Geology topics

O.B. James

Publications and source records attributed to O.B. James.

15 recordsLinked to original sources

USGS2012

Subdivision of the Mg-suite noritic rocks into Mg-gabbronorites and Mg-norites

Mg-suite noritic rocks can be divided into two groups, the Mg-gabbronorites and the Mg-norites. The rocks of these groups differ in ratios of high-Ca pyroxene to total pyroxene, compositions of pyroxene and plagioclase, assemblages of Ti-, Nb-, and Zr-bearing minerals, compositions of chrome spinel, bulk-rock Ti/Sm and Sc/Sm, and measured ages. The two groups probably crystallized from different types of parent magmas. Two hypotheses are offered for the differences in composition of the parent magmas. One hypothesis ascribes the differences to compositional heterogeneity of the mantle source areas. The other hypothesis ascribes the differences to variations in extent of partial melting of the mantle source regions and variations in extent of assimilation of the anorthosite and the highly differentiated residual liquid that were produced during the primordial lunar differentiation.

Journal of Geophysical Research Solid Earth↗

USGS2012

Petrology and geochemistry of alkali gabbronorites from Lunar Breccia 67975

Clasts of an unusual type of lunar highlands igneous rock, alkali gabbronorite, have been found in Apollo 16 breccia 67975. The alkali gabbronorites form two distinct subgroups, magnesian and ferroan. Modes and bulk compositions are highly varied. The magnesian alkali gabbronorites are composed of bytownitic plagioclase (OrAn), hypersthene (WoEn), augite (WoEn), a silica mineral, and trace Ba‐rich K‐feldspar. The ferroan alkali gabbronorites are composed of ternary plagioclase (OrAn), pigeonite (WoEn), augite (WoEn), Ba‐rich K‐feldspar, and a silica mineral. Trace minerals in both subgroups are apatite, REE‐rich whitlockite, and zircon. The magnesian and ferroan alkali gabbronorites appear to have formed by progressive differentiation of the same, or closely related, parent magmas; the compositional data indicate that these magmas were REE‐rich. The ternary plagioclase is probably a high‐temperature metastable phase formed during crystallization. In composition and mineralogy, the 67975 alkali gabbronorites show many similarities to Apollo 12 and 14 alkali norites, alkali gabbronorites, and alkali anorthosites, and all these rocks together constitute a distinctive alkali suite. In addition, the alkali gabbronorites show some similarities to KREEP basalts, Mg‐norites, and some felsites. These data suggest genetic links between some or all of these types of pristine rocks. Two types of relationships are possible. The first is that alkali‐suite rocks crystallized in plutons of KREEP basalt magma, and KREEP basalts are their extrusive equivalents. The second is that the alkali‐suite rocks and some felsites all crystallized in plutons of Mg‐norite parent magmas, and KREEP basalt magmas formed by remelting of these plutons. Additional studies are needed to resolve which of these hypotheses is correct.

Journal of Geophysical Research B: Solid Earth↗

USGS2010

Lunar highland meteorite Dhofar 026 and Apollo sample 15418: Two strongly shocked, partially melted, granulitic breccias

Studies of lunar meteorite Dhofar 026, and comparison to Apollo sample 15418, indicate that Dhofar 026 is a strongly shocked granulitic breccia (or a fragmental breccia consisting almost entirely of granulitic breccia clasts) that experienced considerable post-shock heating, probably as a result of diffusion of heat into the rock from an external, hotter source. The shock converted plagioclase to maskelynite, indicating that the shock pressure was between 30 and 45 GPa. The post-shock heating raised the rock's temperature to about 1200 °C; as a result, the maskelynite devitrified, and extensive partial melting took place. The melting was concentrated in pyroxene-rich areas; all pyroxene melted. As the rock cooled, the partial melts crystallized with fine-grained, subophitic-poikilitic textures. Sample 15418 is a strongly shocked granulitic breccia that had a similar history, but evidence for this history is better preserved than in Dhofar 026. The fact that Dhofar 026 was previously interpreted as an impact melt breccia underscores the importance of detailed petrographic study in interpretation of lunar rocks that have complex textures. The name “impact melt” has, in past studies, been applied only to rocks in which the melt fraction formed by shock-induced total fusion. Recently, however, this name has also been applied to rocks containing melt formed by heating of the rocks by conductive heat transfer, assuming that impact is the ultimate source of the heat. We urge that the name “impact melt” be restricted to rocks in which the bulk of the melt formed by shock-induced fusion to avoid confusion engendered by applying the same name to rocks melted by different processes.

