Behavior of isotopic systematics during deformation and metamorphism: a Hf,Nd and Sr isotopic study of mylonitized granite

A combined compositional and Hf, Nd and Sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the Harquahala Mountains, western Arizona, to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks. The 1.4 Ga Harquahala Granite was deformed during Mesozoic thrusting along the Harquahala thrust. Granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite. In most of these mylonites, the protolith is not megascopically recognizable, and can be discerned only by the progressive transition to undeformed granite. Isotopic analyses of Hf, Nd and Sr from the shear zone document the immobility of the Hf and Nd isotopic systems relative to that of the Sr isotopic system during deformation. The Rb–Sr isotopic data show considerable scatter on an isochron plot, exhibiting both gains and losses of Rb and Sr from the whole-rock systems. In contrast, the Sm–Nd and Lu–Hf isotopic systematics are mostly well behaved on isochron diagrams, plotting mostly in tight clusters or along 1.4 Ga isochrons. These results show that while the Sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance, both Hf and Nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism.     

The eastern boundary of the Baikal collisional belt: Geological,geochronological, and Nd isotopic evidence

New geological. geochronological, and Nd isotopic data are reported for the rocks occurring at the interfluve of the Barguzin, Nomama, and Katera rivers, where the main structural elements of the Early Paleozoic collisional system have been established. The strike-slip and thrust Tompuda-Nomama and Barguzin boundary sutures separate the Svetlaya and the Katera zones of the Baikal-Muya Belt from the Barguzin terrigenous-carbonate terrane. The age estimates of syntectonic (prebatholithic) gneissic granite and gabbrodiorite intrusive bodies (469 ± 4 and 468 ± 8 Ma, respectively) coincide with the age of collisional events in the Ol’khon, Southwest Baikal, and Sayan regions (480–470 Ma). A linear zone with zonal metamorphism and granite-gneiss domes dated at 470 Ma is revealed in the allochthonous fold-nappe packet of the Upper Riphean Barguzin Formation. This zone of Caledonian remobilization marks the collisional front between the Riphean structural units of the Barguzin Terrane consolidated 0.60–0.55 Ga ago and the Baikal-Muya Belt. New data allow us to recognize this zone as the northeastern flank of the Baikal Collisional Belt. The Nd isotopic data for the reference igneous complexes of the collisional zone indicate that the Late Riphean juvenile crust was involved in the Ordovician remobilization in the zone of conjugation of the consolidated Baikalian structural elements at the northeastern flank of the Baikal Belt and likely was a basement of the entire Barguzin Terrane or, at least, its frontal portion. The lateral displacements of the terranes to the northeast during the Early Ordovician collision were constrained by the rigid structural framework of the Baikalides in the Muya segment of the Baikal-Muya Belt, where the Riphean blocks were involved in strike-slip faulting and the Vendian-Cambrian superimposed basin underwent deformation. Finally, it may be concluded that the Early Ordovician was an epoch of collision, complex in kinematics, between heterogeneous blocks of the continental crust: the Baikalides of the Baikal-Muya Belt and polycyclic Barguzin-Vitim Superterrane.     

A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle

The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al₂O₃/TiO₂ and CaO/Al₂O₃ ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb)_N ratios. In contrast, the Al-depleted komatiites show significantly lower Al₂O₃/TiO₂ ratios of 8–12, highly variable CaO/Al₂O₃ (0.19–2.81) ratios combined with (Gd/Yb)_N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. ¹⁴⁷Sm/¹⁴⁴Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite _i,Nd(3.45) values cluster at +1.9±0.7.

Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites.

The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al₂O₃/TiO₂, (Gd/Yb)_N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb)_PMN, (Sm/Yb)_PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites.     

Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico

Sm-Nd ages for garnets in the three Precambrian exposures of eastern and southern Mexico demonstrate that they belong to the Grenville tectonothermal event. The Sm-Nd garnet ages of 0.95 Ga for the Oaxacan Complex and 0.90 Ga for the Huiznopala Gneiss, Molango and the Novillo Gneiss, Ciudad Victoria, are postdated 75 Ma by Rb-Sr ages on biotites. Both sets of data document a cooling history following Grenville metamorphism at or before 1.0 Ga ago. Our garnet data are consistent with a blocking temperature for Sm-Nd in that mineral around 600° C suggested by Humphries and Cliff (1982).The three Precambrian occurrences have Nd chemical ages of separation from depleted mantle (T_DM) grouped in the range 1.40–1.60 Ga. This may result from derivation of the rocks from actual crustal protoliths which had been separated from the mantle 0.5 Ga before the Grenville Orogeny. It is much more likely, however, that crustal materials of 1.7 Ga or older age were mixed with mantle-derived products during Grenville events to produce intermediate T_DM ages and _Nd values around zero 1.0 Ga ago.     

Two-stage contamination during crustal assimilation: isotopic evidence from volcanic rocks in eastern Nevada

Thousands of cubic kilometers of dominantly intermediate composition, metaluminous magma erupted at approximately 35 Ma in eastern Nevada. Two stages of crustal contamination are inferred from detailed study of the earliest vocanic rocks. Fine-grained mafic rocks and rocks with mixed textures, ranging from basalt to rhyolite, were contaminated with a crustal component rich in Nd but poor in Sr. Overlying plagioclaserich andesites and dacites have greater Sr and ⁸⁷Sr/⁸⁶Sr, but less Nd and lower _Nd and are interpreted to have been contaminated by a crustal component with the opposite elemental signature (i.e., poor in Nd but rich in Sr). The first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event, or bulk crust. The net effect is bulk crustal assimilation, but in two bites. The separation of the crustal souree into two elementally distinct contaminants causes divergent trends with respect to the variation of ⁸⁷Sr/ ⁸⁶Sr versus _Nd that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs. Comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contammants representing melt and residue at about 30% melting. The model age of the bulk crust is approximately 2.2 Ga, consistent with an early Proterozoic crustal province inferred by other workers.     

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center,eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have _Nd values of -6.2 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the _Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the ⁸⁷Sr/⁸⁶Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO₂ contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.     

Petrogenesis and tectonics of late Permian felsic volcanic rocks,eastern Qiangtang block,north-central Tibet: Sr and Nd isotopic evidence

The Palaeo-Tethyan tectonic evolution of central Tibet remains a topic of controversy. Two Permian to Late Triassic arc-like volcanic suites have been identified in the eastern Qiangtang (EQ) block of north-central Tibet. Three competing models have been proposed to explain the formation of these volcanic suites, with two models involving a single stage of long-lived subduction but with opposing subduction polarities, while the other model involves a two-stage subduction process. Here, we present new whole-rock geochemistry, including Sr–Nd isotope data, for late Permian felsic volcanics of the Zaduo area. These volcanics are mainly low to middle K calc-alkaline felsic tuffs and rhyolites with SiO₂ concentrations up to 73 wt.%. In primitive mantle-normalized diagrams, the volcanics are typified by large ion lithophile element enrichment and high-field-strength element (e.g. Nb, Ta, P, and Ti) depletion, with slightly negative Eu anomalies. They have initial Sr ratios (⁸⁷Sr/⁸⁶Sr) _i of 0.70319–0.70547, and ?Nd(t) values of +3.4 to +3.5, suggesting derivation by the partial melting of a depleted mantle wedge, followed by assimilation of crustal material. The available geochemical data indicate the presence of two distinctive igneous evolution trends within the Permian to Late Triassic volcanics of the EQ block, consistent with a two-stage subduction model. Permian to Early Triassic arc-like volcanics are formed during northward (present-day orientation) subduction, whereas the Late Triassic volcanics are related to southward (present-day orientation) subduction of mafic crust of the Garze–Litang Ocean.     

