The isotopic and chemical evolution of planets: Mars as a missing link

The study of planetary bodies has advanced to a stage where it is possible to contemplate general models for the chemical and physical evolution of planetary interiors, which might be referred to as UMPES (Unified Models of Planetary Evolution and Structure). UMPES would be able to predict the internal evolution and structure of a planet given certain input parameters such as mass, distance from the sun, and a time scale for accretion. Such models are highly dependent on natural observations because the basic material properties of planetary interiors, and the processes that take place during the evolution of planets are imperfectly understood. The idea of UMPES was particularly unrealistic when the only information available was from the earth. However, advances have been made in the understanding of the general aspects of planetary evolution now that there is geochemical and petrological data available for the moon and for meteorites

Models of Hawaiian volcano growth and plume structure: Implications of results from the Hawaii Scientific Drilling Project

The shapes of typical Hawaiian volcanoes are simply parameterized, and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate. The dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the Hawaiian plume, with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit. The timescale of volcano growth is t = 2 R/ν_plate where R is the radius of the melting zone of the (circular) plume and νplate is the velocity of the Pacific plate. The growth progress of a volcano can be described by a dimensionless time t′ = tν_plate/2R, where t′ = 0 is chosen to be the start of volcano growth and t′ = 1 approximates the end of “shield” growth. Using a melt generation rate for the whole plume of 0.2 km^(3)/yr, a plume diameter of 50 km, and a plate velocity of 10 cm/yr, we calculate that the lifetime of a typical volcano is 1000 kyr. For a volcano that traverses the axis of the plume, the “standard” dimensions are a volume of 57,000 km^3, a summit thickness of 18 km, a summit elevation of 3.6 km, and a basal radius of 60 km. The volcano first breaches the sea surface at t′ ≈ 0.22 when it has attained only 5% of its eventual volume; 80% of the volume accumulates between t′ = 0.3 and t′ = 0.7. Typical lava accumulation rates start out over 50 m/kyr in the earliest stages of growth from the seafloor, and level out at ∼35 m/kyr from t′ ≈ 0.05 until t′ = 0.4. From t′ = 0.4 to t′ = 0.9, the submarine lava accumulation rates decrease almost linearly from 35 m/kyr to ∼0; subaerial accumulation rates are about 30% lower. The lava accumulation rate is a good indicator of volcano age. A volcano that passes over the plume at a distance 0.4R off to the side of the plume axis is predicted to have a volume of about 60% of the standard volcano, a lifetime about 8% shorter, and lava accumulation rates about 15–20% smaller. The depth-age data for Mauna Kea lavas cored by the Hawaii Scientific Drilling Project are a good fit to the model parameters used, given that Mauna Kea appears to have crossed the plume about 15–20 km off-axis. The lifetime of Mauna Kea is estimated to be 920 kyr. Mauna Loa is predicted to be at a stage corresponding to t′ ≈ 0.8, Kilauea is at t′ ≈ 0.6, and Loihi is at t′ ≈0.16. The model also allows the subsurface structure of the volcanoes (the interfaces between lavas from different volcanoes) to be modeled. Radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross-sectional area of the plume; this inference is qualitatively born out by available isotopic data. The model predicts that new Hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano's flank

A Tone-Aided/Dual Vestigial Sideband (TA/DVSB) system for mobile satellite channels

Tone-aided modulation is one way of combatting the effects of multipath fading and Doppler frequency shifts. A new tone-aided modulation format for M-ary phase-shift keyed signals (MPSK) is discussed. A spectral null for the placement of the tone is created in the center of the MPSK signal by translating the upper sideband upwards in frequency by the same amount. The key element of the system is the algorithm for recombining the data sidebands in the receiver, a function that is performed by a specialized phase-locked loop (PLL). The system structure is discussed and simulation results showing the PLL acquisition performance are presented

