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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Garnet crystallization does not drive oxidation at arcs

Arc magmas, the building blocks of continental crust, are depleted in total iron (Fe), have higher ratios of oxidized Fe to total Fe (Fe 3+ /∑Fe), and record higher oxygen fugacities ( f O 2 ’s) compared with magmas erupted at mid-ocean ridges. Garnet crystallization could explain these observations if garnet removes substantial amounts of Fe 2+ , but not Fe 3+ , from magma, yet this model for continental crust generation has never been tested experimentally. Analysis of garnets and melts in laboratory experiments show that the compatibilities of Fe 2+ and Fe 3+ in garnet are of similar magnitudes. Our results indicate that fractional crystallization of garnet-bearing cumulates will remove 22 % of total Fe from primary arc basalts but negligibly alter the Fe 3+ /∑Fe ratio and f O 2 of the melt. Garnet crystallization is unlikely to be responsible for the relatively oxidized nature of basaltic arc magmas or the Fe-depletion trend observed in continental crust.

Science & Technology - Other Topics↗
DOE OSTI2022

Experimental calibration of an Fe3+/Fe2+-in-amphibole oxybarometer and its application to shallow magmatic processes at Shiveluch Volcano, Kamchatka

Abstract Oxygen fugacity is an important but difficult parameter to constrain for primitive arc magmas. In this study, the partitioning behavior of Fe3+/Fe2+ between amphibole and glass synthesized in piston-cylinder and cold-seal apparatus experiments is developed as an oxybarometer, applicable to magmas ranging from basaltic to dacitic composition. The partitioning of Fe2+ is strongly dependent on melt polymerization; the relative compatibility of Fe2+ in amphibole decreases with increasing polymerization. The Fe2+/Mg distribution coefficient between amphibole and melt is a relatively constant value across all compositions and is, on average, 0.27. The amphibole oxybarometer is applied to amphibole in mafic enclaves, cumulates, and basaltic tephra erupted from Shiveluch volcano in Kamchatka with measured Fe3+/FeTotal. An average Fe3+/Fe2+ amphibole-glass distribution coefficient for basalt is used to convert the Fe3+/FeTotal of amphibole in samples from Shiveluch to magmatic oxygen fugacity relative to NNO. The fO2 of primitive melts at the volcano is approximately NNO+2 and is faithfully recorded in amphibole from an amphibole-rich cumulate and the basaltic tephra. Apparently, higher fO2 recorded by amphibole in mafic enclaves likely results from partial dehydrogenation of amphibole during residence in a shallow andesite storage region. We identify three pulses of mafic magma recharge within two weeks of, a month before, and two to three months before the eruption and find that, at each of these times, the host andesite was recharged by at least two magmas at varying stages of differentiation. Application of the amphibole oxybarometer not only gives insight into magmatic fO2 but also potentially details of shallow magmatic processes.

Geochemistry & Geophysics↗
NASA NTRS1972

Morphology and petrography of volcanic ashes.

Study of volcanic ash samples collected from a variety of recent eruptions using petrography, chemical analyses, and scanning electron microscopy to characterize each type and to relate ash morphology to magma composition and the type of eruption. The ashes are placed in the broad genetic categories of magmatic and phreatomagmatic. The morphology of ash particles from magmatic eruptions of high viscosity magma is governed primarily by vesicle density and shape. Ash particles from eruptions of low viscosity magmas are mostly droplets. The morphology of ash particles from phreatomagmatic eruptions is controlled by stresses within the chilled magma which result in fragmentation of the glass to form small blocky or pyramidal glass ash particles.

Heiken, G.↗
NASA NTRS1974

An atlas of volcanic ash

Volcanic ash samples collected from a variety of recent eruptions were studied, using petrography, chemical analyses, and scanning electron microscopy to characterize each ash type and to relate ash morphology to magma composition and eruption type. The ashes are best placed into two broad genetic categories: magnetic and hydrovolcanic (phreatomagmatic). Ashes from magmatic eruptions are formed when expanding gases in the magma form a froth that loses its coherence as it approaches the ground surface. During hydrovolcanic eruptions, the magma is chilled on contact with ground or surface waters, resulting in violent steam eruptions. Within these two genetic categories, ashes from different magma types can be characterized. The pigeon hole classification used here is for convenience; there are eruptions which are driven by both phreatic and magmatic gases.

