Depleted Mantle Reservoir Research Articles

Petrogenesis of olivine-phyric shergottite Yamato 980459, revisited

Primitive magmas provide critical information on mantle sources, but most Martian meteorites crystallized from fractionated melts. An olivine-phyric shergottite, Yamato 980459 (Y-980459), has been interpreted to represent a primary melt, because its olivine megacrysts have magnesian cores (Fo84–86) that appear to be in equilibrium with the Y-980459 whole-rock composition based on Fe–Mg partitioning. However, crystal size distribution (CSD) plots for Y-980459 olivines show a size gap, suggesting a cumulus origin for some megacrysts. Because melting experiments using the Y-980459 whole-rock composition have been used to infer the thermal structure and volatile contents of the Martian mantle, the interpretation that this rock is primitive should be scrutinized. We report major, minor and trace element compositions of Y-980459 olivines and compare them with results from melting experiments (both hydrous and anhydrous) and thermodynamic calculations. Cores of the olivine megacrysts have major and minor element contents identical to those of the most magnesian olivines from the experiments, but they differ slightly from those of thermodynamic calculations. This is probably because the Y-980459 whole-rock composition lies near the limit of the range of liquids used to calibrate these models. The megacryst cores (Fo80–85) exhibit minor and trace element (Mn–Ni–Co–Cr–V) characteristics distinct from other olivines (megacryst rims and groundmass olivines, Fo < 80), implying that the megacryst cores crystallized under more reduced conditions (∼IW + 1). Y-980459 contains pyroxenes with orthopyroxene cores mantled by pigeonite and augite. We also found some reversely zoned pyroxenes that have augite cores (low-Mg#) mantled by orthopyroxenes (high-Mg#), although they are uncommon. These reversely zoned pyroxenes are interpreted to have grown initially as atoll-like crystals with later crystallization filling in the hollow centers, implying disequilibrium crystallization at a moderate cooling rate (3–7 °C/h). The calculated REE pattern of a melt in equilibrium with normally zoned pyroxene is parallel to those of glass and the Y-980459 whole-rock as well as other depleted olivine-phyric shergottites, suggesting that Y-980459 was derived from a depleted mantle reservoir. Considering the CSD patterns of Y-980459 olivines, we propose that the olivine megacrysts are cumulus crystals which probably formed in a feeder conduit by continuous melt replenishment, and the parent melt composition was indistinguishable from the Y-980459 whole-rock with 0–2 wt% of H 2O and 0–5 wt% of CO 2. The final magma pulse entrained these cumulus olivines and then crystallized groundmass olivines and pyroxenes. Although Y-980459 contains small amounts of cumulus olivine (<∼6 vol%), we conclude that the Y-980459 whole-rock composition closely approximates a Martian primary melt composition.

3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry

We present field, petrographic, Sm–Nd whole-rock isochron and whole-rock geochemical data for komatiites from Sargur Group greenstone belts of the western Dharwar craton. Field evidence such as pillow structure indicates their eruption in a marine environment. Petrographic data reveal that the igneous mineralogy has been altered during post-magmatic hydrothermal alteration processes corresponding to greenschist- to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene. A 16-point Sm–Nd whole-rock isochron gives an age of 3352 ± 110 Ma for the timing of eruption of komatiite lavas. About 60% of the studied komatiite samples show Al-depletion whilst the remaining are Al-undepleted. The Al-depleted komatiites are characterised by high CaO/Al 2O 3 ratios (1.01–1.34) and low Al 2O 3/TiO 2 (5–16) whereas Al-undepleted komatiites show lower CaO/Al 2O 3 ratios (0.59–0.99) and higher Al 2O 3/TiO 2 (17–26). Trace element distribution patterns of komatiites suggest that most of the primary geochemical and Nd isotopic compositions are preserved with only minor influence of post-magmatic alteration processes and negligible crustal contamination. The chemical characteristics of Al-depleted komatiites, such as high (Gd/Yb) N together with lower HREE, Y, Zr and Hf, imply their derivation from deeper upper mantle with garnet (majorite?) involvement, whereas lower (Gd/Yb) N slightly higher HREE, Y, Zr and Hf suggest derivation from shallower upper mantle without garnet involvement. The observed chemical characteristics (CaO/Al 2O 3, Al 2O 3/TiO 2, MgO, Ni, Cr, Nb, Zr, Y, Hf, REE) indicate derivation of the komatiite magmas from different depths in a plume setting, whereas sub-contemporaneous felsic volcanism and TTG accretion can be attributed to an arc setting. In order to explain the spatial association of komatiite volcanism with contemporaneous mafic-felsic volcanism and TTG accretion we propose a combined plume-arc setting. Nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early (>4.53 Ga) global differentiation of the silicate Earth as suggested by Boyet and Carlson [Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (>4.53 Ga) global differentiation of silicate Earth. Science 309, 577–581] or extraction of continental crust during the early Archaean.

