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At least 127 records · Page 7Linked to original sources
USGS2007

Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism

Mesozoic continental arcs in the North American Cordillera were examined here to establish a baseline model for Phanerozoic continent formation. We combine new trace-element data on lower crustal xenoliths from the Mesozoic Sierra Nevada Batholith with an extensive grid-based geochemical map of the Peninsular Ranges Batholith, the southern equivalent of the Sierras. Collectively, these observations give a three-dimensional view of the crust, which permits the petrogenesis and tectonics of Phanerozoic crust formation to be linked in space and time. Subduction of the Farallon plate beneath North America during the Triassic to early Cretaceous was characterized by trench retreat and slab rollback because old and cold oceanic lithosphere was being subducted. This generated an extensional subduction zone, which created fringing island arcs just off the Paleozoic continental margin. However, as the age of the Farallon plate at the time of subduction decreased, the extensional environment waned, allowing the fringing island arc to accrete onto the continental margin. With continued subduction, a continental arc was born and a progressively more compressional environment developed as the age of subducting slab continued to young. Refinement into a felsic crust occurred after accretion, that is, during the continental arc stage, wherein a thickened crustal and lithospheric column permitted a longer differentiation column. New basaltic arc magmas underplate and intrude the accreted terrane, suture, and former continental margin. Interaction of these basaltic magmas with pre-existing crust and lithospheric mantle created garnet pyroxenitic mafic cumulates by fractional crystallization at depth as well as gabbroic and garnet pyroxenitic restites at shallower levels by melting of pre-existing lower crust. The complementary felsic plutons formed by these deep-seated differentiation processes rose into the upper crust, stitching together the accreted terrane, suture and former continental margin. The mafic cumulates and restites, owing to their high densities, eventually foundered into the mantle, leaving behind a more felsic crust. Our grid-based sampling allows us to estimate an unbiased average upper crustal composition for the Peninsular Ranges Batholith. Major and trace-element compositions are very similar to global continental crust averaged over space and time, but in detail, the Peninsular Ranges are slightly lower in compatible to mildly incompatible elements, MgO, Mg#, V, Sc, Co, and Cr. The compositional similarities suggest a strong arc component in global continental crust, but the slight discrepancies suggest that additional crust formation processes are also important in continent formation as a whole. Finally, the delaminated Sierran garnet pyroxenites have some of the lowest U/Pb ratios ever measured for silicate rocks. Such material, if recycled and stored in the deep mantle, would generate a reservoir with very unradiogenic Pb, providing one solution to the global Pb isotope paradox.

Earth and Planetary Science Letters↗
USGS1987

Sedimentary processes on the northwestern Iberian continental margin viewed by long-range side-scan sonar and seismic data

The effects of an eastern boundary current in the North Atlantic have been mapped from about 39° north latitude along the Iberian margin to as far north as 43°30 north latitude at the western margin of Galicia Bank. The geostrophic current has produced sediment drifts that are covered with bedforms. The sediment drifts are difficult to detect on Gloria long-range side-scan sonar data but are easily resolved on seismic-reflection records as anomalously thick accumulations of sediment banked against either buried or outcropping basement highs. The bedforms ornamenting the drift surfaces were subdivided into 1,000-m water-depth intervals, and their dimensions were tabulated. There are few bedforms in water depths less han 2,000 m, but from depths between 2,000 and 4,000 m they are numerous and have a mean wavelength of 695 m. Bedforms from depths greater than 4,000 m have a mean wavelength of 999 m. The different wavelengths from different water depths suggest two distinct and separated boundary flows. The wave heights of all bedforms found in water depths greater than 2,000 m are less than 10 m. In order to investigate the continuity of sediment drifting through geological time, the stratigraphic section drilled at DSDP Site 398 was reinterpreted and, using seismic-reflection profiles, was traced throughout the northern Iberian margin. Together, the lithostratigraphic and seismic data indicate that sediment drifting developed along this margin in the Eocene. The lithofacies of the Eocene section is t e oldest to have numerous layers of sand and silt. An unconformity separates the Eocene section from the latest Miocene-Pliocene section. The unconformity is interpreted to be the result of the initial pulses of Mediterranean outflow that followed the Messinian desiccation events. A second period of sediment drifting commenced during the Pliocene once the Mediterranean basin filled and the flow out of the Strait of Gibraltar resumed.

Journal of Sedimentary Petrology↗
USGS1991

Age and petrology of the Tertiary As Sarat volcanic field, southwestern Saudi Arabia

Harrat As Sarat forms the second smallest and southernmost of the basalt fields of western Saudi Arabia and is part of a voluminous Red Sea rift-related continental alkali basalt province. The rocks of the As Sarat were emplaced during the first stage of Red Sea rifting and represent the northernmost extension of the Tertiary Trap Series volcanics that occur mainly in the Yemen Arab Republic and Ethiopia. The field consists of up to 580 m of basalt flows, that are intruded by basaltic plugs, necks, minor dikes, and highly evolved peralkaline trachyte intrusions. K-Ar ages indicate that the As Sarat field formed between 31 and 22 Ma and contains an eruption hiatus of one million years that began about 25 Ma ago. Pre-hiatus flows are primarily hypersthene normative intersertal subalkaline basalt, whereas the majority of post-hiatus flows are nepheline normative alkali basalt and hawaiite with trachytic textures. Normative compositions of the basalts are consistent with their genesis by partial melting at varying depths. Trace element abundances in the basalt indicate that varying degrees of partial melting and fractional crystallization (or crystal accumulation) had major and minor roles, respectively, in development of compositional variation in these rocks. Modeling indicates that the pre-hiatus subalkaline basalts represent 8–10 percent mantle melting at depths of about 70 km and the post-hiatus alkali basalts represent 4–9 percent mantle melting at depths greater than 70 km.

southwestern Saudi Arabia↗
USGS2025

Confocal laser-scanning microscopy (CLSM)-based thermal maturity of Tasmanites and progress in standardization of fluorescence microspectrometry

Evaluation of thermal maturity in vitrinite-free or vitrinite-deficient sediments via fluorescence microspectrometry can provide relevant information related to petroleum exploration and thermal history assessment. However, variation in spectral fluorescence properties of alginite macerals with increasing thermal maturity is largely underexplored. Here, authors of this study have applied confocal laser-scanning microscopy (CLSM) in conjunction with fluorescence microspectrometry to a maturity series of marine Upper Devonian Tasmanites algae from the Ohio Shale (Huron Member) and a single sample from the Marcellus Formation of the Appalachian Basin. Spectral fluorescence properties of Tasmanites were evaluated in relation to orientation, measurement location, and the number of measurements per sample, and were compared to published literature. Emission spectra of Tasmanites from continuous wave laser excitation (405 nm) were acquired from sections perpendicular and parallel to bedding and at different positions within individual Tasmanites bodies. The results showed a progressive red-shift in emission maxima (λ max ) in a large sample sized maturity series ( N = 19), e.g., 493 to 578 nm for the perpendicular section at middle position. Further, blue-shifted apex and mineral-adjacent positions within sections perpendicular to bedding were observed, with the latter being reported here for the first time. While blue-shift at apex positions can be attributed to mechanical deformation-induced reorientation of photoselected fluorophores, the blue-shifted mineral-adjacent positions could result from strain loading and development of a plastic deformation region at the mineral contact zone with Tasmanites . A decrease in standard deviation with increasing number of measured emission maxima is well-observed, and 15 to 20 individual measurements per sample appears sufficient for low standard deviation and coefficient of variance. CLSM-derived thermal maturity parameters indicated that a moderate positive correlation of red/green quotient (Q 650/500 ; R 2 = 0.67) with solid bitumen reflectance (BR o in %) exists. For reproducible results, the determination of λ max and Q 650/500 should be conducted exclusively in the middle position at perpendicular and parallel sections of the polished whole-rock pellets, where the lowest standard deviation in measurement was observed. These results strengthen the suitability and relevance of the CLSM technique in thermal maturity studies of dispersed organic matter (DOM) and contribute to the standardization of fluorescence microspectrometry methods in organic petrology investigation.

Kentucky, Ohio, Virginia, West Virginia↗
USGS1986

Guidelines to classification and nomenclature of Arabian felsic plutonic rocks

Well-defined procedures for classifying the felsic plutonic rocks of the Arabian Shield on the basis of petrographic, chemical and lithostratigraphic criteria and mineral-resource potential have been adopted and developed in the Saudi Arabian Deputy Ministry for Mineral Resources over the past decade. A number of problems with conventional classification schemes have been identified and resolved; others, notably those arising from difficulties in identifying precise mineral compositions, continue to present difficulties. The petrographic nomenclature used is essentially that recommended by the International Union of Geological Sciences. Problems that have arisen include the definition of: (1) rocks with sodic, zoned or perthitic feldspar, (2) trondhjemites, and (3) alkali granites. Chemical classification has been largely based on relative molar amounts of alumina, lime and alkalis, and the use of conventional variation diagrams, but pilot studies utilizing univariate and multivariate statistical techniques have been made. The classification used in Saudi Arabia for stratigraphic purposes is a hierarchy of formation-rank units, suites and super-suites as defined in the Saudi Arabian stratigraphic code. For genetic and petrological studies, a grouping as ‘associations’ of similar and genetically related lithologies is commonly used. In order to indicate mineral-resource potential, the felsic plutons are classed as common, precursor, specialized or mineralized, in order of increasing exploration significance.

Journal of African Earth Sciences↗
USGS2021

Proportions, timing, and re-equilibration progress during the 1959 Summit Eruption of Kīlauea: An example of magma mixing processes operating during OIB petrogenesis

Petrographic and chemical analysis of scoria samples collected during the 1959 Kīlauea summit eruption illustrates the progress of thermal and chemical homogenization of the melts, and the gradual growth and/or re-equilibration of olivine phenocrysts, over the course of the eruption. Glass compositions show that thermal equilibration was largely complete within the span of the eruption, whereas chemical homogenization was a work in progress. The olivine phenocryst population, known to contain conspicuous antecrystic components, is also hybrid within the euhedral population. The bulk of the olivine reached the level of the erupting magma on November 18–19, 1959. Zoning patterns in olivine phenocrysts show that initially unzoned grains developed normal zoning by the end of the eruption. Reverse zoning in relatively Fe-rich olivine phenocrysts (interpreted as cognate to the stored magma) was progressively eliminated from November 21 to December 19, 1959, by diffusive re-equilibration between crystals and melt. Toward the end of the eruption, the only olivine composition in direct contact with the melt was Fo 84–86 , with the original rim compositional heterogeneity gone in 4–5 weeks’ time. Activity in December 1959 differed from that in November, as high fountaining events were more closely spaced and almost all samples were picritic, with bulk MgO ≥16·5 wt%. Three different levels were in play during the 1959 eruption: a deep source for high-MgO melts and forsteritic (Fo 87–89 ) olivines, an intermediate source for the bulk of the stored magma, and a shallower source for the most differentiated magma. This model is consistent with geophysical, petrological and chemical observations. Comparison of the 1959 eruption with results from older explosive deposits suggests that stored and recharge melts and olivine from the deeper parts of Kīlauea’s plumbing are similar in composition to those observed or inferred in the 1959 eruption, so they behave similarly during extrusive and explosive periods alike.

Hawaii↗
USGS1997

Comagmatic A-type granophyre and rhyolite from the Alid volcanic center, eritrea, northeast Africa

Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly crystallized, intrusive equivalent of pumice from the pyroclastic flow. Phenocryst compositions and geochemical characteristics of the pumice and granophyre are virtually identical. Silicate melt inclusions and other geochemical and geological constraints reveal those processes leading to development of the granophyric texture. Rhyolitic (A-type) magma with ∼2.6 wt % dissolved H 2 O and a temperature near 870°C was intruded to within 2–4 km of the surface, causing deformation and structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degassing, which forced undercooling and consequent granophyric crystallization of some of the magma remaining in the intrusion. The most recent eruption from Alid excavated the crystallized granitic wall of the magma chamber, bringing the granophyric clasts to the surface.

Journal of Petrology↗
USGS2017

Petrology and geochronology of Mesoproterozoic basement of the Mount Rogers area of southwestern Virginia and northwestern North Carolina: Implications for the Precambrian tectonic evolution of the southern Blue Ridge province

Results from new geologic mapping, SHRIMP U-Pb geochronology, and petrologic studies indicate that Mesoproterozoic basement in the northern French Broad massif near Mount Rogers consists of multiple, mostly granitic plutons, map- and outcrop-scale xenoliths of pre-existing crustal rocks, and remnants of formerly overlying meta-sedimentary lithologies. Zircon and titanite ages demonstrate that these rocks collectively record nearly 350 m.y. of tectonic evolution including periods of igneous intrusion at ca . 1190 to 1130 Ma (Early Magmatic Suite) and ca . 1075 to 1030 Ma (Late Magmatic Suite) and three episodes of regional metamorphism at ca . 1170 to 1140, 1070 to 1020, and 1000 to 970 Ma. The existence of ca . 1.3 Ga age crust is indicated by (1) orthogranofels of ca . 1.32 Ga age in a map-scale xenolith, (2) inherited zircons of ca . 1.33 to 1.29 Ga age in Early Magmatic Suite plutons, and (3) ca . 1.36 to 1.30 Ga age detrital zircons in meta-sedimentary lithologies. Mineral assemblages developed in amphibolites and granofelses indicate that metamorphism during both Mesoproterozoic episodes occurred at upper amphibolite- to lower granulite-facies conditions. Syn-orogenic Early Magmatic Suite plutons emplaced at ca . 1190 to 1145 Ma are characterized by high-K, variably magnesian, dominantly calc-alkalic compositions, and have trace-element characteristics indicative of continental-arc magmatic origin involving melting of thick continental crust. In contrast, ca . 1140 Ma age quartz syenite displays A-type features indicating derivation from depleted crustal sources with increased mantle input during waning stages of regional contraction. Plutons of the compositionally bimodal Late Magmatic Suite include (1) ca . 1060 Ma meta-granite with geochemical characteristics transitional between silicic rocks of arc systems and post-collisional granites of A-type lineage, and (2) ca . 1055 Ma monzodioritic rocks with A-type compositional characteristics that likely reflect derivation from fertile, mafic sources in the lower crust. Collectively, these data suggest that Mesoproterozoic rocks of the study area preserve evidence of multiple orogenic episodes that likely involved continental-arc development and deformation at ca . 1150 Ma followed by crustal thickening at ca . 1060 Ma. Field relations and geochronologic data indicate that regional uplift and sedimentation occurred at ca . 1100 Ma between the two episodes of overlapping magmatism and orogenesis. The nature and timing of Mesoproterozoic events recorded in basement rocks of the study area illustrate significant differences in the lithologic assemblages and geologic history preserved by Mesoproterozoic basement of the adjacent Shenandoah and French Broad massifs, suggesting that the Blue Ridge massifs occupied different locations within the regional Grenville-age orogen until about 1070 Ma when the effects of Ottawan-age tectonics began to affect both areas. The near ubiquity of Ottawan-age orogenic activity recorded in Mesoproterozoic rocks of the Blue Ridge, other Appalachian inliers such as the New Jersey Highlands, and the Grenville province of Canada, including the Adirondacks, suggests that these formerly disparate terranes were amalgamated to form a common, regional orogen by this time.