Heiken, G.↗
NASA NTRS1973

Partial pressures of oxygen, phosphorus and fluorine in some lunar lavas

Lunar sample 14310 is a feldspar-rich basalt which shows no evidence of shock deformation or recrystallization. Pyroxenes include Mg-rich orthopyroxene, pigeonite and augite; pyroxferroite occurs in the interstitial residuum. Plagioclase feldspars are zoned from An(96) to An(67), and variations in feldspar compositions do not necessarily indicate loss of Na during eruption of the lava. Opaque phases include ilmenite, ulvospinel, metallic iron, troilite, and schreibersite. Both whitlockite and apatite are present, and the interstitial residua contain baddeleyite, tranquillityite and barium-rich sanidine. Theoretical calculations provide estimates of partial pressures of oxygen, phosphorus, and fluorine in lunar magmas. In general, partial pressures of oxygen are restricted by the limiting assemblages of iron-wuestite and ilmenite-iron-rutile; phosphorus partial pressures are higher in lunar magmas than in terrestrial lavas. The occurrence of whitlockite indicates significantly lower fugacities of fluorine in lunar magmas than in terrestrial magmas.

Nash, W. P.↗
NASA NTRS1979

A petrogenetic model of the relationships among achondritic meteorites

Petrological evidence is used to support the hypothesis that although the magma source regions and parent bodies of basaltic achondrite, shergottite, nakhlite, and chassignite meteorites are clearly distinct, they may be simply related. It is proposed that the peridotites which on partial melting generated the parent magmas of the shergottite meteorites differed from those which gave rise to eucritic magmas by being enriched in a component rich in alkalis and other volatiles. Similarly, the source regions of the parent magmas of the nakhlite and chassignite meteorites differed from those on the shergottite parent body by being still richer in this volatile-rich component. These regions could have been related by processes such as mixture of variable amounts of volatile-rich and volatile-poor components in planetary or nebular settings, or alternatively by variable varying degrees of volatile loss from volatile-rich materials.

Stolper, E.↗
NASA NTRS1980

Chemistry of Apollo 11 low-K mare basalts

A reexamination of the bulk major and trace element geochemistry of Apollo 11 low-K mare basalts is presented. New analyses are given for seven previously unanalyzed samples (10003, 10020, 10044, 10047, 10050, 10058, and 10062) and for two low-K basalts (10029 and 10092) and one high-K basalt (10071) for which comprehensive compositional data were previously lacking. The data show that three distinct magma types have been sampled, as proposed by Beaty and Albee (1978), and that these magma types are unrelated by near-surface crystal fractionation. Each magma type is characterized by distinctive magmaphile element ratios, which enable previously unclassified samples (10050 and 10062) to be assigned to an appropriate magma type.

Rhodes, J. M.↗
NASA NTRS1980

A model of early lunar differentiation

The presence of anorthosites and the scarcity of complementary mafic rocks in the lunar crust suggest a body of melt with depth of the order of the depth of the crust itself (about 60 km). This depth is consistent with geophysical and geochemical models which call for a depth of melting of the order of 200-500 km. The major and minor element compositions of the ancient pristine rocks require complex igneous processes if the moon accreted homogeneously and has chondritic interelement ratios among the refractory elements. Anorthosites crystallized from magmas that had assimilated plagioclase and a light REE-enriched component. KREEP as well as most of the troctolites and norites formed from relatively primitive magmas that were mixed with a LIL-enriched, Sc-Ti depleted component. Assimilation and mixing processes inferred for pristine rocks are consistent with crystallization from a convecting magma ocean which initially developed a floating anorthosite crust with a plagioclase-saturated boundary layer beneath it, transitional in temperature and composition to the main body of the magma beneath it.

Longhi, J.↗
NASA NTRS1982

Mechanism for the extrusion of KREEP

The evolution of the final residue of the magma ocean from the time it entered the anorthositic crust until it was extruded is modeled. The magma density first increased as anorthite and orthopyroxene crystallized, but once ilmenite precipitation began the density dropped. The KREEPy magma collected beneath the thinnest parts of the anorthitic crust, and intruded the crust when the density dropped below that of the adjacent crustal materials. The nearly constant degree of incompatible element fractionation moon-wide was maintained because the rapid drop in density during ilmenite crystallization occurred after about the same degree of crystallization at all locations. The high viscosity of the final residue of the magma ocean and its low density contrast with the crust initially resulted in very slow migration through the crust.