Zircon U–Pb age and Hf isotope evidence for 3.8 Ga crustal remnant and episodic reworking of Archean crust in South China

Four populations of zircons are recognized by U–Pb analyses of the Neoproterozoic Liantuo Formation in the northern part of the Yangtze Block, South China. They are grains of detrital zircon older than 3.0 Ga, ca. 2.95 Ga, ca. 1.95 Ga and 820–750 Ma, respectively. The oldest zircon has a U–Pb age of 3802±8 Ma with a εHf(t) value of −0.8 and model Hf ages of 3.96 Ga (TDM1) and 4.00 Ga (TDM2). This demonstrates the existence of 3.8 Ga old crustal remnants in South China, with possible crustal growth as early as 4.0 Ga. A series of 3.3 Ga zircons have positive εHf(t) values as high as 4.2, providing compelling evidence for growth of juvenile crust from a depleted mantle reservoir at Paleoarchean. All the zircons have Archean Hf model ages, with prominent peaks at 3.2 to 3.6 Ga, indicating an important period of crustal growth in this period. The other three zircon populations at ∼2.95 Ga, ∼1.95 Ga and 820–750 Ma have negative εHf(t) values and consistent Archean Hf model ages, suggesting multi-stage episodic reworking of Archean crustal materials. The youngest zircons have U–Pb ages of ∼750 Ma, very close to the deposition time of the Liantuo Formation. This indicates rapid recycling of supracrustal materials in a rift basin, possibly in association with breakup of the supercontinent Rodinia at that time.

A new geochemical model for the Earth's mantle inferred from 146Sm– 142Nd systematics

New measurements of 142Nd/ 144Nd in kimberlites, carbonatites, komatiites, ocean island basalts from Pitcairn, and mid-ocean ridge basalts from the Pacific and Indian Oceans show no deviation from the terrestrial Nd standard within an external reproducibility of 8 ppm. Measurements of mafic rocks from Isua Greenland confirm previous discoveries of an excess of between 0 and 17 ppm in the 142Nd/ 144Nd of these rocks. Since the terrestrial Nd standard has a 142Nd/ 144Nd approximately 20 ppm higher than measured in chondrites and eucrites, all terrestrial rocks so far analyzed have superchondritic 142Nd/ 144Nd. This requires either that the bulk silicate earth has a superchondritic Sm/Nd ratio or that the Earth experienced a global differentiation event within a couple of hundred million years of planet formation that created complementary incompatible element enriched and depleted mantle reservoirs. If that portion of the mantle involved in the production of continental crust was originally characterized by incompatible element depletion (i.e. high Sm/Nd) rather than chondritic relative abundances of refractory lithophile elements, as is commonly assumed, the mass of mantle affected by continent formation may constitute as much as 80% or more of the mantle. The abundances of K, U and Th in this early-formed depleted mantle are 60–70% of those calculated for a “chondritic” mantle, which removes the inference that half of the 40Ar produced over Earth history must still reside in the mantle and explains why the mantle component characterized by high 3He/ 4He has superchondritic Nd and Hf isotopic composition. If these concentrations characterize the whole mantle, this implies a bulk-earth Urey ratio of ∼ 0.3 and hence that much of the heat escaping the Earth's interior is primordial and not the result of radioactive decay over Earth history. If the bulk silicate earth is presumed to have chondritic abundances of refractory lithophile elements, there must be an incompatible element enriched reservoir buried deep enough in to mantle to preclude its involvement in mantle convection and its contribution to surface volcanism because no terrestrial rock with 142Nd/ 144Nd less than chondritic has been identified.

