ABSTRACT Mantle xenoliths rapidly brought to the surface by recent intraplate volcanism are a valuable source of information about the nature and evolution of the mantle. A common feature of many xenolith suites is the presence of... more
ABSTRACT Mantle xenoliths rapidly brought to the surface by recent intraplate volcanism are a valuable source of information about the nature and evolution of the mantle. A common feature of many xenolith suites is the presence of reactions, which have been variously and controversy interpreted both in terms of processes (partial melting, interaction with various melts, mineral breakdown) and in terms of timing/ locations (in the mantle, in crustal magma chambers, during the transport to the surface, or as post-entrainment reactions. In this contribution we review different disequilibrium textures observed in a xenolith suite from the Massif Central. The xenoliths are anhydrous or amphibole (and minor mica)-bearing spinel lherzolites, which have been variously cryptically or modally metasomatized by the percolation of melts prior to their entrainment in the host magma (Wagner and Deloule, 2007). Some "hydrous" xenoliths are totally surrounded by an amphibole selvage whereas other show cross cutting amphibole veins and disseminated amphibole crystals mostly around spinel. A variety of reaction textures will be examined: 1) Spongy/sieved textures developed in clinopyroxene. 2) Spongy/sieved textures developed on spinel rim in contact with glass. The rim may be compositionally zoned with an inner Al-enriched and an outer Cr-enriched rim. 3) Reaction zones developed around amphibole in contact with spinel contain secondary clinopyroxene, olivine, spinel and fresh colorless vesicle-rich glass; sometimes plagioclase is also observed. 4) Glass-rich patches with similar texture of clinopyroxene, olivine and spinel disseminated through anhydrous xenoliths. 5) Reaction zones around orthopyroxene with secondary clinopyroxene, olivine and glass. 6) Symplectites developed around clinopyroxene, amphibole or orthopyroxene. It should be noted that these reactions are not mutually exclusive and that glass is present in most reaction sites. The development of the reactions is independent of the proximity of the host magma contact. The reactions may be well-developed in some xenoliths and totally absent from other samples of the same occurrence. The aim of this contribution is to place constraints on the origin and the timing of formation of the reactions from detailed chemical investigations, thermometry, simulations, and fO2 estimates and taking into account recent experimental studies (e.g. Shaw, 2009; Shaw & Dingwell, 2008; Lo Cascio et al., 2008). In this contribution we evidence the involvement of fluids/melt in the development of the different reactions as well as the role of amphibole, even in some "anhydrous" xenoliths. Wagner and Deloule, 2007, Geochim. Cosmochim. Acta, 71, 4279-4296; Shaw, 2009, Lithos, 110, 215-228; Shaw & Dingwell, 2008, Contrib.Mineral. Petrol., 155, 199-214; Loscacio et al., 2008, Contrib.Mineral. Petrol., 156, 87-102.
Lithium isotopes can be effective tracers of fluid processes in the mantle. However, before interpreting the Li isotope signature, possible isotope fractionation between mantle minerals and zonation within grains have to be investigated.... more
Lithium isotopes can be effective tracers of fluid processes in the mantle. However, before interpreting the Li isotope signature, possible isotope fractionation between mantle minerals and zonation within grains have to be investigated. Li isotopic composition and concentration were determined by ion microprobe (Cameca IMS 3f) at the CRPG, Nancy, on olivine, orthopyroxene (opx), clinopyroxene (cpx) and amphibole (amp) from lherzolite xenoliths from the Massif Central (France). These samples are fertile spinel lherzolites representing various degrees of mantle metasomatism from unmetasomatised anhydrous (3 samples) to amphibole-rich (4 samples) lherzolites . d7Li values range from +9.5 to +14.5 % in olivine, from +5.9 to +11.3 % in opx and from +8.9 to +9.7 % in cpx from the anhydrous lherzolites. In the amphibole-bearing rocks d7Li values both overlap and extend beyond this range of values up to higher values in olivine (+16 %) and opx (+21 %), and negative d7Li value (- 8 %) in cpx. In all samples, except for two, olivine has the heaviest d7Li value, followed by opx and then by cpx to lighter value. d7Li and Li ppm show a negative correlation in olivine, d7Li varies at rather constant Li ppm in opx and amp while both d7Li and Li ppm are highly variable in cpx. Li isotope fractionation is constant and weak (about 1.5 %) between opx and olivine, but variable between cpx and other phases. In the metasomatised samples Li isotope fractionation between opx and cpx is inversely correlated to the temperature. The different phases from the anhydrous and the metasomatised samples show core to rim variations in d7Li. These variations are small in the anhydrous samples (mostly 1-2 %) but may reach 10-16 % in the metasomatised samples. Similarly, amp is isotopically zoned with high d7Li (+21.2 %) in the core of coarse grains to +3.2 % for small grains. Considering that amp d7Li reflects that of the percolating fluid, we assume it has a high positive d7Li signature. While migrating in the mantle this fluid interacts with the different phases, which lowers its isotopic composition. The d7Li of this evolving fluid is then recorded in the variable Li isotopic composition of the different minerals from the lherzolites.