Shirley, D. N.↗
NASA NTRS1982

The 18 May 1980 eruption of Mount St. Helens: The nature of the eruption, with an atmospheric perspective

Mount St. Helens erupted somewhat less than 0.5 cu km of magma (dense rock equivalent) on May 18, 1980. The May 18 event was usually violent. As much 35% of the volume of the airfall material fell outside of the 2.5 mm isopach, which encloses about 88,000 sq km. This extraordinary dispersive power was transmitted by an eruption column which reached heights of more than 20 km. There was a lateral blast (or surge) of unusually large dimensions associated with the onset of the eruption. The magma is dacitic in composition and had a low ( 500 ppm) sulfur content. Distal ashes contain much nonmagmatic (lithic) material, but smaller ( 50 microns m) particles are mostly finely divided magmatic dacite. The grain size distributions of the ash are multimodal, frequently with peaks at 90, 25, and 10 microns. The finer populations fell out faster than their terminal velocities as simple particles would suggest. It is inferred that large proportions of the fine ash fell out as composite particles. This condition greatly reduces the atmospheric burden of silicate particles. Some of the unusual aspects (violence, intense surges, multimodal grain size distributions, lithic content of the ashes) of the eruption may be due to its phreatomagmatic character. The hydrothermal system above the magma may have infiltrated the magma body at the onset of the eruption. An "overprint" of the geochemistry of this hydrothermal system on the geochemistry of the magmatic gas system is likely. One important feature is that reduced gas species may be much more abundant than in many eruptions. Another is that fine ash may form aggregates more readily.

Rose, W. I., Jr.↗
NASA NTRS1982

The mechanisms of fine particle generation and electrification during Mount St. Helens volcanic eruption

Microscopical investigation of volcanic ash collected from ground stations during Mount St. Helens eruptions reveal a distinctive bimodel size distribution with high concentrations of particle ranges at (1) 200-100 microns and (2) 20-0.1 microns. Close examination of individual particles shows that most larger ones are solidified magma particles of porous pumice with numerous gas bubbles in the interior and the smaller ones are all glassy fragments without any detectable gas bubbles. Elemental analysis demonstrates that the fine fragments all have a composition similar to that of the larger pumice particles. Laboratory experiments suggest that the formation of the fine fragments is by bursting of glassy bubbles from a partially solidified surface of a crystallizing molten magma particle. The production of gas bubbles is due to the release of absorbed gases in molten magma particles when solubility decreases during phase transition. Diffusion cloud chamber experiments strongly indicate that sub-micron volcanic fragments are highly hygroscopic and extremely active as cloud condensation nuclei. Ice crystals also are evidently formed on those fragments in a supercooled (-20 C) cloud chamber. It has been reported that charge generation from ocean volcanic eruptions is due to contact of molten lava with sea water. This seems to be insufficient to explain the observed rapid and intense lightning activities over Mount St. Helens eruptions. Therefore, a hypothesis is presented here that highly electrically charged fine solid fragments are ejected by bursting of gas bubbles from the surface of a crystallizing molten magma particles.

Cheng, R. J.↗
NASA NTRS1982

Explosive volcanism on Hecates Tholus, Mars - Investigation of eruption conditions

From a reexamination of the medium and high-resolution Viking images of Amazonian and Hesperian age volcanic centers on Mars, it is believed that an excellent example of well-preserved explosive activity does indeed exist close to the summit of Hecates Tholus. A mantled region to the west of the summit caldera is seen as an example of a geologically very recent plinian air fall ash deposit. Morphological evidence is presented for describing this as explosively generated material, and numerical models of magma ascent and eruption in the Martian environment are used to estimate the physical characteristics (eruption cloud height, magma discharge rate, magma volatile content, duration of activity, and vent size) for this event. Attention is also given to the implication of this eruptive style for the composition of the erupted magma.

Mouginis-Mark, P. J.↗
NASA NTRS1983

Seventh Foray - Whitlockite-rich lithologies, a diopside-bearing troctolitic anorthosite, ferroan anorthosites, and KREEP

Seventeen nonmare samples, most of them pristine, are characterized, and implications of the new data are discussed. Five pristine samples are from Apollo 14, near the center of the KREEP-rich zone in the moon's western hemisphere. Three of them are alkali anorthosites rich in Ca-phosphate (whitlockite), which apparently crystallized from magmas with REE contents roughly 6 x those of high-K KREEP. The alkali anorthosites probably formed from Mg-rich magmas that assimilated large amounts of urKREEP, but some might have formed by metasomatism of ferroan anorthosite by urKREEP. The gabbronorite/norite classification scheme is not well suited to western hemisphere lithologies, probably due mainly to the overriding effects of longitude-petrochemistry correlations. A diopside-bearing Mg-rich lithology indicates that a low degree of melting was not a prerequisite for producing gabbroic (high-Ca pyroxene-rich) Mg-rich magmas, and suggests that some source regions of Mg-rich magmas were relatively Ca-rich. Several pristine KREEP fragments from Apollo 15 station 2 are texturally and compositionally much like other pristine KREEP, and thus reinforce the evidence that KREEP is highly uniform. The pristine anorthosites, on the other hand, demonstrate further that lunar anorthosites are diverse.

