Suite Of Basalts Research Articles

Petrogenesis of continental flood basalts in eastern Parnaiba basin, Brazil: A singular sill occurrence with low- and high-TiO2 tholeiites

The intracratonic Parnaiba basin, in northeast Brazil, records episodes of tholeiitic magmatism characteristic of Large Igneous Provinces (LIP's). They are represented by the Mosquito (Triassic-Jurassic) and Sardinha (Cretaceous) formations related to tension and rupture processes of Pangea and Gondwana, respectively. The Mosquito (mostly basaltic flows) and Sardinha (mostly diabase sills) formations include low- and high-TiO2 suites. This paper presents the results related with the petrogenesis of a 43 m-thick diabase sill sampled from the 1UN-26 borehole within the eastern portion of the Parnaiba basin. This stratified magmatic body comprises three petrographic facies: fine grained basalt (chilled margin); medium-grained diabase (intermediate portion) and coarse-grained gabbro (inner portion of the sill). The rocks are tholeiitic basalts and andesitic basalts which define two chemically distinct groups: high-TiO2 tholeiites (TiO2 > 2 wt %, inner body portion) and low-TiO2 tholeiites (TiO2 < 2 wt %, upper body portion). The former is more evolved (lower MgO and higher Fe2O3t, SiO2, and alkalis) and enriched in a broad spectrum of LILE (Large Ion Lithophile Elements), HFSE (High Field Strength Elements) and REE (Rare Earth Elements) compared to the latter. Geochemical modeling indicates that the different groups are not cogenetic with each other by fractional crystallization or AFC (assimilation with fractional crystallization) processes, having been generated by different mantle sources as depicted by their trace element and Sr–Nd isotope initial ratios. Negative εNdi suggests at least a contribution of the subcontinental lithospheric mantle (SCLM) in these magma genesis. The La/Ybn ratios of least evolved compositions in both suites can be explained by 15–16% partial melting from either a peridotite with 5% of residual garnet and chondritic La/Yb ratio or a peridotite with 2% of residual spinel and a La/Yb ratio twice chondritic values. Small differences in Sr–Nd isotope ratios of representative samples of the two basaltic suites preclude the involvement of rather different mantle sources but may indicate the participation of a differently enriched, vertical SCLM in the petrogenesis of these basalts. As such, local scale, vertical mantle heterogeneities in addition to the lateral heterogeneity normally observed in LIP's elsewhere may be relevant for geodynamic models involving the Parnaiba basalts.

The age, metal source and genesis of the Nifty copper deposit in the context of the geological evolution of the Paterson Province, Western Australia

The Nifty Cu deposit, hosted in Neoproterozoic shales and dolomitic mudstones of the Paleo- to Neoproterozoic Paterson Province, Western Australia, has features that have led to both syngenetic (pre-tectonic) and syn-tectonic genetic models. Here, we report Sm-Nd, Rb-Sr and U-Pb isotope data for vein apatite coeval with Cu minerals which provide absolute age constraints on apatite deposition and, therefore, Cu mineralization. Fluorapatites from two structurally and mineralogically similar vein sets have similar 87Sr/86Sr (0.7213–0.7238) and 208Pb/204Pb (37.90–38.04) ratios, unusual bell-shaped rare-earth element patterns and very high Sm/Nd ratios. A pooled Sm-Nd isochron for the vein apatites yields an age of 791 ± 42 Ma (eNdi = + 0.6 ± 3.7), consistent with a 207Pb-206Pb isochron age of 822 ± 23 Ma for galena and U-rich apatites from one of the samples. These mineral ages, combined with independent age constraints on the sedimentary host rocks and textural observations, constrain the timing of Cu mineralization to a period of ca. 850–800 Ma, that is during basin formation or early stages of basin deformation. Relatively high eNd values in vein apatite suggest that ore fluids were derived from, or reacted with, high-eNd mafic igneous rocks, as represented by a suite of gabbros and basalts (840–815 Ma) emplaced into the Neoproterozoic sequence. By contrast, 87Sr/86Sr ratios (~ 0.723) imply extensive re-equilibration of ore fluids with local sedimentary host rocks. The hypothesis of mafic igneous rocks as the source of copper at Nifty is consistent with inferred Cu sources for other sediment-hosted Cu deposits.