North Carolina, Virginia↗
USGS1979

Petrology, composition, and age of intrusive rocks associated with the Quartz Hill molybdenite deposit, southeastern Alaska

A large porphyry molybdenum deposit (Quartz Hill deposit) was recently discovered in the heart of the Coast Range batholithic complex about 70 km east of Ketchikan, southeastern Alaska. Intrusive rocks associated with the mineral deposit form two composite epizonal to hypabyssal stocks and many dikes in country rocks. The stocks are characterized by a variety of textural rock types varying from equigranular or weakly seriate biotite granite to porphyries with aphanitic or very fine grained and aplitic groundmasses. These rocks contain about equal amounts of quartz, albitic plagioclase, and microperthitic microcline and less than 2.5% biotite. Unaltered rocks contain between 0.2 and 1% CaO, less than 1.7% combined Fe 2 O 3 , FeO, and MgO, and 74.4 to 77.7% SiO 2 . Total alkalis are between 8 and 9%, and K 2 O/Na 2 O is about 1.1. The range of major-oxide variation is small, but it is systematically related to lithology. Many trace-elements, including B, Pb, Sn, and Li have low concentrations. Intrusive rocks associated with the Quartz Hill deposit are more albitic and possibly trace-element depleted compared to some other rocks associated with porphyry molybdenum deposits.All observed metallization and alteration is within the Quartz Hill stock. Molybdenite forms fracture coatings and occurs in veins with quartz. Alteration is widespread and includes development of secondary quartz, pyrite, K-feldspar, biotite, white mica, chlorite, and zeolite. Field relations indicate that the stocks were emplaced after regional uplift and erosion of the Coast Range batholithic complex, and K–Ar data show that intrusion and alteration took place in late Oligocene time, about 27 to 30 Ma ago. Data from the Ketchikan quadrangle indicate that porphyry molybdenum metallization in the Coast Range batholithic complex is associated with regionally extensive but spotty, middle Tertiary or younger, felsic magmatism.

Alaska↗
USGS2010

Thermomagmatic evolution of Mesoproterozoic crust in the Blue Ridge of SW Virginia and NW North Carolina: Evidence from U-Pb geochronology and zircon geothermometry

New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon define three episodes of magmatism: 1327 ± 7 Ma, 1180–1155 Ma, and 1061 ± 5 Ma. Crustal recycling is recorded by inherited igneous cores of 1.33–1.29 Ga age in 1161 ± 7 Ma meta-monzogranite. Overlapping ages of igneous and metamorphic crystallization indicate that plutons of ca. 1170 and 1060 Ma age were emplaced during episodes of regional heating. Local development of hornblende + plagioclase + quartz ± clinopyroxene indicates that prograde metamorphism at 1170–1145 Ma and 1060–1020 Ma reached upper-amphibolite-facies conditions, with temperatures estimated using Ti-in-zircon geothermometry at ~740 ± 40 °C during both episodes. The chemical composition of 1327 ± 7 Ma orthogranofels from migmatite preserves the first evidence of arc-generated rocks in the Blue Ridge, indicating a subduction-related environment that may have been comparable to similar-age systems in inliers of the Northern Appalachians and the Composite Arc belt of Canada. Granitic magmatism at 1180–1155 Ma and ca. 1060 Ma near Mount Rogers was contemporaneous with anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Northern Appalachian inliers and Canadian Grenville Province. Metamorphism at ca. 1160 and 1060 Ma correlates temporally with the Shawinigan orogeny and Ottawan phase of the Grenvillian orogeny, respectively, suggesting that the Blue Ridge was part of Rodinia dating back to ca. 1180 Ma.

North Carolina;Virginia↗
USGS2016

Low-δD hydration rinds in Yellowstone perlites record rapid syneruptive hydration during glacial and interglacial conditions

Hydration of silicic volcanic glass forms perlite, a dusky, porous form of altered glass characterized by abundant “onion-skin” fractures. The timing and temperature of perlite formation are enigmatic and could plausibly occur during eruption, during post-eruptive cooling, or much later at ambient temperatures. To learn more about the origin of natural perlite, and to fingerprint the hydration waters, we investigated perlitic glass from several synglacial and interglacial rhyolitic lavas and tuffs from the Yellowstone volcanic system. Perlitic cores are surrounded by a series of conchoidal cracks that separate 30- to 100-µm-thick slivers, likely formed in response to hydration-induced stress. H 2 O and D/H profiles confirm that most D/H exchange happens together with rapid H 2 O addition but some smoother D/H variations may suggest separate minor exchange by deuterium atom interdiffusion following hydration. The hydrated rinds (2–3 wt% H 2 O) transition rapidly (within 30 µm, or by 1 wt% H 2 O per 10 µm) to unhydrated glass cores. This is consistent with quenched “hydration fronts” where H 2 O diffusion coefficients are strongly dependent on H 2 O concentrations. The chemical, δ 18 O, and δD systematics of bulk glass records last equilibrium between ~110 and 60 °C without chemical exchange but with some δ 18 O exchange. Similarly, the δ 18 O of water extracted from glass by rapid heating suggests that water was added to the glass during cooling at <200 °C. Our observations support fast hydration at temperatures as low as 60 °C; prolonged exposure to high temperature of 175°–225° during water addition is less likely as the glass would lose alkalies and should alter to clays within days. A compilation of low-temperature hydration diffusion coefficients suggests ~2 orders of magnitude higher rates of diffusion at 60–110 °C temperatures, compared with values expected from extrapolation of high-temperature (>400 °C) experimental data. The thick hydration rinds in perlites, measuring hundreds of microns, preserve the original D/H values of hydrating water as a recorder of paleoclimate conditions. Measured δD values in perlitic lavas are −150 to −191 or 20–40 ‰ lower than glass hydrated by modern Yellowstone waters. This suggests that Yellowstone perlites record the low-δD signature of glacial ice. Cooling calculations, combined with the observed high water diffusion coefficients noted for 60–150 °C, suggest that if sufficient hot water or steam is available, any rhyolite flow greater than ~5 m thick can develop the observed ~250-µm hydration rinds within the expected timescale of cooling (weeks–years). As the process of hydration involves shattering of 30- to 100-µm-thick slivers to expose unhydrated rhyolite glass, the time required for hydration may be even shorter. Rapid hydration and formation of relatively thick-walled glass shards allow perlites to provide a snapshot view of the meteoric water (and thus climate) at the time of initial alteration. Perlites retain their initial hydration D/H signal better than thin-walled ash, which in contrast hydrates over many thousands of years with time-averaged precipitation.

Contributions to Mineralogy and Petrology↗
USGS2016

ChemCam activities and discoveries during the nominal mission of the Mars Science Laboratory in Gale crater, Mars

At Gale crater, Mars, ChemCam acquired its first laser-induced breakdown spectroscopy (LIBS) target on Sol 13 of the landed portion of the mission (a Sol is a Mars day). Up to Sol 800, more than 188 000 LIBS spectra were acquired on more than 5800 points distributed over about 650 individual targets. We present a comprehensive review of ChemCam scientific accomplishments during that period, together with a focus on the lessons learned from the first use of LIBS in space. For data processing, we describe new tools that had to be developed to account for the uniqueness of Mars data. With regard to chemistry, we present a summary of the composition range measured on Mars for major-element oxides (SiO 2 , TiO 2 , Al 2 O 3 , FeO T , MgO, CaO, Na 2 O, K 2 O) based on various multivariate models, with associated precisions. ChemCam also observed H, and the non-metallic elements C, O, P, and S, which are usually difficult to quantify with LIBS. F and Cl are observed through their molecular lines. We discuss the most relevant LIBS lines for detection of minor and trace elements (Li, Rb, Sr, Ba, Cr, Mn, Ni, and Zn). These results were obtained thanks to comprehensive ground reference datasets, which are set to mimic the expected mineralogy and chemistry on Mars. With regard to the first use of LIBS in space, we analyze and quantify, often for the first time, each of the advantages of using stand-off LIBS in space: no sample preparation, analysis within its petrological context, dust removal, sub-millimeter scale investigation, multi-point analysis, the ability to carry out statistical surveys and whole-rock analyses, and rapid data acquisition. We conclude with a discussion of ChemCam performance to survey the geochemistry of Mars, and its valuable support of decisions about selecting where and whether to make observations with more time and resource-intensive tools in the rover's instrument suite. In the end, we present a bird's-eye view of the many scientific results: discovery of felsic Noachian crust, first observation of hydrated soil, discovery of manganese-rich coatings and fracture fills indicating strong oxidation potential in Mars' early atmosphere, characterization of soils by grain size, and wide scale mapping of sedimentary strata, conglomerates, and diagenetic materials.

Journal of Analytical Atomic Spectrometry↗
USGS2013

Discerning crystal growth from diffusion profiles in zoned olivine by in situ Mg–Fe isotopic analyses

Mineral zoning is used in diffusion-based geospeedometry to determine magmatic timescales. Progress in this field has been hampered by the challenge to discern mineral zoning produced by diffusion from concentration gradients inherited from crystal growth. A zoned olivine phenocryst from Kilauea Iki lava lake (Hawaii) was selected for this study to evaluate the potential of Mg and Fe isotopes for distinguishing these two processes. Microdrilling of the phenocryst (∼300 μm drill holes) followed by MC-ICPMS analysis of the powders revealed negatively coupled Mg and Fe isotopic fractionations (δ 26 Mg from +0.1‰ to −0.2‰ and δ 56 Fe from −1.2‰ to −0.2‰ from core to rim), which can only be explained by Mg–Fe exchange between melt and olivine. The data can be explained with ratios of diffusivities of Mg and Fe isotopes in olivine scaling as D 2 /D 1 = (m 1 /m 2 )β with β Mg ∼0.16 and β Fe ∼0.27. LA-MC-ICPMS and MC-SIMS Fe isotopic measurements are developed and are demonstrated to yield accurate δ 56 Fe measurements within precisions of ∼0.2‰ (1 SD) at spatial resolutions of ∼50 μm. δ 56 Fe and δ 26 Mg stay constant with Fo# in the rim (late-stage overgrowth), whereas in the core (original phenocryst) δ 56 Fe steeply trends toward lighter compositions and δ 26 Mg trends toward heavier compositions with higher Fo#. A plot of δ 56 Fe vs. Fo# immediately distinguishes growth-controlled from diffusion-controlled zoning in these two regions. The results are consistent with the idea that large isotopic fractionation accompanies chemical diffusion in crystals, whereas fractional crystallization induces little or no isotopic fractionation. The cooling timescale inferred from the chemical-isotope zoning profiles is consistent with the documented cooling history of the lava lake. In the absence of geologic context, in situ stable isotopic measurements may now be used to interpret the nature of mineral zoning. Stable isotope measurements by LA-MC-ICPMS and MC-SIMS can be used as standard petrologic tools to identify samples for diffusion-based geospeedometry.

Geochimica et Cosmochimica Acta↗
USGS1985

Paleomagnetic and petrologic evidence bearing on the age and origin of uranium deposits in the Permian Cutler Formation, Lisbon Valley, Utah

An approximate age for uranium deposits in red beds of the Permian Cutler Formation, Lisbon Valley salt anticline, Utah, was obtained using paleomagnetic techniques. Progressive thermal demagnetization of samples of mineralized sandstone isolates stable magnetization components having high (≳400 °C) unblocking temperatures that define a tilt-corrected mean direction of D = 358.1°, I = 65.5°, α 95 = 3.3°. This direction is close to expected Late Cretaceous to middle Tertiary mean directions at Lisbon Valley. In contrast, thermal cleaning of samples of unmineralized sandstone isolates tilt-corrected southeasterly, shallow mean directions (D = 140.5°, I = −9.5°, α 95 = 8.6°, and D = 155.9°, I = −4.4°, α 95 = 10.9°, for two localities in the Lisbon Valley area). These results are closely similar to results from the Cutler Formation elsewhere on the Colorado Plateau and from other Lower Permian strata in North America. In unmineralized sandstone, the stable remanent magnetization is carried predominantly by martite of postdepositional or detrital origin, whereas in mineralized sandstone, the stable magnetization is carried largely by authigenic specular hematite as uranium- and vanadium-bearing clusters in interstitial areas, and it thus reflects the time of mineralization. Partial dissolution of martite grains and grain-coating ferric oxide pigment prior to formation of abundant interstitial specular hematite has minimized or eliminated the influence of late Paleozoic magnetic components in mineralized sandstone. These results, combined with information on the structural development of the Lisbon Valley area and on nearby uranium deposits in the Triassic Chinle Formation, suggest that the growth of the Lisbon Valley anticline during Late Cretaceous to early or middle Tertiary time promoted oxidative destruction of Chinle orebodies and led to a redistribution of uranium and iron from the Chinle into the Cutler. Acidic solutions generated during destruction of sulfide minerals in Chinle orebodies may account for the simultaneous transport of significant quantities of iron and uranium and for the partial dissolution of martite in the Cutler host beds. Neutralization of these solutions by reaction with host-rock constituents (especially calcite) caused precipitation of amorphous ferric oxide, which incorporated uranium by adsorption and which aged to form specular hematite.

