Silicon in iron meteorite metal

Abstract– We report Si concentrations in the metal phases of iron meteorites. Analyses were performed by secondary ion mass spectrometry using a CAMECA 1270 ion probe. The Si concentrations are low (0.09–0.46 μg g^?1), with no apparent difference in concentration between magmatic and nonmagmatic iron meteorites. Coexisting kamacite and Ni‐rich metal phases have similar Si contents. Thermodynamic calculations show that Fe,Ni‐metal in equilibrium with silicate melts at temperatures where metal crystallizes should contain approximately 100 times more Si than found in iron meteorites in this work. The missing Si may either occur as tiny silicate inclusions in metal or it may have diffused as Si‐metal into surrounding silicates at low temperatures. In both cases, extensive low‐temperature diffusion of Si in metal is required. It is therefore concluded that low Si in iron meteorites is a result of subsolidus reactions during slow cooling.     

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     

The Generation of Continental Crust: An Integrated Study of Crust-Forming Processes in the Archaean of Zimbabwe

The Archaean craton of Zimbabwe includes two major episodesof crust generation at 3.5 and 2.9 Ga recorded in the emplacementof tonalite-gneiss granitoids. A total of 180 samples of representativegneisses and massive tonalites and sills has been collectedfrom three areas in the southern part of the craton, at Mashaba,Chingezi, and Shabani. These rocks have been analysed for major,trace, and rare earth elements to evaluate the effects of thefractional crystallization and partial melting processes inthe generation of this segment of Archaean crust. Three groups are distinguished on the basis of their major andtrace element contents, and they follow two main trends of differentiation:the sodic and the calc-alkaline (sensu stricto) trends. GroupI samples are tonalitic in composition and follow a sodic trendcharacterized by decreasing CaO/Na₂O ratios. Y and Sr behaveas compatible elements and are negatively correlated with Rb.REE patterns are moderately fractionated with La/Ybn=4–23.5.The characteristics of this group have been described only inthe Archaean craton from Swaziland. Group II is an intermediateGroup with a marked decrease in Na₂O/K₂O with increasing differentiation,similar to the Archaean tonalite-trondhjemite-granodiorite suitesfrom Finland or the Pilbara Block, Australia. Samples displaybiotite tonalite and trondhjemite compositions, and Y, Sr, andRb are all incompatible. The REE patterns are strongly fractionated,with La/Ybn=23–44, and with small positive or negativeEu anomalies, as observed in other Archaean tonalite-trondhjemites.Group III is composed mainly of trondhjemites and granites similarto many post-Archaean granitoids: they follow a calc-alkalinetrend (sensu stricto) with decreasing CaO/Na₂O and Na₂O/K₂O.Sr and Y are incompatible, whereas Rb increases with differentiation.REE patterns are variably fractionated, with La/Ybn=6–36,with high REE contents, and marked negative Eu anomalies. The above geochemical features are explained in a three-stagepetrogenetic model. The first stage consists of 6–20%melting of upper-mantle peridotite and the generation of tholeiiticbasalts, as observed in the associated greenstone belts. Thesecond stage involves 4–25% partial melting of metamorphosedbasalts with a Gt amphibolite (15–45% Pl + 30–50%Hb+2–35% Cpx+3– 15% Gt) residue resulting in theGroup I samples, under water-unsaturated conditions at intermediatepressure (16 kbar), or with an eclogite residue to generatethe parental magmas for the Group II rocks. The third stageis lowpressure fractional crystallization (<8 kbar) of liquidsgenerated during this second stage, leaving a 19–20% Qtz+36–42%Pl0–2% HbMt cumulate for the more evolved Group II samples,and 55% fractional crystallization of a 14% Qtz+37.6% Pl (An₂₆)3.3%Bt+0.1% Ilm0.8% Mt cumulate for Group III samples. The highlyfractionated REE patterns of the Group II rocks are inheritedfrom the second stage of partial melting of the metamorphosedbasalt source rocks with an eclogite residue. Thus Group IIand III initial liquids were generated through partial meltingof eclogite and Gt amphibolite, respectively. The genetic relationshipsbetween Group I sodic and Group III calc-alkaline suites areevaluated, with the latter resulting from various stages offractional crystallization processes of parental magmas withinthe sodic suite.