Zircon U–Pb age, Hf and O isotope constraints on protolith origin of ultrahigh-pressure eclogite and gneiss in the Dabie orogen

Zircon U–Pb dating, Hf and O isotope analyses were carried out for ultrahigh-pressure eclogite and felsic gneiss from the Dabie orogen in China. The results provide constraints on their protolith origin, with significance for continental growth by rift magmatism during splitting of the Yangtze Block from the supercontinent Rodinia. Mafic and felsic protoliths formed as a bimodal volcanic suite at about 750 Ma, with incorporation of ca. 2.15 Ga crust into the felsic protolith. δ18O values of −4.1‰ to 4.4‰ suggest differential involvement of meteoric water in protolith magmas. Initial Hf isotope ratios are subdivided into two groups, with positive εHf(t) values of 12.9±0.7 to 5.9±0.9 and neutral εHf(t) values of 2.3±0.3 to −2.7±0.6, respectively. The positive εHf(t) values correspond to depleted mantle Hf model ages of 0.82 to 1.24 Ga, whereas the negative εHf(t) values correspond to crust Hf model ages of 1.82±0.07 Ga. A few zircons from the gneiss have strongly negative εHf(t) values of −22.6±0.6, which are associated not only with consistent depleted mantle and crust Hf model ages of 2.2 Ga but also with the old U–Pb ages falling on a 2147±22 Ma discordia line. Thus the felsic protolith contains Paleoproterozoic crustal relicts, some of which were originally derived from coeval depleted mantle. On the other hand, strongly positive εHf(t) values are associated with young U–Pb ages falling on the ∼750 Ma discordia lines. This suggests that both eclogite and gneiss protoliths principally formed by immediate reworking of mid-Neoproterozoic juvenile crust, with contrasting features only in petrochemistry. The highest εHf(t) values of 12.7 to 14.4 for the eclogite correspond to the youngest Hf model ages of 0.75 to 0.82 Ga relative to the depleted mantle reservoir. These Hf model ages are not only close to the timing of zircon growth from the mafic magma, but also similar to ages for bimodal magmatism in the periphery of the Yangtze Block. This demonstrates new addition of the depleted mantle material to the continental crust by rift magmatism in the northern margin of the Yangtze Block, with coeval crust–mantle interaction in the mid-Neoproterozoic rifting tectonic zone. Therefore, the bimodal magmatism at about 750 Ma transports both heat and material from the mantle to the crust, resulting in reworking of both the Paleoproterozoic old crust and the meteoric-hydrothermally altered juvenile crust along the active rifting zone.

Eocene volcanism above a depleted mantle slab window in southern Alaska

The Caribou Creek volcanic fi eld lies along the continent-side edge of forearc basin rocks in south-central Alaska and consists of over 1000 m of shallow-dipping basalt and andesite lavas with minor mafi c pyroclastic deposits. Dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas. The basalts are tholeiitic and strongly depleted in the light rare earth elements (La/ Yb = 0.18–1.5), with concentrations of high fi eld strength elements (e.g., Zr, Hf, Ti, Y) similar to mid-ocean-ridge basalt and with variable enrichment in fl uid-mobile elements (e.g., Cs, Ba, and Pb). Intermediate and felsic rocks show enrichment in the rare earth elements and fl uid-mobile elements plus Rb and K, but retain low La/Yb ratios (0.48–3.6). A few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites (e.g., Sr/Y up to 52). Ten 40 Ar/ 39 Ar ages for the Caribou Creek volcanic rocks range from 49.4 ± 2.2 to 35.6 ± 0.2 Ma. An adakite-like tuff beneath the other volcanic rocks yields an age of 59.0 ± 0.4 Ma. Caribou Creek basalts were derived from mid-ocean-ridge–like depleted mantle that was emplaced beneath the southern margin of Alaska through a slab window following spreading ridge subduction. Caribou Creek volcanism was coeval with oblique subduction, oroclinal bending, and right-lateral strike-slip faulting in south-central Alaska, all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas. The basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of Jurassic arc basement rocks to form the intermediate and felsic magmas. Enrichment of the basaltic parent magmas in fl uid-mobile elements occurred by contamination from the Jurassic arc rocks and/or by contamination with metasomatic mantle remnant from preceding subduction. High heat fl ow through the slab window induced partial melting of garnet-bearing mafi c parts of the Jurassic arc basement to form the adakite-like rocks. The Caribou Creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime (e.g., crustal extension) and that the infl uence of a slab window on continental margin magmatism can be long-lived (>20 m.y.).