Nd Isotopic variations and petrogenetic models

^(147)Sm ɑ-decays to ^(143)Nd so that ^(143)Nd/^(144)Nd reflects the time-integrated Sm/Nd environment of a sample. The increase in 143/144 in a reservoir with chondritic Sm/Nd is 1.2% in 4.5AE. There exists sufficient variation of Sm/Nd to cause sizeable effects in 143/144. Young samples were measured to elucidate the nature of their source regions. An oceanic high Fe, Ti basalt (113152) and alk. basalt (113031), a continental alk. besalt (BCR-l), an apatite (Khibiny massif) and two reagent "normals", NN1 and NN2, were analyzed. Isotopic ratios of NN2 and BCR-1, normalized to 148/144=0.241572 are tabulated. Following the pioneering work of Lugmair, et al. (EPSL 27, 79) our 143/144 data are presented relative to the total rock value for Juvinas (0.51278). the present value of a chondritic reservoir. Data are given as deviations from this value in parts in 10^4 (є) and show a wide range. Nd in the source regions of the rock samples evolved in an environment of approximately chondritic Sm/Nd (±5%) over the history of the earth. Small variations exist, reflecting long time scale differences of Sm/Nd in the source regions. The low Sm/Nd observed in alkali basalts cannot reflect an ancient source region with low Sm/Nd as є is near zero. REE patterns of alkali basalts must thus reflect relatively recent fractionation from a source with essentially chondritic relative abundances. Study of initial ^(143)Nd/^(144)Nd in conjunction with REE patterns promises to contribute important petrogenetic information

Inferences about magma sources and mantle structure from variations of ^(143)Nd/^(144)Nd

Continental flood basalts and mid-ocean ridge (MOR) tholeiitic basalts have distinctly different ^(143)Nd/^(144)Nd which may permit a priori distinction between "continental" and "oceanic" igneous rocks. Initial ^(143)Nd/^(144)Nd of continental igneous rocks through time fall on a Sm/Nd evolution curve with chondritic REE abundance ratio. These observations indicate that many continental igneous rocks are derived from a reservoir with chondritic REE pattern which may represent primary material remaining since the formation of the earth. Oceanic igneous rocks are derived from a different ancient reservoir which has Sm/Nd higher than chondritic. Initial ^(143)Nd/^(144)Nd and ^(87)Sr/^(86)Sr in young basalts from both oceans and continents show a strong correlation suggesting that Sm-Nd and Rb-Sr fractionation events in the mantle may be correlative and caused by the same process. From this correlation Rb/Sr for the earth is inferred to be 0.029

The sources of island arcs as indicated by Nd and Sr isotopic studies

Island arc lavas from New Britain and the Marianas have ^(143)Nd/^(144)Nd similar to other oceanic basalts and distinctly different from continental flood basalts and thus appear to be derived from a high Sm/Nd, light-REE-depleted reservoir. Consideration of both Nd and Sr isotopes suggests seawater involvement in the generation of some island arc lavas and thus indicates that they may be derived from altered subducted oceanic crust. Other island arc lavas show no evidence of seawater involvement and may be derived from mantle reservoirs with affinities to the sources of ocean island basalts. Andesite and rhyolite from an Andean volcano reflect assimilation of old continental crust. Nd and Sr in basaltic and ultrapotassic continental rocks indicate that some mafic magmas in continental regions may be derived from old low-Sm/Nd reservoirs or are heavily contaminated with old continental crustal material. Fish debris from the ocean floor provides an estimate of ^(143)Nd/^(144)Nd in seawater and indicates that light-REE in the marine environment are derived mainly from continents. Basalts erupted above sea level in oceanic and continental areas are isotopically distinct from those erupted on the ocean floor, suggesting a relationship between parental reservoirs and hydrostatic head

Extremely high He isotope ratios in MORB-source mantle from the proto-Iceland plume

The high <sup>3</sup>He/<sup>4</sup>He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic <sup>3</sup>He/<sup>4</sup>He ratios yet recorded. A strong correlation between <sup>3</sup>He/<sup>4</sup>He and <sup>87</sup>Sr/<sup>86</sup>Sr, <sup>143</sup>Nd/<sup>144</sup>Nd and trace element ratios demonstrate that the <sup>3</sup>He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest <sup>3</sup>He/<sup>4</sup>He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a <sup>3</sup>He-recharged depleted mantle (HRDM) reservoir may be the principal source of high <sup>3</sup>He/<sup>4</sup>He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure