Nancy J. McMillan

The Nevados de Payachata volcanic region (18°S/69°W, N. Chile) II. Evidence for widespread crustal involvement in Andean magmatism

Volcanism extending over 11 Ma is represented in the rocks of the Nevados de Payachata region, culminating in the formation of two large composite stratocones within the last 500 000 years. Chemically distinct mafic magmas are erupted at a number of parasitic centers. These cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones. Two distinct trends are defined by the intermediate and evolved rocks; a high LILE (large ion lithophile element), TiO2 and Ce/Yb lineage among the youngest rocks (including the two major stratocones), and a more typical calc-alkaline trend among the older (>1 Ma) rock types. Within individual volcanic centers, differentiation involves fractionation of plagioclase, pyroxene and hornblende, with biotite and K-feldspar in the more-evolved rock types. Isotopic compositions (Sr, Pb, Nd, O) vary little with differentiation from basaltic andesite to rhyolite, or with age. Contamination during differentiation from basalt to rhyolite may occur, but the most mafic rocks erupted in the region are already enriched in incompatible trace elements and therefore may be insensitive to the effects of interaction with the crust. The majority of data are similar to “baseline” compositions (Cenozoic parental magmas) from other parts of the central Andes and may reflect a relatively homogeneous magma source (or source mixture) throughout this central volcanic zone (CVZ), which is distinct from the southern and northern Andes, and from island-arc volcanic rocks.The detailed study of Nevados de Payachata serves as a useful reference against which to assess magmatism in general in the CVZ. The possibility that central Andean magmas are generated from an enriched subcontinental-lithosphere mantle wedge is rejected on the basis of: (1) thermal considerations (subcontinental mantle lithosphere is probably cold and refractory); (2) lack of consistency between the tectonic history of the region and geochemical variations through time. Instead, parental magmas in the CVZ are thought to be generated by mixing between normal arc magmas originating in the depleted mantle wedge followed by contamination and homogenization with lower crustal melts. In the central Andes, the extent of contamination increased greatly as the crust thickened due to crustal shortening within the last 20 Ma, the thicker crust providing an effective filter to trap and differentiate magma batches repeatedly during ascent.

The Nevados de Payachata volcanic region (18°S/69°W, N. Chile)

Subduction-related volcanism in the Nevados de Payachata region of the Central Andes at 18°S comprises two temporally and geochemically distinct phases. An older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late Miocene age. The Pleistocene to Recent phase (≤0.3 Ma) includes the twin stratovolcanoes Volcan Pomerape and Volcan Parinacota (the Nevados de Payachata volcanic group) and two small centers to the west (i. e., Caquena and Vilacollo). Both stratovolcanoes consist of an older dome-and-flow series capped by an andesitic cone. The younger cone, i. e., V. Parinacota, suffered a postglacial cone collapse producing a widespread debris-avalanche deposit. Subsequently, the cone reformed during a brief, second volcanic episode. A number of small, relatively mafic, satellitic cinder cones and associated flows were produced during the most recent activity at V. Parinacota. At the older cone, i. e., V. Pomerape, an early dome sequence with an overlying isolated mafic spatter cone and the cone-forming andesitic-dacitic phase (mostly flows) have been recognized. The two Nevados de Payachata stratovolcanoes display continuous major- and trace-element trends from high-K2O basaltic andesites through rhyolites (53%–76% SiO2) that are well defined and distinct from those of the older volcanic centers. Petrography, chemical composition, and eruptive styles at V. Parinacota differ between pre- and post-debris-avalanche lavas. Precollapse flows have abundant amphibole (at SiO2 > 59 wt%) and lower Mg numbers than postcollapse lavas, which are generally less silicic and more restricted in composition. Compositional variations indicate that the magmas of the Nevados de Payachata volcanic group evolved through a combination of fractional crystallization, crustal assimilation, and intratrend magma mixing. Isotope compositions exhibit only minor variations. Pb-isotope ratios are relatively low (206Pb/204Pb = 17.95–18.20 and208Pb/204Pb = 38.2–38.5);87Sr/86Sr ratios range 0.70612–0.70707,143Nd/144Nd ratios range 0.51238–0.51230, andγ18OSMOW values range from + 6.8%o to + 7.6%o SMOW. A comparison with other Central Volcanic Zone centers shows that the Nevados de Payachata magmas are unusually rich in Ba (up to 1800 ppm) and Sr (up to 1700 ppm) and thus represent an unusual chemical signature in the Andean arc. These chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components. Primary mantle-derived melts, which are similar to those generated by subduction processes throughout the Andean arc, are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface. These magmas subsequently evolve at shallower levels through assimilation-crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast.