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-Calmalli area, Baja California, Mexico. 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-Vizcaino 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.

Hf isotopes in zircon from the western Superior province, Canada: Implications for Archean crustal development and evolution of the depleted mantle reservoir

U–Pb and Hf isotopic measurements on zircons from the western Superior province confirm that the area contains at least three distinct terrane types. Juvenile terranes that formed mostly within the time span 2.75–2.68 Ga occupy much of the western Wabigoon subprovince as well as granite-greenstone belts to the south. Juvenile 3.0 Ga terranes that were reworked over the time span 2.7–3.0 Ga occupy the south-central part of the Wabigoon subprovince and the North Caribou block in the Sachigo subprovince. Rocks with mantle extraction ages as old as 3.5 Ga and zircon U–Pb ages extending to 3.3 Ga characterize a third type of terrane represented by the Winnipeg River subprovince. This terrane was strongly reworked during the late Archean. Arc-related magmatism was ongoing at 2.71–2.75 Ga in the different terranes, which probably accreted over the time span 2.71–2.68 Ga. Enriched Hf and high O isotopic signatures in late sanukitoid-suite plutons appear to be correlated, which suggests that assimilation of Mesoarchean crust was an important factor in their magmatic evolution. Enriched Hf isotopic signatures in detrital and igneous zircon from parts of the north-central Wabigoon subprovince support previous suggestions that the Winnipeg River terrane extends eastward beyond the Winnipeg River subprovince. The Winnipeg River subprovince was probably being uplifted and eroded into the Quetico sedimentary basin shortly after 2700 Ma, as shown by detrital zircons with enriched Hf isotopic signatures and Meso- to Paleoarchean ages. The pattern of ages and isotopic signatures from the North Caribou block and the south-central Wabigoon subprovince are similar, suggesting that these terranes are correlative. If so, the south-central Wabigoon terrane may have been tectonically transported from the north. Hf isotopic compositions of zircon from juvenile Archean sources are remarkably consistent and define an average ɛ Hf value of +3.5 ± 0.2 for samples with an average age of 2724 Ma and a best estimate of +2.7 ± 0.4 at 3000 Ma. Thus, the Neaoarchean depleted mantle reservoir beneath the Superior province appears to have been isotopically well mixed. ɛ Hf values were calculated using a value of 1.865 × 10 −5 Ma −1 [ Scherer, E., Munker, C., Mezger, K., 2001. Calibration of the Lutetium-Hafnium clock. Science 293, 683–687] for the 176Lu decay constant, which is thus far the best reproduced estimate and the one most consistent with depleted mantle evolution results based on Nd isotopes and Nb/Th ratios. A linear Hf mantle growth curve defined by these values and recent MORB intersects the chondritic Hf growth curve during the early Archean (3.4–4.0 Ga). This could indicate that the earliest formation of significant amounts of enriched crust coincides with ages of the oldest preserved rocks, but such a conclusion is contradicted by evidence from 142Nd and 143Nd in early Archean rocks for significant mantle depletion during the Hadean eon (>4.0 Ga). Both lines of evidence might be reconciled if Hadean enriched crust were largely remixed with its depleted mantle source near the beginning of the Archean, leaving only fragmentary evidence of its existence in the oldest rocks.

A determination of the neon isotopic composition of the deep mantle

The neon isotopic composition of the deep mantle, which allows to investigate formation processes of the Earth, is subject to debate. We have analyzed Ne trapped in fluid inclusions from plume-related Devonian rocks from the Kola Peninsula, Russia. The lower limit for pristine neon in the deep mantle is set at 20Ne/ 22Ne≥13.0±0.2, close to the solar wind (SW) value. We interpret this similarity as evidence that mantle light noble gases were directly incorporated from the solar nebula rather than derived from implanted meteoritic solar component, possibly through gravitational capture of a proto-atmosphere. This constraint is consistent with geochemical observations of rapid accretion of a major part of the growing Earth within the inferred lifetime of the solar nebula. If depleted mantle neon admits an upper limit of 12.5, then the occurrence of plume type neon with 20Ne/ 22Ne≥13.0±0.2 may indicate a primordial Ne heterogeneity in the mantle. The 3He/ 22Ne ratios of terrestrial mantle reservoirs sampled by MORB and plumes vary with the degree of volatile element depletion of the mantle. This correlation was possibly established during magmatic events having led to the chemical characteristics of the depleted mantle reservoir.