Multiple reservoirs of volatiles in the Moon revealed by the isotopic composition of chlorine in lunar basalts

The isotopes of chlorine (37Cl and 35Cl) are highly fractionated in lunar samples compared to most other Solar System materials. Recently, the chlorine isotope signatures of lunar rocks have been attributed to large-scale degassing processes that occurred during the existence of a magma ocean. In this study we investigated how well a suite of lunar basalts, most of which have not previously been analyzed, conform to previous models. The Cl isotope compositions (δ37Cl (‰) = [(37Cl/35Clsample/37Cl/35ClSMOC) − 1] × 1000, where SMOC refers to standard mean ocean chloride) recorded range from ∼+7 to +14‰ (Apollo 15), +10 to +19‰ (Apollo 12), +9 to +15‰ (70017), +4 to +8‰ (MIL 05035), and +15 to +22‰ (Kalahari 009). The Cl isotopic data from the present study support the mixing trends previously reported by Boyce et al. (2015) and Barnes et al. (2016), as the Cl isotopic composition of apatites are positively correlated with bulk-rock incompatible trace element abundances in the low-Ti basalts, inclusive of low-Ti and KREEP basalts. This trend has been interpreted as evidence that incompatible trace elements, including Cl, were concentrated in the urKREEP residual liquid of the lunar magma ocean, rather than the mantle cumulates, and that urKREEP Cl had a highly fractionated isotopic composition. The source regions for the basalts were thus created by variable mixing between the mantle (Cl-poor and relatively unfractionated) and urKREEP. The high-Ti basalts show much more variability in measured Cl isotope ratios and scatter around the trend formed by the low-Ti basalts. Most of the data for lunar meteorites also fits the mixing of volatiles in their sources, but Kalahari 009, which is highly depleted in incompatible trace elements, contains apatites with heavily fractionated Cl isotopic compositions. Given that Kalahari 009 is one of the oldest lunar basalts and ought to have been derived from very early-formed mantle cumulates, a heavy Cl isotopic signature is likely not related to its mantle source, but more likely to magmatic or secondary alteration processes, perhaps via impact-driven vapor metasomatism of the lunar crust.

Early loss, fractionation, and redistribution of chlorine in the Moon as revealed by the low-Ti lunar mare basalt suite

The relative abundances of chlorine isotopes measured in low-Ti basalts from the Moon appear to reflect mixing between two reservoirs: One component representing the urKREEP—the final product of the crystallization of the lunar magma ocean—with δ37Cl=+25‰ (relative to Standard Mean Ocean Chlorine), the other representing either a mare basalt reservoir or meteoritic materials with δ37Cl∼0‰. Using the abundances of other KREEP-enriched elements as proxies for the abundance of Cl in low-Ti mare basalts—which is difficult to constrain due to magmatic processes such as fractional crystallization and degassing—we find that the urKREEP contains ∼28 times higher Cl abundance (25–170 ppm Cl) as compared to the low-δ37Cl end member in the observed mixing relationship. Chlorine—with an urKREEP/C.I. ratio of 0.2 to 1.5—is 500 to 3400 times less enriched than refractory incompatibles such as U and Th, and is consistent with incomplete loss of Cl species taking place during or prior to the magma ocean phase. The preservation of multiple, isotopically distinct reservoirs of Cl can be explained by: 1) Incomplete degassing pre- or syn-giant impact, with preservation of undegassed chondritic Cl and subsequent formation of an enriched and isotopically fractionated reservoir; or 2) Development of both high-concentration, high-δ37Cl and low-concentration, low-δ37Cl reservoirs during the degassing and crystallization of the lunar magma ocean. A range of model bulk lunar Cl abundances from 0.3–0.6 ppm allows us to place Cl in the context of the rest of the elements of the periodic table, and suggests that Cl behaves as only a moderately volatile element during degassing. Chlorine isotope fractionation resulting from loss syn- or pre-magma ocean is characterized by 1000•ln⁡[α]=−3.96 to −4.04. Abundance and isotopic constraints are consistent with the loss of Cl being limited by vaporization of mixtures of Cl salts such as HCl, ZnCl2, FeCl2, and NaCl. These new constraints on the chlorine abundance and isotopic values of urKREEP make it a well-constrained target for dynamic models aiming to test plausible conditions for the formation of the Earth–Moon system.

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhon area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial 187Os/188Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.

Constraining the timing and sources of volcanism at the Apollo 12 landing site using new Pb isotopic compositions and crystallisation ages

The basaltic suites collected at the Apollo 12 landing site have been interpreted as representing a stratigraphic sequence of volcanic flows emplaced in the Oceanus Procellarum region between approximately 3100–3300 Ma. This study presents Secondary Ion Mass Spectrometry (SIMS) Pb isotopic analyses of samples from each of the basaltic suites, which have been used to constrain precise crystallisation ages and initial Pb isotopic compositions. The new crystallisation ages are consistent with the three main basaltic suites (olivine, pigeonite and ilmenite) being emplaced over a period of approximately 60 million years, and the improved precision of these ages has made it possible to reinterpret the stratigraphic sequence of basalt flows underlying the Apollo 12 landing site. Contrary to previous studies, the three ilmenite basalts are determined as having the oldest ages (with a weighted average of 3187 ± 6 Ma; 2σ) and are, therefore, interpreted as representing the lowest unit in the sequence, underlying the olivine and pigeonite basalts (with an age range constrained by the oldest and youngest pigeonite basalts; 3176 ± 6 Ma and 3129 ± 10 Ma; 2σ). The initial Pb isotopic compositions have been compared with recalculated initial Sr and Nd isotopic compositions, and are consistent with the three main basaltic suites originating from magmatic sources that incorporated different proportions of a common primitive mafic cumulate and the residual trapped liquid fraction remaining after a majority of the lunar magma ocean had crystallised. Our data also demonstrate that the feldspathic basalt (12038) is unique, both in terms of its crystallisation age (3242 ± 13 Ma) and its derivation from a distinct mantle reservoir.

Cretaceous basalts of the High Arctic large igneous province at Axel Heiberg Island (Canada): Volcanic stratigraphy, geodynamic setting, and origin

The Cretaceous (Cenomanian, ~96 Ma) basaltic rocks of the Strand Fiord Formation of the Sverdrup Basin of the Canadian Arctic Archipelago, Nunavut, Canada, are a part of the High Arctic large igneous province. The basaltic suite reaches a thickness of almost 1,000 m on Axel Heiberg Island in its depocentre and over 700 m at the studied section. The rocks are variably fractionated tholeiitic basalts, which are geochemically similar to many other continental flood basalts, particularly to low Ti basalts of flood basalt provinces. Geochemical as well as Sr (87Sr/86Srinitial~ 0.7045 to 0.7067) and Nd (ƐNd(t)~ +1.3 to +4.3) isotopic signatures of the basalts were inherited during partial melting of spinel peridotite of an ancient subcontinental lithospheric mantle, which was modified around 0.7–0.9 Ga by a metasomatic event. The melting generating the Cretaceous basalts was probably triggered by a mantle plume but was also associated with basin rifting and stretching. There is evidence of plume‐induced doming and uplift of the crust prior to and during the eruption of the basalts as documented by the progression from marine sedimentation to subaerial/lacustrine and then back to marine sedimentation in the Sverdrup Basin centre. The basalts represent parts of the Early Cretaceous igneous province that is dispersed around the Arctic Ocean due to the opening of the Canada Basin, which is inferred to be related to a mantle plume.

Tracking Hadean processes in modern basalts with 142-Neodymium

The short-lived Sm146→142Nd isotope system (t1/2=103 Ma) provides constraints on the timing and processes of terrestrial silicate fractionation during the early Hadean. Although some Archean terranes preserve variability in 142Nd/144Nd, no anomalies have been resolved previously in young rocks. This study provides high precision 142Nd/144Nd data on a suite of ocean island basalts from Samoa and Hawaii previously shown to have variable depletions in 182W/184W that are inversely correlated with 3He/4He ratios. Improved analytical techniques and multiple replicate analyses of Nd show a variation in μ142Nd values between −1.3 and +2.7 in the suite, relative to the JNdi standard. Given the reproducibility of the standard (±2.9 ppm, 2 SD), two Samoan samples exhibit resolved variability in their 142Nd/144Nd ratios outside of their 95% confidence intervals, suggesting minor variability in the Samoan hotspot. One sample from Samoa has a higher μ142Nd of +2.7, outside the 95% confidence interval (±1.0 ppm) of the average of the JNdi standard. Limited, but resolved, variation in 142Nd/144Nd within the suite suggests the preservation of early Hadean silicate differentiation in the sources of at least some basalts from Samoa. Larger variations of 182W/184W and 3He/4He ratios in the same samples suggest that metal–silicate separation and mantle outgassing left a more persistent imprint on the accessible mantle compared to 142Nd/144Nd ratios which are impacted by early silicate differentiation.

