Abstract A review of northern Victoria Land ultramafic xenoliths, collected and studied over more than 30 years, was carried out. More than 200 samples were gathered and characterized in a coherent and comparative manner, both for mantle-derived and cumulate xenoliths. Almost 2000 analyses of major elements and more than 300 analyses of trace elements of in situ and separated olivine, pyroxenes, amphibole, spinel and glass were taken into consideration. Particular attention was devoted to mantle lithologies in order to emphasize the composition and the evolution of this portion of the subcontinental lithosphere. The three main localities in northern Victoria Land where mantle xenoliths were found (i.e. Mount Melbourne (Baker Rocks), Greene Point and Handler Ridge), over a >200 km distance, were described and compared with ultramafic xenoliths in three other localities (Harrow Peaks, Browning Pass and Mount Overlord) that are mainly cumulate in nature. Altogether, these data enabled us to reconstruct a long evolutionary history, from old depletion to most recent refertilization and metasomatic events, for this large sector of the northern Victoria Land subcontinental lithospheric mantle.

ABSTRACT The petrology of anhydrous peridotite xenoliths hosted in the Cenozoic alkaline volcanic rocks from Handler Ridge (northern Victoria Land, Antarctica) provides new constraints on the characterization of the subcontinental lithospheric mantle beneath the West Antarctic Rift. For most samples, the temperature of equilibration was calculated on the basis of Fe/Mg partitioning among olivine, orthopyroxene, and spinel, at a pressure of 15 kbar. These results revealed a temperature of ~1030 °C and fO 2 ranging from –0.26 to +0.39 with respect to fayalite-magnetite-quartz buffer (ΔFMQ). Compared with other anhydrous and hydrated mantle xenolith suites occurring in northern and southern Victoria Land, these xenoliths represent the highest-temperature and most-oxidized conditions. On the basis of major-element modeling, we suggest that this portion of the mantle represents a residuum after 7%–18% partial melting. Geochemical and isotopic compositional evidence is indicative of significant metasomatism caused by an alkaline melt almost entirely overprinting the residual peridotite composition within a period of 10 2 –10 3 yr.

Based on phase geochemistry and Re-Os isotopic ratios, an exotic (in an oceanic setting) K-rich silicate melt, named kimberlite-type, has been claimed to be the metasomatizing agent interacting with subcontinental lithospheric mantle fragments beneath the Cape Verde Archipelago. On the basis of textural features and major- and trace-element chemistry, we constrain key geochemical indicators able to discriminate percolation at depth of this exotic melt from infiltration of the host magma in Cape Verde mantle xenoliths. Cape Verde type A lherzolites and harzburgites show evidence of dissolution of the primary phases (mainly pyroxenes) and the presence of large patches of secondary mineral (and glass) assemblages, and they do not show textural evidence of host basalt infiltration. Cape Verde type A mantle xenoliths frequently contain almost pure K-feldspar (An 3.8–8.8 , Ab 6–24 , Or 72–89 ) in the secondary mineral assemblage. They have an anomalously high K content (up to 0.49 wt%), and K/Na ratios generally >1, with respect to Cape Verde peridotites clearly affected by host basalt infiltration (type B samples). The dichotomy between Na and K observed in the two textural types suggests that the Na-alkaline host basalt (K/Na <1), which infiltrated at low pressure, was able to modify the whole-rock Na content of the xenoliths (type B samples). In turn, a completely different K-rich alkaline melt, which interacted at depth with the peridotite, imposed its alkali ratio (K/Na >1) on the bulk composition and formed the type A xenoliths. The kimberlite-type metasomatic agent, which reacted with the Cape Verde peridotite assemblage (mainly orthopyroxenes) in those regions where the mantle xenoliths are entrapped in the host basalt ( P = 17 kbar; T = 1092 °C), reasonably tends toward SiO 2 -saturated, K-rich basic magmas (lamproite-type?) with K-feldspars as the “liquidus” phase. Isotopic data on separate clinopyroxenes do not contribute to discrimination between metasomatism and infiltration processes but certainly concur to reinforce the hypothesis that a fragment of subcontinental lithospheric mantle domain was preserved during the opening of the Atlantic Ocean, forming K-rich undersaturated silicate melts that percolated through the peridotite matrix. Whole-rock major- and trace-element and isotopic geochemistry alone would not contribute to the interpretation of the processes occurring in the mantle xenolith. The most reliable tool would be an in situ mineral (and glass) study, which would be valid for Cape Verde mantle xenoliths and others. Small-melting-degree undersaturated silicate melts percolating at depth are olivine-saturated and may form, by reaction and dissolution of pyroxene, type A olivine without substantially modifying the original Fe/Mg peridotite ratio. By contrast, under low-pressure (<1.5 GPa), high-temperature regimes, olivine silicate melts infiltrating the mantle xenoliths form type B olivine, in which Fe/Mg ratios will be controlled by fractionation. Mantle diopsides interact (at depth) with undersaturated silicate melts, rearranging the most fusible elements into a new diopside composition: type A clinopyroxene. By contrast, diopsides that interact with melts at progressively lower pressure react and are locally rearranged in a new chemical structure that is able to accommodate the high diffusive elements (i.e., Fe and Ti): type B aegirine-augites. Fe 3+ in spinel is a key element in the investigation of the processes acting on Cape Verde mantle xenoliths. As a metasomatic product, secondary chromian spinel tends toward a Fe 3+ -buffered composition, mainly depending on pressure and chemistry of the magma. A decompression system is able to change the percolation regime from porous flow to open conduit. At this stage, the chromian spinel would be the low-pressure phase able to accommodate larger amounts of Fe 3+ . Type A glasses have exceptionally high K 2 O content, and, when associated with K-feldspars, they are buffered at ~9 K 2 O wt%, in a silica range of 55.7–66.8 wt%. By contrast, type B glasses follow a hypothetical major-element trend toward the host basanites. In conclusion, the compositional features (in particular major elements) of minerals and glasses in relation to their chemical behavior in mantle systems are the most efficient tools to distinguish metasomatism-related (type A) from infiltration-related (type B) samples and consequently to place the mantle xenoliths in a correct genetic framework.

Abstract Supplementary material: An extended dataset for calatrava xenoliths is available at: http://geolsoc.org.uk/sup18410 . Mantle xenoliths from the Calatrava Volcanic District (CLV), central Spain, are characterized by a wide compositional range that includes lherzolites (prevalent), as well as minor amounts of wehrlite, olivine (ol)-websterite and rare dunites. They generally have a bulk-rock Mg# of less than 89, lower than any primordial mantle estimates. Intra-suite variations in modal proportions are inconsistent with those predicted by melting models irrespective of the starting composition; mineral and bulk-rock variation diagrams show inconsistencies between the CLV compositions (anomalously enriched in Fe–Ti) and those predicted from the partial melting of primordial mantle material. Processes other than pure melt extraction are confirmed by the whole-rock REE (rare earth element) budget, typically characterized by LREE enrichments, with La N /Yb N (up to 6.7), probably related to pervasive metasomatism. CLV mantle clinopyroxenes (cpx) generally display fractionated REE patterns with upwards-convex shapes, characterized by low HREE (Tm–Lu) concentrations (typically <6× chondrite) and enrichments in middle–light REE (MREE–LREE) (Nd N /Yb N up to 7, La N /Yb N up to 5). These ‘enriched’ cpx compositions either result from re-equilibration of primary mantle cpx with an incoming melt, or represent cpx crystallization directly from the metasomatic agent. The latter was plausibly generated at greater depths in the presence of residual garnet (from peridotite or eclogite starting materials). Separated cpx have homogeneous 87 Sr/ 86 Sr compositions between 0.7031 and 0.7032; 143 Nd/ 144 Nd ranges from 0.51288 to 0.51295 (ɛNd 4.74–6.07) and 176 Hf/ 177 Hf is in the range 0.28302–0.28265 (ɛHf −3.6 to 9.0). Unlike mantle xenoliths and alpine-type peridotites from other Iberian occurrences, which range in composition from the depleted mantle (DM) to the enriched mantle (EM), the CLV mantle cpx approach the composition of the HIMU mantle end member, the genesis of which is generally interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalent) via ancient subduction. A model is proposed for the mantle evolution under central Iberia, where sublithospheric convective instabilities – possibly triggered by the neighbouring subduction along the Betic collisional belt – could have remobilized deep domains from the mantle ‘transition zone’ (410–660 km), which may include relicts of older subducted slabs. Within these remobilized domains, characterized by the coexistence of peridotite and eclogite and referred to as a ‘piclogite’ association, the eclogites melt preferentially generating Fe–Ti rich melts characterized by a HIMU isotopic signature that infiltrates and metasomatizes the shallower lithospheric mantle.

We investigated the petrogenetic characteristics of the Paleogene Veneto volcanic province and compared them with other intraplate magmatic occurrences of the Adria–North Africa plate since Late Cretaceous time. Veneto volcanic province magmas were erupted through a transtensional rift system that resulted from intra-plate reactions to the Alpine collisional events. The lavas, mostly basic in composition, encompass a wide range of serial affinities from (mela)-nephelinites to quartz-normative tholeiites. Nephelinites and basanites often carry spinel-peridotite mantle xenoliths that have rheologic and thermobarometric characteristics that indicate an origin from the mechanical boundary layer at depths not exceeding 50–60 km. Incompatible element patterns of the most primitive Veneto magmas, together with their isotopic signature ( 87 Sr/ 86 Sr 0.70315–0.70386; 143 Nd/ 144 Nd 0.51279–0.51298; 206 Pb/ 204 Pb 18.8–19.8), share geochemical characteristics with other magmatic occurrences of Adria–North Africa domains, and they show a clear affinity with intraplate sodic lavas, particularly HIMU (high U/Pb = high µ) and, to a lesser extent, enriched-mantle–ocean-island basalt (EM2-OIB) magmas. An integrated petrogenetic model, generally applicable for Adria–North Africa domains, suggests that most of the magmas were generated within the spinel-peridotite lithospheric mantle, from progressively deeper sources (30–100 km) and with a concomitant decrease in the degrees of partial melting (25%–3%) from quartz-normative tholeiites to nephelinites. The modeled magma sources invariably require enrichments in incompatible elements and metasomatic phases comparable (or equivalent) to those observed in some mantle xenoliths associated with the Veneto volcanic province lavas. Two kinds of mantle sources were identified: lherzolites bearing amphibole ± phlogopite for tholeiites to basanites, and lherzolites bearing amphibole ± phlogopite plus carbonatitic components for nephelinites. The elemental and isotopic characteristics of these mantle sources correspond to variable mixing of HIMU and, to a lesser extent, EM2 metasomatic components with a pristine depleted-mantle (DM) lithosphere. The HIMU metasomatizing agents may possibly be related to the mantle plume that is thought to extend from the eastern Atlantic to Europe and the Mediterranean, including Adria–North Africa domains, since the Late Cretaceous. These components more effectively accumulated in the lower lithospheric portion, i.e., the thermal boundary layer, whereas older metasomatic EM2 components may have been better preserved in the upper, more rigid, mechanical boundary layer.