Young Basalts Research Articles

The Inner Solar System Chronology (ISOCHRON) Lunar Sample Return Mission Concept: Revealing Two Billion Years of History

Abstract We present the Inner SOlar System CHRONology (ISOCHRON) concept to return samples of the youngest extensive mare basalt for age-dating and geochemical analysis. The young basalt is exposed at a site southwest of Aristarchus Plateau, for which complete remote-sensing data are available for thorough landing site analysis. Data from these samples would revolutionize the ability to assign exposure ages to rocky planetary surfaces based on the samples returned by Apollo and Luna. Their petrology and geochemistry will enable assessment of the most recent voluminous lunar magmatism. Regolith evolution and mixing models such as ballistic sedimentation would be directly testable to provide crucial ground truth that would enhance the science value of current and future remotely sensed data sets. ISOCHRON’s science goals support NASA’s Artemis program to return to the Moon and its related robotic programs currently in planning.

Petrogenesis of the Cenozoic volcanic rocks in Baekdu volcano in northeastern Asia and the expected depth of the magma chamber based on geochemistry, mineral chemistry, and Sr-Nd-Pb isotope chemistry

Baekdu volcano is a Cenozoic composite volcano in northeastern Asia that consists of a basaltic plateau, a basaltic shield, and a trachytic to comenditic stratovolcano. In this study, the petrogenesis and tectonic setting of Baekdu volcano are studied based on the whole-rock geochemistry and Sr-Nd-Pb isotope compositions of the Baekdu volcanic rocks. The K–Ar age and mineral chemistry are also investigated to estimate the evolution of the magma chamber depth beneath Baekdu volcano. The Toudao and Baishan basalts formed ca. 2.5 Ma and 1.1 Ma and erupted from magma chambers located at depths of 11.5 ± 2.2 km and 24.9 ± 3.8 km, respectively. The Xiaobaishan and Middle trachytes and the comendites formed during the Pleistocene from magma chambers located at depths of 2.9 ± 0.2 km, 5.4 ± 0.45 km, and 4.4 ± 0.3 km, respectively. These data suggest that the trachytes and comendites erupted from magma chambers located at depths of approximately 3–5 km. The Baishan basalts have geochemical characteristics comparable with those of ocean island basalt (OIB), whereas the Toudao basalts show enriched mid-ocean ridge basalt (E-MORB) or OIB characteristics. The trends of several components, such as the CaO/Al2O3 ratio and the TiO2 and Sc concentrations, are different between older basalts (>2 Ma, including the Toudao basalts) and younger basalts (<1.5 Ma, including the Baishan basalts), indicating that the magma-forming process changed ca. 2–1.5 Ma. The enrichment in Th/Yb ratios in the Toudao basalts indicates that they experienced crustal contamination. Trachytes and comendites are considered to be products of the fractional crystallization of the Baishan basalt based on the similar geochemical trends of the two rocks. Crustal assimilation (up to 20%) also occurred with fractional crystallization (more than 70%) during the formation of peralkaline silicic volcanic rocks. All volcanic rocks in Baekdu volcano have Sr-Nd-Pb isotope compositions comparable with the enriched mantle (EM1)-like signature. This EM1-like signature may have originated from the contamination of asthenospheric mantle with Paleoproterozoic lithosphere that sank into the asthenospheric mantle due to delamination caused by regional extensional events in northeastern Asia between the Cretaceous and the Cenozoic. The characteristics of trace element compositions indicate a within-plate tectonic setting for Baekdu volcano. These data imply that Baekdu volcano origin may have been related to a plume of EM1-type upper mantle under the North China Craton.