Geochronology, stratigraphy and geochemistry of Cambro-Ordovician, Silurian and Devonian volcanic rocks of the Saxothuringian Zone in NE Bavaria (Germany)—new constraints for Gondwana break up and ocean–island magmatism

Stratigraphically well-defined volcanic rocks in Palaeozoic volcano-sedimentary units of the Frankenwald area (Saxothuringian Zone, Variscan Orogen) were sampled for geochemical characterisation and U–Pb zircon dating. The oldest rock suite comprises quartz keratophyre, brecciated keratophyre, quartz keratophyre tuff and basalt, formed in Upper Cambrian to Tremadocian time (c. 497–478 Ma). Basaltic volcanism continued until the Silurian. Quartz keratophyre shows post-collisional calc-alkaline signature, the Ordovician–Silurian basalt has alkaline signature typical of continental rift environments. The combined datasets provide evidence of Cambro-Ordovician bimodal volcanism and successive rifting until the Silurian. This evolution very likely resulted from break-up of the northern Gondwana margin, as recorded in many terranes throughout Europe. The position at the northern Gondwana margin is supported by detrital zircon grains in some tuffs, with typical Gondwana-derived age spectra mostly recording ages of 550–750 Ma and minor age populations of 950–1100 and 1700–2700 Ma. The absence of N-MORB basalt in the Frankenwald area points to a retarded break-off of the Saxothuringian terrane along a continental rift system from Uppermost Cambrian to Middle Silurian time. Geochemical data for a second suite of Upper Devonian basalt provide evidence of emplacement in a hot spot-related ocean-island setting south of the Rheic Ocean. Our results also require partial revision of the lithostratigraphy of the Frankenwald area. The basal volcanic unit of the Randschiefer Formation yielded a Tremadocian age and, therefore, should be attributed to the Vogtendorf Formation. Keratophyre of the Vogtendorf Formation, previously assigned to the Tremadoc, is most likely of Upper Devonian age.

Geochemical and isotopic evidence for Carboniferous rifting: mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

Abstract In this paper, we present detailed field observations, chronological, geochemical and Sr–Nd isotopic data and discuss the petrogenetic aspects of two types of mafic dykes, of alkaline to subalkaline nature. The alkaline mafic dykes exhibit a cumulate to foliated texture and strike NW–SE, parallel to the main trend of the region. The 40Ar/39Ar amphibole age of 321.32 ± 0.55 Ma from an alkaline mafic dyke is interpreted as an indication of Carboniferous cooling through ca. 550 °C after intrusion of the dyke into the granitic Galeh-Doz orthogneiss and Amphibolite-Metagabbro units, the latter with Early Carboniferous amphibolite facies grade metamorphism and containing the Dare-Hedavand metagabbro with a similar Carboniferous age. The alkaline and subalkaline mafic dykes can be geochemically categorized into those with light REE-enriched patterns [(La/Yb)N = 8.32–9.28] and others with a rather flat REE pattern [(La/Yb)N = 1.16] and with a negative Nb anomaly. Together, the mafic dykes show oceanic island basalt to MORB geochemical signature, respectively. This is consistent, as well, with the (Tb/Yb)PM ratios. The alkaline mafic dykes were formed within an enriched mantle source at depths of ˃ 90 km, generating a suite of alkaline basalts. In comparison, the subalkaline mafic dykes were formed within more depleted mantle source at depths of ˂ 90 km. The subalkaline mafic dyke is characterized by 87Sr/86Sr ratio of 0.706 and positive ɛNd(t) value of + 0.77, whereas 87Sr/86Sr ratio of 0.708 and ɛNd(t) value of + 1.65 of the alkaline mafic dyke, consistent with the derivation from an enriched mantle source. There is no evidence that the mafic dykes were affected by significant crustal contamination during emplacement. Because of the similar age, the generation of magmas of alkaline mafic dykes and of the Dare-Hedavand metagabbro are assumed to reflect the same process of lithospheric or asthenospheric melting. Carboniferous back-arc rifting is the likely geodynamic setting of mafic dyke generation and emplacement. In contrast, the subalkaline mafic sill is likely related to the emplacement of the Jurassic Darijune gabbro.