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N isotopes in rocks can trace past biological activity, but it implies a deep knowledge of the N trapping sites and of the associated geo-biochemical N fractionation processes. The studied chert (PB 458) belongs to the 3.2 Ga Marble Bar... more
N isotopes in rocks can trace past biological activity, but it implies a deep knowledge of the N trapping sites and of the associated geo-biochemical N fractionation processes. The studied chert (PB 458) belongs to the 3.2 Ga Marble Bar complex, Pilbara, Western Australia. Isotopic analyses showed two N components with d15N of 6.7±1.6% and 10.0±1.6%, respectively. New mineralogical and REE analyses distinguished three microenvironments: 1) a silicified basalt having preserved its porphyric texture, with euhedral hydrothermal Fe-sulfides precipitated in the protomafic minerals; (2) Fe-Mn oxyhydroxide (BIFs) laminae and associated Ni-Cu-Mn-Au-Pd alloys and HREE-Y phosphates alternate with cryptocrystalline quartz, hosting magnetite inclusions. BIFs are initially composed of magnetite and carbonates, segregated from a Fe-Ca-Mg-enriched fluid, resulting from water-basalt interaction; (3) massive cryptocrystalline quartz with interstitial K-feldspar, Ba-K-mica and Fe-sulfides representing the residual fluids. K-Al-silicates incorporated 130 to 2540 ppm of N, probably NH4+ and 2990 to 6970 ppm of C from hydrothermal fluids. Oxidizing and slightly acid fluids later infiltrated the chert, and quartz veins were formed during diagenesis. Goethite and hydrous Mn-Fe minerals replaced carbonates, sulfides and magnetite, incorporating 500 to 4550 ppm of N and 3440 to 6000 ppm of C. N might occur as NH4+, replacing K+ in the Fe-Mn oxyhyroxide structure. The oxyhydroxide texture is vermicular and filamentous Their heterogeneous chemical composition is caused possibly by microbial activity. This is in agreement with measured N/C ratios of 0.06-0.6, similar to those known for marine bacteria. The d15N signatures of N are similar to those measured in marine sediments and derived from metabolic activity of bacteria (d15N=7.6%). The higher measured d15N values of 10% could be due to post-emplacement devolatilization of the sample or alternatively to different redox conditions during N fixation into sediments by organisms.
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Research Interests: Geology and Geological
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This work presents a procedure developed for trace element analysis using the electron microprobe (EMP). The method is demonstrated by analysis of seven glasses prepared from reference rock powders, with compositions of granite,... more
This work presents a procedure developed for trace element analysis using the electron microprobe (EMP). The method is demonstrated by analysis of seven glasses prepared from reference rock powders, with compositions of granite, granodiorite, andesite, diabase, and basalt. The melting process was adapted to prevent the loss of volatile elements and to obtain homogeneous samples. A routine procedure for analysis of major and minor (above 1000 ppm) elements yielded results in fairly good agreement with published values. The methods presented in this study produced detection limits as low as: (1) 6-8 ppm when the Ka peaks of transition metals of the first row (Cr and Ni) were used; (2) 23 ppm with soft La peaks (Y, Zr, and Sr); (3) 15 ppm with high-energy La peaks (rare earth elements); and (4) 35 ppm with the Ma peaks of heavy elements (Pb and Th). These limits were achieved after total counting times (peak 1 two background measurements) of about 15 min with a beam of 35 kV and 500 nA...
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K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID... more
K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO2 and <0.5 wt% Cr2O3. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO2, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al2O3) and contain 0.1–0.2 wt% TiO2, 0.9–1.1 wt% Na2O and 1.3–1.7 wt% Cr2O3. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr2O3 and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism.