Unfractionated Rare Earth Elements Research Articles

The Gharuchah-Sofla intrusion (northwestern Sanandaj-Sirjan zone, Iran): Age, petrology, geochemistry and tectonomagmatic implications

The Cretaceous Gharuchah-Sofla intrusion is a composite batholith consisting of a variety of mafic to felsic rocks in the northwestern Sanandaj-Sirjan zone, Iran. Both its age and the relationship between various rock types in the intrusion are unknown. To address these issues, we conducted a detailed age, petrologic, and geochemical study of the intrusion, their country rocks, and mafic microgranular enclaves (MMEs) hosted therein. Plutonic rocks from the main mass of the intrusion, dated at ~110 Ma by in-situ zircon UPb method, are dominated by gabbroic, dioritic, and quartz dioritic with subordinate granodioritic rocks. Magmatic rocks occurring mostly along margins of the intrusion have Late Ediacaran zircon UPb ages (~560 Ma). Both the Early Cretaceous and the Ediacaran magmatic rocks are sub-alkaline; the former has I-type affinity whereas the latter has S-type affinity with a tendency towards I-type. Both sets of rocks are characterized by continental crust-like trace element patterns showing negative anomalies in Nb, Ta, Sr, P, Eu, and Ti of variable intensity. Early Cretaceous gabbroic rocks exhibit unfractionated rare-earth element patterns (La/Yb = 1.9–3.0) and negative ZrHf anomalies. In contrast, other rocks of the Early Cretaceous and Ediacaran magmatic rocks show fractionated rare-earth element patterns (La/Yb = 11.0–16.8) along with the lack of such anomalies. MMEs, which occur within Early Cretaceous plutonic rocks, have major and trace element compositions similar to the gabbroic rocks. Contrasting Nd isotopic ranges are present between the Early Cretaceous plutonic rocks and the MMEs (ɛNd = +2.0 to +4.1) and the Ediacaran magmatic rocks (ɛNd = −5.8 to −0.4). Based on the geochemical and isotopic data obtained, we suggest that (i) the Ediacaran magmatic rocks formed by melting of Cadomian basement rocks below the blocking temperature of UPb exchange between zircon and melt, (ii) the MMEs represent frozen melts (i.e., calc-alkaline basalts) that formed the gabbroic rocks in the Early Cretaceous instruction, and (iii) fractional crystallization with slightly concomitant assimilation of Cadomian basement rocks was a key process forming intermediate and silicic rocks of the intrusion. The ~110 Ma Gharuchah-Sofla intrusion was likely associated with a continental margin arc related to the subduction of the Neotethys, providing solid evidence that this segment of the arc was built directly on the Cadomian basement.

Geochemical, Sr-Nd isotopic and U-Pb zircon study of 1.88 Ga gabbro-wehrlite from north-eastern Singhbhum Craton, India: Vestiges of Precambrian oceanic crust?