Magnesium isotope constraints on subduction contribution to Mesozoic and Cenozoic East Asian continental basalts

To better understand the influence of subduction on geochemical heterogeneity of the mantle, Mg isotope ratios were measured in a suite of continental basalts from East Asia. In contrast to global oceanic basalts (δ26Mg=−0.25±0.07‰), these continental basalts display considerable Mg isotope variation that can be roughly related to the time and place of eruption. The Cenozoic basalts have slightly lower δ26Mg values (n=32; −0.46±0.05 to −0.25±0.06‰; average −0.38‰) than the Mesozoic basalts (n=12; −0.41±0.08 to −0.23±0.05‰; average −0.30‰) and display a decreasing δ26Mg trend with increasing distance from the present subduction trench, particularly for the basalts from South China Craton. Such variations may be related to westward subduction of Pacific oceanic plate beneath East Asia. Some outliers of Mg isotopic data in the North China Craton probably involve additional contributions from earlier subduction and collision from the south (Tethyan ocean and Yangtze Craton) and north (Paleo-Asian ocean). The correlations between δ26Mg and element indices (e.g., TiO2, Li/Y) of contributions of slab melt indicate input of a component from the slab that is progressively dehydrating in the source region. Highly variable Sr, Nd and Pb isotope ratios for a given δ26Mg value suggest continuous interaction rather than a simple binary mixing of the mantle with subduction components. Intra-continental magmatism in East Asian is thus derived from a mantle wedge affected by the ongoing subduction of the Pacific Plate. Mixing with the uniform Mg isotopic compositions of MORB and OIB sources, global mantle convection likely erases the enriched components, whereas mantle wedges might be able to retain some Mg isotope heterogeneity.

Mantle transition zone-derived EM1 component beneath NE China: Geochemical evidence from Cenozoic potassic basalts

The isotopic characteristics of the sub-oceanic mantle are well established, but in continental regions these properties are usually obscured, and therefore controversial, because of the potential effects of crustal contamination together with lithospheric mantle metasomatism and melting. The so-called EM1 (Enriched Mantle-1) signature, characterized by low 206Pb/204Pb and 143Nd/144Nd ratios, is particularly problematic in this respect because EM1-type OIB sources are commonly attributed to recycled crust and/or lithospheric mantle. In this paper we show that a suite of Cenozoic potassic basalts from NE China displays many previously unrecognized correlations between chemical and isotopic parameters, which tightly constrain the isotopic characteristics of an extreme EM1-type mantle source located in the asthenosphere. Its radiogenic isotopes are similar to, but even more extreme than, those of the oceanic endmember composition represented by the Pitcairn hotspot, namely 206Pb/204Pb ≤ 16.5, 143Nd/144Nd ≤ 0.5123 (or εNd≤−6.4), 176Hf/177Hf ≤ 0.2825 (or εHf≤−10.1). These characteristics require a source of recycled crustal material of Precambrian age (∼2.2 Ga). An important new constraint is the Mg isotopic composition of δ26Mg (≤−0.6‰), which is lower than normal mantle (δ26Mg=−0.25±0.07‰) and lower crustal values (δ26Mg=−0.29±0.15‰), but consistent with sedimentary carbonate (δ26Mg=−5.57‰ to −0.38‰). The endmember EM1 source produced high-SiO2 melts with low MgO, CaO/Al2O3 and δ26Mg values, exceptionally high K/U ≅ 50,000, Ba/Th ≅ 400, low U/Pb ≅ 0.06, and positive Zr and Hf anomalies. The chemical and isotopic parameters of this potassic basalt suite form binary mixing arrays, one end point of which is the inferred asthenospheric EM1 reservoir, whereas the other is a more ordinary, depleted mantle component, which is also sampled by local lithospheric mantle xenoliths. These binary arrays include well-developed correlations between Sr, Nd, Hf, Pb and Mg isotopes, negative correlations of 206Pb/204Pb with K2O, K/U, Hf/Hf⁎, positive correlations of δ26Mg with MgO, and 143Nd/144Nd with Fe2O3T and U/Pb.We propose that the EM1 reservoir contains recycled ancient carbonate-bearing sediments, subducted into the mantle transition zone, where K, Rb, Ba and Pb are sequestered by K-hollandite as suggested by Murphy et al. (2002) for the Gaussberg lamproites. Loss of small amounts of carbonate melt extracted Th, U and some of the LREE, while retaining K, Rb, Ba, Pb, Zr and Hf in the residue, thereby generating the observed trace element anomalies. In Cenozoic time, this deep EM1 reservoir ascended into the shallow asthenosphere and underwent low-degree partial melting, at pressures below the stability field of K-hollandite, thereby releasing K, Rb and Ba into the melt. The partial melts ascended through subcontinental lithosphere and were progressively modified by interaction with the lithospheric mantle, thus accounting for the linear chemical and isotopic trends noted above. This interaction imposed a progressively more depleted character on the erupted melt, both in terms of isotopic composition and trace element enrichment.

Mineralogy and chemistry of San Carlos high-alkali basalts: Analyses of alteration with application for Mars exploration

The discovery of Fe, Mg, and Al phyllosilicates on Mars using visible and short-wave infrared (VSWIR) spectroscopy from orbit indicates aqueous alteration of basaltic rocks. Analyses at Gusev Crater by the Spirit rover and Gale Crater by the Curiosity rover have discovered alkaline basaltic rocks. In this work, multiple methods—VSWIR spectroscopy, X-ray diffraction (XRD), and chemical analyses—were used to study a suite of alkaline basalts from San Carlos, Arizona, which have been altered by water in an oxidative, semi-arid environment. As an analog for the weathering of alkaline basaltic rocks on Mars, a suite of rocks visually identified to have different degrees of alteration were characterized to understand the spectral, mineralogical, and chemical trends in alteration as sensed by multiple techniques. Samples with strong 1.9 μm H_2O-related absorptions in VSWIR commonly exhibited absorption bands at 1.4, 2.2, and/or 2.3 μm, indicating the presence of clay minerals or silica as well as features at 0.5–0.9 μm indicative of ferric iron oxides. Primary mineralogy for all samples, as determined by point analyses with the microprobe and XRD, consisted of olivine, plagioclase, nepheline, augite, and titanomagnetite. Compositional imaging and spot analyses with the microprobe revealed distinct alteration textures and phases, suggesting weathering pathways involving the oxidation of iron in olivine and primary Fe^(2+) oxides to form Fe^(3+) oxides as well as the formation of aluminum phyllosilicates and magnesium phyllosilicates from feldspars and olivines, respectively, while pyroxene remained relatively unaltered. Bivariate plots of major oxides both from bulk-chemical analysis and microprobe measurements also revealed trends in alkali and silica depletion and calcium enrichment, but there was little chemical fractionation in most of the major oxides. The strength of the 1.9 μm H_2O absorption, loss on ignition, and depletion in silica and sodium, correlated with increasing alteration. The data sets provide an analog for understanding possible weathering pathways in martian alkaline basalts and thresholds for the detection of aqueous alteration in multiple data sets.

Evidence for the solar wind in lunar magmas: A study of slowly cooled samples of the Apollo 12 olivine basalt suite

Evidence for the solar wind in lunar magmas: A study of slowly cooled samples of the Apollo 12 olivine basalt suite

A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment

The pattern of trace element enrichment and variability found in differentiated suites of basalts is a simple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystallization of a single primary melt. Explanations for this over-enrichment have focused on the differentiation processes in crustal magma chambers. Here we consider an additional mechanism and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high-MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallization (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crystallization. Using a simple statistical model to explore the effects of concurrent mixing and crystallization on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO content, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]–MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, their MgO contents and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can result from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk Kd. Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallization, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply.

Boron isotopes reveal multiple metasomatic events in the mantle beneath the eastern North China Craton

Linkages inferred between the geochemical heterogeneity of the mantle beneath eastern Eurasia and the stagnant Pacific slab documented geophysically in its mantle transition zone are as yet not clearly characterized. In this paper we report new elemental and isotopic data for boron (B) on a suite of well-characterized Cenozoic basalts (alkali basalts, basanites and nephelinites), with ocean island basalt (OIB)-like trace element signatures from western Shandong of the eastern North China Craton (NCC). Correlations between major elements (e.g., FeOT versus SiO2), trace elements (e.g., CeN/PbN versus BaN/ThN) and radiogenic isotopes (e.g., 206Pb/204Pb versus 87Sr/86Sr) suggest these basalts are derived via the mixing of melts from two mantle components: a fluid mobile element (FME; such as Ba, K, Pb and Sr) enriched component, which is most evident in the alkali basalts, and a FME depleted mantle component that is more evident in the basanites and nephelinites. The alkali basalts in this study have lower B concentrations (1.4–2.2μg/g) but higher δ11B (−4.9 to −1.4) values than the basanites and nephelinites (B=2.1–5.0μg/g; δ11B=−6.9 to −3.9), and all the samples have nearly constant B/Nb ratios between 0.03 and 0.07, similar to the observed range in B/Nb for intraplate lavas. Our high-SiO2 samples have higher δ11B than that of our low SiO2 samples, indicating that the B isotopic differences among our samples do not result from the addition of a continental crustal component in the mantle source, or direct crustal assimilation during the eruption process. The positive B versus Nb correlation suggests the B isotopic compositions of the western Shandong basalts primarily reflect the pre-eruptive compositions of their mantle sources. Correlations among B, Nd and Sr isotope signatures of the western Shandong basalts differ from those among basalts from plume settings (e.g., Azores and Hawaii), and are inconsistent with models suggesting single-step metasomatic additions to the mantle. We thus call upon multiple metasomatic events in the mantle beneath the eastern NCC in order to interpret its observed radiogenic and boron isotopic variability. The heavier δ11B, FME-enriched mantle source developed during an older event, while metasomatism by melts from the stagnant Pacific plate in the MTZ led to the development of a FME-depleted mantle source at greater depths with a lower δ11B.

An analysis of Apollo lunar soil samples 12070,889, 12030,187, and 12070,891: Basaltic diversity at the Apollo 12 landing site and implications for classification of small‐sized lunar samples

AbstractLunar mare basalts provide insights into the compositional diversity of the Moon's interior. Basalt fragments from the lunar regolith can potentially sample lava flows from regions of the Moon not previously visited, thus, increasing our understanding of lunar geological evolution. As part of a study of basaltic diversity at the Apollo 12 landing site, detailed petrological and geochemical data are provided here for 13 basaltic chips. In addition to bulk chemistry, we have analyzed the major, minor, and trace element chemistry of mineral phases which highlight differences between basalt groups. Where samples contain olivine, the equilibrium parent melt magnesium number (Mg#; atomic Mg/[Mg + Fe]) can be calculated to estimate parent melt composition. Ilmenite and plagioclase chemistry can also determine differences between basalt groups. We conclude that samples of approximately 1–2 mm in size can be categorized provided that appropriate mineral phases (olivine, plagioclase, and ilmenite) are present. Where samples are fine‐grained (grain size &lt;0.3 mm), a “paired samples t‐test” can provide a statistical comparison between a particular sample and known lunar basalts. Of the fragments analyzed here, three are found to belong to each of the previously identified olivine and ilmenite basalt suites, four to the pigeonite basalt suite, one is an olivine cumulate, and two could not be categorized because of their coarse grain sizes and lack of appropriate mineral phases. Our approach introduces methods that can be used to investigate small sample sizes (i.e., fines) from future sample return missions to investigate lava flow diversity and petrological significance.

Geochronology and geochemistry of the Triassic bimodal volcanic rocks and coeval A-type granites of the Olzit area, Middle Mongolia: Implications for the tectonic evolution of Mongol–Okhotsk Ocean

The Olzit volcanism in Middle Mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the Main Mongolia Lineament. The basalts are characterized by enrichment in LILE and LREE, and depletion in HFSE with typical Sr–Nd isotopic signatures (εNd(t)=−2.50 to −0.38 and (87Sr/86Sr)i=0.7058–0.7063), indicating they were likely derived from partial melting of an enriched lithospheric mantle, modified by subducted slab-derived fluids. The rhyolites show a close affinity to A-type granites with enrichment in LILE and LREE, and depletion in Nb, Ta and Ti. They also show a significant negative Eu anomaly, and have εNd(t) values ranging from 0.50 to 1.38 and initial 87Sr/86Sr ratios ranging from 0.7022 to 0.7200, suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt. The rhyolite porphyry yields a SHRIMP zircon U–Pb age of 207±2Ma (MSWD=1.42), indicating the bimodal volcanic suite formed in the Late Triassic. The miarolitic per-alkaline granite and biotite-bearing granite, which are associated with the bimodal volcanic rocks, show typical A-type granitic geochemical affinity with εNd(t)=0.89–0.91 and (87Sr/86Sr)i=0.7021–0.7043, indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end-member of bimodal suite. The miarolitic per-alkaline granite and biotite-bearing granite yielded SHRIMP zircon U–Pb ages of 209±2Ma (MSWD=0.91) and 213±3Ma (MSWD=1.65) respectively, which are nearly coeval with the age of the bimodal volcanic suites. In view of the new geochemical and chronological data in this study, we suggest the Olzit Late Triassic bimodal volcanic rocks together with the coeval A-type granites represent a back-arc basin extensional environment, which probably related to the roll-back of Mongol–Okhotsk oceanic plate during the southward subduction under the Central Mongolia microcontinent.

Petrogenesis and timing of mafic magmatism, South Taimyr, Arctic Siberia: A northerly continuation of the Siberian Traps?

