Eastern Pyrenees) to the steady-state equilibration under spinel-facies conditions and is related to the early evolution of peridotites. This stage is broadly related to the timing of lithospheric accretion.

Contrasting bulk and mineral chemistry in depleted mantle peridotites: evidence for reactive porous flow

A series of recent papers have indicated that reconstructed bulk compositions of abyssal peridotites define chemical correlations, namely increasing FeOtot and decreasing SiO2 with increasing MgO, which cannot be produced by simple extraction of partial melts. However, no general consensus exists on the reliability of these trends, because they could be artifacts of the adopted calculations, and on the origin of abyssal peridotites. Specifically, it has been inferred that abyssal peridotite compositions are consistent with combined histories of partial melting and subsequent melt migration which caused either olivine addition, or dissolution and precipitation reaction, by equilibrium porous flow [Niu et al., Earth Planet. Sci. Lett. 152 (1997) 251–265; Asimow, Earth Planet. Sci. Lett. 169 (1999) 303–319]. We report combined bulk-rock and mineral chemical data for ophiolitic peridotites from the Erro-Tobbio (ET) Unit (Voltri Massif, Ligurian Alps), which represent lithosphere remnants of the Jurassic Ligurian Tethys embryonic ocean. These peridotites include sp-lherzolites and sp-harzburgites, and display overall depleted geochemical signature. However, comparison between bulk rock and corresponding mineral compositions reveals that these rocks cannot be residues of simple (equilibrium or fractional) melt extraction. Mineral compositions are similar in all the samples. By contrast, the bulk rock compositions define striking correlations, i.e. increasing FeOtot, Ni, Co, and decreasing Al2O3, SiO2, CaO, Sc, Cr, YbN, with increasing MgO: the MgO–FeOtot and MgO–SiO2 correlations are similar to those recognized in abyssal peridotites. Thus, the ET peridotites provide evidence that the above trends are indeed consistent with similar variations in on-land peridotites; also, these trends cannot simply result from progressive melt depletion, because constituent minerals in the different ET samples have rather uniform composition. Calculated bulk modes indicate that the observed chemical variations are coupled to systematic modal changes, namely decrease in cpx and opx, and increase in olivine, at increasing bulk MgO. The ET peridotites also display decrease of the cpx/opx ratio at increasing bulk MgO, and this argues against a process of simple olivine addition. By contrast, some peculiar bulk-mineral compositional features – e.g. (i) nearly constant olivine Mg* [=Mg/(Mg+Fetot)] values, at increasing modal olivine, (ii) opposite bulk Cr and Ni correlations, (iii) bulk Cr and Yb decrease, and parallel Ni increase, strikingly correlated with progressive modal cpx decrease and concomitant modal olivine increase – are consistent with the expected chemical and modal effects, during a process of interaction between depleted peridotites and melts migrating by equilibrium porous flow (involving pyroxene dissolution and olivine precipitation reactions). Bulk-rock and mineral chemistry data in the ET peridotites thus indicate that the major element correlations inferred by Niu et al. for the abyssal peridotites are reliable, and most likely result from a combined history of partial melting and melt interaction by reactive porous flow.

Peridotite clinopyroxene chemistry reflects mantle processes rather than continental versus oceanic settings

Abstract Comparison of 360 trace element analyses of clinopyroxenes from peridotites indicates that clinopyroxene composition cannot discriminate between suboceanic and subcontinental mantle. Clinopyroxenes from abyssal and fore-arc peridotites have the lowest incompatible element concentrations and record melting and basalt extraction as the dominant process. Clinopyroxenes from continental peridotite massifs partially overlap the composition of clinopyroxenes from abyssal peridotites and, in general, behave as a less depleted component in the chemical variation trends defined by the latter. In the peridotite massifs, subsolidus re-equilibration involving phase transitions (spinel to plagioclase facies) may cause significant element variations in clinopyroxene (e.g. Sr and Eu/Eu * decrease, REE, Ti increase). Metasomatic processes cause only comparatively minor trace element variations. Clinopyroxenes from mantle xenoliths in OIB and continental alkali basalts have the highest incompatible trace element concentrations and the largest compositional variations. They primarily record metasomatic enrichment processes, which are similar in suboceanic and subcontinental mantle. These processes induced the highest incompatible element enrichment in the clinopyroxenes from the most refractory peridotites, supporting the chromatographic nature of infiltration metasomatism. This enrichment, however, does not affect Ti, which is in the same concentration range in clinopyroxenes from both xenoliths and abyssal peridotites. The apparent Ti immobility may be due to several causes, such as reduced Ti solubility in hydrous fluids, fractionation of Ti-rich phases from percolating silicate melts, reaction with carbonatite melts formerly equilibrated with amphibole-peridotite. In general, clinopyroxene geochemistry does not allow a clear distinction between different metasomatic agents. The similarity between the geochemical characteristics of xenoliths from continental and oceanic environments supports previous results that the compositions of the pre-metasomatic lithosphere and of the asthenosphere, from where metasomatic agents derive, do not differ in the two environments.

