Silica-rich granites and rhyolites are components of igneous rock suites found in many tectonic environments, both continental and oceanic. Silica-rich magmas may arise by a range of processes ...including partial melting, magma mixing, melt extraction from a crystal mush, and fractional crystallization. These processes may result in rocks dominated by quartz and feldspars. Even though their mineralogies are similar, silica-rich rocks retain in their major and trace element geochemical compositions evidence of their petrogenesis. In this paper we examine silica-rich rocks from various tectonic settings, and from their geochemical compositions we identify six groups with distinct origins. Three groups form by differentiation: ferroan alkali-calcic magmas arise by differentiation of tholeiite, magnesian calc-alkalic or calcic magmas form by differentiation of high-Al basalt or andesite, and ferroan peralkaline magmas derive from transitional or alkali basalt. Peraluminous leucogranites form by partial melting of pelitic rocks, and ferroan calc-alkalic rocks by partial melting of tonalite or granodiorite. The final group, the trondhjemites, is derived from basaltic rocks. Trondhjemites include Archean trondhjemites, peraluminous trondhjemites, and oceanic plagiogranites, each with distinct geochemical signatures reflecting their different origins. Volcanic and plutonic silica-rich rocks rarely are exposed together in a single magmatic center. Therefore, in relating extrusive complements to intrusive silica-rich rocks and determining whether they are geochemically identical, comparing rocks formed from the same source rocks by the same process is important; this classification aids in that undertaking.
Spectroscopic photoacoustic imaging has the potential to discriminate between normal and lipid-rich atheromatous areas of arterial tissue by exploiting the differences in the absorption spectra of ...lipids and normal arterial tissue in the 740 to 1400 nm wavelength range. Identification of regions of high lipid concentration would be useful to identify plaques that are likely to rupture (vulnerable plaques). To demonstrate the feasibility of visualizing lipid-rich plaques, samples of human aortas were imaged in forward mode, at wavelengths of 970 and 1210 nm. It was shown that the structure of the arterial wall and the boundaries of lipid-rich plaques obtained from the photoacoustic images were in good agreement with histology. The presence of lipids was also confirmed by comparing the photoacoustic spectra (740 to 1400 nm) obtained in a region within the plaque to the spectral signature of lipids. Furthermore, a lipid-rich plaque was successfully imaged while illuminating the sample through 2.8 mm of blood demonstrating the possibility of implementing the photoacoustic technique
.
Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and ...serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mössbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric 'brucite' may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96-98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite.
Dunite from New Caledonia displays three types of serpentine-dominated veins. The earliest, type 1 veins are narrow (50–100μm wide) and rarely extend across more than a single olivine grain. They are ...lizardite, contain abundant brucite and never contain magnetite. Type 2 veins are 0.01 to 0.1mm wide, extend across several olivine grains and cut across the type 1 veins. They are lizardite, contain magnetite, often in vein interiors, and contain less brucite than type 1 veins. Type 3 veins are dominantly chrysotile, cm-scale, have a magnetite-rich core, and extend for meters or more. Analyses of two representative samples indicate that the type 1 veins have relatively Fe-rich serpentine (XMg=0.92) and brucite (XMg=0.82). These minerals are less magnesian than those in the type 2 veins; serpentine has XMg=0.93–0.94 and brucite has XMg=0.84. In the magnetite-rich core to the type 3 vein both serpentine (XMg=0.94–0.97) and one of the two brucite populations (XMg=0.94) are Mg-rich. Opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0.05mm wide. The serpentine veins after Opx lack talc or magnetite and, as with veins cutting olivine, the older veins are more Fe rich (XMg=0.84) than the younger veins (XMg=0.90). We conclude that the formation of magnetite was accompanied by the extraction of iron from the early-formed serpentine and brucite.
Thermodynamic calculations suggest that the type 1 veins formed in a rock-dominated system where the activities of FeO, MgO, and SiO2 were dictated by the compositions of olivine and orthopyroxene. In contrast the type 2 veins were formed in a more fluid-dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite–serpentine assemblage. Reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen. By producing magnetite, this reaction extracted iron from brucite and serpentine, making them both more magnesian. This would drive the brucite–serpentine–magnetite assemblage to higher oxygen fugacity, progressively decreasing the efficiency of the magnetite-forming reactions.
► Five types of serpentine veins are found in dunite from New Caledonia. ► Early veins, with Fe-rich brucite and serpentine cut single grains of olivine. ► Later through-going veins contain magnetite and Mg-rich brucite and serpentine. ► Early veins were rock-dominated, later veins were fluid-dominated. ► Reduction of fluid was accompanied by production of magnetite and hydrogen.
Serpentinites have the lowest silica activity of common crustal rocks. At the serpentinization front, where olivine, serpentine, and brucite are present, silica activities (relative to quartz) are of ...the order of 10−2·5 to 10−5, depending on the temperature. Here we argue that this low silica activity is the critical property that produces the unusual geochemical environments characteristic of serpentinization. The formation of magnetite is driven by the extraction of silica from the Fe3Si2O5(OH)4 component of serpentine, producing extremely reducing conditions as evinced by the rare iron alloys that partially serpentinized peridotites contain. The incongruent dissolution of diopside to form Ca2+, serpentine, and silica becomes increasingly favored at lower T, producing the alkalic fluids characteristic of serpentinites. The interaction of these fluids with adjacent rocks produces rodingites, and we argue that desilication is also part of the rodingite-forming process. The low silica activity also explains the occurrence of low-silica minerals such as hydrogrossular, andradite, jadeite, diaspore, and corundum in serpentinites or rocks adjacent to serpentinites. The tendency for silica activity to decrease with decreasing temperature means that the presence of certain minerals in serpentinites can be used as indicators of the temperature of serpentinization. These include, with decreasing temperature, diopside, andradite and diaspore. Because the assemblage serpentine + brucite marks the lowest silica activity reached in most serpentinites, the presence and distribution of brucite, which commonly is a cryptic phase in serpentinites, is critical to interpreting the processes that lead to the hydration of any given serpentinite.
Serpentinization is an important geochemical process that affects the chemistry and petrophysical properties of the oceanic lithosphere and supports life through abiogenic formation of hydrogen. ...Here, we document through detailed mineralogical evidence and equilibrium thermodynamic models the importance of water (H
2
O) and silica (SiO
2
) activities on mineral assemblages produced during progressive serpentinization of a harzburgite. We describe a harzburgite from the Santa Elena Ophiolite in Costa Rica that is ~30 % serpentinized. Serpentine + brucite ± magnetite veins occur in olivine, Al-rich serpentine + talc veins occur in orthopyroxene, and Al-rich serpentine ± talc ± brucite veins occur at the boundary of orthopyroxene and olivine. Bulk vein chemistry and element distribution maps demonstrate distinct chemical zonations within veins and chemical gradients between orthopyroxene- and olivine-dominated areas. Specifically, the sample records (1) varying brucite composition depending on whether or not it is associated with magnetite, (2) formation of magnetite from Fe-rich brucite (±Fe-rich serpentine) during olivine hydration, where magnetite coexists with brucite Mg#96 and serpentine Mg#99, (3) chemical gradients in Si, Al, Cr, and Ca within and between orthopyroxene- and olivine-hosted veins, and 4) local (different) equilibrium assemblages within different zones of veins. The studied sample preserves rarely observed textures documenting continuous replacement of olivine, rather than individual vein generations and overprinting that is typically observed in more intensely serpentinized peridotites. Furthermore, the presence of a discrete sequence of vein textures and mineralogy allows direct comparison between mineral textures and equilibrium thermodynamic models and permits new insights into mineral reactions during serpentinization.
Abstract
The distribution of pegmatitic rocks in the zoned gabbro–diorite plutons of the late Jurassic Smartville Complex is counter-intuitive. Whereas pegmatitic gabbros are common in mafic cumulate ...olivine gabbros within the zoned plutons, they appear to be absent from the more evolved gabbros and diorites. We argue that this paradox is resolved by examining the crystallization and temperature history of the rocks in question. The evolved rocks in the plutons consist of two-pyroxene hornblende biotite gabbro and diorite. The amphibole in these rocks occurs as a partial replacement of pyroxene that forms as a consequence of the incongruent crystallization reaction generalized as amphibole ± quartz ± plag1 = pyroxene + hydrous melt ± plag2 reaction (1). This is a vapor-absent reaction that can buffer melt water content during crystallization and conceivably preclude water saturation altogether. Experimental evidence places the onset of the incongruent formation of amphibole at c. 900°C. In contrast, amphibole and other hydrous phases occur only in trace amounts in the cumulate olivine gabbros. The water-buffering reaction is not encountered during crystallization, the intercumulus melt proceeds to water saturation, and high-temperature plagioclase–pyroxene pegmatites form. We argue that the primary role for undercooling favored for the formation of granitic pegmatites is not likely to be applicable to the Smartville gabbro pegmatites. As crystallization of the gabbro pegmatite proceeds, the amphibole-in boundary is eventually reached and incongruent crystallization of amphibole occurs. Nevertheless, once water saturation occurs, it is irreversible, and is eventually manifested by the low-temperature deuteric alteration associated with the pegmatites. In short, we argue that pegmatite formation in the Smartville gabbros, and probably many other gabbroic plutons, reflects water saturation of a magma at high temperature, estimated here at >900°C. In systems where reaction (1) is attained prior to water saturation, pegmatite formation may be delayed until late in the crystallization history or precluded altogether. MELTS modeling of basalt crystallization suggests a solution to the paradox, indicating that the difference between the two systems can reflect equilibrium versus fractional crystallization. Equilibrium crystallization of trapped, intercumulus melt favors high-temperature crystallization and water saturation and, hence, pegmatite formation. Fractional crystallization, for example in an open-system magma chamber, favors lower-temperature crystallization and permits reaction (1) to proceed, inhibiting water saturation and preventing pegmatite formation. The open-system nature of the fractionating magma chamber might also allow escape of volatiles at its margins, with the lower volatile content then communicated to the magma as a whole as the system convects or otherwise mixes.
