Aurora Pun

Xenolithic evidence for Proterozoic crustal evolution beneath the Colorado Plateau

Tertiary diatremes of the Navajo volcanic field brought a wide variety of Proterozoic xenoliths to the surface of the Colorado Plateau. Examination of crustal xenoliths from the Navajo volcanic field diatremes permits reconstruction of Proterozoic pressure-temperature ( P-T ) histories beneath the Colorado Plateau. Diatremes from the northwest part of the Navajo volcanic field carry the greatest variety of xenoliths, including metasedimentary rocks, amphibolites, felsic gneisses, mafic granulites, and crustally derived eclogites; these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories. In contrast, diatremes from the southeast part of the Navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration; metasedimentary and eclogitic xenoliths are absent in these diatremes. The P-T paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 °C and pressures equal to or greater than 10 kbar. Later hydrous alteration occurred around 500 °C and 8–12 kbar. Xenoliths from the southeast diatremes preserve little evidence of their P-T evolution, but paths involve heating and/or decompression. The differing rock types, P-T paths, and alteration histories of the northwest and southeast populations suggest that Proterozoic tectonism juxtaposed two distinct crustal blocks beneath the Colorado Plateau. Previous workers have postulated that the boundary between the Proterozoic Yavapai and Mazatzal provinces occurs in the region; our data support the existence of a northeast-trending boundary beneath the Four Corners area. If the eclogitic metamorphism and hydrous alteration are Proterozoic in age, their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during Proterozoic accretion of the Mazatzal province onto North America.

Unequilibrated eucrites and the equilibrated Juvinas eucrite; pyroxene REE systematics and major, minor, and trace element zoning

Abstract In unequilibrated eucrites, pyroxene is a sensitive recorder of variables that affect their crystallization histories. Therefore, major, minor, and trace element systematics of pyroxene were evaluated from several unequilibrated eucrites and an equilibrated eucrite, Juvinas, by EMP and SIMS techniques. The Cr, Al, and Ti zoning trends in pyroxene reflect its igneous crystallization history, including the sequence of crystallization of pyroxene and plagioclase. The primary substitutional couples in unequilibrated pyroxene among Cr, Al, and Ti are [6]Cr3+-[4]Al3+, [6]Al3+-[4]Al3+, and [6]Ti4+-2[4]Al3+, with the [6]iTi4+-2[4]Al3+ couple dominating over [6]IA3+-[4]Al3+ following the onset of plagioclase crystallization. Reequilibration erases the primary igneous zoning trends of Cr, Al, and Ti, and the dominant substitutional couples in homogenized pyroxene are [16]Ti4+-2[4]Al3+ and [6]Cr3+-[4]Al3+ in equal proportions. Trace element compositions in unequilibrated eucrites are variable because of igneous zoning, but averaged analyses show core-rim differences that reflect their Ca-rich rims relative to Ca-poor cores. The pyroxene REE abundances in the unequilibrated samples are as much as ten times chondrite in their rims, whereas homogenized Juvinas pyroxenes have abundances between the core and rim values of pyroxene from unequilbrated eucrites. The ratio of REEs (homogenized pyroxene/bulk composition) for Juvinas is significantly higher than the ratio of REEs (pyroxene cores/bulk composition) for the unequilibrated eucrites. This is especially true for the HREEs. The higher observed REE concentrations in the equilibrated pyroxene result from the intracrystalline exchange between Ca-poor cores and Ca-rich rims and also from intercrystalline exchange with other phases, including plagioclase, phosphates, and mesostasis. Use of the equilibrated pyroxene REE concentrations and igneous D values leads to an incorrect estimate of the parental melt composition. The estimated chondrite-normalized melt REE concentrations are too high by a factor of two to four. These studies suggest that great care must be used to take into account subsolidus elemental redistributions when attempting to estimate parental melt compositions from measured mineral trace element concentrations.

Ion microprobe investigation of exsolved pyroxenes in cumulate encrites: Determination of selected trace-element partition coefficients

We have examined the partitioning of selected minor and trace elements in exsolved pyroxenes from cumulate eucrites by secondary ion mass spectrometry (SIMS). Solid-solid partition coefficients (Ds) for Al, Cr, Ti, seven REEs, Sr, Y, and Zr have been determined between low Ca- and high-Ca pyroxenes from Moore County, Binda, Serra de Mage, and Moama. Equilibrium temperatures of the co-existing pyroxenes were calculated to constrain the relevant temperatures for the partition coefficients. Trace-element abundance patterns are similar for pyroxenes from the four cumulate eucrites and thus indicate similar distribution behaviors of the trace-elements during exsolution. There is a significant HREE enrichment over LREEs in the low-Ca pyroxenes and abundances of the REEs in augite are greater than those found in the low-Ca pyroxene hosts. This REE behavior reflects the pyroxene crystal structures. The calculated solid-solid partition coefficients are found to vary systematically as expected from pyroxene crystal chemical arguments. Calculated subsolidus equilibration temperatures are within the expected range produced by pyroxene solvus temperature-composition relations, and consistent with petrographic observations of the cumulates. The trace-element patterns and abundances indicate that three of the four cumulates examined (Binda, Moama, Serra de Mage) were formed from the same melt or similar melts crystallizing under similar conditions in their parent body. Moore County has much higher trace-element concentrations and this suggests that it either formed from a more evolved melt than the other three cumulates and crystallized later in the fractionation sequence, or alternatively, it was derived from a different melt system.

Deep metastable eutectic condensation in Al-Fe-SiO-H2-O2 vapors : Implications for natural Fe-aluminosilicates

Abstract Vapors of Al-Fe-SiO-O2-H2 having two different compositions produced ferroaluminosilica grains as a function of agglomeration and fusion along mixing lines in the Al2O3-FeO-SiO2 system that are defined by the predictable, deep metastable eutectic (DME) compositions of the smallest condensate grains. Disorder of these amorphous grains is higher than in quenched glass of identical composition, which is the very property of dissipative structures (Prigogine 1978, 1979) that are states of organization of matter where disequilibrium becomes a source of order. Iron-oxidation states control ferrosilica condensate compositions. We present the first magnetic measurements showing a high Fe3+ content in condensed ferrosilica grains. The Fe-cordierite grain composition is primarily the result of predictable non-equilibrium condensation, not the bulk gas phase composition. Natural terrestrial and anthropogenic (e.g., smelters, coal fly ash) Fe-cordierite might well be a metastable phase due to kinetically controlled processes. Amorphous Mg,Fe-bearing aluminosilica dust in chondritic interplanetary dust aggregates and (rare) Mg,Fe-aluminosilicates in meteorites might have condensed via similar processes.

Subsolidus REE partitioning between pyroxene and plagioclase in cumulate eucrites: An ion microprobe investigation

Abstract In an attempt to elucidate the relationship between the cumulate eucrites and the noncumulate (main series) eucrites, we have examined the trace-element systematics of plagioclase grains that coexist with inverted pigeonites of the cumulate eucrites. Analyses were done by secondary ion mass spectrometry (SIMS). Plagioclase compositions are uniform in trace-element abundances within grains from each cumulate eucrite, but are variable between the four analyzed: Moama, Moore County, Serra de Mage, and Binda. Shapes of CI-normalized trace-element abundance patterns of the plagioclase grains of the cumulates are typical of plagioclase, showing a LREE-enriched pattern with positive Eu anomalies reflecting the site preference of Eu 2+ in the feldspar structure. Using trace-element concentrations of the pyroxenes (orthopyroxene, pigeonite, augite; Pun and Papike, 1995) and plagioclase (this study), we calculate the subsolidus partition coefficients (Ds) for the REEs and Y between plagioclase and pyroxenes in the examined cumulate eucrites. Trace-element systematics suggest that igneous trace-element signatures have been altered by subsolidus exchange for both the plagioclase and pyroxene. Calculated hypothetical parental-melt patterns determined from pyroxene and plagioclase are different and can differ by a factor of 4–5 for individual REEs.