Os isotopes and highly siderophile elements (HSE) in the Ligurian ophiolites, Italy

The Os isotopic and highly siderophile element (HSE) concentration systematics of the upper mantle have been the focus of much recent interest. However, little systematic study has addressed the combined HSE and Os isotopes in mantle rocks from MOR ophiolites. The Ligurian ophiolites in northern Italy represent an important class of ophiolites representing, like Zabargad or the Galicia margin, crust with clear ocean ridge affinity floored by older mantle with arguably more continental affinities [Rampone et al., J. Petrol. 36, 18–105, 1995; Rampone et al., Contrib. Mineral. Petrol. 123, 61–67, 1996]. We have studied a suite of 15 geochemically and geologically well characterized mantle peridotites from this ophiolite. The samples have Os isotopic and HSE concentration characteristics in agreement with similar measurements made in abyssal peridotites [Snow and Schmidt, Nature 391, 166–169, 1998]. The observations are consistent with the notion that the depleted mantle shows a HSE signature consistent with the influence of core-derived metal. Robust age information could not be obtained for either body, however the data from the External unit are consistent with a Proterozoic melting age similar to the Ronda ultramafic massif. Os–Nd isotopic correlations observed are not, however, consistent with the evolution of variably depleted mantle. Such Os–Nd correlations could be modeled by addition of recycled sediment to depleted mantle, however this disagrees with current notions of the origin of the depleted mantle. Variable 187Os/188Os with essentially constant Al2O3 in mantle rocks can be explained by recent remelting of ancient mantle.

Melt migration and intrusion in the Erro-Tobbio peridotites (Ligurian Alps, Italy): Insights on magmatic processes in extending lithospheric mantle

The Alpine/Apennine ophiolites are largely thought to represent lithosphere sectors mostly formed at an ocean/continent transition and allow the direct observation of petrologic and geodynamic processes in extensional systems evolving from continental rifting to ultraslow spreading. The Alpine/Apennine peridotites experienced multiple melt/rock interaction and melt intrusion events occurred at different lithospheric depths, thus providing insights on mantle dynamics and lithosphere-asthenosphere interactions during progressive lithosphere extension. Here we present an overview of this multi-stage melt migration and intrusion history, as recorded in the Erro-Tobbio (ET) peridotites (Ligurian Alps, Italy). In the ET spinel peridotites, the oldest intrusion event is documented by the diffuse occurrence of cm-scale folded pyroxenite bands. They display variably fractionated REE spectra, marked by LREE depletion and absent Eu N anomaly. Unusual trace element signature (high Sc,V contents and low MREE/HREE ratios) in clinopyroxenes from one pyroxenite layer is witness of a precursor garnet-bearing magmatic assemblage. Spinel pyroxenites likely originated as high- P (> 15–20 kbar) intrusions that preceded the extension-related peridotite exhumation. In the spinel peridotites, field, textural and chemical evidence ( e.g . olivine embayment replacing pyroxene porphyroclasts, increasing modal olivine at constant bulk Mg values), points that they experienced open-system melt migration by reactive porous flow, subsequent to pyroxenite intrusion and folding. Melt/rock interaction (causing olivine crystallization and pyroxene dissolution) occurred at high melt volumes at deep lithospheric levels. At shallower lithospheric depths, the ET peridotites were impregnated by melts causing significant plagioclase enrichment and crystallization of poikilitic orthopyroxene replacing mantle olivine and clinopyroxene. Reacted clinopyroxenes preserve strong LREE depletion, indicating that impregnating melts originated as depleted melt fractions. After impregnation, peridotites underwent multiple gabbroic intrusion events. Structural and geochemical features of melt impregnation and melt intrusion products point to a progressive change in melt composition and dynamics. Peridotite impregnation was caused by diffuse migration of opx-saturated depleted melts, and is consistent with cooling and crystallization of migrating melts when the peridotites, due to lithosphere extension and thinning, became part of shallower and colder lithospheric environments. The subsequent intrusion events originated by MORB-type aggregated magmas that had not experienced significant compositional modifications during ascent. The transition from porous flow melt migration to emplacement of magmas in fractures reflects progressive change of the lithospheric mantle rheology, across the ductile to brittle transition, during extension-related uplift and cooling of the ET mantle.

Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN∼0.5) and HREE (GdN/ YbN∼1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN ∼0.3) and lesser HREE (GdN/YbN<1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.

Meter-scale Nd isotopic heterogeneity in pyroxenite-bearing Ligurian peridotites encompasses global-scale upper mantle variability

Pyroxenites embedded in peridotite are often invoked as a major cause of short-length scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence. We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy, with depleted mantle compositions, representing a surface exposure of veined upper mantle, a potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and adjacent mantle rocks results in centimeter-scale chemical modifi cations in the host perido- tites, systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host peridotite at >0.1 m scale to acquire an isotopic heterogeneity larger than the range defi ned by the peridotite and pyroxenite end-members. Moreover, the 143 Nd/ 144 Nd variation of a single outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites. Such pyroxenite-peridotite veined mantle domains may represent the enriched component rarely found in abyssal peridotites, but often invoked to account for the low end of 143 Nd/ 144 Nd