Warren, P. H.↗
NASA NTRS1984

Pristine highland clasts in consortium breccia 14305 Petrology and geochemistry

Data are presented on the petrography and mineral chemistry of six pristine highland clasts chipped from the polymict lunar breccia 14305. Major and trace elements in the clasts were determined by instrumental neutron activation analysis, and mineral analyses were performed by electron microprobe. Mg-suite clasts have 'eastern' geochemical affinities, reaffirming the importance of local variations in geochemistry. These local variations are superimposed on the moon-wide, longitudinal variations noted by Warren and Wasson (1980). Alkali anorthosites and Mg-suite troctolites and anorthosites are not comagmatic, and cannot be related to a single parent magma by either fractional crystallization or variable assimilation of KREEP. Both magma suites may have assimilated varied amounts of KREEP into distinct parent magmas. Alternatively, alkali anorthosites may have crystallized directly from a KREEP-basalt parent magma. A thick crust of ferroan anorthosite probably never existed on the western lunar nearside, or was removed by basin-forming impacts prior to intrusion of later plutonic suites.

Shervais, J. W.↗
NASA NTRS1983

Density constraints on the formation of the continental Moho and crust

As part of a general review of recent high-resolution seismological data, constraints are established for the distribution of mass in the continental crust and upper mantle of the earth. It is shown that the densities of mantle magmas such as MORB-like tholeiites, oicrites, and komatites at 10 kilobars are greater than densities in the continental crust. In order to explain the deviation, a mechanism is proposed for the control of mass transfer during igneous and metamorphic processes throughout geological time. Within the context of a description of the mechanism, the continental crust is characterized as a density filter through which only highly evolved magmas with H2O may pass. It is shown that the removal of less evolved magmas can lead to an over accretion (penetration) or an under accretion (underplating) of magmas in the continental Moho. Some implications of the proposed mechanism for current theoretical models of crust formation are discussed in detail.

Herzberg, C. T.↗
NASA NTRS1987

Petrogenesis of basalts from the Archean Matachewan Dike Swarm Superior Province of Canada

The Matachewan Dike swarm of eastern Ontario comprises Archean age basalts that were emplaced in the greenstone, granite-greenstone, and metasedimentary terrains of the Superior Province of Canada. The basalts are Fe-rich tholeiites, characterized by the near ubiquitos presence of large, compositionally uniform, calcic plagioclase. Major and trace element whole-rock compositions, along with microprobe analyses of constituent phases, from a group of dikes from the eastern portion of the province, were evaluated to constrain petrological processes that operated during the formation and evolution of the magmas. Three compositional groupings, were identified within the dikes. One group has compositional characteristics similar to modern abyssal tholeiites and is termed morb-type. A second group, enriched in incompatible elements and light-REE enriched, is referred to as the enriched group. The third more populated group has intermediate characteristics and is termed the main group. The observation of both morb-type and enriched compositions within a single dike strongly argues for the contemporaneous existence of magmas derived through different processes. Mixing calculations suggest that two possibilities exist. The least evolved basalts lie on a mixing line between the morb-type and enriched group, suggesting mixing of magmas derived from heterogeneous mantle. Mixing of magmas derived from a depleted mantle with heterogeneous Archean crust can duplicate certain aspects of the Matachewan dike composition array.

Nelson, Dennis O.↗
NASA NTRS1988

The petrogenesis of oceanic kimberlites and included mantle megacrysts: The Malaitan alnoite

The study of unambiguous suboceanic mantle was facilitated by the occurrence of anomalous kimberlite-type intrusives on Malaita in the Solomon Islands. The pseudo-kimberlites were termed alnoites, and are basically mica lamprophyres with melilite in the ground mass. Alnoitic magmas were explosively intruded into the Ontong Java Plateau (OJP) 34 Ma ago. The OJP is a vastly overthickened portion of the Pacific plate which now abuts the Indo-Australian plate. Malaita is considered to be the obducted leading edge of the OJP. Initial diapiric upwelling beneath the OJP produced the proto-alnoite magma. After impingement on the rigid lithosphere, megacrysts fractionation occurred, with augites precipitating first, representing the parental magma. Sea water-altered oceanic crust, which underplated the OJP, was assimilated by the proto-alnoite magma during megacrysts fractionation (AFC).

Neal, Clive R.↗
NASA NTRS1988

Continental crustal formation and recycling: Evidence from oceanic basalts

Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.

Saunders, A. D.↗