New Lunar Samples Returned by Chang’e-5: Opportunities for New Discoveries and International Collaboration

On December 16th, 2020 at 17:59 UTC, the sample capsule of Chang’e-5 successfully landed at Dorbod Banner, Inner Mongolia, China. It is a milestone for the Chinese Lunar Exploration Program (CLEP), which has achieved the goals of its first three phases: orbiting, landing, and sample return. China becomes the third country to return samples from the Moon after the United States and the Soviet Union. Forty-four years since the Luna 24 mission in 1976, new lunar samples have been returned to Earth. Chang’e-5 was launched from Wenchang Satellite Launch Center, Hainan Island, on November 23rd, 2020 at 20:30 UTC and landed on the Moon in a region known as Oceanus Procellarum (Figure 1) on December 1st, 2020. After 19 hours of lunar surface activities, sample collection and packaging were completed on December 2nd. Then, the ascent vehicle took off with the samples from atop the lander on December 3rd to rendezvous with the orbiting spacecraft (returner vehicle). The sample capsule reentered the Earth's atmosphere on December 16th, 2020. Chang’e-5 collected the lunar samples in two ways: soil core drilling, and soil and rock fragment scooping (Figure 1). The lander was equipped with a panoramic camera, a landing camera, a visible and near-infrared imaging spectrometer, and a ground-penetrating radar. These payloads obtained in situ reflection spectra of the surface materials, and the topography and shallow structure of the landing site, providing vital geological context for the returned samples. The samples returned from the Moon are invaluable for planetary sciences. For example, the discovery of pure anorthosite fragments in the Apollo samples revealed a global magma ocean in the Moon's early history.3Wood J.A. Dickey Jr., J.S. Marvin U.B. Powell B.N. Lunar anorthosites.Science. 1970; 167: 602-604Crossref PubMed Scopus (93) Google Scholar When calibrated using the absolute ages of the returned samples, the impact craters on the Moon provide an approximate relative chronology for other planetary surfaces in the inner solar system.4Stöffler D. Ryder G. Ivanov B.A. et al.Cratering history and lunar chronology.Rev. Mineral. Geochem. 2006; 60: 519-596Crossref Scopus (267) Google Scholar The compositional relationship between the Earth and the Moon offers constraints on various lunar formation hypotheses.5Canup R.M. Dynamics of lunar formation.Annu. Rev. Astron. Astrophys. 2004; 42: 441-475Crossref Scopus (198) Google Scholar The landing site of Chang’e−5 is located far from the six Apollo and three Luna sampling sites. The crater counting method indicates that the lava flows at the Chang’e-5 landing site are much younger than the basalts collected by the Apollo and Luna missions. Furthermore, the remote sensing data show that these younger lavas are enriched in K, REE (rare earth elements), and P components (so-called KREEP, the proposed last dregs of the Lunar Magma Ocean) with high abundances of radiogenic elements, such as K, U, and Th (Figure 1). Therefore, these new lunar samples will allow a better understanding of the formation and evolution of the Moon. For example, calibration with the isotope chronological data can make the crater counting method more accurate for younger geological events at the Moon and the rest of the inner solar system. These young basalt rocks can reveal the late history of lunar volcanism and the geochemical features of the lunar mantle reservoir. The drill core of lunar soil will benefit the understanding of the effects and mechanisms of space weathering on airless bodies and the secular evolution of the solar wind. The CLEP consists of five subsystems: the spacecraft system, the rocket system, the launch site system, the tracking and control system, and the ground application system. The ground application system is responsible for curating the lunar samples. The China National Space Administration (CNSA) established a lunar sample management committee to evaluate the requests for the samples. To promote collaboration and interaction between Chinese and European scientists in the area of lunar science, CNSA and the European Space Agency (ESA) co-organized four workshops, in-person in Beijing (2017), Amsterdam (2018), and Zhuhai (2019), and an online one (2020) due to the COVID-19 pandemic. Both sides have established the CNSA-ESA joint research team to analyze lunar samples and participate in lunar exploration. Chang’e-5 is the first sample return mission of the long-planned sequence of space exploration by China. Its technology and experience will be utilized for future sample return missions. Chang’e-6 will attempt to return samples from the Moon's south pole region. Sample return missions to Mars and an asteroid as the quasi-satellite of the Earth are also planned for this decade. As the world's second-largest economy, China looks forward to contributing to human deep space exploration. Chinese scientists look forward to collaborating with scientists worldwide to contribute to planetary sciences.

