Richard F. Wendlandt

Rare earth partitioning between immiscible carbonate and silicate liquids and CO2 vapor: Results and implications for the formation of light rare earth-enriched rocks

Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt.Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions.Minimum REE partition coefficients for CO2 vapor/carbonate melt and CO2 vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO2 vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO2 vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar.Mantle metasomatism by a CO2-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO2-rich fluids under both mantle and crustal conditions.

A comparative study of the Rio Grande and Kenya rifts

Abstract Since they are two of the prominent continental rifts which are active today, the Kenya and Rio Grande rifts have been the subject of many recent studies. There are many gaps in our knowledge, but the data available make a comparative analysis worthwhile. Although they are part of much larger extensional regimes, these rifts are of similar dimensions and extent. In addition to these obvious similarities, geophysical data suggest the crustal and upper mantle structures of these features are also similar. They both are associated with relatively localized crustal thinning and much broader zones of lithospheric thinning. Seismic and gravity data indicate that the basement structure beneath both rift valleys is very complex, and recent studies have stressed the role of low-angle normal faulting in their structural development. However, geologic data and earthquake focal mechanisms indicate that high-angle normal faults and strike-slip faulting are also important. However, as one looks at the tectonic evolution and volcanic histories of the rifts, many differences are apparent. Volcanism has been virtually continuous in the Kenya rift area for over 20 Ma and the total volume of extrusives is over 200,000 km 3 . Unlike the Kenya rift, the Rio Grande rift formed in a region which was already both tectonically and magmatically active. Thus, there is uncertainty about what constitutes rift volcanism in the Rio Grande rift. However, the volume of extrusives generally accepted to be associated with this rift is about 5–10% that of the Kenya rift. The compositions of the volcanics are also very different in these rifts, and the Kenya rift displays migration of volcanic activity, and compositional variations in time which have not been recognized in the Rio Grande rift. The volcanic activity in the Rio Grande rift post-dated most of the faulting, whereas faulting and volcanism display complex interactions in Kenya. The nature and timing of uplift are important but difficult questions in both rifts.

The origin of Kenya rift plateau‐type flood phonolites: Results of high‐pressure/high‐temperature experiments in the systems phonolite‐H2O and phonolite‐H2O‐CO2

Near-liquidus melting relations have been determined for a mafic, plateau-type, flood phonolite from the Kenya rift at 0.5, 0.7, 0.9, and 1.2 GPa, with H2O added through saturation, and at 0.7 GPa with H2O and CO2 added. Mixed-volatile experiments at 0.7 GPa delineate a near-liquidus multiple saturation of augite, andesine, phlogopite, oxides, and apatite at 1000°C, XCo2 = 0.42, with calcic amphibole melting above 975°C. The multiple saturation and phase assemblage are interpreted to indicate that plateau phonolites were in equilibrium with the residuum of a parental alkali basaltic composition at 0.7 GPa consisting of augite, andesine, titanomagnetite, and olivine (a product of incongruent melting of phlogopite and, possibly, amphibole). This evidence for lower crustal equilibration refutes suggestions that plateau phonolites are low-pressure differentiates. Their enormous volumes (about 50,000 km3), restricted eruptive period (14–11 Ma), uniform major element compositions, and the paucity of associated mafic-intermediate rocks also argue against a deep origin by fractional crystallization. A two-stage process for the origin of the phonolites is consistent with the thermal evolution of the rift. The lower crust was pervasively injected by alkali basaltic magmas during the period of voluminous eruption of early to middle Miocene basalts. Rising isotherms during rift evolution caused subsequent partial melting of this predominantly basaltic lower crust in the late Miocene, generating the plateau phonolites.