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

 Single zircon U–Pb dating combined with ²⁰⁷Pb/²⁰⁶Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial e_Nd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K₂O/Na₂O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc. Received: 29 March 1999 / Accepted: 25 August 1999     

Chemical and isotopic (C, O, Sr, Nd) characteristics of the Xiluvo carbonatite (central-western Mozambique)

Summary The Xiluvo complex of central Mozambique is made up of coarse- to fine-grained calciocarbonatites (sövites and alvikites), heavily altered lamprophyres and syenitic rocks that intruded the Precambrian basement ca. 120Ma ago. The carbonatites have fractionated rare earth element patterns (chondrite-normalized La/Yb=30–80) and markedly negative Rb, K, P, Zr and Ti anomalies in mantle-normalized incompatible element diagrams. The ¹⁸O (+7 to +8), ¹³C (–5), and the age-corrected ⁸⁷Sr/⁸⁶Sr (0.7032–0.7033) and ¹⁴³Nd/¹⁴⁴Nd (0.51263–0.51262) indicate an origin in the mantle. A few carbonatitic samples have higher ¹⁸O (+13), indicating interaction with high-¹⁸O crust or late-stage fluids. The chemical and isotopic compositions of the Xiluvo carbonatites and nearby carbonatites of similar age in Malawi indicate very similar sources, characterized by time-integrated depletion of Rb with respect to Sr and of Sm with respect to Nd. These characteristics point to a source similar in many respects to those of other East African carbonatites and to those of some ocean island basalts, with the additional influx of components possibly located in the African lithospheric mantle.     

Relationships between the chemical and isotopic (Sr,Nd, Hf,and Pb) heterogeneity of the mantle

The reasons for the isotopic heterogeneity of the mantle are analyzed in this paper on the basis of published isotopic data. It was shown that the observed variations in the Sr, Nd, Hf, and Pb isotopic compositions of oceanic basalts cannot be explained by mixing of a finite number of homogeneous reservoirs (components). The considerable variations in the contents of Rb, Sr, Sm, Nd, Lu, Hf, U, Th, and Pb and ratios of these and other trace elements in tholeiitic basalts indicate that the chemical heterogeneity of mantle-derived rocks is inherited in part from their sources. Oceanic tholeiitic basalts show a tight correlation between the variances of Nd, Hf, Sr, and Pb isotopic ratios and the variances of respective radiogenic additions that could be accumulated in these rocks over a time period of 〈t〉 = 1.8 Gyr. This paradox clearly indicates that variations in all the mentioned isotopic systems in the mantle cannot be understood without the analysis of the geochemical heterogeneity of rocks.The close to lognormal distributions of lithophile trace elements in oceanic tholeiitic basalts and the character of correlations between them suggest that magmatic differentiation was the major mechanism of the formation of chemical heterogeneity in the mantle. The role of metasomatism in the global transport of trace elements and formation of the geochemically heterogeneous mantle is probably rather limited. Intrusive processes within the mantle could result in the development of chemical and, after a period of time, isotopic anomalies in the mantle. Simple calculations show that long-lived geochemical anomalies related to alkaline magmatism could be responsible for EM-I type isotopic anomalies, and geochemical anomalies produced in the mantle by enriched tholeiitic melts could be sources of EM-II type isotopic anomalies. The analysis of the distribution of the isotopic compositions of mantle-derived igneous rocks in various “isochron” coordinates suggested that the formation of geochemical anomalies in the mantle is a long-term process lasting for hundreds of millions of years. Nonetheless, trends approaching 4.5 Ga were never observed in such diagrams, i.e., the mantle is in general rejuvenated in all isotopic systems. Both on global and local scales, there are no mantle domains that have remained geochemically closed and isolated since the Earth’s formation. The entire mantle is involved in material exchange processes.The development of isotopic systems in the mantle was explored by means of statistical modeling accounting for the tendency of a continuous increase in the chemical heterogeneity of the mantle source and the tendency of obliteration of the isotopic heterogeneity owing to the convective mixing in the mantle. The modeling demonstrated that the character of the isotopic heterogeneity of the mantle is statistically consistent with the character of its chemical heterogeneity. The mantle isotopic anomalies HIMU, EM-I, and EM-II were generated by two simultaneous processes: the magmatic differentiation of mantle material and its not very efficient mixing.     