Arc abandonment as a cause for passive continental rifting: Comparison of the Jurassic Mexican Borderland rift and the Cenozoic Rio Grande rift

Two rift systems, one of late Mesozoic age and the other of Tertiary age, in the southern Cordillera of North America formed along the inner flanks of former continental arcs. Both rift systems were initiated when arc magmatism abandoned its former inboard extent as a result of retrograde motion of the subducted slab. Similarities in stratigraphy and geochemistry preceding and during crustal extension of each rift system suggest a three-phase magmatic-depositional model for the formation of passive continental rifts above a foundering subducted slab. Continental arc magmatism associated with normal subduction weakens the continental crust during phase 1. Phase 2 involves incipient retrograde motion of the slab, or slab foundering, initiating mantle return into the wedge-shaped volume between slab and overlying continental lithosphere. This causes crustal extension, lithospheric melting, and deposition of conglomerate in nascent rift basins stratigraphically above and adjacent to the extinguished arc. Caldera-related silicic volcanism defines an ignimbrite flare-up, accompanied by extrusion of lithosphere-derived basalt. Mafic volcanism, block faulting, and extensional sedimentary-basin formation continue after the end of silicic volcanism. During phase 3, decompression partial melting of convecting asthenosphere creates basalts with ocean-island chemical affinities intercalated with alluvial or marine sedimentary rocks in extensional basins.

Evolution of magma source regions in the Rio Grande rift, southern New Mexico

Early magmatism of the southern Rio Grande rift was strongly controlled by the thermal history of the lithosphere. The compositions of igneous rocks in southern New Mexico record a general shift in magma source regions through the Cenozoic from lithosphere to asthenosphere and from upper crust to lower crust and, finally, to little crustal involvement. Extension began as early as 36 Ma, coincident with the onset of bimodal volcanism of the middle Tertiary ignimbrite flare-up and production of half grabens after a short episode of post-Laramide subduction-related volcanism. Oligocene mafic magmas have incompatible trace element patterns similar to modern continental-arc basalts (low Nb, Ta, and Ti; high Rb/Nb, Ba/Nb, and La/Nb). The most mafic magma has 87Sr/86Sr and ϵNd values near bulk Earth (0.704771 and +0.2, respectively). The Oligocene mafic to intermediate-composition suites evolved toward slightly higher 87Sr/86Sr, lower ϵNd, and nonradiogenic Pb isotopic compositions (87Sr/86Sr = 0.70440–0.70785; ϵNd = −2.2 to −4.8; 206Pb/204Pb = 17.039–18.084, 207Pb/204Pb = 15.387–15.498; 208Pb/204Pb = 37.094–38.130) and are interpreted as partial melts of slightly hydrated lithosphere that were contaminated in the lower crust. Contemporaneous rhyolitic magmas have more radiogenic Sr and Pb isotopic compositions (87Sr/86Sr = 0.7111; ϵNd = −4.5; 206Pb/204Pb = 18.435; 207Pb/204Pb = 15.538; 208Pb/204Pb = 38.607) and record the involvement of an upper-crustal component. Silicic volcanism ceased abruptly at 28.5 Ma, but mafic to intermediate-composition lithospheric magmatism persisted until 24 Ma. Although extension continued, forming half grabens with sedimentary fill, the period between 24 and 10 Ma was amagmatic. This lack of volcanism and the end of lithosphere-dominated magma genesis reflect the effective scavenging of the hydrated parts of the lithospheric mantle by middle Tertiary magmatism, which produced a refractory, infertile, subcontinental lithosphere. Volcanism resumed at 10 Ma with sporadic eruption of tholeiitic and alkalic basalts having trace element patterns similar to oceanic-island basalts (high Nb, Ta, and Ti; low Ba/Nb and La/Nb), depleted Sr and Nd isotopic compositions (87Sr/86Sr = 0.70297–0.70396; ϵNd = +4.7 to +7.3), and more-radiogenic Pb isotopic compositions (206Pb/204Pb = 18.460–19.698; 207Pb/204Pb = 15.461–15.684; 208Pb/204Pb = 38.091–39.411) than the lithosphere-derived suites. The composition of these late Cenozoic basalts records a dramatic shift in source region to upwelling asthenosphere that melted by adiabatic decompression.