The Siberian large igneous province (LIP) forms the world's most extensive continental exposure of basalt and has several sub-provinces surrounding it, which may be genetically related. The Taimyr peninsula of north Siberia is one of these sub-provinces and is frequently assumed to be the northerly continuation of the basalts exposed at Noril'sk, the best-studied area of the Siberian LIP. However, the correlation is uncertain.We present new major and trace element data from 35 samples of extrusive and intrusive rocks from Taimyr, with Sr and Nd isotope data from a subset of ten. The Taimyr rocks fall into two groups with low (~7wt.%) and elevated (~9wt.%) MgO concentrations. The high-MgO rocks display a restricted range of initial 87Sr/86Sr (0.705 to 0.706) and 143Nd/144Nd (0.5122 to 0.5124) ratios, and share bulk silicate earth normalised rare earth element patterns strikingly similar to data observed in the ore-related Noril'sk intrusions. The remaining low-MgO group samples have a broader range with higher Sr and lower Nd isotope values and higher incompatible trace element ratios (e.g., Th/Ta>5.3 and La/Smn>1.7) similar to the crustally-contaminated Nadezhdinsky and Morongovsky suite basalts of the Noril'sk region. The major and trace element data for both groups are consistent with a process of fractional crystallisation coupled with small degrees of assimilation of incompatible-element-enriched lower crust involving different contaminants. Trace element model calculations indicate a process of magma formation at large degrees of partial melting and at pressures of less than 3GPa, probably within the garnet–spinel transition zone or the spinel stability field of the asthenospheric mantle. We obtained an argon plateau age of ~252 (252.7±1.5) Ma and a ~239Ma total fusion age from a Taimyr lava and intrusive sample, respectively, confirming that volcanism is only partly contemporaneous with the activity of the Siberian LIP. Although this is in agreement with previous interpretations, we argue that the age difference between both events is only ~13Ma and probably less (~5Ma) although further investigation of the relationship is required. Our data allow correlation with distinct Noril'sk members and most importantly to the ore-bearing (Ni–Cu) intrusions implying that whole rock chemistry could have value as a prospecting tool in Taimyr.

Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: Discrimination of contributions from the subducted Pacific slab

Contributions from fluid and melt inputs from the subducting Pacific slab to the chemical makeup of intraplate basalts erupted on the eastern Eurasian continent have long been suggested but have not thus far been geochemically constrained. To attempt to address this question, we have investigated Cenozoic basaltic rocks from the western Shandong and Bohai Bay Basin, eastern North China Craton (NCC), which preserve coherent relationships among the chemistries of their melt inclusions, their hosting olivines and their bulk rock compositions. Three groups of samples are distinguished: (1) high-Si and (2) moderate-Si basalts (tholeiites, alkali basalts and basanites) which were erupted at ∼23–20Ma, and (3) low-Si basalts (nephelinites) which were erupted at <9Ma. The high-Si basalts have lower alkalies, CaO and FeOT contents, lower trace element concentrations, lower La/Yb, Sm/Yb and Ce/Pb but higher Ba/Th ratios, and lower εNd and εHf values than the low-Si basalts. The olivines in the high-Si basalts have higher Ni and lower Mn and Ca at a given Fo value than those crystallizing from peridotite melts, and their corresponding melt inclusions have lower CaO contents than peridotite melts, suggesting a garnet pyroxenitic source. The magmatic olivines from low-Si basalts have lower Ni but higher Mn at a given Fo value than that of the high-Si basalts, suggesting more olivine in its source. The olivine-hosted melt inclusions of the low-Si basalts have major elemental signatures different from melts of normal peridotitic or garnet pyroxenitic mantle sources, pointing to their derivation from a carbonated mantle source consisting of peridotite and garnet pyroxenite. We propose a model involving the differential melting of a subduction-modified mantle source to account for the generation of these three suites of basalts. Asthenospheric mantle beneath the eastern NCC, which entrains garnet pyroxenite with an EM1 isotopic signature, was metasomatized by carbonatitic melts from carbonated eclogite derived from subducted Pacific slab materials present in the deeper mantle. High degree melting of garnet pyroxenites from a shallower mantle source produced the early (∼23–20Ma) higher-Si basalts. Mixing of these materials with deeper-sourced melts of carbonated mantle source produced the moderate-Si basalts. A thicker lithosphere after 9Ma precluded melting of shallower garnet pyroxenites, so melts of the deeper carbonated mantle source are responsible for the low-Si basalts.