Engineering topics

Snyder, Gregory A.

Publications and source records attributed to Snyder, Gregory A..

14 records

NASA NTRS1995

Processes involved in the formation of magnesian-suite plutonic rocks from the highlands of the Earth's Moon

The earliest evolution of the Moon likely included the formation of a magma ocean and the subsequent development of anorthositic flotation cumulates. This primary anorthositic crust was then intruded by mafic magmas which crystallized to form the lunar highlands magnesian suite. The present study is a compilation of petrologic, mineral-chemical, and geochemical information on all pristine magnesian-suite plutonic rocks and the interpretation of this data in light of 18 'new' samples. Of these 18 clasts taken from Apollo 14 breccias, 12 are probably pristine and include four dunites, two norites, four troctolites, and two anorthosites. Radiogenic isotopic whole rock data also are reported for one of the 'probably pristine' anorthositic troctolites, sample 14303,347. The relatively low Rb content and high Sm and Nd abundances of 14303,347 suggest that this cumulate rock was derived from a parental magma which had these chemical characteristics. Trace element, isotopic, and mineral-chemical data are used to interpret the total highlands magnesian suite as crustal precipitates of a primitive KREEP (possessing a K-, rare earth element (REE)-, and P-enriched chemical signature) basalt magma. This KREEP basalt was created by the mixing of ascending ultramafic melts from the lunar interior with urKREEP (the late, K-, REE-, and P-enriched residuum of the lunar magma ocean). A few samples of the magnesian suite with extremely elevated large-ion lithophile elements (5-10x other magnesian-suite rocks) cannot be explained by this model or any other model of autometasomatism, equilibrium crystallization, or 'local melt-pocket equilibrium' without recourse to an extremely large-ion lithophile element-enriched parent liquid. It is difficult to generate parental liquids which are 2-4 x higher in the REE than average lunar KREEP, unless the liquids are the basic complement of a liquid-liquid pair, i.e., the so-called 'REEP-fraction,' from the silicate liquid immiscibility of urKREEP. Scarce age information on lunar rocks suggests that magnesian-suite magmatism was initiated at progressively more recent time from the northeast to the southwest on the lunar nearside from 4.45 to 4.25 Ga.

Snyder, Gregory A.↗

NASA NTRS1994

Basalt generation at the Apollo 12 site. Part 1: New data, classification, and re-evaluation

New data are reported from five previously unanalyzed Apollo 12 mare basalts that are incorporated into an evaluation of previous petrogenetic models and classification schemes for these basalts. This paper proposes a classification for Apollo 12 mare basalts on the basis of whole-rock Mg# (molar 100*(Mg/(Mg+Fe))) and Rb/Sr ratio (analyzed by isotope dilution), whereby the ilmenite, olivine, and pigeonite basalt groups are readily distinguished from each other. Scrutiny of the Apollo 12 feldspathic 'suite' demonstrates that two of the three basalts previously assigned to this group (12031, 12038, 12072) can be reclassified: 12031 is a plagioclase-rich pigeonite basalt; and 12072 is an olivine basalt. Only basalt 12038 stands out as a unique sample to the Apollo 12 site, but whether this represents a single sample from another flow at the Apollo 12 site or is exotic to this site is equivocal. The question of whether the olivine and pigeonite basalt suites are co-magmatic is addressed by incompatible trace-element chemistry: the trends defined by these two suites when Co/Sm and Sm/Eu ratios are plotted against Rb/Sr ratio demonstrate that these two basaltic types cannot be co-magmatic. Crystal fractionation/accumulation paths have been calculated and show that neither the pigeonite, olivine, or ilmenite basalts are related by this process. Each suite requires a distinct and separate source region. This study also examines sample heterogeneity and the degree to which whole-rock analyses are representative, which is critical when petrogenetic interpretation is undertaken. Sample heterogeneity has been investigated petrographically (inhomogeneous mineral distribution) with consideration of duplicate analyses, and whether a specific sample (using average data) plots consistently upon a fractionation trend when a number of different compostional parameters are considered. Using these criteria, four basalts have been identified where reported analyses are not representative of the whole-rock composition: 12005, an ilmenite basalt; 12006 and 12036, olivine basalts; and 12031 previously classified as a feldspathic basalt, but reclassified as part of the pigeonite suite.