Crustal sources involved in continental arc magmatism: A case study of volcan Mocho-Choshuenco, southern Chile

Volcan Mocho-Choshuenco, a Pleistocene-Holocene basaltic andesite to dacite shield volcano located at lat 40°S in the Southern Volcanic Zone (SVZ) of the Andes, is built on the thinnest SVZ continental crust (≈30 km), which consists largely of Paleozoic sedimentary and metamorphic rocks and Mesozoic plutons. Sr, Pb, Nd, and O isotopic ratios of Mocho-Choshuenco lavas ( 87 Sr/ 86 Sr = 0.70399 to 0.70422; 206 Pb/ 204 Pb =18.57 to 18.61; 207 Pb/ 204 Pb = 15.60 to 15.63; 208 Pb/ 204 Pb = 38.45 to 38.57; 143 Nd/ 144 Nd = 0.512787 to 0.512881; δ 18 O = +5.8‰ to +6.8‰ Standard Mean Ocean Water) are similar to those of other SVZ centers and exhibit no correlation with indices of differentiation. However, incompatible trace-element systematics indicate that evolved lavas have been contaminated by crustal plutonic rocks by 5% to 15%. Rb/Ba, K/Ba, Rb/La, and K/La ratios increase with increasing SiO 2 content as a result of assimilation of orogenic calc-alkalic plutonic rocks enriched in Rb and K relative to Ba and La. Xenoliths of plutonic rocks recovered from Mocho-Choshuenco lavas are slightly more radiogenic in 87 Sr/ 86 Sr, less radiogenic in 143 Nd/ 144 Nd, and comparable in Pb isotopic ratios, and they have variable 18 O contents ( 87 Sr/ 86 Sr = 0.70482 to 0.70557; 206 Pb/ 204 Pb = 18.60 to 18.64; 207 Pb/ 204 Pb = 15.60 to 15.62; 208 Pb/ 204 Pb = 38.53 to 38.56; 143 Nd/ 144 Nd = 0.512686; δ 18 O = +3.3‰ to +5.5‰ SMOW). Contamination of Mocho-Choshuenco parental magmas by crustal melts with small isotopic contrast imposed little variation on isotopic heterogeneities existing in the parental compositions. These heterogeneities probably were produced during magma-crust interaction at the crust-mantle boundary or in the lower crust.

Use of laser induced breakdown spectroscopy in the determination of gem provenance: beryls.

The provenance of gem stones has been of interest to geologists, gemologists, archeologists, and historians for centuries. Laser induced breakdown spectroscopy (LIBS) provides a minimally destructive tool for recording the rich chemical signatures of gem beryls (aquamarine, goshenite, heliodor, and morganite). Broadband LIBS spectra of 39 beryl (Be(3)Al(2)Si(6)O(18)) specimens from 11 pegmatite mines in New Hampshire, Connecticut, and Maine (USA) are used to assess the potential of using principal component analysis of LIBS spectra to determine specimen provenance. Using this technique, beryls from the three beryl-bearing zones in the Palermo #1 pegmatite (New Hampshire) can be recognized. However, the compositional variation within this single mine is comparable to that in beryls from all three states. Thus, a very large database with detailed location metadata will be required to routinely determine gem beryl provenance.

Correlation of limestone beds using laser-induced breakdown spectroscopy and chemometric analysis

Correlation of limestone beds is commonly based on a variety of features, including the age of the bed, the fossil assemblage, internal sedimentary structures, and the relationship to other units in the stratigraphy. This study uses laser-induced breakdown spectroscopy (LIBS) to correlate 16 limestone beds from Kansas, USA, using three multivariate techniques: (1) soft independent modeling of class analogy (SIMCA) classification, (2) a partial least squares regression, 1 variable (PLS-1) model in which the spectra are regressed against a matrix of the indicator variables 1 through 16, and (3) a matching algorithm that consists of a sequence of binary PLS-1 models. Each gravel-sized limestone particle was analyzed by one LIBS shot; ten spectra were averaged into a single spectrum for chemometric analysis. The entire spectrum (198-969 nm wavelength) is used for multivariate analysis; the only preprocessing is averaging. The SIMCA and PLS-1 models fail to discriminate among the beds, which are chemically similar. In contrast, the matching algorithm has a success rate of 95% to 96%, using half of the spectra to train the model and the other half of the spectra to validate it. However, 100% success can be accomplished by accepting the classification of the majority of spectra for a given bed as the correct classification. This study indicates that LIBS can be applied to complex geologic correlation problems and provide rapid, accurate results.