Fluid evolution and transport during metamorphism: evidence from the Llano Uplift,Texas

A field, petrologic and stable isotopic investigation of the marbles and calc-silicates of the 1.15 b.y. Valley Spring Gneiss documents the dilution of internally evolved CO₂-rich fluids by externally derived aqueous fluids introduced along channelways. Reaction textures within calcsilicates record the evolution through time of initially CO₂-rich fluids toward increasingly more aqueous compositions. Assemblage zonations within calc-silicates require equilibration within local gradients of the mole fraction of CO₂ in the fluid, and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies. Localized depletions in ¹³C and ¹⁸O corroborate petrologic evidence for channelized infiltration. Isotopic compositions reflect both devolatilization and the introduction of low- ¹⁸O fluids; estimated minimum oxygen-equivalent fluid-to-rock ratios are near unity. Both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc-silicate formation. The calc-silicate assemblages which equilibrated with fluids of the lowest mole fraction of CO₂ record isotopic exchange equilibrium with fluids of ¹⁸O typical of those derived from normal granites, as do the granitic aplites and pegmatites which transect most calcsilicate occurrences. Thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism.     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.     

Origin and evolution of 2 Ga old carbonatite complex (Ihouhaouene,Ahaggar, Algeria): Nd and Sr isotopic evidence

Associated syenitic rocks and carbonatites from Ihouhaouene, Algeria, have been investigated for their Sr and Nd isotope and trace element geochemistry. A zircon U-Pb emplacement age (1994 +22 -17 Ma) has been obtained from the carbonatites. The REE characteristics, among which the significant and approximately constant negative Eu anomaly and the evolution of (La/Ce)N and (Yb/Lu)N ratios which increase sympathetically with total REE abundances, are considered to be of purely magmatic origin. They are used to constrain the genetic links between syenites and carbonatites. Sr and Nd isotopes suggest a similar source for carbonatites and syenites, which is enriched compared to a chondritic reference: Nd(T)=-6.4 to -8.6 and ISr(T)=0.7097. These features are interpreted as evidence of contamination of a mantle-derived magmatic precursor by continental crust, occurring in a magmatic chamber.     

Strontium isotopic equilibration during metamorphism of tillites from the Ogcheon Belt,South Korea

The NE/SW trending Ogcheon Belt formed during the Mesozoic by deformation and metamorphism of a sedimentary sequence which includes tillite formations of late Precambrian or Lower Palaeozoic age. In this study Rb-Sr isotopic data are reported from cm-scale sampling of clasts and matrix in these tillites, which underwent metamorphism at grades ranging from the biotite zone to the kyanite zone. Sr-isotopic disequilibrium between clasts and matrix persists up to the highest grade but in four out of five rocks the matrix samples approximate to isochrons (MSWD's<12). The mechanism for this selective isotopic equilibration is considered to be related to the metamorphic reactions in the matrix, which were probably accompanied by a substantial fluid flow. The matrix isochrons for three of the rocks, together with separated white micas from associated schists, suggest that metamorphism took place in the Trias, approximately 200 Myr ago. Metamorphism was followed by slow cooling and biotite closure temperatures were not reached until 190-150 Myr. — These results suggest that where high fluid/rock ratios can be demonstrated on independent petrological grounds, the analysis of small-scale whole-rock samples may allow effective Rb/Sr dating of metamorphism.     

Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements,REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)_N ∼ 2–3 times chondrite, (Gd/Yb)_N ∼ 1). The ε^Nd(t) values vary from +1.23 to -3.27 whereas δ¹⁸O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable ε^Nd, low Sr_i(0.702) and low δ¹⁸O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.     