Neal, Clive R.↗

NASA NTRS1994

Basalt generation at the Apollo 12 site. Part 2: Source heterogeneity, multiple melts, and crustal contamination

The petrogenesis of Apollo 12 mare basalts has been examined with emphasis on trace-element ratios and abundances. Vitrophyric basalts were used as parental compositions for the modeling, and proportions of fractionating phases were determined using the MAGFOX prograqm of Longhi (1991). Crystal fractionation processes within crustal and sub-crustal magma chambers are evaluated as a function of pressure. Knowledge of the fractionating phases allows trace-element variations to be considered as either source related or as a product of post-magma-generation processes. For the ilmenite and olivine basalts, trace-element variations are inherited from the source, but the pigeonite basalt data have been interpreted with open-system evolution processes through crustal assimilation. Three groups of basalts have been examined: (1) Pigeonite basalts-produced by the assimilation of lunar crustal material by a parental melt (up to 3% assimilation and 10% crystal fractionation, with an 'r' value of 0.3). (2) Ilmenite basalts-produced by variable degrees of partial melting (4-8%) of a source of olivine, pigeonite, augite, and plagioclase, brought together by overturn of the Lunar Magma Ocean (LMO) cumulate pile. After generation, which did not exhaust any of the minerals in the source, these melts experienced closed-system crystal fractionation/accumulation. (3) Olivine basalts-produced by variable degrees of partial melting (5-10%) of a source of olivine, pigeonite, and augite. After generation, again without exhausting any of the minerals in the source, these melts evolved through crystal accumulation. The evolved liquid counterparts of these cumulates have not been sampled. The source compositions for the ilmenite and olivine basalts were calculated by assuming that the vitrophyric compositions were primary and the magmas were produced by non-modal batch melting. Although the magnitude is unclear, evaluation of these source regions indicates that both be composed of early- and late-stage Lunar Magma Ocean (LMO) cumulates, requiring an overturn of the cumulate pile.

Neal, Clive R.↗

NASA NTRS1993

Rare earth element selenochemistry of immiscible liquids and zircon at Apollo 14 - An ion probe study of evolved rocks on the moon

Results are presented of trace-element analyses of three lunar zircons. The major-element and REE compositions were determined using electron microprobes, and a correction was made for zircon for Zr-Si-O molecular interferences in the La to Pr mass region. The three zircons were found to exhibit similar REE abundances and patterns. Results of the analyses confirm earlier studies (Hess et al., 1975; Watson, 1976; Neal and Taylor, 1989) on the partitioning behavior of trace elements in immiscible liquid-liquid pairs. The results also support the postulated importance of silicate liquid immiscibility in the differentiation of the upper mantle and crust of the moon.

Snyder, Gregory A.↗

NASA NTRS1993

On the composition of neuKREEP: QMD contamination at Apollo 11?

The Group A basalts of Apollo 11 differ in many respects from other high-Ti basalts of the region. Chemically, they are the only high-K (greater than 2000 ppm K) variety of high-Ti basalt and are enriched in incompatible trace elements relative to other basalts from both the Apollo 11 and Apollo 17 sites. In addition, Group A basalts are the youngest of all high-Ti basalts, with an age of 3.56 +/- 0.02 Ga. The cluster of compositions is consistent with the Apollo 11 Group A basalts representing a single flow. Papanastassiou et al. have also indicated the uniqueness of these basalts, based particularly on relatively young Rb-Sr model ages (3.8 - 3.9 Ga). A model for the formation of the Group A basalts was presented by Jerde et al., wherein the Apollo 17 orange volcanic glass is the parent liquid. Fractionation of this composition, coupled with the assimilation of incompatible-element-rich material, results in compositions akin to those of the Apollo 11 Group A basalt population. Orange glass of similar major-element composition is present at the Apollo 11 site as well, although complete trace element analyses are not available. New modelling results using the Apollo 11 orange glass major elements are grossly similar to those obtained using the Apollo 17 orange glass, indicating approximately 30 percent fractionation.

Jerde, Eric A.↗

NASA NTRS1993

Primary differentiation in the early Earth: Nd and Sr isotopic evidence from diamondiferous eclogites for both old depleted and old enriched mantle, Yakutia, Siberia