Utah↗
USGS2011

Petrologic, tectonic, and metallogenic evolution of the Ancestral Cascades magmatic arc, Washington, Oregon, and northern California

Present-day High Cascades arc magmatism was preceded by ∼40 m.y. of nearly cospatial magmatism represented by the ancestral Cascades arc in Washington, Oregon, and northernmost California (United States). Time-space-composition relations for the ancestral Cascades arc have been synthesized from a recent compilation of more than 4000 geochemical analyses and associated age data. Neither the composition nor distribution of ancestral Cascades magmatism was uniform along the length of the ancestral arc through time. Initial (>40 to 36 Ma) ancestral Cascades magmatism (mostly basalt and basaltic andesite) was focused at the north end of the arc between the present-day locations of Mount Rainier and the Columbia River. From 35 to 18 Ma, initial basaltic andesite and andesite magmatism evolved to include dacite and rhyolite; magmatic activity became more voluminous and extended along most of the arc. Between 17 and 8 Ma, magmatism was focused along the part of the arc coincident with the northern two-thirds of Oregon and returned to more mafic compositions. Subsequent ancestral Cascades magmatism was dominated by basaltic andesite to basalt prior to the post–4 Ma onset of High Cascades magmatism. Transitional tholeiitic to calc-alkaline compositions dominated early (before 40 to ca. 25 Ma) ancestral Cascades eruptive products, whereas the majority of the younger arc rocks have a calc-alkaline affinity. Tholeiitic compositions characteristic of the oldest ancestral arc magmas suggest development associated with thin, immature crust and slab window processes, whereas the younger, calc-alkaline magmas suggest interaction with thicker, more evolved crust and more conventional subduction-related magmatic processes. Presumed changes in subducted slab dip through time also correlate with fundamental magma composition variation. The predominance of mafic compositions during latest ancestral arc magmatism and throughout the history of modern High Cascades magmatism probably reflects extensional tectonics that dominated during these periods of arc magmatism. Mineral deposits associated with ancestral Cascades arc rocks are uncommon; most are small and low grade relative to those found in other continental magmatic arcs. The small size, low grade, and dearth of deposits, especially in the southern two-thirds of the ancestral arc, probably reflect many factors, the most important of which may be the prevalence of extensional tectonics within this arc domain during this magmatic episode. Progressive clockwise rotation of the forearc block west of the evolving Oregon part of the ancestral Cascades magmatism produced an extensional regime that did not foster significant mineral deposit formation. In contrast, the Washington arc domain developed in a transpressional to mildly compressive regime that was more conducive to magmatic processes and hydrothermal fluid channeling critical to deposit formation. Small, low-grade porphyry copper deposits in the northern third of the ancestral Cascades arc segment also may be a consequence of more mature continental crust, including a Mesozoic component, beneath Washington north of Mount St. Helens.

California, Oregon, Washington↗
USGS2003

Volcano seismology

A fundamental goal of volcano seismology is to understand active magmatic systems, to characterize the configuration of such systems, and to determine the extent and evolution of source regions of magmatic energy. Such understanding is critical to our assessment of eruptive behavior and its hazardous impacts. With the emergence of portable broadband seismic instrumentation, availability of digital networks with wide dynamic range, and development of new powerful analysis techniques, rapid progress is being made toward a synthesis of high-quality seismic data to develop a coherent model of eruption mechanics. Examples of recent advances are: (1) high-resolution tomography to image subsurface volcanic structures at scales of a few hundred meters; (2) use of small-aperture seismic antennas to map the spatio-temporal properties of long-period (LP) seismicity; (3) moment tensor inversions of very-long-period (VLP) data to derive the source geometry and mass-transport budget of magmatic fluids; (4) spectral analyses of LP events to determine the acoustic properties of magmatic and associated hydrothermal fluids; and (5) experimental modeling of the source dynamics of volcanic tremor. These promising advances provide new insights into the mechanical properties of volcanic fluids and subvolcanic mass-transport dynamics. As new seismic methods refine our understanding of seismic sources, and geochemical methods better constrain mass balance and magma behavior, we face new challenges in elucidating the physico-chemical processes that cause volcanic unrest and its seismic and gas-discharge manifestations. Much work remains to be done toward a synthesis of seismological, geochemical, and petrological observations into an integrated model of volcanic behavior. Future important goals must include: (1) interpreting the key types of magma movement, degassing and boiling events that produce characteristic seismic phenomena; (2) characterizing multiphase fluids in subvolcanic regimes and determining their physical and chemical properties; and (3) quantitatively understanding multiphase fluid flow behavior under dynamic volcanic conditions. To realize these goals, not only must we learn how to translate seismic observations into quantitative information about fluid dynamics, but we also must determine the underlying physics that governs vesiculation, fragmentation, and the collapse of bubble-rich suspensions to form separate melt and vapor. Refined understanding of such processes—essential for quantitative short-term eruption forecasts—will require multidisciplinary research involving detailed field measurements, laboratory experiments, and numerical modeling.