Triatomine bugs, the vectors of Chagas disease, associate with vertebrate hosts in highly diverse ecotopes. It has been proposed that occupation of new microhabitats may trigger selection for ...distinct phenotypic variants in these blood-sucking bugs. Although understanding phenotypic variation is key to the study of adaptive evolution and central to phenotype-based taxonomy, the drivers of phenotypic change and diversity in triatomines remain poorly understood.
We combined a detailed phenotypic appraisal (including morphology and morphometrics) with mitochondrial cytb and nuclear ITS2 DNA sequence analyses to study Rhodnius ecuadoriensis populations from across the species' range. We found three major, naked-eye phenotypic variants. Southern-Andean bugs primarily from vertebrate-nest microhabitats (Ecuador/Peru) are typical, light-colored, small bugs with short heads/wings. Northern-Andean bugs from wet-forest palms (Ecuador) are dark, large bugs with long heads/wings. Finally, northern-lowland bugs primarily from dry-forest palms (Ecuador) are light-colored and medium-sized. Wing and (size-free) head shapes are similar across Ecuadorian populations, regardless of habitat or phenotype, but distinct in Peruvian bugs. Bayesian phylogenetic and multispecies-coalescent DNA sequence analyses strongly suggest that Ecuadorian and Peruvian populations are two independently evolving lineages, with little within-lineage phylogeographic structuring or differentiation.
We report sharp naked-eye phenotypic divergence of genetically similar Ecuadorian R. ecuadoriensis (nest-dwelling southern-Andean vs palm-dwelling northern bugs; and palm-dwelling Andean vs lowland), and sharp naked-eye phenotypic similarity of typical, yet genetically distinct, southern-Andean bugs primarily from vertebrate-nest (but not palm) microhabitats. This remarkable phenotypic diversity within a single nominal species likely stems from microhabitat adaptations possibly involving predator-driven selection (yielding substrate-matching camouflage coloration) and a shift from palm-crown to vertebrate-nest microhabitats (yielding smaller bodies and shorter and stouter heads). These findings shed new light on the origins of phenotypic diversity in triatomines, warn against excess reliance on phenotype-based triatomine-bug taxonomy, and confirm the Triatominae as an informative model system for the study of phenotypic change under ecological pressure .
When main-sequence stars expand into red giants, they are expected to engulf close-in planets
. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning ...red giants
has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars
. Here we present the discovery that the giant planet 8 Ursae Minoris b
orbits a core-helium-burning red giant. At a distance of only 0.5 AU from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 AU. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet
. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems.
Serpentinization of olivine-rich troctolite from core 227, Integrated Ocean Drilling Program (IODP) Hole U1309D ranges from <10% to >90%. Two episodes of serpentinization are recognized. The first, ...dominant in weakly serpentinized samples, is an approximately isochemical (except for water) replacement of olivine (Fo84–85) by a mixture of serpentine (antigorite, Mg-number 92) and brucite (amakinite-rich; Mg-number 65). The compositions of the minerals in type 1 veins are a reflection of Fe–Mg exchange between olivine and the brucite + serpentine formed during early serpentinization. The early serpentinite veins (type 1) are thin (< 0·05 mm), irregular, and exploit pre-existing cracks in olivine. The presence of antigorite suggests that early serpentinization occurred at T > 300°C. Type 1 veins reflect rock-dominated serpentinization, became isolated early in their history, and persist as relicts in all but the most altered samples. The main episode of serpentinization is manifested by through-going lizardite (average Mg-number 96)–magnetite veins (type 2). Type 2 veins define an anastomosing foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins. Type 2 veins reflect open-system serpentinization. Magnetite in these veins formed by oxidation of the Fe in brucite and serpentine, whereas addition of silica to the system converted the Mg-component of the brucite to serpentine. Magnetite forms one or more distinct bands in the interior of the vein and is never in direct contact with relict olivine. A brucite–serpentine mixture, similar to that found in type 1 veins, but with lizardite instead of antigorite, is commonly present at the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine). We interpret type 2 veins as a steady-state system where brucite continually forms at the olivine–vein contact and then reacts out in the interior of the vein. This continual formation and destruction of brucite imposes an exceptionally low aSiO2 on the system. Magnetite and olivine are never in contact in type 2 veins (or anywhere else) because the olivine-out reaction yields ferroan brucite and not magnetite. The desilication of serpentine in the type 2 veins is a reflection of the inherent instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine in equilibrium with magnetite in serpentinites is a function of serpentine–magnetite and not serpentine–olivine equilbria.
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