Oxygen diffusion in basalt and andesite melts: experimental results and discussion of chemical versus tracer diffusion

Chemical diffusion coefficients for oxygen in melts of Columbia River basalt (Ice Harbor Dam flow) and Mt. Hood andesite have been determined at 1 atm. The diffusion model is that of sorption or desorption of oxygen into a sphere of uniform initial concentration from a constant and semi-infinite atmosphere. The experimental design utilizes a thermogravimetric balance to monitor the rate of weight change arising from the response of the sample redox state to an imposed fO2. Oxygen diffusion coefficients are approximately an order-ofmagnitude greater for basaltic melt than for andesitic melt. At 1260° C, the oxygen diffusion coefficients are: D=1.65×10−6cm2/s and D=1.43×10−7cm2/s for the basalt and andesite melts, respectively. The high oxygen diffusivity in basaltic melt correlates with a high ratio of nonbridging oxygen/tetrahedrally coordinated cations, low melt viscosity, and high contents of network-modifying cations. The dependence of the oxygen diffusion coefficient on temperature is: D=36.4exp(−51,600±3200/RT)cm2/s for the basalt and D=52.5exp(−60,060±4900/RT)cm2/s for the andesite (R in cal/deg-mol; T in Kelvin). Diffusion coefficients are independent of the direction of oxygen diffusion (equilibrium can be approached from extremely oxidizing or reducing conditions) and thus, melt redox state. Characteristic diffusion distances for oxygen at 1260° C vary from 10-2 to 102 m over the time interval of 1 to 106 years. A compensation diagram shows two distinct trends for oxygen chemical diffusion and oxygen tracer diffusion. These different linear relationships are interpreted as supporting distinct oxygen transport mechanisms. Because oxygen chemical diffusivities are generally greater than tracer diffusivities and their Arrhenius activation energies are less, transport mechanisms involving either molecular oxygen or vacancy diffusion are favored.

Solubility and stability of beryl in granitic melts

The petrogenesis of granitic pegmatites may be one of the greatest enigmas of igneous petrology at the present time. Pegmatites are typically defined by the presence of large (centimeter scale) to extraordinarily large (several meters across) crystals. The presence of such large crystals suggests that pegmatites cooled slowly and that crystals grew from melts in which diffusion was rapid. However, pegmatites are small bodies, rarely more than 50 m long (Cameron et al. 1949), and commonly intrude cool upper crustal rocks at low temperatures. Consequently, cooling is rapid, requiring only hundreds to thousands of years, at most, to reach solidus temperatures (Chakoumakos and Lumpkin 1990; London 1992; Webber et al. 1997). Additionally, diffusion of silicate components in pegmatite melts is expected to be slow based upon their similarity to granitic compositions (Baker 1991, 1992) and their low liquidus temperatures, near 923 K (Chakoumakos and Lumpkin 1990). If one were to choose a melt composition and conditions of emplacement best suited to the production of small crystals in an intrusive rock, one would choose a pegmatite. Instead, we find that conditions thought to produce small crystals result in exactly the opposite. Most granitic pegmatite bodies are internally zoned, either symmetrically or asymmetrically, in mineral texture and element distribution. A general zoning pattern has been recognized in many pegmatites. This pattern consists of a lower zone (footwall) composed of a fine grained, albite-rich aplite (in ABSTRACT

Origin of Kenya Rift Plateau‐type flood phonolites: Integrated petrologic and geophysical constraints on the evolution of the crust and upper mantle beneath the Kenya Rift