Archean crustal evolution in the northern Yilgarn Craton: U–Pb and Hf-isotope evidence from detrital zircons

The integrated application of U–Pb dating, Hf-isotope analysis and trace-element analysis to detrital zircon populations offers a rapid means of assessing the geochronology and crustal evolution history of different terranes within a composite craton. In situ U–Pb and Hf-isotope analyses of 550 zircons from 21 modern drainages across the northern part of the Yilgarn Craton and the adjacent Capricorn Orogen provide a broad view of crustal evolution in Archean and Proterozoic time.The oldest crustal components (3.7Ga) are identified in the Yeelirrie geophysical domain [A.J. Whitaker, Proceedings of Fourth International Archaean Symposium Ext. Abstracts AGSO-Geoscience Australia Record, vol. 37, 2001, p. 536] that runs N–S down the middle of the craton; these components are represented by ancient zircons and also are reflected in the Hf model ages of younger magmas. Ancient (>3.4Ga) crust contributed to the generation of younger magmas in the Narryer Province, and the proportion of ancient recycled material increases from east to west across the Murchison Province. In contrast, the Hf-isotope data provide no evidence for crust older than 2.9–3.0Ga in the Southern Cross or Eastern Goldfields (including the Marymia Inlier) domains. The Yeelirrie domain and the composite Narryer–Murchison block are interpreted as ancient microcontinents, sandwiched with the juvenile terranes of the Southern Cross and Eastern Goldfields domains.There is little evidence for the existence of a Depleted Mantle reservoir beneath the Yilgarn Craton prior to 3.1–3.2Ga, but this reservoir is a major contributor to crustal generation from 3.1 to 2.6Ga; this suggests that much of the continental crust in the craton was generated after ca. 3.2Ga. 1.8–2.3Ga magmatism, associated with the Capricorn Orogen, involved the recycling of older crust with little obvious contribution from the Depleted Mantle. A significant (and previously unrecognised) 540Ma episode in the NE part of the craton involved metamorphism or remelting of the 2.7–3.0Ga crust of the Eastern Goldfields Province.

Early history of Earth's crust-mantle system inferred from hafnium isotopes in chondrites.

The 176Lu to 176Hf decay series has been widely used to understand the nature of Earth's early crust-mantle system. The interpretation, however, of Lu-Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu (lambda176Lu), as well as bulk-Earth reference parameters. A recent calibration of the lambda176Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9 Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3 Gyr ago, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu-Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu-Hf isochron with an initial 176Hf/177Hf ratio of 0.279628 +/- 0.000047, corresponding to lambda176Lu = 1.983 +/- 0.033 x 10-11 yr-1 using an age of 4.56 Gyr for the chondrite-forming event. This lambda176Lu value indicates that Earth's oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment. The depletion event must have occurred no later than 320 Myr after planetary accretion, consistent with timing inferred from extinct radionuclides.

Sm-Nd dating and Nd-Sr isotopic characteristics of the Shimian ophiolite suite, Sichuan Province

By measuring the Sm-Nd and Rb-Sr isotopic compositions of harzburgite and gabbro from Shimian ophiolite suite, we got the whole rock Sm-Nd isochron age of (938±30) Ma (2σ), and the eNd of 7.6±0.8 (2σ), which shows that the ophiolite was formed at the Early Neoproterozoic. The obvious change (0.70209–0.70708) of ISr values of the ophiolite is caused by the meteoric hydrothermal alteration. The high eNd values indicate that the primitive magma was derived from the intense depleted mantle reservoir. It is suggested that this area was in a back-arc basin environment during the Early Neoproterozic.

Crustal heat production and the thermal evolution of Mars

The chemical composition of soils and rocks from the Pathfinder site and Phobos‐2 orbital gamma‐ray spectroscopy indicate that the Martian crust has a bulk composition equivalent to large‐ion lithophile (LIL) and heat‐producing element (K, Th, U) enriched basalt, with a potassium content of about 0.5%. A variety of radiogenic isotope data also suggest that separation of LIL‐enriched crust and depleted mantle reservoirs took place very early in Martian history (&gt;4.0 Ga). Accordingly, if the enriched Martian crust is ≥30 km thick it is likely that a large fraction (up to at least 50%) of the heat‐producing elements in Mars was transferred into the crust very early in the planet's history. This would greatly diminish the possibility of early widespread melting of the Martian mantle.

Nd and Pb isotopes from the Lake of the Woods greenstone belt, northwestern Ontario: implications for mantle evolution and the formation of crust in the southern Superior Province

Nd and Pb isotopes from the Lake of the Woods greenstone belt indicate the presence of three distinct reservoir sources: old enriched crust (&gt;3.0 Ga); pre-2.7 Ga, homogeneous depleted mantle; and post-2.70 Ga heterogeneous mantle. EpsilonNd values of +1.1 to +2.3 for ultramafic to felsic metavolcanic rocks (2.74–2.72 Ga) indicate derivation from depleted mantle. The εNd value of –0.9 for younger turbidite (2.71 Ga), in conjunction with detrital zircon ages ranging from 2.72 to 3.0 Ga, indicates detritus from local greenstone belt sources (depleted mantle) mixed with an older crustal source. Post-2.70 Ga heterogeneity is demonstrated by εNd values ranging from –0.4 to +0.4 in shoshonitic to calc-alkaline metavolcanic rocks and +2.1 in a coeval ultrapotassic pluton. Pb isotopes from the pluton indicate derivation from a depleted mantle reservoir with an initial 207Pb/204Pb of 14.52, an initial 206Pb/204Pb of 13.29, and µ1 of 7.86. Isotopic comparison with post-2.70 Ga potassic suites from across the Superior Province indicates widespread mixing between depleted mantle and enriched end members. The enriched end member has isotopic characteristics of rocks derived from old crustal terrains, such as the Winnipeg River and Opatica subprovinces. This type of isotopic heterogeneity could be the result of crustal contamination or derivation from metasomatized mantle. Contamination of the mantle wedge by influx of fluids derived from partial melting of isotopically evolved, subducted sediments is favoured for the Superior Province potassic suite, because elevated concentration of Sr, Nd, and Pb in conjunction with primitive Mg#s suggest only limited crustal contamination has occurred.

Nd and Pb isotopes from the Lake of the Woods greenstone belt, northwestern Ontario: implications for mantle evolution and the formation of crust in the southern Superior Province

Nd and Pb isotopes from the Lake of the Woods greenstone belt indicate the presence of three distinct reservoir sources: old enriched crust (>3.0 Ga); pre-2.7 Ga, homogeneous depleted mantle; and post-2.70 Ga heterogeneous mantle. EpsilonNd values of +1.1 to +2.3 for ultramafic to felsic metavolcanic rocks (2.74–2.72 Ga) indicate derivation from depleted mantle. The εNd value of –0.9 for younger turbidite (2.71 Ga), in conjunction with detrital zircon ages ranging from 2.72 to 3.0 Ga, indicates detritus from local greenstone belt sources (depleted mantle) mixed with an older crustal source. Post-2.70 Ga heterogeneity is demonstrated by εNd values ranging from –0.4 to +0.4 in shoshonitic to calc-alkaline metavolcanic rocks and +2.1 in a coeval ultrapotassic pluton. Pb isotopes from the pluton indicate derivation from a depleted mantle reservoir with an initial 207Pb/204Pb of 14.52, an initial 206Pb/204Pb of 13.29, and µ1 of 7.86. Isotopic comparison with post-2.70 Ga potassic suites from across the Superior Province indicates widespread mixing between depleted mantle and enriched end members. The enriched end member has isotopic characteristics of rocks derived from old crustal terrains, such as the Winnipeg River and Opatica subprovinces. This type of isotopic heterogeneity could be the result of crustal contamination or derivation from metasomatized mantle. Contamination of the mantle wedge by influx of fluids derived from partial melting of isotopically evolved, subducted sediments is favoured for the Superior Province potassic suite, because elevated concentration of Sr, Nd, and Pb in conjunction with primitive Mg#s suggest only limited crustal contamination has occurred.

Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time

The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation ε Hf = 1.40 ε Nd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of ε Hf = 1.36 ε Nd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in ε Nd values ( −4.4 to +4.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The ε Hf values for these rocks are consistently positive and have much less variation (0 to +3.4) than their ε Nd counterparts. The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks. Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive ε Hf and ε Nd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.

Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California

High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas.

Noble gas constraints on the evolution of the atmosphere-mantle system

We present a model on the evolution of atmosphere-mantle system assuming a significant degassing from the less depleted mantle, based on the recent results of the He-Ar systematics proposed by Matsuda and Marty (1995). The degassing fluxes of noble gases are represented by the concentrations in the mass flow, which follows the model of Porcelli and Wasserburg (1995a, Porcelli and Wasserburg 1995b). However, we have not assumed the steady-state, and have assumed the mass transfer as an exponential function of time. The degassed amount from the less depleted mantle is related to the mixed amount of mass between the less depleted mantle and the depleted mantle. The amount of mixing is constrained from the K abundances in each mantle reservoir. Consequently, the degassed amount from the less depleted mantle is also constrained. The following conclusions are obtained from this model. The initial elemental ratio 3He/ 36Ar is likely to be in cosmic abundances, and the initial concentration of 36Ar in the Earth is constrained to be ∼1 × 10 12 atoms/g. The present fraction of 36Ar in the depleted mantle reservoir is calculated to be at most 2.4 × 10 −3 of the initial inventory. The 40Ar/ 36Ar ratio in this reservoir is estimated to be less than (1.3–1.4) × 10 5. The less depleted mantle reservoir also degassed and the present amount of 36Ar is calculated to be at most 1.3 × 10 −1 of the initial inventory. The 40Ar/ 36Ar ratio in the less depleted mantle reservoir is estimated to be less than 3.7 × 10 4. To explain the difference between the neon isotopic ratios in the atmosphere and those observed in the mantle rocks, isotopic fractionation during the escape from the atmosphere and/or significant contribution of late accreted materials possessing planetary Ne is necessary. In our model, Ne degassing from both mantle reservoirs would have provided at least 50–90 times the amount of 22Ne in the present atmosphere. Therefore, the fraction supplied by late-accreted material would be negligible, which is contrary to the model proposed by Porcelli and Wasserburg (1995b). We conclude that isotopic fractionation model during escape from the atmosphere provides the best fit to the observations.