On the vertical growth of continents in deep depressions of the Earth’s peridotite mantle

Research subject. The geological structure and evolution of the Earth’s continents.Methods. This article is based on a long-term study and review of geological, geophysical and bathymetric published data, as well as on an analysis of the major geological discoveries of the 20th century.Results and conclusions. It is established that all the continents on the Earth, except for Antarctica, constitute a single Northen megamaterik, which was being formed during a prolonged period of time (4.4 billion years) in a deep three-beam cavity on the surface of the peridotite mantle. The ancient Hadean– Archean basement of the megacontinent was being formed during the period of 3 billion years, which comprises about 70% of the Earth’s geological history. In the Late Proterozoic and Phanerozoic, periodically formed local depressions were flooded with sedimentary material leading to the formation of sedimentary basins and folded rock structures. As a result, the thickness of the megacontinent’s crust steadily increased reaching a large size of 15–40 or 60–70 km. During this period, the primary oceanic (peridotite) crust with a thickness of 3–5 km remained unchanged until the Mesozoic–Cenozoic, when it was covered with a layer of younger basalts and loose rock sediments with a thickness of 1–2 km.

Regionally continuous Miocene rhyolites beneath the eastern Snake River Plain reveal localized flexure at its western margin: Idaho National Laboratory and vicinity

The eastern Snake River Plain (ESRP) is a northeast-trending topographic basin interpreted to be the result of the time-transgressive track of the North American plate above the Yellowstone hotspot. The track is defined by the age progression of silicic volcanic rocks exposed along the margins of the ESRP. However, the bulk of these silicic rocks are buried under 1 to 3 kilometers of younger basalts. Here, silicic volcanic rocks recovered from boreholes that penetrate below the basalts, including INEL-1, WO-2 and new deep borehole USGS-142, are correlated with one another and to surface exposures to assess various models for ESRP subsidence. These correlations are established on U/Pb zircon and 40Ar/39Ar sanidine age determinations, phenocryst assemblages, major and trace element geochemistry, δ18O isotopic data from selected phenocrysts, and initial εHf values of zircon. These data suggest a correlation of: (1) the newly documented 8.1 ± 0.2 Ma rhyolite of Butte Quarry (sample 17KS03), exposed near Arco, Idaho to the upper-most Picabo volcanic field rhyolites found in borehole INEL-1; (2) the 6.73 ± 0.02 Ma East Arco Hills rhyolite (sample 16KS02) to the Blacktail Creek Tuff, which was also encountered at the bottom of borehole WO-2; and (3) the 6.42 ± 0.07 Ma rhyolite of borehole USGS-142 to the Walcott Tuff B encountered in deep borehole WO-2. These results show that rhyolites found along the western margin of the ESRP dip ~20º south-southeast toward the basin axis, and then gradually tilt less steeply in the subsurface as the axis is approached. This subsurface pattern of tilting is consistent with a previously proposed crustal flexural model of subsidence based only on surface exposures, but is inconsistent with subsidence models that require accommodation of ESRP subsidence on either a major normal fault or strike-slip fault.

Chronology of young basalt flows from lava tube gypsum U-series ages

Basaltic lavas provide a window into their mantle sources and the associated geodynamic processes associated with them. Obtaining absolute chronology on young basalt flows is often challenging. Some gypsum crusts in pyro-duct (lava tube) caves crystallize during the last “cooling” stages of tube formation probably at relatively low temperatures (i.e., <100 °C). Here we propose uranium-series dating of these ‘speleogenetic’ gypsum crusts as a way to establish the ages of young basalt flows. We demonstrate the technique using cave crusts from four young basalt flows in El Malpais National Monument, New Mexico, three of which are moderately well-dated by alternative methods ( 14 C, 36 Cl, and 3 He ages). They all exhibit well-formed lava tube systems that contain gypsum crust speleothems interpreted to be remnant of the volcanic sulfur or sulfate processes when the tube systems were active during their terminal stages. δ 18 O SO4 , δ 34 S, 87 Sr/ 86 Sr, and δ 234 U values for the gypsum crusts have near-basaltic signatures, and the crusts contain uranium concentrations suitable for uranium-series dating in three of these four basalt flows. Analyses of gypsum from caves of two well-dated basalt flows yielded uranium-series ages that agree with the ages determined by other techniques showing that absolute ages of young basalt flows can be achieved by this method. The fourth flow, the Hoya de Cibola basalt, previously undated by other absolute means has a U–Th gypsum crust isochron age of 16.9 ± 0.9 ka. The uranium-series age and isotopic analyses of these gypsum crusts should be useful in geologic applications involving young basalt flows.

Melting of recycled ancient crust responsible for the Gutenberg discontinuity

A discontinuity in the seismic velocity associated with the lithosphere-asthenosphere interface, known as the Gutenberg discontinuity, is enigmatic in its origin. While partial mantle melts are frequently suggested to explain this discontinuity, it is not well known which factors critically regulate the melt production. Here, we report geochemical evidence showing that the melt fractions in the lithosphere-asthenosphere boundary were enhanced not only by accumulation of compacted carbonated melts related to recycled ancient marine sediments, but also by partial melting of a pyroxene-rich mantle domain related to the recycled oceanic eclogite/pyroxenites. This conclusion is derived from the first set of Mg isotope data for a suite of young petit-spot basalts erupted on the northwest Pacific plate, where a clearly defined Gutenberg discontinuity exists. Our results reveal a specific linkage between the Gutenberg discontinuity beneath the normal oceanic regions and the recycling of ancient subducted crust and carbonate through the deep Earth.

Authigenic Mineral Texture in Submarine 1979 Basalt Drill Core, Surtsey Volcano, Iceland

AbstractMicrometer‐scale maps of authigenic microstructures in submarine basaltic tuff from a 1979 Surtsey volcano, Iceland, drill core acquired 15 years after eruptions terminated describe the initial alteration of oceanic basalt in a low‐temperature hydrothermal system. An integrative investigative approach uses synchrotron source X‐ray microdiffraction, microfluoresence, micro‐computed tomography, and scanning transmission electron microscopy coupled with Raman spectroscopy to create finely resolved spatial frameworks that record a continuum of alteration in glass and olivine. Microanalytical maps of vesicular and fractured lapilli in specimens from 157.1‐, 137.9‐, and 102.6‐m depths and borehole temperatures of 83, 93.9, and 141.3 °C measured in 1980, respectively, describe the production of nanocrystalline clay mineral, zeolites, and Al‐tobermorite in diverse microenvironments. Irregular alteration fronts at 157.1‐m depth resemble microchannels associated with biological activity in older basalts. By contrast, linear microstructures with little resemblance to previously described alteration features have nanocrystalline clay mineral (nontronite) and zeolite (amicite) texture. The crystallographic preferred orientation rotates around an axis parallel to the linear feature. Raman spectra indicating degraded and poorly ordered carbonaceous matter of possible biological origin are associated with nanocrystalline clay mineral in a crystallographically oriented linear microstructure in altered olivine at 102.6 m and with subcircular nanoscale cavities in altered glass at 137.9‐m depth. Although evidence for biotic processes is inconclusive, the integrated analyses describe the complex organization of previously unrecognized mineral texture in very young basalt. They provide a foundational mineralogical reference for longitudinal, time‐lapse characterizations of palagonitized basalt in oceanic environments.

Significance of high field strength and rare earth element distributions in deciphering the evolution of the inner solar system

Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings.Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (>4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir (<3 Ga), perhaps located in the two LLSVPs commonly sampled by oceanic island basalts. Between 3 and 2 Ga, the recycled enriched mantle reservoir became established in Earth, possibly in response to the widespread propagation of subduction. On Mars enriched mantle shows depleted radiogenic isotopic signatures and requires a multistage process to decouple trace element and isotopic systems.Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (<15) characteristic of some planetary and asteroid basalts may reflect separation of a metal-sulfide melt enriched in Nb, which may or may not produce a core. This fractionation must occur early during a relatively reduced stage of planetary evolution (IW-3 to IW-5) such that Nb behaves as a chalcophile or siderophile element. If the average Nb/Ta ratio of both primitive and depleted mantle is equal to 15, production of basaltic magma in the terrestrial mantle through time has not fractionated Nb from Ta. On the other hand, if the Nb/Ta in primitive mantle equals 17, Nb must be fractionated from Ta before 4 Ga, perhaps by partitioning into the core during or soon after planetary accretion when reducing conditions may have existed.