The process of glauconitization: chemical and isotopic evidence

Sequential leaching experiments were made on Recent glauconies and clay fractions of the associated mud from off-shore Africa near the estuary of the Congo River. Analyses of major/rare earth elements (REE) and Nd isotopic compositions on the resulting leachate and residue pairs allow identification of at least three important and isotopically distinct components which contributed to the glauconitization process: (1) a detrital component with relatively high ⁸⁷Sr/⁸⁶Sr and relatively low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios; (2) a phosphate phase rich in REE and Sr with sea water Sr and Nd isotopic characteristics; (3) a component rich in organic matter and Ca with a sea water Sr isotopic signature, a relatively low Nd isotopic composition and elevated Sm/Nd ratios. This latter component probably represents the suspended organic and carbonate-rich river load. The detrital and the river components were mixed up in the muddy off-shore sediment, ingested by worms, and integrated into faecal pellets. The resulting material has Sr and Nd isotopic signatures intermediate between those of the detrital and river components, and represents the precursor of the glaucony minerals. During the subsequent dissolution-crystallization process, the glauconitic pellets remain isotopically closed to any external supply, but expulsion of Sr and Nd with increasing degree of maturation is observed without any effect on the Sr and Nd isotopic compositions. At a higher maturation stage (K₂O>4.5%), the Sr and Nd isotopic compositions tend to decrease and increase, respectively, approximating the isotopic composition values of the phosphate-rich phase. Because the Sr and Nd concentrations decrease, the evolution of the glauconies toward lower Sr and higher Nd isotopic compositions can only be explained by expulsion of Sr and Nd of the detrital component with high Sr and low Nd isotopic signatures. Dissolution of the chemically unstable, wormdigested clay material from mud may be responsible for the liberation of these elements. Consequently, the phosphate-rich phase with sea water Sr and Nd isotopic signatures becomes increasingly important for the isotopic characteristics of the maturing glauconite grains, and sea water isotopic signatures can be reached during the stage of mature glauconite (K₂O>6.5%), without chemical exchange with the depositional environment.     

The role of the grain boundary at chemical and isotopic fronts in marble during contact metamorphism

Carbon and oxygen isotopic profiles around a low pressure metasomatic wollastonite reaction front in a marble of the Hida metamorphic terrain, central Japan, display typical metamorphic fluid-enhanced isotopic zonations. Isotopic profiles obtained from detailed microscale analyses perpendicular to the chemical reaction front in calcite marble show that diffusion-enhanced isotopic exchange may control these profiles. Carbon and oxygen isotopic behaviour in grain boundaries is remarkably different. Oxygen isotopic troughs (¹⁸O depleted rims) around the calcite-grain boundaries are widely observed in this contact aureole, demonstrating that diffusion of oxygen in calcite grain boundary dominates over lattice diffusion in calcite. In contrast, no difference is observed in carbon isotopic profiles obtained from grain cores and rims. There is thus no specific role of the grain boundary for diffusion of carbonic species in the metamorphic fluid during transportation. Carbon chemical species such as CO₂ and CO₃ ions in metamorphic fluid migrate mainly through lattice diffusion. The carbon and oxygen isotope profiles may be modelled by diffusion into a semi-infinite medium. Empirically lattice diffusion of oxygen isotopes is almost six times faster than that of carbon isotopes, and oxygen grain-boundary diffusion is ten times faster than oxygen lattice diffusion. Oxygen isotopic results around the wollastonite vein indicate that migration of the metamorphic fluid into calcite marble was small and was parallel to the aquifer. From the stability of wollastonite and the attainment of oxygen isotopic equilibrium, we suggest that diffusion of oxygen occurred through an aqueous fluid phase. The timescale of formation of the oxygen isotopic profile around the wollastonite vein is calculated to be about 0.76 × 10⁶ years using the experimentally determined diffusion constant. Received: 14 January 1997 / Accepted: 23 April 1998     

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)

The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmallí area, Baja California, México. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaíno region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.