Meteoritics and Planetary Science↗

USGS2010

Comment on: “New” lunar meteorites: Impact melt and regolith breccias and large‐scale heterogeneities of the upper lunar crust, by P. H. Warren, F. Ulff‐Møller, and G. W. Kallemeyn

We described lunar meteorite Dhofar 026 (Cohen et al. 2004) and interpreted this rock as a strongly shocked granulitic breccia (or fragmental breccia consisting almost entirely of granulitic‐breccia clasts) that was partially melted by post‐shock heating. Warren et al. (2005) objected to many aspects of our interpretation: they were uncertain whether or not the bulk rock had been shocked; they disputed our identification of the precursor as granulitic breccia; and they suggested that mafic, igneous‐textured globules within the breccia, which we proposed were melted by post‐shock heating, are clasts with relict textures. The major evidence for shock of the bulk rock is the fact that the plagioclase in the lithologic domains that make up 80–90% of the rock is devitrified maskelynite. The major evidence for a granulitic‐breccia precursor is the texture of the olivine‐plagioclase domain that constitutes 40—45% of the rock; Warren et al. apparently overlooked or ignored this lithology. Textures of the mafic, igneous‐textured globules, and especially of the vesicles they contain, demonstrate that these bodies were melted and crystallized in situ. Warren et al. suggested that the rock might have originally been a regolith breccia, but the textural homogeneity of the rock and the absence of solar wind—derived noble gases preclude a regolith‐breccia precursor. Warren et al. classified the rock as an impact‐melt breccia, but they did not identify any fraction that was impact melt.

Meteoritics and Planetary Science↗

USGS2010

Osmium isotope and highly siderophile element systematics of the lunar crust

Coupled 187 Os/ 188 Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ?? 0.3 pg g - 1 Os, 1.5 ?? 0.6 pg g - 1 Ir, 6.8 ?? 2.7 pg g - 1 Ru, 16 ?? 15 pg g - 1 Pt, 33 ?? 30 pg g - 1 Pd and 0.29 ?? 0.10 pg g - 1 Re (??? 0.00002 ?? CI) and Re/Os ratios that were modestly elevated ( 187 Re/ 188 Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (??? 0.00007 ?? CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle-crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust-mantle concentration ratios (D-values) must be ??? 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust-mantle HSE partitioning for the Earth and Moon are somewhat surprising. Low HSE abundances in the lunar crust, coupled with estimates of HSE concentrations in the lunar mantle implies there may be a 'missing component' of late-accreted materials (as much as 95%) to the Moon if the Earth/Moon mass-flux estimates are correct and terrestrial mantle HSE abundances were established by late accretion. ?? 2009 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2008

Osmium isotope and highly siderophile element systematics of lunar impact melt breccias: Implications for the late accretion history of the Moon and Earth