Dismembered, kilometer-scale slices of gabbroid and serpentinized wehrlite rocks are found emplaced within metapelites of Chaibasa Formation, belonging to the North Singhbhum Mobile Belt, Singhbhum Craton, India. Zircon grains from the gabbroid body yield an average U-Pb crystallization age of 1881.1 ± 8.4 Ma. The undeformed gabbroid body exhibits modal and cryptic layering, and both units exhibit cumulate texture. The presence of dismembered cumulate and layered mafic–ultramafic rocks within a mobile belt assemblage is enigmatic and has potentially far-reaching implications. The gabbroid rocks are moderately evolved with Mg# (100 Mg/Mg + Fe) ranging from 48 to 63, low alkali, TiO2, U, Th, Zr, and high Al2O3, Pb, Ba, Rb, with unfractionated rare earth elements patterns. The serpentinized wehrlite (Mg# 65–71) hosts secondary minerals (clinopyroxene rims, amphibole blebs and interstitial Mg-chlorite) and has LREE enriched rare earth element patterns, which suggest complex fluid-rock interaction history. The gabbroid rocks have 87Sr/86Sr(t) 0.70600–0.71099, 143Nd/144Nd(t) 0.510130–0.510294, and ƐNd(t) 1.8 to −1.4. The serpentinized wehrlite displays extreme enrichment with 87Sr/86Sr(t) 0.71045–0.72640, 143Nd/144Nd(t) 0.509120–0.509706, and almost upper crust-like ƐNd(t) of −9.7 to −21.2. The cumulate nature of the mafic–ultramafic rocks with other petrographical and geochemical evidences suggest formation in a magma chamber. Additionally, their field characteristics, stratigraphic constraints and E-MORB like character of calculated equilibrium melts of the gabbroid rocks, indicates affinity towards formation in an oceanic magma chamber and possible emplacement by obduction. Mineral chemistry, whole-rock geochemistry and clinopyroxene trace elements of the serpentinized wehrlite reveal progressive fluid-rock interaction from very high-temperature to low-temperature regimes, and deep crustal affinity of the modeled metasomatic fluid. Isotopic character of the gabbroid rocks can be explained by 30%-50% mixing between a depleted and an enriched domain in the lithospheric mantle, whereas isotopic nature of the wehrlite has been perturbed by the prolonged metasomatic event. Their occurrence within the Chaibasa intra-continental rift basin indicates rift-drift transition and formation of incipient oceanic crust at 1.88 Ga. A 1.86 Ga orogenic event caused closure of the basin and obduction of these remnant oceanic fragments onto the mobile belt assemblage. This is the first discovery of oceanic crustal fragments in Singhbhum Craton, which fits into the model of rift-dominated tectonics, widespread continental amalgamation, and ophiolite emplacement during the Columbia supercontinent cycle in Paleoproterozoic.

Irradiation origin of 10Be in the solar nebula: Evidence from Li-Be-B and Al-Mg isotope systematics, and REE abundances of CAIs from Yamato-81020 CO3.05 chondrite

We have performed in situ analyses of Li-Be-B and Al-Mg isotope systematics, and abundances of rare earth elements (REEs) in two Ca-Al-rich inclusions (CAIs) from the Ornans-like carbonaceous chondrite Yamato-81020 (CO3.05). The present CO CAIs are depleted in ultra-refractory heavy REEs (group II REE pattern), suggesting condensation of these CAIs or their precursors from the solar nebula. Initial 26Al/27Al ratios, (26Al/27Al)0, of these CO CAIs are found to be (4.8 ± 0.5) × 10–5 and (4.9 ± 0.3) × 10–5 (95% confidence), indicating their contemporaneous formation with a majority of CAIs from Vigarano-like carbonaceous (CV) chondrites. Melilite grains in the present CO CAIs show clear excesses in 10B, ranging from ∼370 to ∼4300‰ relative to the chondritic B isotopic composition, which are correlated well with 9Be/11B ratios. The correlation indicates in situ decay of 10Be in the present CO CAIs and yields initial 10Be/9Be ratios, (10Be/9Be)0, for the individual CAIs of (2.9 ± 0.6) × 10–3 and (2.2 ± 1.0) × 10–3 (95% confidence), which are significantly greater than the average (10Be/9Be)0 = ∼0.7 × 10–3 recorded in CAIs from CV chondrites. The apparent variation in (10Be/9Be)0 between the CO and CV CAIs, despite having indistinguishable (26Al/27Al)0 of ∼5 × 10–5, provides evidence for heterogeneous distribution of 10Be in the CAI forming-regions at the very beginning of the Solar System. The elevated (10Be/9Be)0 and group II REE patterns in the CO CAIs may reflect that compared with the CV CAIs having unfractionated REEs the present CO CAIs have formed closer to the Sun where 10Be was produced more efficiently through solar cosmic ray irradiation caused by solar flares. Alternatively, if the present CO CAIs and CV CAIs formed in the same region, and 26Al was distributed homogeneously at the CAI-forming region, our results indicate that solar cosmic ray fluxes at the forming region have fluctuated by a factor of six within a short duration (∼0.2 million years) inferred from the Al-Mg chronology.