Ancient, stable, continental cratons possess thick, subcontinental-lithospheric mantle 'keels' which favor particularly the emplacement of diamondiferous kimberlites and included peridotites and eclogites. These refractory mantle samples of the roots provide hard constraints on the theories of formation, growth, and evolution of these cratons. Xenoliths containing only primary garnet and clinopyroxene (eclogites), although rare in most kimberlites, can retain the geochemical signatures of their parent protoliths (e.g., subducted oceanic crust, ancient mantle) thus offering the opportunity to address mantle processes which may have taken place at earlier times in the Earth's history. In fact, it has been postulated that some eclogites are residues from the accretion of the early Earth. Nd and Sr isotopic data are presented which may be interpreted as evidence of an early (greater than 4 Ga) mantle differentiation event. The kimberlites of Yakutia are located both marginal and central to the Siberian craton, and a wide variety of xenoliths are present within them. The Siberian mantle samples have received little attention in the western world, largely because suitable suites of Yakutian samples have not been readily available. Importantly, there is evidence that metasomatism of the Siberian lithosphere has been considerably less intense or extensive than for the Kaapvaal craton. Therefore, it should be considerably easier to elicit the igneous/metamorphic histories of Siberian kimberlitic xenoliths. One of the notable features of the Siberian eclogites is the common appearance of diamonds, especially in the Mir and Udachnaya pipes. In all, eight eclogite samples (eight garnet separates and eight clinopyroxene separates) have been analyzed to date on the Udachnaya pipe, seven from our group.

Snyder, Gregory A.↗

NASA NTRS1993

Chronology and genesis of high-Ti mare volcanism: Melting of cogenetic depleted and enriched reservoirs

An understanding of the chronology and petrogenesis of volcanic rocks is paramount to unravelling the complexities of the composition and structure of the mantle of the Moon. High-Ti mare basalts represent the best-sampled suite of lunar volcanic rocks and likely represent melts of the uppermost mantle of the Moon. Compiled Nd and Sr isotopic data for high-Ti basalts, combined with weighted average ages determiend by various techniques, yield a complex melting history for the high-Ti mantle source. Melting occurred during three distinct episodes (3.84 Ga, 3.75-3.69 Ga, and 3.56 Ga) separated by hiatuses of 100 Ma or more. The first episode included the melting of a relatively trapped liquid (KREEPy)-rich source beneath the A-11 landing site. Later melting of this source produced magmas which were relatively trapped liquid-free at the A-11 site. Synchronous melting at the A-17 landing site produced magmas with variable proportions of this evolved trapped liquid component. The final phase of volcanism occurred at the A-11 site and involved the assimilation of an evolved 'neuKREEP' component. Continued melting of a similar source, which was nearly exhausted of its ilmenite component, beneath the A-12 landing site may have lead to production of magmas parental to the A-12 ilmenite basalts.

Snyder, Gregory A.↗

NASA NTRS1993

Geochronology and petrogenesis of the western highlands alkali suite: Radiogenic isotopic evidence from Apollo 14

Several rocks of alkalic affinity, from the western highlands of the Moon, have been analyzed for their Nd and Sr isotopic compositions. One sample yields a Sm-Nd mineral isochron of 4110 = 41 Ma. This age, in conjunction with U-Pb zircon ages on two other alkalic rocks from the Apollo 14 landing site suggests a distinct western highlands 'event' which was approximately 100 Ma in duration. Since the last dregs of the lunar magma ocean likely crystallized prior to 4.3 Ga, this alkalic 'event' may have included the re-melting of evolved plutons or the remobilization of urKREEP trapped liquid from upper mantle cumulates. Alkalic lithologies such as granites and felsites have been known from the Moon since the earliest days of the Apollo lunar sample returns. However, not until 1977 were alkali-rich rocks recognized from typical highlands suites such as ferroan anorthosites (FAN) and norites and Mg-suite rocks. In the intervening years, several other alkali suite samples have been discovered and characterized, mostly through labor-intesive breccia pull-apart studies of clasts and analyses of coarse-fine fractions of soils. We will speculate on the origins of this suite of lunar highlands rocks.

Snyder, Gregory A.↗

NASA NTRS1993

Magnesian anorthosites from the western highlands of the Moon: Isotope geochemistry and petrogenesis

Breccias from the Apollo 14 landing site have provided a wealth of information on the genesis of the lunar highlands. Various pristine rock-types have been discovered in relative abundance including rare ferroan anorthosites and alkali-suite and magnesian-suite rocks. Mineral-chemical and radiogenic isotopic data are reported here for a newly discovered Mg-suite anorthosite from Apollo 14, sample 14303,347. Meyer et al. reported U-Pb zircon analyses of Mg-suite highlands rocks from the western limb of the Moon. We have compiled these ages and generated a weighted average age of 4211 = 6 Ma; some 200 Ma younger than ferroan anorthosites. Utilizing this age for Mg-anorthosite 14303,347, our data results in an initial epsilon(sub Nd) value of -1.0 and initial Sr-87/Sr-86 of 0.69915. Based on trace-element, isotopic, and mineral-chemical data, the western highlands Mg-suite is interpreted to be crustal precipitates of a picritic magma, which assimilated KREEPy trapped liquid from upper-mantle cumulates during its transport to the crust.