To characterize the compositions of materials accreted to the Earth-Moon system between about 4.5 and 3.8 Ga, we have determined Os isotopic compositions and some highly siderophile element (HSE: Re, Os, Ir, Ru, Pt, and Pd) abundances in 48 subsamples of six lunar breccias. These are: Apollo 17 poikilitic melt breccias 72395 and 76215; Apollo 17 aphanitic melt breccias 73215 and 73255; Apollo 14 polymict breccia 14321; and lunar meteorite NWA482, a crystallized impact melt. Plots of Ir versus other HSE define excellent linear correlations, indicating that all data sets likely represent dominantly two-component mixtures of a low-HSE target, presumably endogenous component, and a high-HSE, presumably exogenous component. Linear regressions of these trends yield intercepts that are statistically indistinguishable from zero for all HSE, except for Ru and Pd in two samples. The slopes of the linear regressions are insensitive to target rock contributions of Ru and Pd of the magnitude observed; thus, the trendline slopes approximate the elemental ratios present in the impactor components contributed to these rocks. The 187Os/188Os and regression-derived elemental ratios for the Apollo 17 aphanitic melt breccias and the lunar meteorite indicate that the impactor components in these samples have close affinities to chondritic meteorites. The HSE in the Apollo 17 aphanitic melt breccias, however, might partially or entirely reflect the HSE characteristics of HSE-rich granulitic breccia clasts that were incorporated in the impact melt at the time of its creation. In this case, the HSE characteristics of these rocks may reflect those of an impactor that predated the impact event that led to the creation of the melt breccias. The impactor components in the Apollo 17 poikilitic melt breccias and in the Apollo 14 breccia have higher 187Os/188Os, Pt/Ir, and Ru/Ir and lower Os/Ir than most chondrites. These compositions suggest that the impactors they represent were chemically distinct from known chondrite types, and possibly represent a type of primitive material not currently delivered to Earth as meteorites. ?? 2008 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS2005

187Os/188Os And Highly Siderophile Element Systematics Of Apollo 17 Aphanitic Melt Rocks

Generally chondritic relative abundances and high absolute abundances of the highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt, Au) in Earth's upper mantle provide strong evidence that these elements were added to the Earth following the last major interaction between its metallic core and silicate fraction. So called "late accretion" may have added materials comprising as much as 0.8% of the total mass of the Earth and possibly a similar proportion of mass to the Moon. We have begun to study the chemical nature of late accreted materials to the Earth-Moon system by examining the HSE contained in lunar impact-melt rocks. The HSE contained in melt rocks were largely added to the Moon during the period of time from the origin of the lunar highlands crust (4.4-4.5 Ga) to the end of the late bombardment period (ca. 3.9 Ga). These materials provide the only direct chemical link to the late accretionary period. The chemical fingerprints of the HSE in late accreted materials may enable us to ascertain under what conditions and where in the solar system the late accreted materials formed. The 1870s/1880s ratios (reflecting long-term Re/Os), coupled with ratios of other HSE, can be diagnostic for identifying the nature of the impactor. A critical issue, however, will be deconvolving the exogenous from indigenous components. Herein we examine the Os isotopic and HSE systematics of Apollo 17 aphanitic melt rocks 73215 and 73255. The HSE in these rocks were likely added at ~3.9 Ga from the impactor that formed the Serenitatis basin.

Conference Paper↗

USGS2002

Rare earth element variations resulting from inversion of pigeonite and subsolidus reequilibration in lunar ferroan anorthosites

We present results of a secondary ion mass spectrometry study of the rare earth elements (REEs) in the minerals of two samples of lunar ferroan anorthosite, and the results are applicable to studies of REEs in all igneous rocks, no matter what their planet of origin. Our pyroxene analyses are used to determine solid-solid REE distribution coefficients (D = C REE in low-Ca pyroxene/C REE in augite) in orthopyroxene-augite pairs derived by inversion of pigeonite. Our data and predictions from crystal-chemical considerations indicate that as primary pigeonite inverts to orthopyroxene plus augite and subsolidus reequilibration proceeds, the solid-solid Ds for orthopyroxene-augite pairs progressively decrease for all REEs; the decrease is greatest for the LREEs. The REE pattern of solid-solid Ds for inversion-derived pyroxene pairs is close to a straight line for Sm-Lu and turns upward for REEs lighter than Sm; the shape of this pattern is predicted by the shapes of the REE patterns for the individual minerals. Equilibrium liquids calculated for one sample from the compositions of primary phases, using measured or experimentally determined solid-liquid Ds, have chondrite-normalized REE patterns that are very slightly enriched in LREEs. The plagioclase equilibrium liquid is overall less rich in REEs than pyroxene equilibrium liquids, and the discrepancy probably arises because the calculated plagioclase equilibrium liquid represents a liquid earlier in the fractionationsequence than the pyroxene equilibrium liquids. “Equilibrium” liquids calculated from the compositions of inversion-derived pyroxenes or orthopyroxene derived by reaction of olivine are LREE depleted (in some cases substantially) in comparison with equilibrium liquids calculated from the compositions of primary phases. These discrepancies arise because the inversion-derived and reaction-derived pyroxenes did not crystallize directly from liquid, and the use of solid-liquid Ds is inappropriate. The LREE depletion of the calculated liquids is a relic of formation of these phases from primary LREE-depleted minerals. Thus, if one attempts to calculate the compositions of equilibrium liquids from pyroxene compositions, it is important to establish that the pyroxenes are primary. In addition, our data suggest that experimental studies have underestimated solid-liquid Ds for REEs in pigeonite and that REE contents of liquids calculated using these Ds are overestimates. Our results have implications for Sm-Nd age studies. Our work shows that if pigeonite inversion and/or subsolidus reequilibration between augite and orthopyroxene occurred significantly after crystallization, and if pyroxene separates isolated for Sm-Nd studies do not have the bulk composition of the primary pyroxenes, then the Sm-Nd isochron age and ε Nd will be in error.