Origins of mass-dependent and mass-independent Ca isotope variations in meteoritic components and meteorites

The Ca isotope composition of meteorites and their components may vary due to mass-dependent and/or -independent isotope effects. In order to evaluate the origin of these effects, five amoeboid olivine aggregates (AOAs), three calcium aluminum inclusions (CAIs), five chondrules (C), a dark inclusion from Allende (CV3), two dark fragments from North West Africa 753 (NWA 753; R3.9), and a whole rock sample of Orgueil (CI1) were analyzed. This is the first coupled mass-dependent and -independent Ca isotope dataset to include AOAs, a dark inclusion, and dark fragments. Where sample masses permit, Ca isotope data are reported with corresponding petrographic analyses and rare earth element (REE) relative abundance patterns. The CAIs and AOAs are enriched in light Ca isotopes (δ44/40Ca −5.32 to +0.72, where δ44/40Ca is reported relative to SRM 915a). Samples CAI 5 and AOA 1 have anomalous Group II REE patterns. These REE and δ44/40Ca data suggest that the CAI 5 and AOA 1 compositions were set via kinetic isotope fractionation during condensation and evaporation. The remaining samples show mass-dependent Ca isotope variations which cluster between δ44/40Ca +0.53 and +1.59, some of which are coupled with unfractionated REE abundance patterns. These meteoritic components likely formed through the coaccretion of the evaporative residue and condensate following Group II CAI formation or their chemical and isotopic signatures were decoupled (e.g., via nebular or parent-body alteration). The whole rock sample of Orgueil has a δ44/40Ca +0.67 ± 0.18 which is in agreement with most published data. Parent-body alteration, terrestrial alteration, and variable sampling of Ca-rich meteoritic components can have an effect on δ44/40Ca compositions in whole rock meteorites.Samples AOA 1, CAI 5, C 2, and C 4 display mass-independent 48/44Ca anomalies (ε48/44Ca +6 to +12) which are resolved from the standard composition. Other samples measured for these effects (AOA 5, CAI 1, CAI 2, C 3, D 1, D 2, D 3) possess the same 48/44Ca isotope composition as the standard within analytical uncertainty. These data indicate a heterogeneous distribution of 48Ca in the early solar nebula during formation of CAIs, AOAs, and chondrules. In a ε48/44Ca vs. δ44/40Ca plot, no strong correlation is evident which suggests that the thermal processing event which caused a heterogeneous distribution of ε48/44Ca in the solar nebula is unlikely to be directly related to the thermal processing event that caused coupled REE and Ca mass-dependent isotopic fractionation in meteoritic components.

Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U–Pb ages, petrography, geochemistry and Nd–Sr–Hf isotopes

Whole rock major and trace element, Nd–Sr and zircon Hf isotopic compositions and secondary-ion mass spectrometry zircon U–Pb ages of eleven granitoid intrusions and dioritic rocks from the East Junggar (NW China) were analyzed in this study. The East Junggar granitoids were emplaced during terminal Early to Late Carboniferous (325–301Ma) following volcanic eruption of the Batamayi Formation. Zircons from the East Junggar granitoids yielded 210 concordant 206Pb/238U ages which are all younger than 334Ma and exhibit εHf(t) values distinctly higher than Devonian arc volcanic-rocks. Seismic P-wave velocities of deep crust of the East Junggar proper resemble those of oceanic crust (OC). These characteristics suggest absence of volcanic rock and volcano-sedimentary rock of Devonian and Early Carboniferous from the source region. The East Junggar granitoids show εNd(t) and initial 87Sr/86Sr values substantially overlapping those of the Armantai ophiolite in the area. The Early Paleozoic OC with seamount-like composition as the Zhaheba–Armantai ophiolites remained in the lower crust and formed main source rock of the East Junggar granitoids. Based on petrography and geochemistry, the East Junggar granitoids are classified into peralkaline A-type in the northern subarea, I-type (I1 and I2 subgroups) mainly in the north and A-type in the south of the southern subarea. The perthitic or argillated core and oligoclasic rim with an argillated boundary of feldspar phenocrysts and inclusion of perthites or its overgrowth by matrix plagioclase, in the monzogranites (northern subarea), suggest mixing of peralkaline granitic magma with mafic magma. In the north of the southern subarea, the presence of magmatic microdioritic enclaves (MMEs) in the I1 subgroup granitoids, transfer of plagioclase phenocrysts and hornblendes between host granodiorite and the MME across the boundary and a prominent resorption surface in the plagioclase phenocrysts indicate mixing of crustal magma (I2 subgroup granitoids) with mafic magma. Magma mixing shifted (87Sr/86Sr)i of the I1 subgroup granitoids towards the mantle array. Two generations of hornblende with zonal distribution and similar mineral and geochemical compositions of quartz monzodiorite and hosted MME with unfractionated rare earth elements (REE) suggest extended magma mixing with onset probably at or near source region. These observations imply concurrency of mantle input and the crustal melting and, hence, a causal relationship between underplating/intraplating and the lower OC/upper OC melting. The I-type granitoids experienced plagioclase and hornblende fractionations, whereas fractionated phases of the two groups of A-type granites were alkali feldspar and albite–oligoclase with significant involvement of F−-rich fluid. Granodioritic parent magmas of the I2 subgroup granitoids stemmed from the hydrous upper OC. Parent magmas of the two A-type groups possess syenogranitic or quartz syenitic compositions. The peralkaline A-type granites stemmed from the lower OC, whereas the A-type granites from dehydrated upper OC left behind after extensive partial melting and extraction of I-type granitoids. Based on comparison in the ternary system Mg2SiO4–CaAl2SiO6–SiO2, most of the Batamayi volcanic rocks with affinity to ocean-island basalts were derived from asthenospheric upwelling. The gabbro–dioritic rocks with higher light to heavy REE ratios stemmed from metasomatized lithospheric mantle. Both of the above mafic rocks contain subducted slab component.Loci of ponding of mafic magma and crustal melting migrated upward and southeastward from the lower OC in the northern subarea (yielding the peralkaline A-type granites) to the upper OC in the southern subarea. The latter evolved from hydrous (yielding the I-type granitoids in the north of the subarea) to dehydrated (yielding the A-type granites in the south of the subarea). Sequence of these events resulted in increasingly-later formation of the East Junggar granitoids southeastwards with an age span of ~20Ma. Minor contribution from Late Paleozoic OC to the source of the southern granitoid subarea caused the latter showing distinctly higher εNd(t) than the northern subarea. In conclusion, underplating caused OC remelting which, together with significant magma mixing, produced the East Junggar granitoids in a non-subduction zone environment.

Fractionation of rare earth elements in refractory inclusions from the Ningqiang meteorite: Origin of positive anomalies in Ce, Eu, and Yb

Ion microprobe analyses of rare earth elements (REEs), Ba, and Hf were performed for various types of refractory inclusions including amoeboid olivine aggregates (AOAs) from the Ningqiang ungrouped carbonaceous chondrite to search for possible relationships between REE abundance patterns and bulk chemical compositions of the inclusions. Four types of CI-normalized REE patterns were recognized: (1) nearly flat (unfractionated) pattern with or without Eu (and Yb) anomalies (Groups I, III, or V), (2) depletions of ultrarefractory heavy REEs (HREEs) relative to light REEs (LREEs), and depletions of Eu and Yb ( Group II, but without depletion of Yb in some cases), (3) depletions of ultrarefractory HREEs with positive anomalies in Ce, (Eu), and Yb ( Modified Group II), and (4) nearly flat pattern with positive anomalies in Ce, (Eu), and Yb ( Modified Group I). No systematic correlation was found between bulk chemical compositions and REE patterns of the inclusions. This suggests that the observed REE fractionations occurred prior to condensation of major elements (e.g., Mg and Si) which defined bulk chemical compositions of the inclusions. It is remarkable that 7 out of 19 inclusions show positive anomalies in Ce, Yb, and in some cases, Eu as well (Modified Group I and Modified Group II), suggesting that such anomalies are rather common among inclusions in the Ningqiang and possibly in other primitive meteorites. Two possible mechanisms are considered for the formation of Modified Group II and Modified Group I patterns. In Model 1, Modified Group II is formed by a process similar to that produced Group II but removal of ultrarefractory dust occurred at slightly lower temperatures, where not only ultrarefractory HREEs but some fraction of LREEs had been condensed and removed from the system. Modified Group I may be explained by addition of an unfractionated component to the Modified Group II component, or alternatively, by partial removal of ultrarefractory dust from the system. In Model 2, Modified Group II is formed by later addition of Ce, (Eu), and Yb onto fine-grained dust or inclusions having HREE-depleted, Group II-like REE patterns. Similarly, Modified Group I is explained by later addition of Ce, (Eu), and Yb onto those with almost unfractionated REE patterns. The observed REE data show that both the degree of HREE-depletion (e.g., Er-depletion) and that of fractionation among HREEs (e.g., depletion in the Er/Gd ratio) for Modified Group II are very similar to those for Group II. Model 1 predicts almost complete removal of ultrarefractory HREEs from the system, resulting in much higher HREE-depletion for Modified Group II, which is not consistent with the present observations. Addition of an unfractionated component may explain moderate depletion of HREEs in Modified Group II, but it will diminish fractionation among HREEs, which is not consistent with the present observations. In contrast, Model 2 predicts no correlations between Ce–(Eu)–Yb-enrichment and HREE-depletion or between Ce–(Eu)–Yb-enrichment and fractionation among HREEs, consistent with the present observations. Hence, Model 2 seems more likely. If this is the case, at least two distinct regions with different REE characteristics are required for the formation of Modified Group II inclusions: one is a high temperature region where Group II-like (HREE-depleted) inclusions or their precursors are formed by condensation from a fractionated gas after removal of ultrarefractory dust, and another is a low temperature region enriched in Ce, Eu, and Yb in the gas phase. Abundant occurrence of positive Ce–(Eu)–Yb anomalies suggests that migration of solid materials from one region to another occurs rather frequently in the solar nebula. The most likely place satisfying such conditions for the formation of these inclusions may be the innermost part of the protoplanetary disk.

