Basaltic Reservoir Research Articles

Elastostatic effects around a magma reservoir and pathway due to historic earthquakes: a case study of Mt. Fuji, Japan

We discuss elastostatic effects on Mt. Fuji, the tallest volcano in Japan, due to historic earthquakes in Japan. The 1707 Hoei eruption, which was the most explosive historic eruption of Mt. Fuji, occurred 49 days after the Hoei earthquake (Mw 8.7) along the Nankai Trough. It was previously suggested that the Hoei earthquake induced compression of a basaltic magma reservoir and unclamping of a dike-intruded region at depth, possibly triggering magma mixing and the subsequent Plinian eruption. Here, we show that the 1707 Hoei earthquake was a special case of induced volumetric strain and normal stress changes around the magma reservoir and pathway of Mt. Fuji. The 2011 Tohoku earthquake (Mw 9), along the Japan Trench, dilated the magma reservoir. It has been proposed that dilation of a magma reservoir drives the ascent of gas bubbles with magma and further depressurization, leading to a volcanic eruption. In fact, seismicity notably increased around Mt. Fuji during the first month after the 2011 Tohoku earthquake, even when we statistically exclude aftershocks, but the small amount of strain change (< 1 μ strain) may have limited the ascent of magma. For many historic earthquakes, the magma reservoir was compressed and the magma pathway was wholly clamped. This type of interaction has little potential to mechanically trigger the deformation of a volcano. Thus, Mt. Fuji may be less susceptible to elastostatic effects because of its location relative to the sources of large tectonic earthquakes. As an exception, a possible local earthquake in the Fujikawa-kako fault zone could induce a large amount of magma reservoir dilation beneath the southern flank of Mt. Fuji.

Influence of relative permeability on injection pressure and plume configuration during CO2 injections in a mafic reservoir

Carbon capture and sequestration projects have traditionally targeted deep sedimentary basins; however, mafic reservoirs may also be attractive targets for CO2 disposal on the basis of permanent mineral trapping over relatively short time scales (101 to 102yr). Nevertheless, CCS development in mafic reservoirs is hampered by substantial uncertainty in fracture-controlled reservoir characteristics, particularly with respect to the effects of multi-phase fluid flow, e.g., relative permeability and capillary pressure. The present study quantifies uncertainty surrounding relative permeability effects in a basalt reservoir by developing a numerical modeling experiment on the basis of site characterization data from the Slack Canyon #2 flow top, which is one of three flow tops comprising the injection zone at the Wallula Basalt Sequestration Pilot Project in southeast Washington State. This numerical modeling experiment controls for the effects of curvature in the relative permeability models by performing an ensemble of 399 CO2 injection simulations with constant geometry and reservoir properties, while systematically varying the phase interference parameter (λ) and residual CO2 saturation (Sgr), which govern wetting and non-wetting phase relative permeability, respectively. The relative permeability parameter space is defined by selecting combinations of λ and Sgr that cover a wide range of experimental laboratory measurements. For each simulation, CO2 is injected into the reservoir for 10 years at a constant rate of 2.78kgs−1 (87,856metrictons(MT)yr−1), which is 10% of the annual injection rate proposed for one injection scenario at the Wallula Site. Results from the ensemble of simulations show that relative permeability alone can account for >50MPa of variability in the injection pressure and a two-fold difference in lateral CO2 plume migration. Additionally, this work shows that curvature in the wetting phase relative permeability model is the stronger influence on reservoir pressure accumulation, while curvature in the non-wetting phase relative permeability model strongly governs CO2 plume geometry.

Impact of gas injection IOR on YURIHARA basalt reservoir

Impact of gas injection IOR on YURIHARA basalt reservoir

Hydraulic Fracturing Technology Design in Natural Fracture Reservoirs

Nowadays, hydraulic fracturing has become the mainly treatment in low permeability reservoirs, but the hydraulic fracturing design technology in different reservoirs still use common methods. Natural fracture reservoirs mainly include granite reservoir, basalt reservoir and igneous rock reservoir which its hydrocarbon pore volume is fracture system. As the existing of natural fracture, hydraulic fracturing treatment always counting some problems, such as difficult sand pumping, easily screen-out and limited scale. In this paper, from the point of the reservoir characteristics, the mainly problems were analyzed and the corresponding methods were put forward .the core technique in this kind reservoir include communicating the distance nature fracture and meanwhile protecting the conductivity nature fracture. Production can reach 90% from natural fractures using numerical simulation.