Geochimica et Cosmochimica Acta↗

USGS1998

Lunar ferroan anorthosite petrogenesis: clues from trace element distributions in FAN subgroups

The rare earth elements (REE) and selected other trace elements were measured in plagioclase and pyroxene from nine samples of the lunar ferroan anorthosite (FAN) suite of rocks. Samples were selected from each of four FAN subgroups previously defined by James et al. (1989). Plagioclase compositions are homogeneous within each sample, but high- and low-Ca pyroxenes from lithic clasts typically have different REE abundances from their counterparts in the surrounding granulated matrices. Measured plagioclase/low-Ca pyroxene concentration ratios for the REE have steeper patterns than experimentally determined plagioclase/low-Ca pyroxene partition coefficients in most samples. Textural and trace element evidence suggest that, although subsolidus equilibration may be responsible for some of the discrepancy, plagioclase compositions in most samples have been largely unaffected by intermineral redistribution of the REE. The REE systematics of plagioclase from the four subgroups are broadly consistent with their deviation through crystallization from a single evolving magma. However, samples from some of the subgroups exhibit a decoupling of plagioclase and pyroxene compositions that probably reflects the complexities inherent in crystallization from a large-scale magmatic system. For example, two anorthosites with very magnesian mafic minerals have highly evolved trace element compositions; major element compositions in plagioclase also do not reflect the evolutionary sequence recorded by their REE compositions. Finally, a noritic anorthosite breccia with relatively ferroan mafic minerals contains several clasts with high and variable REE and other trace element abundances. Although plagioclase REE compositions are consistent with their derivation from a magma with a KREEPy trace element signature, very shallow REE patterns in the pyroxenes suggest the addition of a component enriched in the light REE.

Geochimica et Cosmochimica Acta↗

USGS1989

Ferroan anorthosite from lunar breccia 64435: Implications for the origin and history of lunar ferroan anorthosites

A composite clast of ferroan-anorthosite-suite rocks from lunar breccia 64435 is shown to contain three lithologies. The results suggest that the structures of the 64435 composite clast were produced in an early deformation that consisted of extensive granulation followed by pervasive recrystallization, and that some granulitic breccias are nearly monomict. The REE concentrations in the coarse-grained troctolitic anorthosite are explained in terms of mass balance among the equilibrated phases.

Conference Paper↗

USGS1972

Lunar anorthosite 15415: Texture, mineralogy, and metamorphic history

Lunar anorthosite 15415 consists almost entirely of anorthite (homogeneous anorthite 96.6 molecule percent), with accessory diopsidic augite and traces of hypersthene, ilmenite, and a silica mineral. The rock has had a complex metamorphic history. The texture reflects at least two episodes of shearing (followed by intense and partial recrystallization, respectively), one episode of cataclastic deformation, and one or more episodes of shattering and fragmentation.

Science↗

USGS1971

Origin and emplacement of the ultramafic rocks of the Emigrant Gap area, California

The ultramafic bodies of the Emigrant Gap area are part of a mafic complex within a large composite pluton of the northern Sierra Nevada. The pluton was magmatically emplaced and is surrounded by an aureole of hornblende-hornfels facies rocks. Inclusions of country rock in ultramafic rock are of pyroxene-hornfels facies and appear to have been partly melted. Gravity studies indicate that the ultramafic bodies have near-vertical contacts extending to depths of at least 1½ to 2½ km. The mafic complex shows rough concentric zoning of rock types: ultramafic bodies occur at the core; gabbro forms a discontinuous intermediate unit; and diorite, tonalite, and granodiorite occur at the margins. Within the ultramafic bodies, unserpentinized wehrlitic peridotite is dominant; dunite and olivine clinopyroxenite are present but greatly subordinate. The ultramafic rocks consist almost entirely of olivine (FO 80 ) and diopside (Ca 46 Mg 46 Fe 8 ); orthopyroxene, hornbolende, and plagioclase occur locally. The gabbro, diorite, tonalite, and granodiorite contain both ortho- and clinopyroxene. Both ultramafic and two-pyroxene-bearing rocks were emplaced nearly simultaneously, as partly crystallized magmas and magmatic crystal mushes that had similar temperature. In all the rocks the structures are dominantly magmatic and were produced by sorting and orientation of crystals by magmatic flow. Structures produced by post-consolidation deformation and replacement are minor and local. The structural and chemical relations within the mafic complex suggest that all the rocks are derived from a single gabbroic magma by crystal fractionation, with the ultramafic rocks formed by mechanical accumulation of early crystallized mafic minerals, and the two pyroxene-bearing granodiorite crystallized from a felsic differentiate. It is likely that flowage differentiation was the dominant process of crystal segregation. The Emigrant Gap mafic complex is similar in structure, rock texture, and mineralogy to zoned ultramafic complexes, such as those of south-eastern Alaska, and is very different from either stratiform or alpine-type bodies. Though unlike the Alaskan bodies in detail, it appears that this complex should be classed with the zoned complexes in any broad grouping of ultramafic occurrences.

California↗

USGS1970

Petrology of unshocked crystalline rocks and shock effects in lunar rocks and minerals

On the basis of rock modes, textures, and mineralogy, unshocked crystalline rocks are classified into a dominant ilmenite-rich suite (subdivided into intersertal, ophitic, and hornfels types) and a subordinate feldspar-rich suite (subdivided into poikilitic and granular types). Weakly to moderately shocked rocks show high strain-rate deformation and solid-state transformation of minerals to glasses; intensely shocked rocks are converted to rock glasses. Data on an unknown calcium-bearing iron metasilicate are presented.

Science↗

USGS1969

Jadeite: Shock-induced formation from oligoclase, Ries crater, Germany

Jadeite (high-pressure sodium aluminum pyroxene) has been identified in a shock-phase assemblage of oligoclase. The shock assemblage consists of minute particles with high refractive indices that contain at least two phases: one (identified by x-ray) is a jadeite that is nearly pure NaAlSi 2 O 6 ; the other has the chemical composition of oligoclase minus jadeite and appears to be largely amorphous.

Ries crater↗

USGS1969

Shock and thermal metamorphism of basalt by nuclear explosion, Nevada test site

Olivine trachybasalt metamorphosed by nuclear explosion is classified into categories of progressive metamorphism: (i) Weak. Plagioclase is microfractured, and augite cotainis fine twin lamellae. (ii) Moderate. Plagioclase is converted to glass, and mafic minerals show intragranular deformation (undulatory extinction, twin lamellae, and, possibly, deformation lamellae), but rock texture is preserved. (iii) Moderately strong. Plagioclase glass shows small-scale flow, mafic minerals are fractured and show intragranular deformation, and rocks contain tension fractures. (iv) Strong. Plagioclase glass is vesicular, augite is minutely fractured, and olivine is coarsely fragmented, shows mosaic extinction, distinctive lamellar structures, and is locally recrystallized. (v) Intense. Rocks are converted to inhomogeneous basaltic glass.

Nevada↗