Geochemical analysis and paleoecological implications of phosphatic microspherules (otoliths?) from Frasnian–Famennian boundary strata in the Great Basin, USA

Phosphatic microspherules (<1 mm diameter) recovered from the Upper Devonian (uppermost Frasnian) Guilmette Limestone in eastern Nevada are interpreted here to be fish otoliths and thus provide insight into ocean water chemistry just prior to the Frasnian–Famennian mass extinction boundary. Analysis by scanning electron microscope (SEM) and electron microprobe indicates that they are apatite (francolite) and consist of radially aligned, concentrically banded crystals around a central nucleus. This type of microspherule, previously interpreted to be conodont pearls, has compositions more similar to fish teeth derived from the same unit than to conodonts. Microspherules and fish teeth have consistently lower concentrations in wt% of P2O5 (31.88–36.32), F (3.05–5.12), SrO (0.15–0.34), and analysis totals (indicating higher concentrations of OH and/or CO3; 90.66–96.09) and higher CaO (51.67–55.15), SO2 (0.50–0.90), MgO (0.09–0.15), and Fe2O3 (0.11–0.21) than the associated conodonts (P2O5: 37.32–40.01; F: 4.99–6.89; SrO: 0.32–1.79; totals: 96.07–100.55; CaO: 52.32–53.06; SO2: 0.05–0.21; MgO: 0.01–0.05; Fe2O3: 0.02–0.11). These differences in composition are consistent from core to rim in all microfossils analyzed and reflect primary biogenic compositions rather than diagenetic or metamorphic signatures. We interpret the apatite microspherules to be genetically related to the fish teeth rather than to the conodonts based on the geochemical analysis. The microspherules are morphologically similar to modern teleost fish otoliths. The fish teeth found associated with the microspherules are from Acanthodian or Actinopterygian fish, which possessed otoliths in the Devonian and are the distant ancestors to modern teleost fish. Modern fish otoliths normally have a calcium carbonate composition, but their trace element composition is highly sensitive to ambient water temperature and chemistry. We speculate that the stratigraphically restricted range and phosphatic composition of the Devonian otoliths reflects secretion by fish under conditions of excess dissolved reactive phosphorus in the water column that was most likely associated with upwelling and cooler-water conditions on the shelf during maximum transgression.

Provenance determination of sapphires and rubies using laser-induced breakdown spectroscopy and multivariate analysis

Abstract Determination of gem provenance is a topic of research in the gemological community for financial, security, and societal reasons. Laser-induced breakdown spectroscopy (LIBS) and multivariate analysis have the potential to revolutionize the field of gem provenance. This study acquired LIBS spectra from 569 rough sapphire and ruby specimens from 21 localities in 11 countries. The spectra were analyzed using the multivariate technique partial least-squares regression (PLSR) in separate algorithms for sapphires and rubies. Each algorithm consists of a series of PLS models. Each model compares the spectra from a locality of interest to the spectra from all other localities in the database. Success rates, as determined by the percent of correct provenance identifications, are 98.9% (sapphire) and 96.0% (ruby) for country of origin and 97.9% (sapphire) and 95.4% (ruby) for deposit of origin. Individual deposits are not recognized by the concentrations of a few elements; rather, the unique compositional signature of each deposit consists of the ratios of many elements, primarily Ca, Zr, Fe, Ba, Mt, Ti, Sr, Si, Cr, H, C, and Li, some of which may reside in inclusions. This work demonstrates that determination of country or deposit of origin may be related to a quantitative measure with a high level of success.

A comparative study of in situ biosignature detection spectroscopy techniques on planetary surfaces

We demonstrate the biosignature detection capabilities of several classes of instruments, including a compact laser desorption/ionization time-of-flight mass spectrometer, an acousto-optic tunable filter IR point spectrometer, a laser-induced breakdown spectrometer, and a scanning electron microscope. We collected biotic and abiotic calcite, gypsum, and manganese oxide samples from Fort Stanton Cave to identify the presence of biomarkers with each instrument class. We find evidence of biologic activity in these samples including the presence of organic molecules, macroscopic and microscopic morphological features consistent with fossilized mircobes, and the presence of trace elements consistent with the biotic precipitation of minerals. The identification of extant or extinct microbial life is best supported by a suite of biosignatures, rather than a single observation. We demonstrate the unique biosignature detection results of each instrument class and discuss the importance of developing an instrument suite for future landed astrobiology missions on other planetary surfaces.