Primary Igneous Graphite in Ultramafic Xenoliths: I Petrology of the Cumulate Suite in Alkali Basalt Near Tissemt (Egg?r?, Alegerian Sahara)

Ultramafic xenoliths (harzburgite, olivine-orthopyroxenite,orthopyroxenite, websterite and clinopyroxenite) in a Plio-Quaternarystrombolian cone near Tissemt (Egg?r?, Hoggar, Algerian Sahara)contain large (up to 1 mm in diameter) euhedral flakes of graphite.These xenoliths are associated with mafic granulites free ofgraphite. Petrological, mineralogical, and geochemical dataindicate that these rocks have been scavenged from a Precambrianlayered intrusion emplaced in the deep crust. Textural evidencesuggests that the graphite could have crystallized relativelyearly from a silica-saturated melt: following cumulus crystallizationof olivine and orthopyroxene, the graphite crystallized, togetherwith olivine, orthopyroxene, and spinel, as a component of theintercumulus assemblage. The crystallization of graphite directlyfrom the melt is related to relatively high pressure (c. 5 kb)of carbon-rich fluid (CO+CO₂+H₂O) at relatively low oxygen fugacity(–logf_o2, 10 at 1200 ?C).     

Corundum-bearing Garnet Clinopyroxenites at Beni Bousera (Morocco): Original Plagioclase-rich Gabbros Recrystallized at Depth within the Mantle?

The Beni Bousera ultramafic massif, Morocco, is composed ofperidotite with subordinate garnet pyroxenitc units which belongto two different families: (1) the Type I pyroxenites, whichare characterized by an Fe-enrichment trend; and (2) the TypeII pyroxenites, which are characterized by high but nearly constantMg/Fe ratios and highly variable concentrations of Ca and Al;the latter family includes corundum-bearing garnet pyroxeniteswhich resemble the peraluminous eclogites and grospydites describedas xenoliths in kimberlite diatremes. The Type II pyroxenites appear as layered sheets in the peridotite,and have granuloblastic metamorphic texture. They contain aprimary association of a coarse-grained assemblage (cpx + gt;cpx + gt + sp; cpx + gt + co), and a variety of secondary andtertiary associations includ ng clinopyrox-ene, orthopyroxene,olivine, spinel, corundum, sapphirine, plagioclase, and amphibole.The primary assemblage in the corundum-bearing pyroxenite ischaracterized by clinopyroxene rich in A1₂O₃ (up to 20 wt%),and poor in Na₂O (generally less than 2 wt.%). The clinopyroxenephase is therefore richer in the Ca-Ts molecule than in thejadeite molecule. On the other hand, the composition of theprimary and secondary clinopyroxene and garnet phases showsstrong variation across the pyroxenite sheets. These variationsexpress compositional variations of the rock system across thesheets. The cpx-gt associations indicate high temperatures (1200–1350?C) in the central parts of the sheets. The crystallizationpressure may have reached at least 20 kb in the corundum-bearingassemblages. The bulk-rock composition and the compatible element's behaviourin the Type II pyroxenite sheets suggest that the modal andcryptic layering mainly resulted from igneous fractionationprocesses. The REE patterns of corundum-bearing Type II pyroxeniteare characterized by low concentrations of HREE and by significantEu anomalies. These, together with the high bulk-rock Sr/Ndratios, suggest that plagioclase segregation may have playeda significant part in the rock genesis. These geochemical featuresare similar to those described, in the literature, in some low-pressure,plagioclase-bearing adcumulates (e.g., in the crustal sequenceof the Oman ophiolite). They are quite different from thoseobserved in the Type I pyroxenite sheets in the Beni Bouseramassif, whose geochemistry suggests that plagioclase playedno part in the fractionation process, whereas garnet probablyfractionated as an early igneous phase. The Type II pyroxenitesheets have a primary isotopic signature similar to MORB, basedon the composition of leached clinopyroxene. It is concluded that the Mg-rich Type II pyroxenite sheets resultedultimately from the fractionation of a basaltic melt at lowpressure, and from the accumulation of olivine, clinopyroxene,and plagioclase along dykes cross-cutting the surrounding peridotite.The close similarities with the geochemical features in theOman ophiolite lead us to suggest that these processes may havebeen operative in an oceanic crustal environment. The high-pressureand high-temperature crystallization of the ‘primary’cpx+gt + co assemblage was achieved deep in the mantle, aftersubduction and/or dragging down in convection currents of thisparticular piece of the (oceanic?) lithosphere. Further ascentmay have resulted in partial melting of peridotite and/or pyroxenite,and in the emplacement of the Type I pyroxenite sheets.     

Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin

Ultramafic (lherzolites, metasomatized peridotites, harzburgites,websterites and clinopyroxenites) and mafic igneous (basalts,dolerites, diorites and gabbros) rocks exposed at the sea-flooralong the West Iberia continental margin represent a rare opportunityto study the transition zone between continental and oceaniclithosphere. The igneous rocks are enriched in LREE, unlikeNorth Atlantic MORB. A correlation between their ¹⁴³Nd/¹⁴⁴Ndisotopic composition and Ce/Yb ratio suggests that they originatefrom mixing between partial melts of a depleted mantle sourcesimilar to DMM and of an enriched mantle source which may residewithin the continental lithosphere. Clinopyroxenes and amphibolesin the ultramafic rocks are LREE depleted and have flat HREEpatterns with concentrations higher than those of abyssal peridotites.Clinopyroxenes in the harzburgites are less LREE depleted buthave lower HREE concentrations. The clinopyroxenes in the GaliciaBank (GB) lherzolites have radiogenic Nd (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512937 to 0·513402) and unradiogenic Sr(⁸⁷Sr/⁸⁶Sr ranging from 0·702100 to 0·702311)isotopic ratios similar to, or higher than, DMM (Depleted MORBMantle) whereas the clinopyroxenes in the Iberia Abyssal Plainwebsterites have low-Nd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512283 to 0·512553) with high-Sr isotopicratios (⁸⁷Sr/⁸⁶Sr ranging from 0·704170 to 0·705919).Amphiboles in Galicia Bank lherzolites and diorites have Nd–Srisotopic compositions (¹⁴³Nd/¹⁴⁴Nd from 0·512804 to 0·512938and ⁸⁷Sr/⁸⁶Sr from 0·703243 to 0·703887) intermediatebetween those of the clinopyroxenes from the Galicia Bank andthe Iberia Abyssal Plain, but similar to the clinopyroxenesin the 5100 Hill harzburgite (¹⁴³Nd/¹⁴⁴Nd = 0·512865and ⁸⁷Sr/⁸⁶Sr = 0·703591) and to the igneous rocks (¹⁴³Nd/¹⁴⁴Ndranging from 0·512729 to 0·513121 and ⁸⁷Sr/⁸⁶Srranging from 0·702255 to 0·705109). The majorand trace element compositions of cpx in the Galicia Bank spinellherzolites provide evidence for large-scale refertilizationof the lithospheric upper mantle by MORB-like tholeiitic melts.The associated harzburgites did not undergo partial meltingduring the rifting stage, but, in earlier times, probably during,or even before, the Hercynian orogeny. Iberia Abyssal Plainwebsterites are interpreted as high-pressure cumulates formedin the mantle. Their high Sm/Nd ratios (from 0·43 to0·67) coupled with very low-Nd isotopic compositionsare best explained by a two-stage history: formation of thecumulates from the percolation of enriched melts long beforethe rifting, followed by low-degree partial melting of the pyroxenites,accounting for their LREE depletion. This last event probablyoccurs during the rifting episode, 122 Myr ago. The isotopicheterogeneities observed in the ultramafic rocks of the Iberiamargin were already present at the time of the rifting event.They reflect a long and complex history of depletion and enrichmentevents in an old part of the mantle, and provide strong argumentsfor a sub-continental origin of this part of the upper mantle. KEY WORDS: Iberia margin; mantle peridotites; igneous rocks; petrology; geochemistry     