Pure and Applied Geophysics↗
USGS1986

The evolution of young silicic lavas at Medicine Lake Volcano, California: Implications for the origin of compositional gaps in calc-alkaline series lavas

At Medicine Lake Volcano, California, the compositional gap between andesite (57-62 wt.% SiO2) and rhyolite (73-74 wt.% SiO2) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO2 andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO2 andesite. Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition gap). At Mt. Mazama and Mt. St. Helens, USA and Aso Caldera and Shikotsu, Japan the amphibole-bearing assemblage was important. At Krakatau, Indonesia and Katmai, USA, an augite+orthopyroxene-bearing assemblage was important. In addition to its role in the production of a compositional gap between intermediate and rhyolitic lavas, the crystallization process increases the H2O content of the residual liquid. This rapid increase in residual liquid volatile content which results from the precipitation of a large proportion of crystalline solids may be an important factor among several that lead to explosive silicic eruptions. ?? 1986 Springer-Verlag.

Contributions to Mineralogy and Petrology↗
USGS2012

Lead and strontium isotopes and related trace elements as genetic tracers in the Upper Cenozoic rhyolite-basalt association of the Yellowstone Plateau volcanic field

Supported by various field geologic and petrologic data, the contents of Pb, U, Th, Rb, and Sr and the isotopic compositions of Pb and Sr for upper Cenozoic volcanic rocks of the Yellowstone Plateau volcanic field are consistent with the hypothesis of derivation of the basaltic and rhyolitic magmas by partial melting of distinct source regions in the upper mantle and lower crust, respectively. All the basalt samples analyzed but one have systematically lower values of 207 Pb/ 204 Pb and 87 Sr/ 86 Sr than the rhyolites. The values of 206 Pb/ 204 Pb are smaller, and 87 Sr/ 86 Sr are mostly larger than known values in oceanic basalts. In all but one case, the values of 207 Pb/ 204 Pb are higher than expected from an extrapolation of known values in oceanic basalts to less radiogenic values of 206 Pb/ 204 Pb. Because there are no xenoliths, phenocrysts are only moderate to sparse in abundance, REE patterns are low and flat at the radiogenic end of lead isotopic compositions, several values of Rb/Sr are low, and 80% of the basalt samples form a well-developed secondary isochron separate from the rhyolites, we favor an interpretation for basalt genesis wherein isotopic signatures of most mafic magmas were attained in a continental ‘keel’ of mantlelike character about 2.6 b.y. old or somewhat older attached to the crust, and these signatures were unaltered by magma passage through the crust. At the very least, the current data continue to cast serious doubt as to the inevitability of crustal contamination for basaltic magma intruding the continental environment and postulate that much can be learned about the mantle under continents through the study of continental basalts. One basalt unit with an unusually low value of 207 Pb/ 204 Pb and an 87 Ar/ 86 Ar less than 0.704 may represent subcontinental ‘keel’-derived magma that rose unaltered to the surface. Our data also are not consistent with formation of this rhyolite-basalt association primarily by such processes as crystal fractionation, separation of immiscible silicate liquids from a common parental magma, or fractional melting of a homogeneous source. Rather as a conceptual model, we envision large mafic intrusions to have been injected into the lower crust resulting in rhyolite generation through partial anatexis of the adjacent wall rocks which probably had a 206 Pb/ 204 Pb < 17 and 87 Sr/ 86 Sr > 0.709; a model that has much in common with that proposed by Holmes (1931). All the other hypotheses listed have the necessary added complication that either the basalt or the rhyolite or both become contaminated after the two magma types separated, have problems accounting for the lack of igneous rocks of intermediate compositions or production of such large volumes of rhyolitic material (∼5000 km 3 ), and fail to explain why rhyolitic magma is not a more common occurrence in the ocean basin. We appeal to bouyancy of rhyolites to generate a barrier for basalt magma migration and account for the great preponderance of rhyolite relative to basalt at the surface. Furthermore, the complex isotopic picture in the rhyolites indicates that many of these magmas interacted with the upper crustal geologic units that they traversed. The interactions involved diverse processes, probably including reacton with hydrothermal fluids or hydrothermally altered rocks at high levels as well as by contamination with Phanerozoic sedimentary and Precambrian crystalline rocks at deeper levels. At the very least, we feel our study adds a cautionary note to the currently increasingly popular hypothesis that differentiation of basalt or gabbro magmas to rhyolite or granite (as distinct from tonalite or dacite) is a common occurrence and is therefore an important continential building process. Models for formation of rhyolite and granite predominantly by reworking of crust (anatexis) must still be considered. The primitive Archean mantle of the region was characterized by higher Rb/Sr, U/Pb, and Th/U values than are typical of modern suboceanic mantle. The mantle residuum within the continental subcrustal lithosperic ‘keel’ that resulted from the Archean crustal differentiation event probably was depleted in Rb/Sr and U/Pb, and the crust was correspondingly enriched in these ratios. The crust probably was further differentiated by an Archean high-grade metamorphism, during or after the primary event, into a granulitic lower crust depleted in U/Pb and Rb/Sr and a lower-grade upper crust enriched in these ratios.

Journal of Geophysical Research Solid Earth↗