A gravity profile across the Kenya Rift at the equator was modeled with recent results of petrologic and seismic investigations as constraints. This profile is dominated by a 600-km-wide gravity low of 100 × 10−5 m/s2 (100 mGal). The model incorporates the important concept of modification of the lower crust by magmatic processes. Alkali basaltic magmas are believed to have modified the lower crust in southern Kenya during early rift-related basaltic volcanism (23–14 Ma) on the basis of (1) synchronous voluminous eruption of alkali basalt in northern Kenya; (2) Kenya Rift International Seismic Project 1990 (KRISP 90) seismic lines which delineate an ∼10-km-thick layer of basal lower crust in southern Kenya which is absent or very thin to the north; (3) results from high pressure/temperature experiments on a late Miocene Plateau phonolite and related geochemical modeling which indicate a lower crustal origin for these voluminous lavas (14–11 Ma) by fusion of alkali basaltic material; and (4) Plateau phonolite distributions that are conspicuously limited to the southern Kenya Rift above this anomalous lower crust. The gravity model features a lens of mafic intrusives (3000 kg/m3; constrained by petrologic arguments) at the base of the crust. It extends from 25 to 34 km depth beneath the rift valley and pinches out at a distance of about 250 km on each side of the rift. Surrounding lower crust (2850 kg/m3) is bounded by upper crust (2700 kg/m3) at about 15 km depth. The gravitational effect of the positive density contrast in the lower crust due to the lens of mafic intrusives is offset by an underlying wedge of anomalously low density mantle (3150 kg/m3). This wedge is about 300 km wide at the Moho and is relatively steep sided, in agreement with KRISP teleseismic results. East of the rift, this anomalous mantle is bounded by normal upper mantle (3260 kg/m3) extending to 100 km depth. West of the rift, normal upper mantle extends to 90 km depth. Within the rift valley, shallow horst and graben are indicated by the gravity data.

Geochemical interactions resulting from carbon dioxide disposal on the seafloor

Abstract The storage of CO 2(liquid) on the seafloor has been proposed as a method of mitigating the accumulation of greenhouse gases in the Earths atmosphere. Storage is possible below 3000 m water depth because the density of CO 2(liquid) exceeds that of seawater and, thus, injected CO 2(liquid) will remain as a stable, density stratified layer on the seafloor. The geochemical consequences of the storage of CO 2(liquid) on the seafloor have been investigated using calculations of chemical equilibrium among complex aqueous solutions, gases, and minerals. At 3000 m water depth and 4°C, the stable phases are CO 2(hydrate) and a brine. The hydrate composition is CO 2 ·6.3H 2 O. The equilibrium composition of the brine is a 1.3 molal sodium-calcium-carbonate solution with pH ranging from 3.5 to 5.0. This acidified brine has a density of 1.04 g cm −3 and will displace normal seawater and react with underlying sediments. Seafloor sediment has an intrinsic capacity to neutralize the acid brine by dissolution of calcite and clay minerals and by incorporation of CO 2 into carbonates including magnesite and dawsonite. Large volumes of acidified brine, however, can deplete the sediments buffer capacity, resulting in growth of additional CO 2(hydrates) in the sediment. Volcanic sediments have the greatest buffer capacity whereas calcareous and siliceous oozes have the least capacity. The conditions that favor carbonate mineral stability and CO 2(hydrates) stability are, in general, mutually exclusive although the two phases may coexist under restricted conditions. The brine is likely to cause mortality in both plant and animal comunities: it is acidic, it does not resemble seawater in composition, and it will have reduced capacity to hold oxygen because of the high solute content. Lack of oxygen will, consequently, produce anoxic conditions, however, the reduction of CO 2 to CH 4 is slow and redox disequilibrium mixtures of CO 2 and CH 4 are likely. Seismic or volcanic activity may cause conversion of CO 2(liquid) to gas with potentially catastrophic release in a Lake Nyos-like event. The long term stability of the CO 2(hydrate) may be limited: once isolated from the CO 2(liquid) pool, either through burial or through depletion of the CO 2 pool, the hydrate will decopose, releasing CO 2 back into the sediment-water system.