The Thickness and Volume of Young Basalts Within Mare Imbrium

AbstractBasaltic volcanism is one of the most important geologic processes of the Moon. Research on the thickness and volume of late‐stage basalts of Mare Imbrium helps better understand the source of lunar volcanism and eruption styles. Based on whether apparent flow fronts exist or not, the late‐stage basalts within Mare Imbrium were divided into two groups, namely, Upper Eratosthenian basalts (UEm) and Lower Eratosthenian basalts (LEm). Employing the topographic profile analysis method for UEm and the crater excavation technique for LEm, we studied the thickness and distribution of Eratosthenian basalts in Mare Imbrium. For the UEm units, their thicknesses were estimated to be ~16–34 (±2) m with several layers of individual lava (~8–13 m) inside. The estimated thickness of LEm units was ~14–45(±1) m, with a trend of reducing thickness from north to south. The measured thickness of late‐stage basalts around the Chang'E‐3 landing site (~37 ± 1 m) was quite close to the results acquired by the lunar penetrating radar carried on board the Yutu Rover (~35 m). The total volume of the late‐stage basalts in Mare Imbrium was calculated to be ~8,671 (±320) km3, which is 4 times lower than that of Schaber's estimation (~4 × 104 km3). Our results indicate that the actual volume is much lower than previous estimates of the final stage of the late basaltic eruption of Mare Imbrium. Together, the area flux and transport distance of the lava flows gradually decreased with time. These results suggest that late‐stage volcanic evolution of the Moon might be revised.

Diversity and the environmental drivers of spatial variation in Bacteria and micro-Eukarya communities from the Hawaiian anchialine ecosystem

Little is currently known regarding microbial community structure, and the environmental factors influencing it, within the anchialine ecosystem, defined as near-shore, land-locked water bodies with subsurface connections to the ocean and groundwater aquifer. The Hawaiian Archipelago is home to numerous anchialine habitats, with some on the islands of Maui and Hawaii harboring unique, laminated orange cyanobacterial–bacterial crusts that independently assembled in relatively young basalt fields. Here, benthic and water column bacterial and micro-eukaryotic communities from nine anchialine habitats on Oahu, Maui, and Hawaii were surveyed using high-throughput amplicon sequencing of the V6 (Bacteria-specific) and V9 (Eukarya-biased) hypervariable regions of the 16S- and 18S-rDNA genes, respectively. While benthic communities from habitats with cyanobacterial–bacterial crusts were more similar to each other than to ones lacking it on the same island, each habitat had distinct benthic and water column microbial communities. Analyses of the survey data in the context of environmental factors identified salinity, site, aquifer, and watershed as having the highest explanatory power for the observed variation in microbial diversity and community structure, with lesser drivers being annual rainfall, longitude, ammonium, and dissolved organic carbon. Our results epitomize the abiotic and biotic uniqueness characteristic of individual habitats comprising the Hawaiian anchialine ecosystem.

Dissolution and secondary mineral precipitation in basalts due to reactions with carbonic acid