Geology, Petrology, and Controls on PGE Mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada

The Lac des Iles Pd mine, with reserves of 88 million metric tons (Mt) containing 1.51 g/t Pd, is hosted by the 2.69 Ga Lac des Iles intrusive complex in the southern Wabigoon subprovince of the Superior province of Canada. The known economic concentration of Pd occurs in the Mine Block intrusion, the central body of the intrusive complex, where gabbroic rocks range from leucogabbro to pyroxenite and show complicated textures, such as breccias, magma mingling, and pods and veins of pegmatite. The ore is characterized by low concentrations of sulfide (typically less than 3 vol %) and exceptionally high Pd contents (Pd/Pt ~10, Pd/Au ~13). Detailed mapping of the southern Roby and Twilight zones shows that early leucocratic rocks are barren and that the bulk of Pd was introduced by late melanocratic magmas. The average concentration of Pd in the melanocratic rocks, excluding the High-Grade zone, is estimated to be ~4 ppm. The concentrations of sulfur correlate positively with those of base metals, platinum group elements (PGE), and Au. Furthermore, sulfide grains commonly show exsolution textures. The evidence suggests a magmatic origin of the PGE mineralization where the PGE were concentrated in an immiscible sulfide melt in the parental magma. Bulk chemical compositions suggest that all mafic igneous rocks in the mineralized zones, except for late clinopyroxenite, are cogenetic. The hypothetical parental magmas have high MgO and low (~15× chondrite), unfractionated rare earth elements (REE) with (Ce/Yb)chondrite 35% of Pd in the deposit) on the eastern margin of the Roby zone have much higher concentrations of Pd than any other rocks and do not show correlations between sulfur and precious and base metals. Furthermore, the rocks are intensely and pervasively altered to actinolite, talc, anthophyllite, hornblende, chlorite, sericite, calcite, and quartz. These observations suggest subsolidus enrichment of Pd and mobility of S. The lack of apparent fluid pathways within the High-Grade zone and the distribution of the zone are consistent with the enrichment of Pd at high temperatures by fluids that originated from the mafic magmas. The textures of the Lac des Iles deposit are similar to those of contact-type PGE deposits, but there are fundamental differences between the two. The Lac des Iles deposit is not localized near the contact between the host intrusion and the country rocks and evidence of the assimilation of the host rocks is lacking. Instead, the mineralization at Lac des Iles has many features in common with layered intrusion-hosted deposits, in which pulses of primitive magma introduced the PGE. Unlike the quiescent magma chambers of most layered deposits, the magmas at Lac des Iles were intruded energetically, forming breccias and magma-mingling textures.

Archean Nb-enriched basalts in the northern Superior Province

Geochemical data from the ∼2.9-Ga Northern Pickle assemblage of the Pickle Lake greenstone belt in the northern Superior Province distinguish three distinct volcanic suites. Suite I, from the north of the assemblage, comprises tholeiitic basalts with unfractionated rare earth elements (REE). Suite II basalts are light REE (LREE)-enriched with negative mantle-normalized Nb anomalies relative to Th and La, similar to the CA suite of Sajona et al. [J. Petrol. 37 (1996) 693]. Basalts from Suite III are also LREE-enriched but lack negative Nb anomalies. Normalized Nb abundances are either similar to or higher than Th and La, comparable to Nb-enriched basalts (NEB) from Phanerozoic arcs. Variable high field strength elements and heavy REE (HREE) systematics support the subdivision of Suite III basalts into two spatially distinct suites. Suite IIIa is interpreted to have melted at relatively shallow depths in the presence of Nb–Ti-bearing silicates whereas Suite IIIb melted in the presence of garnet. Variations in the geochemistry of the three suites can be accounted for by interaction among primitive mantle, adakite melts, and subduction-modified mantle. The association of the three suites is interpreted to be the result of rifting in a back-arc environment, characterized by Suite I tholeiites, permitting asthenospheric upwelling of subduction-modified NEB and associated arc-like volcanic rocks (Suites II and III). The association of arc-like basalts (Suite II) with NEB (Suite III) in the Northern Pickle assemblage extends the known occurrence of Archean NEB beyond the ∼2.7-Ga age of previously recognized examples.

Rhenium-osmium systematics of calcium-aluminium-rich inclusions in carbonaceous chondrites

The Re-Os isotopic systematics of calcium-aluminium-rich inclusions (CAIs) in chondrites were investigated in order to shed light on the behavior of the Re-Os system in bulk chondrites, and to constrain the timing of chemical fractionation in primitive chondrites. CAIs with relatively unfractionated rare earth element (REE) patterns (groups I, III, V, VI) define a narrow range of 187Re/188Os (0.3764–0.4443) and 187Os/188Os (0.12599–0.12717), and high but variable Re and Os abundances (3209–41,820 ppb Os). In contrast, CAIs that show depletions in highly refractory elements and strongly fractionated REE patterns (group II) also show a much larger range in 187Re/188Os (0.409–0.535) and 187Os/188Os (0.12695–0.13770), and greater than an order of magnitude lower Re and Os abundances than other groups (e.g., 75.7–680.2 ppb Os).Sixteen bulk CAIs and CAI splits plot within analytical uncertainty of a 4558 Ga reference isochron, as is expected for materials of this antiquity. Eight samples, however, plot off the isochron. Several possible reasons for these deviations are discussed. Data for multiple splits of one CAI indicate that the nonisochronous behavior for at least this CAI is the result of Re-Os reequilibration at approximately 1.6 Ga. Thus, the most likely explanation for the deviations of most of the nonisochronous CAIs is late-stage open-system behavior of Re and Os in the asteroidal environment.The 187Os/188Os-Os systematics of CAIs are consistent with previous models that indicate group II CAIs are mixtures of components that lost the bulk of their highly refractory elements in a previous condensation event and a minor second component that provided refractory elements at chondritic relative proportions. The high Re/Os of group II CAIs relative to other CAIs and chondrite bulk rocks may have been caused by variable mobilization of Re and Os during medium- to low-temperature parent body alteration ∼4.5 Ga ago. This model is favored over nebular models, which pose several difficulties.The narrow range of 187Os/188Os in group I, III, V, and VI bulk CAIs, and the agreement with 187Os/188Os of whole rock carbonaceous chondrites suggest that on a bulk inclusion scale, secondary alteration only modestly fractionated Re/Os in these CAIs. The average of 187Os/188Os for group I, III, V, and VI CAIs is indistinguishable from average CI chondrites, indicating a modern solar system value for 187Os/188Os of 0.12650, corresponding to a 187Re/188Os of 0.3964.

Petrography and chemistry of fine-grained dark rims in the Mokoia CV3 chondrite: Evidence for an accretionary origin

The matrix and fine-grained dark rims (FGDRs) around a type-A Ca-, Al-rich inclusion (CAI) and a silicate aggregate in the Mokoia CV3 chondrite differ in their major, minor, and trace element abundances. The matrix contains more Nb, Sr, Ba, and V but less Ca than average CI chondrite and has an unfractionated rare earth element (REE) pattern. The two FGDRs are enriched in Na, Al, K, and Ti and depleted in Fe and Mn compared to the matrix. The rim surrounding the CAI is enriched in Hf, Y, and Sc relative to the matrix and has a REE pattern that reflects a complex history. The rim surrounding the silicate aggregate has a REE pattern that suggests a different history and, additionally, is complementary to those of the enclosed enstatite and mesostasis glass. The trace element abundances indicate that Mokoia FGDRs are a mixture of components derived from both high- and low-temperature processes, while the matrix consists of only relatively low-temperature materials. These FGDRs in Mokoia have an accretionary origin and were derived from multiple dust reservoirs.

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Proterozoic Mantle Heterogeneity: Geochemical Evidence from Contrasting Basic Dykes

Summary Two swarms of early Proterozoic basic dykes occur in the Archaean craton of SW Greenland. The MD dykes are tholeiitic dolerites, while the BN dykes are norites. Virtually identical norites occur with tholeiitic dolerites in the Scourie swarm of NW Scotland and in the Laramie, Bighorn and Beartooth ranges of Wyoming, USA. Close geochemical similarities between typical coarse-grained norites and quenched varieties in SW Greenland and Wyoming suggest that the former type has not been strongly modified by crystal accumulation. The mineralogy and geochemistry of the quenched norites closely resemble those of boninites. Most of the norites have high MgO ( c. 13–18 wt%) and correspondingly high Cr and Ni, but they also have relatively high silica, alkali and large ion lithophile (LIL) element abundances. They are all light rare-earth element (REE) enriched (La = 40 times average CI chondrite, La N /Lu N c. 7). In contrast, most of the tholeiitic MD dolerites have fiat, unfractionated REE distributions. The tholeiites were probably derived from a fertile mantle source. However, the norites originated from a depleted harzburgitic mantle which had earlier borne Archaean komatiitic and tholeiitic magmas but which had been metasomatically replenished in LIL elements by fluids derived from Archaean oceanic and lower continental crustal material returned to the mantle during late-Archaean crustal thickening. Some major layered basic igneous intrusions, such as Bushveld and Stillwater, were derived by the mixing of two different basic magmas, which were equivalent to the tholeiitic and noritic (boninitic) dyke magmas of SW Greenland, NW Scotland and Wyoming. The apparently ubiquitous occurrence of tholeiite-norite associations suggests that similar large-scale mantle heterogeneities, due to the voluminous extraction of Archaean komatiitic and tholeiitic magmas, had become well established by the early Proterozoic.

On the Origin of High-Alumina Arc Basalt and the Mechanics of Melt Extraction

Most models of high-alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements, in particular the relatively unfractionated Rare Earth Element (REE) patterns and the unusually high concentrations of K, Rb, Sr, and Ba are incompatible with a garnet-bearing subducted oceanic crustal (quartz eclogite) source rock. We have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent. The weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near-surface environment. We expand on such models by monitoring the concentrations of R EE and major and trace elements during initial melting, ascent, and extraction of magma. This is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria. The amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids. With quartz eclogite, initial melting initiates gravitational instability of the entire source material (melt plus solids) before melt extraction can occur. During ascent of this mush, melting increases until the solids can be repacked to free the melt. This extraction takes place some 15-20 km above the slab, after about 50 per cent melting, at which point the melt has a pattern of REE and other trace element concentrations almost identical to those observed in high-alumina arc basalts, assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment. Sr abundances are the only ones which are not well-matched by this process. The major element concentrations of the extracted melt also closely match those of high-alumina arc basalt. A similar, but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the REE and other trace element concentrations assuming a LREE-enriched source rock strongly enriched in K, Rb, Sr, and Ba. Ni, Cr, and Co abundances are satisfied only through substantial low pressure fractionation of mafic phases, in particular olivine. Though not rigorously tested, such a process may be compatible with the observed major element concentrations of high-alumina basalt. However, the experimentally verified fact that high-alumina basalts could never have been in equilibrium with either an olivine-beari ng magma or source rock eliminates this possibility altogether. Thus, the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major, trace, and REE characteristics of high-alumina arc basalts.

Experimental study on rare-earth element partitioning in olivine and clinopyroxene formed at 10 and 20kb for basaltic systems.