The origin of Kenya rift plateau‐type flood phonolites: Evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism

Geochemical investigations support the petrogenesis of Kenya rift plateau-type flood phonolites (14–11 Ma) by partial melting of an alkali basaltic material at lower crustal pressures. High-pressure/high-temperature experiments on a natural plateau phonolite (Hay and Wendlandt, this issue) document multiple saturation of augite, andesine, titanomagnetite, and phlogopite at 0.7 GPa, 1000°C, XCO2 = 0.42, with amphibole appearing at 975°C. A least squares solution to major element modeling, involving subtraction of the compositions of near-liquidus augite, andesine, titanomagnetite, and olivine (Fo67; hypothesized product of incongruent melting of hydrous phases) from a Kenya Miocene alkali basalt composition, indicates that plateau phonolites can be derived by 15 wt % fusion of this hypothetical parental material (∑R2 = 0.07). Alkali basaltic magmas may have injected and/or underplated the lower crust in southern Kenya during prior rift-related basaltic volcanism (23–14 Ma). Bulk Earth values of (87Sr/86Sr)i and eNd near zero for four plateau phonolite samples are consistent with a mantle-derived parental composition. Three of these four samples reflect little, if any, postmelting modification; one sample may have evolved by fractional crystallization (high Rb/Sr, low Ba, Sr and Mg #). A fifth sample may show evidence of assimilation and fractional crystallization processes (elevated radiogenic Sr and Pb, large negative Eu anomaly, and low Ba, Sr, and Mg #). Much of the geochemical variation among plateau phonolite lavas, however, can be ascribed to melting of a predominantly alkali basaltic source with contributions from a lower crustal protolith. A mantle-derived source is also supported by Sr-Nd-Pb isotope data for the phonolites, which indicate that the alkali basaltic source can be described in terms of high U/Pb (HIMU) and enriched mantle (EM1 and EM2) components.

Using neural networks to locate edges and linear features in satellite images

Abstract Edges and linear features are manifestations of discontinuities. In geologic applications of satellite imagery, edges and linear features are used to identify faults, lineaments, or lithology changes. Two techniques for detecting these types of features in satellite imagery are skilled human interpretation and mathematical manipulation (e.g. Fourier analysis). The former approach has the advantage of being able to learn, combined with a high degree of fault tolerance. Disadvantages to this approach are that it is labor intensive (slow) and somewhat arbitrary in its decision process. The latter approach is purely mathematical and, when implemented on a computer, is fast and consistent. The disadvantages of the mathematical approach are the lack of both fault tolerance and learning ability. A third approach, applied here, is to use neural networks (NNs). NNs combine the speed and accuracy of computers with the fault tolerance of human beings. NNs are arrays of highly interconnected, simple processing units which excel at pattern recognition. Because of this ability, they are appropriate tools for recognizing edges and linear features in two-dimensional (2-D) data, such as satellite imagery, aeromagnetic, and gravity data. Image interpretation is comprised of two phases. The syntactic phase identifies image primitives such as lines and edges, and the semantic phase resolves the meaning of the lines and edges. This study focused on the first aspect of locating syntactical information in an image. We determined that NNs are of only limited use for detecting linear features, but they are capable of detecting edges at various scales.

Stability of sanidine + forsterite and its bearing on the genesis of potassic magmas and the distribution of potassium in the upper mantle

Experimental determination of the reaction KAlSiO 4 + 4MgSiO 3 ⇌KAlSi 3 O 8 + 2Mg 2 SiO 4 between 17.5 and 30 kbar and 900–1300°C defines an average slope of 33 bar/°C. The reaction delineates the maximum pressure stability of sanidine in a forsterite-bearing assemblage; sanidine and anorthoclase megacrysts in alkalic magmas must have crystallized at lower pressures. The reaction occurs at slightly lower pressures than the transition from silicate liquids (extremely silica-undersaturated) to carbonate-normative liquids as observed in partial melting studies of the systems KAlSiO 4 MgO SiO 2 CO 2 and KAlSiO 4 MgO SiO 2 H 2 O CO 2 . Both the reaction and the change in melt compositions reflect the rapid expansion of the enstatite stability volume with increasing pressure. Suggestions that potassium resides in sanidine or a hollandite structure in the deep upper mantle need to be revised. The stable K Al silicate at high pressures in anhydrous lherzolite compositions is kalsilite.