AbstractOne of the leading hydrothermal alteration processes in volcanic environments is when rock‐forming minerals with high concentrations of iron, magnesium, and calcium react with CO2 and water to form carbonate minerals. This is used to the advantage of geologic sequestration of anthropogenic CO2. Here we experimentally investigate how mineral carbonation processes alter the rock microstructure due to CO2‐water‐rock interactions. In order to characterize these changes, CO2‐water‐rock alteration in Auckland Volcanic Field young basalts (less than 0.3 Ma) is studied before and after a 140 day reaction period. We investigate how whole core basalts with similar geochemistry but different porosity, permeability, pore geometry, and volcanic glass content alter due to CO2‐water‐rock reactions. Ankerite and aluminosilicate minerals precipitate as secondary phases in the pore space. However, rock dissolution mechanisms are found to dominate this secondary mineral precipitation resulting in an increase in porosity and decrease in rigidity of all samples. The basalt with the highest initial porosity and volcanic glass volume shows the most secondary mineral precipitation. At the same time, this sample exhibits the greatest increase in porosity and permeability, and a decrease in rock rigidity post reaction. For the measured samples, we observe a correlation between volcanic glass volume and rock porosity increase due to rock‐fluid reactions. We believe this study can help understand the dynamic rock‐fluid interactions when monitoring field scale CO2 sequestration projects in basalts.

The scale of late-stage basalts in Mare Imbrium

The basaltic volcanism is one of the most important geologic processes of the Moon, which reveals its interior evolution. As lunar basalts are formed from the partial melting of mantle materials, the volume of basalts reflects the size and the degree of partial melting of the source region. The Eratosthenian volcanism deposited large scale of high/medium-Ti basalts in Mare Imbrium, which could provide key information for studying the late-stage lunar volcanism in Mare Imbrium. Using a variety of remote sensing data and techniques (topographic profile and crater excavation method), we studied the scale of the Upper and Lower Eratosthenian (UEm and LEm) basalts in Mare Imbrium. The results show that the UEm is 13–33 m thick with several layers of individual lava (7–15 m) inside, and the thickness of LEm is 14.49–43.61 m. Adding the volume of exposed UEm basalts (~539.3 km3), the volume of exposed LEm basalts (~5198 km3) and the volume of young basalts underlying UEm (~55 6.2 km3) together, the total volume of the late-stage basalts in Mare Imbrium is ~6479 km3. It’s far lower than that of the Schaber’s estimation (~4×104 km3). The volume of the exposed UEm basalts accounts for only about 1/8 of the volume of the exposed LEm basalts, which means that basaltic eruption decreases with time during the Eratosthenian Period in Mare Imbrium. Besides, we estimated the decrease of the areal flux of Em5–Em7: Em5 (249.56 km2/Ma), Em6 (39.49 km2/Ma) and Em7 (46.89 km2/Ma). These results indicate that the basaltic volcanism receded with time during the Eratosthenian Period.

Corundum (sapphire) and zircon relationships, Lava Plains gem fields, NE Australia: Integrated mineralogy, geochemistry, age determination, genesis and geographical typing

AbstractGem minerals at Lava Plains, northeast Queensland, offer further insights into mantle-crustal gemformation under young basalt fields. Combined mineralogy, U-Pb age determination, oxygen isotope and petrological data on megacrysts and meta-aluminosilicate xenoliths establish a geochemical evolution in sapphire, zircon formation between 5 to 2 Ma. Sapphire megacrysts with magmatic signatures (Fe/Mg ∼100–1000, Ga/Mg 3–18) grew with ∼3 Ma micro-zircons of both mantle (δ18O 4.5–5.6%) and crustal (δ18O 9.5–10.1‰) affinities. Zircon megacrysts (3±1 Ma) show mantle and crustal characteristics, but most grew at crustal temperatures (600–800°C). Xenolith studies suggest hydrous silicate melts and fluids initiated from amphibolized mantle infiltrated into kyanite+sapphire granulitic crust (800°C, 0.7 GPa). This metasomatized the sapphire (Fe/Mg ∼50–120, Ga/Mg ∼3–11), left relict metastable sillimanite-corundum-quartz and produced minerals enriched in high field strength, large ion lithophile and rare earth elements. The gem suite suggests a syenitic parentage before its basaltic transport. Geographical trace-element typing of the sapphire megacrysts against other eastern Australian sapphires suggests a phonolitic involvement.

Environmental isotopes meet 3D geological modelling: Conceptualising recharge and structurally-controlled aquifer connectivity in the basalt plains of south-western Victoria, Australia

To develop a conceptual model of recharge and inter-aquifer connectivity for a complex aquifer system in southwestern Victoria, Australia, a three-dimensional (3D) geological model was constructed and used to examine the influence of the regional geology on groundwater composition and age, as well as recharge mechanisms. The major aquifers are three basalts, differentiated on age and geomorphological features, and an underlying palaeovalley (deep lead) system. The upper fine-grained sediments of the deep lead form an aquitard that separates the basalt from the permeable sands and gravels beneath. Recharge calculations (chloride mass balance and water-table fluctuation methods), salinity, stable isotope, tritium and radiocarbon data show that volcanoes and the youngest basalts, although volumetrically minor, form preferential recharge areas for all the basalts (up to ∼30 mm/year), and also recharge the deep lead sediments through volcanic necks that penetrate the aquitard. Infiltration through the thick, clay-rich soils on the older basalts is small (typically ⩽ 1–2 mm/year) and saline, resulting in a progressive increase of groundwater salinity along the flow path in these basalts. The comparison of fault location with groundwater age in the deep lead aquifer suggests that in some areas, faults exert significant structural control on groundwater flow. This study demonstrates the value of integrating geological, hydrogeological and hydrochemical data to identify preferential recharge areas, inter-aquifer connectivity and the influence of faults on groundwater flow in a complex aquifer system.

Volcanic history of the Imbrium basin: A close-up view from the lunar rover Yutu

We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ∼5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite-rich mantle reservoir and then assimilated by 10-20% of the last residual melt of the lunar magma ocean.

A geophysical strategy for measuring the thickness of the critical zone developed over basalt lavas

Estimates of the thickness variation in lateritic weathering profiles (LWPs) are important in tropical areas underlain by young basalt lavas like those found in Hawaii. Seismic shear-wave velocity data were obtained by a new application of multichannel analysis of surface waves (MASW) to map variations in the LWP and to derive the downward rate of advance of the weathering front in basaltic lavas. The MASW technique proved highly capable of imaging the internal structure and base of the critical zone, as confirmed by borehole data and direct field measurements. Profile thickness thus obtained, rapidly and without drilling, has applications to engineering and geochemical studies. The rate of advance of the weathering front derived from MASW in Oahu ranged from 0.010 m/ka to 0.026 m/ka in mesic zones (∼1500 mm/a rainfall), whereas an area with ∼800 mm/a revealed rates from 0.005 m/ka to 0.011 m/ka. These rates are comparable to those derived from recent solute-based mass balance studies of ground and surface water. Conventional P-wave seismic reflection did not perform as well for detecting boundaries due to a gradational seismic velocity structure within the weathering profile. Shear-wave velocity models showed internal variations that may be caused by textural differences in parental lava flows. Limitations in imaging depth were overcome by innovative experiment designs. Increasing source-receiver offsets and merging surface-wave dispersion curves allowed for a more objective derivation of velocity-frequency relations. Further improvements were made from a recently developed form of the combined active and passive source technique. These advances allowed for more detailed and deeper imaging of the subsurface with greater confidence. Velocity models derived from MASW can thus describe the LWP in terms of depth and variability in stiffness.

Timescales and mechanisms of formation of amorphous silica coatings on fresh basalts at Kīlauea Volcano, Hawai'i