Partition coefficients of rare-earth elements (REE), Ba and Sr were precisely determined for crystal glass pairs prepared by partial melting and partial solidification of alkali olivine basalt at 10kb and of picrite basalt at 10 and 20kb. Mutually unfractionated REE partition patterns were observed for olivine-glass pairs prepared by partial melting and partial solidification experiments on alkali olivine basalt. A similar unfractionated REE pattern was observed for (clinopyroxene + olivine)/glass formed by 10kb partial melting of picrite basalt. On the other hand, clinopyroxene was produced in 10kb partial solidification of the same picrite basalt and the clinopyroxene showed fractionated, terrace-shaped REE partition pattern. Partial solidification experiments on picrite basalt produced only clinopyroxene as a mineral coexisting with melt, and the resultant REE partition pattern was a guyot-shaped one. It should be noted that the REE partition pattern (terrace-shaped or guyot-shaped) of clinopyroxene bears on the mutual relationship concerning the relative magnitudes of partition values of Fe and heavy REE. Finally, it is worth while to note that clinopyroxene is prevailingly formed in partial solidification process.

Rare-earth abundances in chondritic meteorites

Fifteen chondrites, including eight carbonaceous chondrites, have been analyzed for rare earth element (REE) abundances by isotope dilution. These analyses complement and extend earlier isotope dilution REE determinations in chondrites, performed in other laboratories, so that coverage of major chondrite classes is now complete. An examination of this body of precise and comparable REE data from individual chondrites reveals that only a small proportion of the analyses have flat, unfractionated REE patterns within experimental error. A statistical procedure is used to derive revised chondritic abundances of REE by selection of unfractionated patterns. A number of the remaining analyses show Eu anomalies and fractionated patterns consistent with magmatic fractionation as encountered in the products of planetary differentiation. However, many patterns exhibit features not readily explicable by known magmatic processes; in particular, positive Ce anomalies are often encountered. Abundance anomalies can be quantitatively determined by the use of a least-squares curve fitting procedure. The wide variety of anomalous patterns and the uncertainties in model parameters preclude detailed modeling of the origin of anomalies, but it is probable that at least some arise from fractional condensation in the solar nebula, as has been demonstrated for Allende inclusions. Elemental abundance anomalies are found in all major chondrite classes. If these anomalies are ignored, the range and nature of variation within chondrite classes are consistent with a parent body model, in which solid-liquid or solid-solid equilibria induce variations from an unfractionated bulk composition. Absolute abundances in the H, L and LL parent bodies are almost twice those of the E parent body. The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibration or were introduced at a late stage of chondrite formation. Large scale segregation of gas and condensate is also implied, and raises the possibility of bulk variations in REE abundances between planetary bodies.

A high [formula omitted] mantle source for low alkali tholeiite, northern Great Basin

Olivine tholeiites, the youngest Tertiary units (about 8–11 m.y. old) at five widely spaced localities in northeastern Nevada, are geologically related to the basalts of the Snake River Plain, Idaho, to the north and are similar in major element and alkali chemistry to mid-ocean ridge basalts (MORB) and island arc tholeiites. The measured K (1250–3350 ppm), Rb (1·9–6·2 ppm) and Sr (140–240 ppm) concentrations overlap the range reported for MORB. Three of the five samples have low, unfractionated rare earth element (REE) patterns, the other two show moderate light-REE enrichment. Barium concentration is high and variable (100–780 ppm) and does not correlate with the other LIL elements. The rocks have 87 Sr/ 86 Sr = 0·7052–0·7076, considerably higher than MORB (~0·702–0·703). These samples are chemically distinct (i.e. less alkalic) from the olivine tholeiites from the adjacent Snake River Plain, but their Sr isotopic compositions are similar. They contain Sr that is distinctly more radiogenic than the basalts from the adjacent Great Basin. About 10 b.y. would be required for the mean measured Rb/Sr (~ 0·02) of these samples to generate, in a closed system, the radiogenic Sr they contain. The low alkali content of these basalts makes crustal contamination an unlikely mechanism. If the magma is uncontaminated, the time-averaged Rb/Sr of the source material must have been ~0·04. A significant decrease in Rb/Sr of the source material (a factor 2̆) thus most probably occurred in the relatively recent (1̌0 9 yr) past. Such a decrease of Rb/Sr in the mantle could accompany alkali depletion produced by an episode of partial melting and magma extraction. In contrast, low 87Sr 86Sr ratios indicate that the source material of the mid-ocean ridge basalts may have been depleted early in the Earth's history.