Snyder, Gregory A.↗

NASA NTRS1992

neoKREEP: A new lunar component at Apollo 17

The Apollo 11 (Mare Tranquillitatis) and Apollo 17 (Mare Serenitatis) landing sites are important as the only sources of high-Ti basalt visited by the Apollo missions. The lunar high-Ti basalts (greater than 6 percent TiO2) have no volumetrically comparable analogs among terrestrial basalts and require the presence of ilmenite in the source region, probably representing cumulates produced late in the crystallization of the lunar magma ocean. Six principal groups of high-Ti basalts are described, three from each of the two sites.

Jerde, Eric A.↗

NASA NTRS1992

Melting of cognetic depleted and enriched reservoirs and the production of high Ti Mare basalts

Implicit in current understanding of the location of terrestrial enriched and depleted reservoirs is the notion that they are spatially separated. The depleted reservoir on Earth is situated in the upper mantle, and the complementary enriched reservoir is located in the crust. However, Earth reservoirs are continually being modified by recycling driven by mantle convection. The Moon is demonstrably different from Earth in that its evolution was arrested relatively early - effectively with 1.5 Ga of its formation. It is possible that crystallized trapped liquids (from the late stages of a magma ocean) have been preserved as LILE-enriched portions of the lunar mantle. This would lead to depleted (cumulate) and enriched (magma ocean residual liquid) reservoirs in the lunar upper mantle. There is no evidence for significant recycling from the highland crust back into the mantle. Therefore, reservoirs created at the Moon's inception may have remained intact for over 4.0 Ga. The topics discussed include the following: (1) radiogenic isotopes in high-Ti mare basalts; (2) formation of cogenetic depleted and enriched reservoirs; and (3) melting of the source to achieve high-Ti mare basalts.

Snyder, Gregory A.↗

NASA NTRS1992

A chemical model for generating the sources of mare basalts - Combined equilibrium and fractional crystallization of the lunar magmasphere

A chemical model for simulating the sources of the lunar mare basalts was developed by considering a modified mafic cumulate source formed during the combined equilibrium and fractional crystallization of a lunar magma ocean (LMO). The parameters which influence the initial LMO and its subsequent crystallization are examined, and both trace and major elements are modeled. It is shown that major elements tightly constrain the composition of mare basalt sources and the pathways to their creation. The ability of this LMO model to generate viable mare basalt source regions was tested through a case study involving the high-Ti basalts.

Snyder, Gregory A.↗

NASA NTRS1992

Imperfect fractional crystallization of the lunar magma ocean and formation of the lunar mantle: A realistic chemical approach

It is generally considered that lunar mare basalts were generated by the melting of a cumulate mantle formed in an early Moon-wide magma ocean or magmasphere. However, the nature and chemistry of this cumulate mantle and the logistics of its origin have remained elusive. Extensive studies of terrestrial layered mafic intrusions over the past sixty years have emphasized the imperfection of fractional crystallization and attendant crystal-crystal and crystal-liquid separation in a convecting magma chamber. These separations were similarly inefficient during evolution of the lunar magmasphere, allowing for the trapping of interstitial melt and entrainment of a small proportion of less-dense plagioclase into the more-dense mafic cumulate mush. Indeed, petrography of lunar highlands samples demonstrates this well for anorthosites (with 1-10 percent olivine). Therefore, we propose a 'realistic' model for the evolution of the lunar mantle, which takes these observations into consideration, by the imperfect fractional crystallization of an early lunar magma ocean.

Snyder, Gregory A.↗

NASA NTRS1992

Petrogenesis of the western highlands of the moon - Evidence from a diverse group of whitlockite-rich rocks from the Fra Mauro formation

A group of KREEPy basalts has been discovered in Apollo 14 soils. These samples exhibit similarities to both HA and VHK basalts, albeit with much higher REE abundances, and contain up to 2 vol pct whitlockite and can be explained by assimilation of a K-, REE- and P-rich fluids by an original HA or VHK basalt. This KREEP component could have been produced late in the evolution of the lunar magma ocean and is similar in composition to QMD at Apollo 14. Two rocks have trace element compositions that are representative of actual KREEP. One of the samples appears to be petrographically pristine and could represent an actual KREEP basalt rock. Five subophitic high-Al basalts represent sampling of either a slowly cooled impact melt sheet or, more likely, the same basalt flow. Two 'quasi-pristine' highland rocks confirm the postulate of a connection between KREEP and the alkali suite. A newly discovered alkali anorthosite is a plagioclase cumulate with about 15 percent trapped KREEPy liquid.

Snyder, Gregory A.↗