Oxygen Thermobarometry of Orogenic Lherzolite Massifs

The oxidation state has been determined for spinel peridotitesfrom 13 orogenic lherzolite massifs including Beni Bousera,Ronda, and 11 smaller massifs in the French Pyrenees. The oxygenfugacity (f_o2) was calculated for 67 samples from microprobeanalyses using a set of secondary spinel standards to correctthe ferric iron content in the spinels. The utility of thismethod is confirmed by the good agreement between the calculatedvalues and those determined by Mossbauer spectroscopy on 28samples. The spinel peridotites of Ronda and Beni Bousera are relativelyreduced, averaging -11 and -1.5 log units relative to fayalite-magnetite-quartz(FMQ) respectively, which is in agreement with values from abyssalperidotites and mid-ocean ridge basalt (MORB) glasses. The Pyreneanmassifs are relatively oxidized and are intermediate between the abyssal peridotites and continentalxenolith suites. No systematic gradients are observable. Instead,variations of up to 2 log units in f_o2 occur at a localizedscale. This type of variation is also observed for trace elementsand radiogenic isotopes. The harzburgites at Beni Bousera recordthe most reduced conditions. Local oxidation coincides withthe appearance of amphibolc, indicating that metasomatizingfluids or melts are generally oxidized compared with the hostperidotites. Partial re-equilibration in the plagioclase peridotitefades has occurred at Ronda, causing the spinels to become Crrich. Re-equilibration is extremely heterogeneous. Mild oxidationappears to attend the crystallization of fine-grained plagioclase. The similarity in fo₂ values at Beni Bousera and Ronda indicatesa fairly uniform oxidation state at a scale of {small tilde}200 km. This scale of homogeneity is also observed in the Pyrenees,where no significant variation in f_o2 is apparent along 200km of strike in the Northern Pyrenean Zone.     

Primary Igneous Graphite in Ultramafic Xenoliths: II Isotopic Composition of the Carbonaceous Phases Present in Xenoliths and Host Lava at Tissemt (Egg?r?, Alegerian Sahara)

¹³C/¹²C ratios have been analysed in graphite adcumulate crystalsassociated with the olivine and orthopyroxene of a suite ofultramafic xenoliths in the alkali basalts of Tissemt (Egg?r?,Algerian Sahara). The ¹³C of these graphites varies from –24?6to – 14?4 per milk with internal variations of up to 1?6per mille within a given sample. Carbonates, linked to the volcanicphase and precipitated at medium temperatures have ¹³C valuesfrom –0?8 to + 3?5 and ¹⁸O values from 21?6 to 31?5 Thehost lava has a high carbon content of 675 p.p.m. with a global¹³C of –4?2. It can be separated into three parts: a majorpeak is obtained between 670 and 985 ?C and also liberated byslow acid attack with a ¹³C of up to +7?3 and representing CO₂trapped in the natrolite; a high temperature peak (above 1160?C with 47 p.p.m at –25?8) will represent carbon trappedin silicates and there is an additional carbon not well determinedextracted from 300 ?C to high temperature which represent 215p.p.m. at –23?0 with a possible fraction as low as –68?0.The graphite is interpreted as the result of the precipitationof carbon from an interstitial liquid previously outgassed thensupersaturated by the ongoing crystallization process. The volcanicevent can be considered to a first approximation as remobilizingthe carbon from the xenoliths by oxidation to CO₂, subsequentpartial reduction upon cooling and precipitation of carbonateand trapping of residual CO₂ within zeolite. Only volatilereducedspecies can be significantly outgassed so that the total ¹³Cof –4?2 is probably close to (or a little higher than)the original ¹³C.