The geochemistry of gem opals as evidence of their origin

Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si⁴⁺ by Al³⁺ and Fe³⁺. The induced charge imbalance is compensated chiefly by Ca²⁺, Mg²⁺, Mn²⁺, Ba²⁺, K⁺, and Na⁺. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence.     

Extensional tectonics in the Gorringe Bank rocks, Eastern Atlantic ocean: evidence of an oceanic ultra-slow mantellic accreting centre

The Gorringe Bank corresponds to an upper mantle peridotite ridge enclosing a 500-m thick/ 50-km long laccolith-like body of gabbro, locally cut and poorly covered by tholeiitic rocks. Strain and kinematic analysis of orientated gabbros and peridotites sampled during the GORRINGE diving cruise (1996) provides new kinematic constraints on extensional high-temperature deformation recorded at deep levels during stretching, near an accreting centre axis of a mantle-dominant oceanic lithosphere. It is argued that the Gorringe Bank lithosphere formed at an oceanic ultra-slow, N010°–020°-trending accreting centre, mostly by passive tectonic denudation of the mantle, without any synchronous large magmatism. This peculiar lithosphere may be representative of the Iberia oceanic domain located between the continent and the J anomaly ridge, which likely marks the beginning of true spreading at an oceanic spreading ridge.     

Continental Lithospheric Contribution to Alkaline Magmatism: Isotopic (Nd, Sr, Pb) and Geochemical (REE) Evidence from Serra de Monchique and Mount Ormonde Complexes

Isotopic results (Sr, Nd, Pb), as well as concentrations ofmajor and trace elements (REE) are reported for whole-rock samplesand mineral separates from the onland alkaline complex of Serrade Monchique (South Portugal) and the offshore alkali basaltvolcanic suite of Mount Ormonde (Gorringe Bank). These two geneticallyrelated alkaline complexes were emplaced at the east Atlanticcontinent–ocean boundary during the Upper Cretaceous,i.e. 66–72 m.y. ago. Taken together, Serra de Monchiqueand Mount Ormonde may be seen as one of the few examples ofwithin-plate magmatism that straddles the continent–oceanboundary. Major and trace element compositions fail to revealany significant differences between onland and offshore complexes.This is particularly true regarding less differentiated samples(mg-number 0.40) which show the same progressive and continuousenrichment of their trace element patterns, with no specificanomaly (e.g. negative Nb anomaly) being present in samplesfrom the onland complex. Initial Pb and Sr isotopic compositionsalso do not allow any distinction to be made between Serra deMonchique and Mount Ormonde samples. Initial Pb isotope ratiosare moderately high (19.1 < ²⁰⁶Pb/²⁰⁴Pb < 19.8; ²⁰⁷Pb/²⁰⁴Pb= 15.6) in both cases. Moreover, once the effects of Sr contaminationby seawater are taken into account and the most contaminatedsamples discarded using data from fresh clinopyroxene separatesand results of leaching experiments, the initial Sr isotopiccompositions of Mount Ormonde samples are found to be unradiogenic(⁸⁷Sr/⁸⁶Sr = 0.7031±1) and identical to those obtainedat Serra de Monchique (⁸⁷Sr/⁸⁶Sr = 0.7032±1). In contrast,a systematic mean difference of 2 _Nd units is observed betweenSerra de Monchique [_Nd(T) = +4.8] and Mount Ormonde [_Nd(T) =+6.6] whole-rock samples. Surprisingly, a variation is alsoobserved at Mount Ormonde between the whole-rock samples andone of the two analysed clinopyroxene separates. Whereas MountOrmonde whole-rock samples invariably yielded _Nd(T) = +6.6 (meanvalue), a value of +0.5 is obtained for one clinopyroxene separate,whereas another gives +6.0. The above geochemical and isotopicresults make it possible to assign respective roles to the asthenosphere,lithosphere and crust in the petrogenesis of Serra de Monchiqueand Mount Ormonde complexes. We propose that both complexesshare a common mantle source whose isotopic characteristicsare very similar to the source of oceanic island basalts. Continentalmantle lithosphere, already characterized isotopically by studiesof peridotite massifs within the Iberian peninsula, acts asa contaminant which is evident onland on the whole-rock scale,and also present offshore as discrete clinopyroxene xenocrysts.The continental crust appears to play no role in the petrogenesisof the Serra de Monchique alkaline rocks. KEY WORDS: alkaline complexes; continental lithosphere; isotope geochemistry; passive continental margin; within-plate volcanics     

Cretaceous seamounts along the continent-ocean transition of the Iberian margin: U-Pb ages and Pb-Sr-Hf isotopes

To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the “Tore Madeira” cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2σ) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for ²⁰⁶Pb/²⁰⁴Pb, 15.544-15.828 for ²⁰⁷Pb/²⁰⁴Pb, 38.750-39.936 for ²⁰⁸Pb/²⁰⁴Pb, 0.70231-0.70340 for ⁸⁷Sr/⁸⁶Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high ²⁰⁷Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30° anti-clockwise rotation of Iberia.     

First alkaline magmatism during Iberia–Newfoundland rifting

The age and origin of alkaline rocks emplaced into the sediments of the rift‐related continental Lusitanian basin were investigated to constrain earliest magmatic activity occurring prior to oceanic plate formation between Iberia and Newfoundland. The U–Pb titanite ages are 146.5 ± 1.6 (2σSTERR), 145.3 ± 1.4 and 142.3 ± 1.0 Ma, and initial Pb isotopic ratios of feldspars lie at 18.418–18.978 for ²⁰⁶Pb/²⁰⁴Pb, at 15.594–15.925 for ²⁰⁷Pb/²⁰⁴Pb and at 37.105–39.216 for ²⁰⁸Pb/²⁰⁴Pb. Initial ⁸⁷Sr/⁸⁶Sr ratios measured in the same feldspar fractions lie at 0.705409–0.706462. This episode of magmatic activity lasting for at least 4.2 ± 2.6 Myr most likely marks a phase of maximum lithospheric thinning during which zones of weakness were created to allow deep magmas to reach the surface. Such zones are preferentially re‐activated Palaeozoic faults of the Iberian plate. The isotope data suggest that the dominant volume of alkaline magmas was generated by partial melting of the metasomatized subcontinental Iberian mantle.     

40Ar/39Ar ages for the alkaline volcanism and the basement of Gorringe Bank,North Atlantic ocean

Gorringe Bank is situated on the Europe-Africa plate boundary at the eastern end of the Azores-Gibraltar fracture zone. It has two summits, Gettysburg Bank to the Southwest and Ormonde Bank to the northeast.We applied the⁴⁰Ar/³⁹Ar stepwise heating method to date six samples of the alkaline volcanic rocks, two gabbros from the Ormonde Bank and a dolerite from the Gettysburg Bank. The results that the alkaline volcanism lasted probably for less than 6 Ma(66-60 Ma).Although the nature of this volcanism precludes any subduction feature during its setting, the alkaline volcanism of Ormonde is probably linked to Upper Cretaceous/Eocene compressive tectonic events.The basement rocks of Gorringe Bank reveal distrubed⁴⁰Ar/³⁹Ar age spectra. One plagioclase and one biotite from a gabbro give evidence for a thermic event whose age is tentatively estimated at about 75 Ma, and related to a variation in the direction of the relative movement between Europe and Africa. The more probable age given by a plagioclase of another gabbro and by a dolerite (110 Ma) corresponds to tilting northeastward of the Gorringe massif.     

Basalts, underplated gabbros and pyroxenites record the rifting process of the West Iberian margin

Summary New petrological data on magmatic rocks obtained from the Iberia Abyssal Plain and from the Gorringe Bank, combined with those already known on the Galicia Bank, allow to better constrain the formation of the West Iberian Margin. These three zones were sampled along East-West transects of the ocean-continent transition, immediately West or at the foot of the last tilted continental block of the margin.These sections expose similar lithological successions including scarce basalts, locally pillowed, resting directly on peridotites (predominant) which themselves include layers of pyroxenites and rare lenses of gabbros and gabbroic differentiates. The latter are locally strongly sheared and metamorphosed e.g. to chlorite schists.At the Galicia Bank the exposed undepleted lherzolites are considered to be typical of a sub-continental lithospheric mantle environment. The mantle sections exposed at both the Iberia Abyssal Plain and the Gorringe Bank are clearly different. The presence of plagioclase-bearing websterite lenses and of small alkaline pyroxenites within the predominant harzburgites, is unknown in pure oceanic environments and supports their lithospheric sub-continental origin.Along each transect, the mineralogy of the gabbroic series documents various depths of emplacement and deformation. The highest pressures near 0.8 GPa ( 24 km depth) at Galicia Bank and Iberia Abyssal Plain are estimated from Al-rich pyroxenes and from the occurrence of metamorphic spinels in the sheared gabbros (Iberia Abyssal Plain). These gabbros are interpreted as deriving from underplated magmas at the base of a slightly thinned continental crust. The lower pressures ( 0.6 GPa) registered in the Gorringe Bank gabbroic rocks and in some reexamined gabbros from the Galicia Bank, suggest the existence of successive intrusions during the stretching process.The few basalts sampled on top of the exposed peridotites of the sea floor are inferred to be among the first post-rift extrusive magmas. They differ slightly from the North to the South of the margin: enriched and transitional tholeiites occur at Galicia Bank and in the Iberia Abyssal Plain, transitional and depleted tholeiites at the Gorringe Bank. The trace elements and the isotope data available show that the basalts and the gabbros derived from similar heterogeneous mantle sources (10.3 Nd_T 3.6). The behavior of some incompatible elements (Nb in particular) documents locally either a possible fractionation of rutile in shallow lithospheric mantle or some contamination by the continental crust. A very slow lithospheric stretching, assisted by the intrusion, underplating, and the shearing of a few gabbroic sills, followed by the unroofing of heterogeneous subcontinental mantle can account for the formation of the entire West Iberian margin.

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Résumé De nouveaux résultats obtenus à partlr de l'étude des roches magmatiques dans la plaine Abyssale Ibérique et sur le Bane de Gorrlnge permettent avec ceux obtenus sur le Bane de Galice de mieux contraindre la formation de la marge Ouest Ibérique. Ces trois zones ont été échantillonnées selon des coupes Est-Ouest de la transition océan-continent, à partir du pied on immédiatement à l'Ouest des derniers blocs continentaux basculés.La croûte de chacun de ces secteurs est composée de rares basaltes, localement en coussin, reposant directement sur des péridotites (prédominantes) qui elles-mêmes contiennent des niveaux de pyroxénites et de rares lentilles de gabbros localement différenciés. Ces derniers sont souvent fortement cisaillés et métamorphises et parfois transformés en schistes chloriteux.Les péridotites fertiles (lherzolites) du Bane de Gallce sont considérées comme représentatives d'un manteau lithosphérique sous-continental. Cette origine est étendue aux sections mantelliques à dominante harzburgitique de la plaine Abyssale Ibérique et du Bane de Gorringe. En effet elles exposent de rares pyroxénites alcalines et d'abondantes lentilles de webstérite à plagioclase, inconnues en domaine purement océanique.La minéralogie des séries gabbroïques indique des conditions variées de mise en place des magmas. Les pyroxènes riches en aluminium du Bane de Galice et de la Plaine Abyssale Ibérique et la présence locale de spinelle dans les gabbros cisaillés (Plaine Abyssale Ibérique) supposent une cristallisation sous des pressions proches de 0.8 GPa ( profondeur de 24 km). Ces gabbros ne peuvent done pas avoir cristallisé sous une croûte océanique normale et sont interprétés comme provenant de magmas sous-plaqués à la base d'une croûte continentale faiblement amincie. Quelques gabbros du Bane de Gallce et ceux du Bane de Gorringe ont enregistré des conditions de cristallisation plus superficielles (à molns de 18 km), donc au sein de péridotites situées sous une croûte continentale plus mince ou inexistante. Dans un processus d'étirement crustal la mlse en place de ces gabbros apparaît ainsi échelonnée avec des venues précoces dans le cas de la Plaine Abyssale Ibérique et plus tardives dans le cas du Banc de Gorringe.Les basaltes reposant directement sur les péridotites du fond océanique correspondent aux premiers magmas extrusifs post-rift. Ils different sensiblement du Nord au Sud de la marge: depuis des tholeiites enrichies et transitionnelles exposees sur le Banc de Gallce et dans la Plaine Abyssale Ibérique, jusqu' à des tholeiites transitionnelles et appauvries sur le Banc de Gorringe. Les éléments traces et les données isotopiques disponibles montrent que les basaltes et les gabbros dérivent de sources mantelliques similaires et hétérogènes (10.3 Nd_T 3.6). Le comportement de quelques éléments incompatibles (Nb en particulier) témoigne localement d'un possible fractionnement de rutile à faible profondeur dans le manteau lithosphérique on d'une contamination par de la croûte continentale. Un amincissement lithosphérique très lent, partiellement aidé par l'injection, le sous-plaquage et le cisaillement de quelques sills de gabbro et suivi de la dénudation d'un manteau sous-continental hétérogène, rend compte de la formation de l'ensemble de la marge Ouest Ibérique.

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With 11 Figures