Xenolith Evidence for Melt-Rock Reaction at the Lithosphereplume Boundary

Poikiloblastic harzburgite xenoliths from Borne, French Massif Central have been interpreted as the deepest spinel facies mantle segments sampled by alkali basalts (Mercier, 1977). However, there exists considerable debate about their petrogenesis (selective recrystallisation of the granular peridotites within a diapir vs precipitation from tholeiitic melts). We thus undertook a comprehensive study on these particular rocks with a strategy of understanding the nature of processes in the lower lithosphere which is imparted by an upwelling plume (Granet et al., 1995). Petrography and geochemical characteristics. The Borne xenoliths can be classified into two groups. The G-type samples are mainly spinel lherzolites and are similar to Group I peridotites (Frey and Prinz 1978) with a textural variation from protogranular, porphyroclastic to equigranular. The equilibrium temperatures are 850-1000~ The P-type samples are harzburgites which are divided into two subgroups in terms of the degree of recrystallisation/deformation. The first subgroup, referred to as non-recrystallised P-type, is characterised by large (>1 cm) and essentially unstrained olivines, whereas the second subgroup, the recrystallised P-type, is characterised by the presence of two generations of olivine and orthopyroxene with some samples being mylonitic in texture. The P-type peridotites are equilibrated under a high temperature (majority > 1200~ The olivines in the two groups of peridotites are similar in Mg# and in NiO content. However, distinctive compositions are observed in P-type and G-type spinels. The P-type spinels display inhomogeneous composition at grain-scale and have much higher Cr contents and calculated ferric ion contents than G-type spinels. In Cr#sp-Mg#ol diagram, the Gtype and non-recrystallised P-type samples follow the progressive partial melting trend, whereas the recrystallised P-type samples tend to plot below the trend. The deviation from the progressive partial melting trend is even more obvious for the Ti contents in all P-type samples which are significantly higher than the normal mantle values. Clinopyroxenes in the G-type peridotites display R E E pattems similar to those commonly observed in peridotites from world-wide occurrences. A strong correlation between R E E patterns and texture is observed for the P-type samples. The recrystallised samples have bell shaped R E E patterns with apex at Nd and Pr. In contrast, cpxs in non-recrystallised samples have lower total R E E and Ti contents, and display a relatively flat pattern. The isotopic compositions of cpxs correlate with texture despite a restricted range in isotopes. G-type peridotites have slightly higher ~Na (11.8-14.9) and low S7Sr/S6Sr (0.7022-0.7030) than the P-type p e r i d o t i t e s (ENd = 6 .0--8 .3 ; S7Sr/86Sr = 0.7031-0.7035), defining a depleted mantle reservoir. The P-type peridotites plot between the MORB field and bulk Earth composition. A percolation-reaction model for the origin of Borne P-type harzburgites. The previous cumulative model is inadequate when following lines of evidences are taken into account. (1) In terms of major element chemistry, the 'refractory' characteristics and homogeneous silicate phases of the P-type peridotites are different from those magmatic segregates which are commonly pyroxene-rich and high in CaO and A1203; (2) The relatively constant Mg# and NiO contents of P-type samples are not consistent with the hypothesis of large volume of cumulates; (3) The concentrations of hypothetical melts in equilibrium with P-type samples are highly variable, attesting to a very strong fractionation processes associated with the formation of P-type xenoliths. This is not consistent with the lack of fractionation revealed from the major chemistry; and (4) A small, but significant difference in Sr-Nd

Constraints on Martian differentiation processes from RbSr and SmNd isotopic analyses of the basaltic shergottite QUE 94201

Isotopic analyses of mineral, leachate, and whole rock fractions from the Martian shergottite meteorite QUE 94201 yield RbSr and SmNd crystallization ages of 327 ± 12 and 327 ± 19 Ma, respectively. These ages are concordant, although the isochrons are defined by different fractions within the meteorite. Comparison of isotope dilution Sm and Nd data for the various QUE 94201 fractions with in situ ion microprobe data for QUE 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite. This component is likely to have been added to QUE 94201 by secondary alteration processes on Mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions. Initial 87Sr/ 86Sr of 0.701298 ± 14, ϵ Nd 143 of +47.6 ± 1.7, and whole rock ϵ Nd 142 of +0.92 ± 0.11 indicate that QUE 94201 was derived from a source that was strongly depleted in 87Rb/ 86Sr and enriched in 147Sm/ 144Nd early in its history. Modeling demonstrates that the SmNd isotopic compositions of QUE 94201 can be produced by either four episodes of melting at 327 Ma of cumulates crystallized from a magma ocean at 4.525 Ga or five episodes of melting of an initially solid Mars at 4.525 Ga and 327 Ma. The neodymium isotopic systematics of QUE 94201 are not consistent with significant melting between 4.525 Ga and 327 Ma. The estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the QUE 94201 parental magma. Rb-Sr-based partial melting models are unable to reproduce the composition of QUE 94201 using the same model parameters employed in the SmNd-based models, implying a decoupling of RbSr and SmNd isotopic systems. The initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate Rb from Sr compared to Sm from Nd. The fact that all Martian meteorites analyzed so far define a RbSr whole rock isochron age of 4.5 Ga suggests that virtually all Rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time. Thus, the Martian mantle cumulates and restites are not expected to evolve past 87 Sr 86 Sr of 0.700 and could not have been significantly enriched in incompatible elements by crustal recycling processes. All Martian meteorites have initial 87 Sr 86 Sr values that are higher than ∼0.700 and are, therefore, likely to be produced by mixing between evolved crustal-like and depleted mantle reservoirs. The absence of crustal recycling processes on Mars may preserve the geochemical evidence for decoupling of the RbSr and SmNd isotopic systems, underscoring one of the fundamental differences between geologic processes on Mars and the Earth.