Physical constraints on geologic CO2 sequestration in low-volume basalt formations

Research Article| March 01, 2014 Physical constraints on geologic CO2 sequestration in low-volume basalt formations Ryan M. Pollyea; Ryan M. Pollyea † 1Department of Geology & Environmental Geosciences, Northern Illinois University, DeKalb, Illinois 60115, USA †E-mail: rpollyea@niu.edu Search for other works by this author on: GSW Google Scholar Jerry P. Fairley; Jerry P. Fairley 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA Search for other works by this author on: GSW Google Scholar Robert K. Podgorney; Robert K. Podgorney 3Energy Resource Recovery and Sustainability, Idaho National Laboratory, Idaho Falls, Idaho 83415, USA Search for other works by this author on: GSW Google Scholar Travis L. Mcling Travis L. Mcling 4Center for Advanced Energy Studies, Idaho National Laboratory, Idaho Falls, Idaho 83415, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Ryan M. Pollyea † 1Department of Geology & Environmental Geosciences, Northern Illinois University, DeKalb, Illinois 60115, USA Jerry P. Fairley 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA Robert K. Podgorney 3Energy Resource Recovery and Sustainability, Idaho National Laboratory, Idaho Falls, Idaho 83415, USA Travis L. Mcling 4Center for Advanced Energy Studies, Idaho National Laboratory, Idaho Falls, Idaho 83415, USA †E-mail: rpollyea@niu.edu Publisher: Geological Society of America Received: 17 Feb 2013 Revision Received: 19 Jul 2013 Accepted: 15 Nov 2013 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2014 Geological Society of America GSA Bulletin (2014) 126 (3-4): 344–351. https://doi.org/10.1130/B30874.1 Article history Received: 17 Feb 2013 Revision Received: 19 Jul 2013 Accepted: 15 Nov 2013 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ryan M. Pollyea, Jerry P. Fairley, Robert K. Podgorney, Travis L. Mcling; Physical constraints on geologic CO2 sequestration in low-volume basalt formations. GSA Bulletin 2014;; 126 (3-4): 344–351. doi: https://doi.org/10.1130/B30874.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Deep basalt formations within large igneous provinces have been proposed as target reservoirs for carbon capture and sequestration on the basis of favorable CO2-water-rock reaction kinetics that suggest carbonate mineralization rates on the order of 102–103 d. Although these results are encouraging, there exists much uncertainty surrounding the influence of fracture-controlled reservoir heterogeneity on commercial-scale CO2 injections in basalt formations. This work investigates the physical response of a low-volume basalt reservoir to commercial-scale CO2 injections using a Monte Carlo numerical modeling experiment such that model variability is solely a function of spatially distributed reservoir heterogeneity. Fifty equally probable reservoirs are simulated using properties inferred from the deep eastern Snake River Plain aquifer in southeast Idaho, and CO2 injections are modeled within each reservoir for 20 yr at a constant mass rate of 21.6 kg s–1. Results from this work suggest that (1) formation injectivity is generally favorable, although injection pressures in excess of the fracture gradient were observed in 4% of the simulations; (2) for an extensional stress regime (as exists within the eastern Snake River Plain), shear failure is theoretically possible for optimally oriented fractures if Sh ≤ 0.70SV; and (3) low-volume basalt reservoirs exhibit sufficient CO2 confinement potential over a 20 yr injection program to accommodate mineral trapping rates suggested in the literature. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dynamic numerical modeling of the usage of groundwater for cooling in north east Jordan – A geothermal case study

Geothermal energy has the potential to significantly contribute to the cooling of buildings. A shallow aquifer system in north east Jordan was proven as a geothermal resource for its efficiency for cooling utilization. A numerical 3D model was developed in order to predict the future performance of the geothermal cooling reservoir. Different possible geothermal installations were studied, using various approaches. The study shows that a geothermal utilization of the respective basaltic reservoir is feasible. It features sufficient hydraulic and thermal properties to be utilized for cooling purposes. The developed model has proven to be robust and flexible. It can be easily extended for analyzing other sites.

High-pressure aragonite phenocrysts in carbonatite and carbonated syenite xenoliths within an alkali basalt

We describe the first observation of primary magmatic aragonite in carbonatite and carbonated syenite, occurring as xenoliths in a Pliocene basaltic diatreme located near the Hungary-Slovakia border. The aragonite-hosting matrix consists of disordered P-rich calcite, occasionally associated with trachyte glass. We interpret the aragonite growth as evidence of supra-lithostatic overpressure in the magmatic plumbing system that connected the crustal basaltic reservoir with the partial melting zone of the lithospheric mantle, and the disordered calcite ± trachyte as quenched residual, immiscible melts, generated close to the solidus of the carbonated alkali basalt differentiated in the crustal reservoir. The quenching event was a phreato-magmatic eruption within the stability field of the low-pressure calcite; this was triggered by advective overpressure, caused by expanding gas bubbles in a quasiincompressible silicate melt system. The high-pressure, pre-eruption origin of aragonite is indicated by enrichment in 13C compared to the associated calcite interpreted as a record of CO2 degassing at T > 500 °C. The oxygen (δ18O ranges of 22.1-24.5‰ V-SMOW in aragonite, 21.6-22.7‰ in calcite) and carbon (δ13C ranges of -4.4 to -5.9‰ V-PDB in aragonite, -11.9 to -12.7‰ in calcite) isotope signatures are consistent with a degassed carbonatite melt primarily derived from a subduction zone.

Sub-seafloor Carbon Dioxide Storage Potential on the Juan de Fuca Plate, Western North America

The Juan de Fuca plate, off the western coast of North America, has been suggested as a site for geological sequestration of anthropogenic carbon dioxide because of its many attractive characteristics (high permeability, large storage capacity, reactive rock types). Here we model CO2 injection into fractured basalts comprising the upper several hundred meters of the sub-seafloor basalt reservoir, overlain with low-permeability sediments and a thick saline water column, to examine the feasibility of this reservoir for CO2 storage. Our simulations indicate that the sub-seafloor basalts of the Juan de Fuca plate may be an excellent CO2 storage candidate, as multiple trapping mechanisms (hydrodynamic, density inversions, and mineralization) act to keep the CO2 isolated from terrestrial environments. Questions remain about the lateral extent and connectivity of the high permeability basalts; however, the lack of wells or boreholes and thick sediment cover maximize storage potential while minimizing potential leakage pathways. Although promising, more study is needed to determine the economic and sociological viability of this option.

Petrophysical and geochemical properties of Columbia River flood basalt: Implications for carbon sequestration

This study presents borehole geophysical data and sidewall core chemistry from the Wallula Pilot Sequestration Project in the Columbia River flood basalt. The wireline logging data were reprocessed, core‐calibrated and interpreted in the framework of reservoir and seal characterization for carbon dioxide storage. Particular attention is paid to the capabilities and limitations of borehole spectroscopy for chemical characterization of basalt. Neutron capture spectroscopy logging is shown to provide accurate concentrations for up to 8 major and minor elements but has limited sensitivity to natural alteration in fresh‐water basaltic reservoirs. The Wallula borehole intersected 26 flows from 7 members of the Grande Ronde formation. The logging data demonstrate a cyclic pattern of sequential basalt flows with alternating porous flow tops (potential reservoirs) and massive flow interiors (potential caprock). The log‐derived apparent porosity is extremely high in the flow tops (20–45%), and considerably overestimates effective porosity obtained from hydraulic testing. The flow interiors are characterized by low apparent porosity (0–8%) but appear pervasively fractured in borehole images. Electrical resistivity images show diverse volcanic textures and provide an excellent tool for fracture analysis, but neither fracture density nor log‐derived porosity uniquely correlate with hydraulic properties of the Grande Ronde formation. While porous flow tops in these deep flood basalts may offer reservoirs with high mineralization rates, long leakage migration paths, and thick sections of caprock for CO2 storage, a more extensive multiwell characterization would be necessary to assess lateral variations and establish sequestration capacity in this reservoir.

储存库空间的不确定性对在美国爱达荷州东斯内克河平原进行CO2封存的影响

Basalt-hosted hydrogeologic systems have been proposed for geologic CO2 sequestration based on laboratory research suggesting rapid mineralization rates. However, despite this theoretical appeal, little is known about the impacts of basalt fracture heterogeneity on CO2 migration at commercial scales. Evaluating the suitability of basalt reservoirs is complicated by incomplete knowledge of in-situ fracture distributions at depths required for CO2 sequestration. In this work, a numerical experiment is used to investigate the effects of spatial reservoir uncertainty for geologic CO2 sequestration in the east Snake River Plain, Idaho (USA). Two criteria are investigated: (1) formation injectivity and (2) confinement potential. Several theoretical tools are invoked to develop a field-based approach for geostatistical reservoir characterization and their implementation is illustrated. Geologic CO2 sequestration is simulated for 10 years of constant-rate injection at ~680,000 tons per year and modeled by Monte Carlo simulation such that model variability is a function of spatial reservoir heterogeneity. Results suggest that the spatial distribution of heterogeneous permeability structures is a controlling influence on formation injectivity. Analysis of confinement potential is less conclusive; however, in the absence of confining sedimentary interbeds within the basalt pile, rapid mineralization may be necessary to reduce the risk of escape.

A feasibility study of time-lapse seismic monitoring of CO2 sequestration in a layered basalt reservoir

Abstract We investigate the potential of scattered seismic waves to remotely sense geological sequestration of CO 2 in basalt. Numerical studies in horizontally layered models suggest that strong scattering quickly complicates the wave fields, but also provides a sensitive tool to monitor physical changes in and around the reservoir. These results go hand-in-hand with recent laboratory work and rock-physics modeling that has shown significant changes in the seismic properties of a reservoir undergoing CO 2 sequestration, due to fluid substitution and mineral precipitation.

Evaluation of ocean crustal Sites 1256 and 504 for long-term CO2sequestration

[1] Geologic storage of CO2 in ocean basalt reservoirs is a potentially long-term solution to offset anthropogenic greenhouse gas emissions. Prior drilling at Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Sites 1256 and 504 provides evidence that the extrusive ocean crust has sufficient porosity and permeability to enable storage of large volumes of CO2. Six intervals are identified as potential reservoirs in the shallow crust at Sites 1256 and 504, with new and reprocessed estimates of porosity from electrical resistivity logs ranging from 6% to 14% and permeability from ∼10−14 to 10−15 m2. Calculations using specific reservoir thickness and porosity estimates suggest that even the smallest reservoir could provide storage capacity for decades of global anthropogenic carbon emissions. In situ hydrologic experiments and pilot injection studies are needed to confirm high permeability and porosity estimates at Sites 1256 and 504, as well as the potential for CO2 injection and retention in these basalt reservoirs.

Lithofacies and reservoir properties of Tertiary igneous rocks in Qikou Depression, East China

SUMMARY Tertiary igneous rocks in Qikou Depression, East China are primarily basic rocks. Two types of igneous reservoirs including the effusive basaltic reservoir and the near-surface intrusive doleritic reservoir have been identified so far and each has different reservoir spaces. In this paper, the lithofacies and reservoir properties of Tertiary igneous rocks are described through a systematic analysis of the characteristics of seismic reflection and well logs and selected cores observation. Both of the two types of igneous rocks present strong and low-frequency reflection events on seismic profiles, however, intrusive diabases exhibit the characteristic of strata penetration with a small intersection angle. Effusive basaltic lavas are divided into upper, middle and lower subfacies. This can be distinguished from the well log curves. Observation of sampled cores finds that primary vesicles and amygdales are abundant in the upper and lower subfacies of basaltic lavas. These vesicles can be favourable reservoir space if connected by cracks formed in the late-stage. Intrusive diabases can also be separated into inner subfacies as well as the upper and lower marginal subfacies, of which the former is well crystallized because of slowly cooling, whereas the latter is vitreous due to fast descending temperature at the contact zone between intrusive rocks and host rocks. Hydrothermal metamorphism left the surrounding mudstones metamorphosed to form shrunk fissures and structural fractures as reservoir space, making mudstones of poor porosity and permeability to be metamorphic and have certain reservoir potential.

EFFECTS OF COMBINED TEMPERATURE- AND DEPTH-DEPENDENT VISCOSITY AND HALL CURRENT ON AN UNSTEADY MHD LAMINAR CONVECTIVE FLOW DUE TO A ROTATING DISK

The present study investigates the effects of mixed temperature- and depth-dependent viscosity and Hall current on an unsteady flow of an incompressible electrically conducting fluid on a rotating disk in the presence of a uniform magnetic field. We assume that the fluid viscosity strongly depends on temperature and depth, which may be directly applicable to the earth's mantle and a uniform mid-ocean ridge basalt reservoir in whole mantle flow. The system of axial symmetric nonlinear partial differential equations governing the unsteady flow and heat transfer is written in cylindrical polar coordinates and reduced to nonlinear ordinary differential equations by introducing suitable similarity parameters. Solutions for the flow and temperature fields are obtained numerically assuming large Prandtl number by using Runge-Kutta and shooting methods. The nature of radial, tangential, and axial velocities and temperature in the presence of a uniform magnetic field is presented for changing various nondimensional parameters at different layers of the medium. The coefficients of skin frictions and the rate of heat transfer are calculated at different parameters. Comparison has been made for steady flow (C = 0) and shows excellent agreement with Sparrow and Gregg (1959), hence encouragement for the use of the present numerical computations.

Time constraints on the origin of large volume basalts derived from O-isotope and trace element mineral zoning and U-series disequilibria in the Laki and Grímsvötn volcanic system

The 1783–1784 AD fissure eruption of Laki (Iceland) produced 15 km 3 of homogeneous basaltic lavas and tephra that are characterized by extreme (3‰) 18O-depletion relative to normal mantle. Basaltic tephra erupted over the last 8 centuries and as late as in November 2004 from the Grímsvötn central volcano, which together with Laki are a part of a single volcanic system, is indistinguishable in δ 18O from Laki glass. This suggests that all tap a homogeneous and long-lived low- δ 18O magma reservoir. In contrast, we observe extreme oxygen isotope heterogeneity (2.2–5.2‰) in olivine and plagioclase contained within these lavas and tephra, and disequilibrium mineral-glass oxygen-isotope fractionations. Such low- δ 18O glass values, and extreme 3‰ range in δ 18O olivine have not been described in any other unaltered basalt. The energy constrained mass balance calculation involving oxygen isotopes and major element composition calls for an origin of the Laki–Grímsvötn quartz tholeiitic basaltic melts with δ 18O = 3.1‰ by bulk digestion of low- δ 18O hydrated basaltic crust with δ 18O = − 4‰ to + 1‰, rather than magma mixing with ultra-low- δ 18O silicic melt. The abundant Pleistocene hyaloclastites, which were altered by synglacial meltwaters, can serve as a likely assimilant material for the Grímsvötn magmas. The ( 226Ra / 230Th) activity ratio in Laki lavas and 20th century Grímsvötn tephras is 13% in-excess of secular equilibrium, but products of the 20th century Grímsvötn eruptions have equilibrium ( 210Pb / 226Ra). Modeling of oxygen isotope exchange between disequilibrium phenocrysts and magmas, and these short-lived U-series nuclides yields a coherent age for the Laki–Grímsvötn magma reservoir between 100 and 1000 yrs. We propose the existence of uniquely fingerprinted, low- δ 18O, homogeneous, large volume, and long-lived basaltic reservoir beneath the Laki–Grímsvötn volcanic system that has been kept alive in its position above the center of the Icelandic mantle plume. Melt generation, crustal assimilation, magma storage and homogenization all took place in only a few thousands of years at most.

Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc

Geologic, chronologic, and U–Th isotope data from Pleistocene–Recent basaltic to rhyolitic lavas and their phenocrysts constrain the long-term evolution of magmatic processes at Seguam Island, Aleutian Island arc, and suggest a model in which erupted magmas were derived from a single, deep seated reservoir for ∼130 kyrs of its eruptive history. The monotonic evolution in ( 230Th / 232Th) 0 ratios is consistent with radiogenic ingrowth of 230Th in a long-lived magma reservoir between 142 and 9 ka. Internal U–Th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40Ar / 39Ar dating, which implies a short period (i.e., 10 3 yrs or less) of crystal residence in the magma prior to eruption. These results can be reconciled if small batches of magma are repeatedly extracted from a deep, thermally buffered, basaltic reservoir, followed by decompression-driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption. Stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering ∼12 km 2. The ignimbrite, as well as subsequent rhyolitic and basaltic lava flows, signal a decrease in ( 230Th / 232Th) 0 and 87Sr / 86Sr ratios. The abrupt change in magma composition at 9 ka most likely reflects disruption of the long-lived reservoir by rapid ascent of basaltic magma with low ( 230Th / 232Th) and 87Sr / 86Sr ratios and excess 226Ra. While the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored, the shift in isotopic composition and evidence for a protracted > 100 kyr period of magma storage, would have been missed had the focus been on only the most recent historical eruptions. Our results suggest that: not all arc basalt produced in the mantle wedge separates and ascends rapidly, magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano, and U–Th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs, provided a sufficiently long period of volcanic activity is examined.

Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pincina village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An20–55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6–7% of total iron as Fe3+ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8–10 mol% Fe2O3 and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm3) primary CO2 inclusions with minor H2, CH4, H2S, CO and N2 ( 0.705–0.706, δ18O>9‰ V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10–11 km. Low water activity in the pincinite parental melt was caused by CO2-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.

The oxygen fugacity of olivine‐phyric martian basalts and the components within the mantle and crust of Mars

Abstract— The oxygen fugacity of olivine‐phyric martian basalts is estimated using olivine‐pyroxene‐spinel equilibria, supported by detailed petrography. Results are plotted, along with previous oxygen fugacity estimates, against La/Yb, which is used as a proxy for long‐term incompatible‐element depletion or enrichment in martian basalt reservoirs. In general, the correlation between oxygen fugacity and La/Yb observed by Herd et al. (2002a) holds for the olivine‐phyric basalts. The implications of the correlation are re‐evaluated in light of work by Borg et al. (Forthcoming), which indicates that the variations in radiogenic isotopic composition can be modeled by mixing of mantle sources established by 4.5 Ga through crystallization of a magma ocean in lieu of assimilation of crustal material. The results demonstrate that the crust‐like component, interpreted as trapped liquid in a magma ocean cumulate pile, must be oxidized to explain the oxygen fugacity of the martian basalts. Consequently, the pre‐eruptive water contents of the more oxidized basalts are expected to be higher, although water is not called upon as the cause of the oxidation. Unmixing of mantle components provides an important context for the interpretation of oxygen isotopes, demonstrated here, and of samples returned from the martian surface.

Constraints on mantle evolution from 187Os/ 188Os isotopic compositions of Archean ultramafic rocks from southern West Greenland (3.8 Ga) and Western Australia (3.46 Ga)

Initial 187Os/ 188Os isotopic compositions for geochronologically and geologically well -constrained 3.8-Ga spinel peridotites from the Itsaq Gneiss Complex of southern West Greenland and chromite separates from 3.46-Ga komatiites from the Pilbara region of Western Australia have been determined to investigate the osmium isotopic evolution of the early terrestrial mantle. The measured compositions of 187Os/ 188Os(0) = 0.10262 ± 2, from an olivine separate, and 0.10329 ± 3, for a spinel separate from ∼3.8-Ga peridotite G93/42, are the lowest yet reported from any terrestrial sample. The corrections for in situ decay over 3.8 Ga for these low Re/Os phases are minimal and change the isotopic compositions by only 0.5 and 2.2% for the spinel and the olivine, respectively, resulting in 187Os/ 188Os (3.8 Ga) = 0.1021 ± 0.0002 and 0.1009 ± 0.0002, respectively. These data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle. Analyses of Pilbara chromites yield 3.46-Ga mantle compositions of 0.1042 ± 0.0002 and 0.1051 ± 0.0002. These new data, combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, are compatible with the mantle, or at least portions of it, evolving from a solar system initially defined by meteorites to a modern composition of 187Os/ 188Os(0) = 0.1296 ± 0.0008 as previously suggested from peridotite xenolith data ( Meisel et al., 2001); the associated 187Re/ 188Os(0) = 0.435 ± 0.005. Thus, chondritic 187Os/ 188Os compositions were a feature of the upper mantle for at least 3.8 billion years, requiring chondritic Re/Os ratios to have been a characteristic of the very early terrestrial mantle. In contrast, nonchondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature of plume materials, reflecting the development of variable Re/Os mantle sources early in Earth history. The lower average 187Os/ 188Os = 0.1247 for abyssal peridotites ( Snow and Reisberg, 1995) indicate that not all regions of the modern mantle have evolved with the same Re/Os ratio. The relative sizes of the various reservoirs are unknown, although mass balance considerations can provide some general constraints. For example, if the unradiogenic 187Os/ 188Os modern abyssal peridotite compositions reflect the prevalent upper mantle composition, then the complementary high Re/Os basaltic reservoir must represent 20 to 40% by mass of the upper mantle (taken here as 50% of the entire mantle), depending on the mean storage age. The difficulties associated with efficient long-term storage of such large volumes of subducted basalt suggest that the majority of the upper mantle is not significantly Re-depleted. Rather, abyssal peridotites sample anomalous mantle regions. The existence of 3.8-Ga mantle peridotites with chondritic 187Os/ 188Os compositions and with Os concentrations similar to the mean abundances measured in modern peridotites places an upper limit on the timing of a late accretionary veneer. These observations require that any highly siderophile element -rich component must have been added to the Earth and transported into and grossly homogenized within the mantle by 3.8 Ga. Either large-scale mixing of impact materials occurred on very short (0–100 myr) timescales or (the interpretation preferred here) the late veneer of highly siderophile elements is unrelated to the lunar terminal cataclysm estimated to have occurred at ∼3.8 to 3.9 Ga.

Implications of Nb/U, Th/U and Sm/Nd in plume magmas for the relationship between continental and oceanic crust formation and the development of the depleted mantle

The Nb/U and Th/U of the primitive mantle are 34 and 4.04 respectively, which compare with 9.7 and 3.96 for the continental crust. Extraction of continental crust from the mantle therefore has a profound influence on its Nb/U but little influence on its Th/U. Conversely, extraction of midocean ridge-type basalts lowers the Th/U of the mantle residue but has little influence on its Nb/U. As a consequence, variations in Th/U and Nb/U with Sm/Nd can be used to evaluate the relative importance of continental and basaltic crust extraction in the formation of the depleted (Sm/Nd enriched) mantle reservoir. This study evaluates Nb/U, Th/U, and Sm/Nd variations in suites of komatiites, picrites, and their associated basalts, of various ages, to determine whether basalt and/or continental crust have been extracted from their source region. Emphasis is placed on komatiites and picrites because they formed at high degrees of partial melting and are expected to have Nb/U, Th/U, and Sm/Nd that are essentially the same as the mantle that melted to produce them. The results show that all of the studied suites, with the exception of the Barberton, have had both continental crust and basaltic crust extracted from their mantle source region. The high Sm/Nd of the Gorgona and Munro komatiites require the elevated ratios seen in these suites to be due primarily to extraction of basaltic crust from their source regions, whereas basaltic and continental crust extraction are of subequal importance in the source regions of the Yilgarn and Belingwe komatiites. The Sm/Nd of modern midocean ridge basalts lies above the crustal extraction curve on a plot of Sm/Nd against Nb/U, which requires the upper mantle to have had both basaltic and continental crust extracted from it. It is suggested that the extraction of the basaltic reservoir from the mantle occurs at midocean ridges and that the basaltic crust, together with its complementary depleted mantle residue, is subducted to the core-mantle boundary. When the two components reach thermal equilibrium with their surroundings, the lighter depleted component separates from the denser basaltic component. Both are eventually returned to the upper mantle, but the lighter depleted component has a shorter residence time in the lower mantle than the denser basaltic component. If the difference in the recycling times for the basaltic and depleted components is ∼1.0 to 1.5 Ga, a basaltic reservoir is created in the lower mantle, equivalent to the amount of basalt that is subducted in 1.0 to 1.5 Ga, and that reservoir is isolated from the upper mantle. It is this reservoir that is responsible for the Sm/Nd ratio of the upper mantle lying above the trend predicted by extraction of continental crust on the plot of Sm/Nd against Nb/U.