Young basalts from Kīlauea Volcano, Hawai'i, frequently feature opaque surface coatings, 1–80μm thick, composed of amorphous silica and Fe–Ti oxides. These coatings are the product of interaction of the basaltic surface with volcanically-derived acidic fluids. Previous workers have identified these coatings in a variety of contexts on Hawai'i, but the timescales of coating development, coating growth rates, and factors controlling lateral coating heterogeneity were largely unconstrained. We sampled and analyzed young lava flows (of varying ages, from hours to ~40years) along Kīlauea's southwest and east rift zones to characterize variation in silica coating properties across the landscape. Coating thickness varies as a function of flow age, flow surface type, and proximity to acid sources like local fissure vents and regional plumes emitted from Kīlauea Caldera and Pu'u Ō'ō. Silica coatings that form in immediate proximity to acid sources are more chemically pure than those forming in higher pH environments, which contain significant Al and Fe. Incipient siliceous alteration was observed on basalt surfaces as young as 8days old, but periods of a year or more are required to develop contiguous coatings with obvious opaque coloration. Inferred coating growth rates vary with environmental conditions but were typically 1–5μm/year. Coatings form preferentially on flow surfaces with glassy outer layers, such as spatter ramparts, volcanic bombs, and dense pahoehoe breakouts, due to glass strain weakening during cooling. Microtextural evidence suggests that the silica coatings form both by in situ dissolution–reprecipitation and by deposition of silica mobilized in solution. Thin films of water, acidified by contact with volcanic vapors, dissolved near-surface basalt, then precipitated amorphous silica in place, mobilizing more soluble cations. Additional silica was transported to and deposited on the surface by silica-bearing altering fluids derived from the basalt interior.

Mineralogy of young lunar mare basalts: Assessment of temporal and spatial heterogeneity using M3 data from Chandrayaan-1

A comparative assessment of the mineralogy of young basalts (∼1.2Ga to ∼2.8Ga) from the western nearside, Moscoviense basin, and the Orientale basin of the Moon has been made using Level 2 Moon Mineralogy Mapper (M3) data from the Chandrayaan-1 mission. Spectral data characteristics of the individual units have been generated from fresh small craters to minimize the complications due to space weathering. Representative spectra for individual units and the derived spectral parameters (band centers and integrated band depth ratio) have been used to study composition of these young basalts. A modified approach of Gaffey et al. (Gaffey, M.J., Cloutis, E.A., Kelley, M.S., Reed, K.L. [2002]. Mineralogy of asteroids. In: Asteroids III. The University of Arizona Press, Tucson, pp. 183–204) (for olivine–pyroxene mixtures) and the methodology of Adams (Adams, J.B. [1974]. J. Geophys. Res. 79, 4829–4836. http://dx.doi.org/10.1029/JB079i032p04829) (for interpreting pyroxene type) have been used to improve our understanding of the spectral behavior of these basalts. Most of the young basalts of Oceanus Procellarum are characterized by abundant olivines and they show complex volcanic history. Vast exposures of olivine concentrated units having higher abundance of olivine content than high-Ca pyroxenes are emplaced in the northern Oceanus Procellarum region. Mostly, they show distinct stratigraphic gradation with the immediately underlying units of relatively lower olivine content. The Moscoviense unit shows signatures of Fe-rich glasses along with clinopyroxenes. The basalts of Orientale basin are typically devoid of olivine and are rich in high-Ca pyroxene. Thus, mineralogy of these mare basalts which erupted during the late stage volcanism vary across the Moon’s surface; however, broader observations reveal apparently higher FeO content in the younger basalts of western nearside and Orientale region.

High precision multi-collector 40Ar/39Ar dating of young basalts: Mount Rouse volcano (SE Australia) revisited

The acquisition of high precision geochronological data for young basalts, particularly low potassium examples <500 ka old, is a non-trivial task. This is due to low 40Ar radiogenic argon yields, high atmospheric argon levels and, sometimes, extraneous argon contamination. In this study we present new, high precision argon isotopic data for a basalt flow from the Mount Rouse volcano of the Newer Volcanic Province, southeast Australia. These data were generated using a new generation, multi-collector, ARGUSVI mass spectrometer. We show that the multi-collector system achieves an order of magnitude improvement in analytical precision and a concomitant improvement in our ability to detect minor isotopic disturbances, compared to ‘conventional’ single-collector mass spectrometry. We report a revised eruption age of 284.4 ± 1.8 ka (0.6%, 95% CI) for Mount Rouse, based on laser step-heating experiments of several groundmass aliquants (50–100 mg) from a single sample (180–250 μm size fraction). This result is considerably more precise than our previous estimate of 303 ± 13 ka (4.3%, 2σ), obtained using an older generation VG3600 mass spectrometer and larger (∼200 mg) sample aliquants.