I. Stewart McCallum

Eocene potassic magmatism in the Highwood Mountains, Montana: Petrology, geochemistry, and tectonic implications

Potassic volcanics and intrusives of Eocene age (52±1 Ma) in the Highwood Mountains of north-central Montana provide evidence for interaction of asthenospheric magmas with Archean mantle lithosphere of the Wyoming Craton. Diverse rock types were produced by shallow level degassing, fractional crystallization and magma mixing within two separate magma series whose parental liquids were latite and olivine minette. Halogen systematics of apatite microphenocrysts and other evidence have established that shallow degassing of the phlogopite-diopside-phyric minettes to yield heteromorphic leucite-salite-phyric mafic phonolites and shonkinites was an important process. Geochemical and mineralogical data suggest that fractional crystallization of olivine, clinopyroxene, mica and leucite, accompanied by widespread magma mixing, produced a spectrum of more evolved magmas such as leucite phonolites, malignites, alkali syenites and trachytes along with mica clinopyroxenite cumulates. Sr, Nd and Pb isotopic compositions and trace element data for the primitive olivine minette magmas are explicable by a multistage model involving three source components. One component is ancient Ba-LREE-enriched, U-Th-HFSE-depleted subcontinental lithospheric mantle, which has been recognized in other alkalic rocks of the region, including those from the Crazy Mountains and Smoky Butte. Glimmerite-veined harzburgite and phlogopite dunite xenoliths (one with -eNd of −33 at 52 Ma) found in the most primitive Highwood olivine minette are probably samples of this material. The other two components are asthenospheric mantle with isotopic composition near that of bulk earth (and dominant in Montana alnoitic diatremes) and a young subduction-related component (probably Eocene, but possibly as old as late Cretaceous), which is required to explain the Rb/Sr-87Sr/86Sr systematics of the Highwood rocks. A consistent model for the petrogenesis of the Highwood parental mafic magmas involves partial melting of asthenospheric mantle wedge triggered by infiltration of melts released from the metasomatized carapace above a low-angle subducted slab of Farallon Plate lithosphere, followed by assimilative interaction of these melts with ancient, phlogopite-rich, metasome veins upon ascent through the Wyoming Craton mantle keel.

Comparative study of equilibrated and unequilibrated eucrites: Subsolidus thermal histories of Haraiya and Pasamonte

Abstract Thermal histories of non-cumulate eucrites, Pasamonte and Haraiya, were determined from Ca/(Ca + Fe + Mg) profiles of host/lamellae pyroxene pairs, from compositional profiles of pyroxene rims affected by metasomatism, and from lamellae thicknesses using a rate law for lamellar coarsening. Pasamonte, a Type 2 eucrite, experienced “partial” equilibration during a relatively short period of thermal annealing. This equilibration is expressed texturally by metasomatized rims in Mg-rich and Fe-rich pyroxene grains and by coarsening of nanometer-scale exsolution lamellae. A cooling rate of ~10.2 °C/day corresponds to post-impact mixing and reburial of the polymict Pasamonte assemblage to a depth similar to that of the initial lava flow. Haraiya, a Type 7 eucrite, has experienced a complex thermal history involving multiple brecciation events and mixing of a single flow to produce a monomict assemblage containing clasts of variable grain size and texture. A period of thermal annealing, ~50 000 years in duration at temperatures between ~1000 and 700 ºC, corresponding to a cooling rate of ~10-4 to 10-5 ºC /day, produced augite exsolution up to 3 μm thick in pigeonite. Rapid burial by successive flows during a period of high magma productivity and high heat flow due to 26Al decay in the crust and mantle was a possible source of heat for metamorphism in the lower crust. However, cooling rates during relaxation of the lower crustal thermal gradient are much less than the mineralogically determined cooling rates. Heat derived from impact or from mid- to upper-crustal level intrusions is a plausible heat source for metamorphism and is consistent with observed cooling rates.

Luna 20 pyroxenes: exsolution and phase transformation as indicators of petrologic history

Pyroxenes from our sample of Luna 20 soil are predominantly orthopyroxene with subordinate pigeonite. The orthopyroxenes are chromium-rich bronzites and contain submicroscopic lamellae of augite in a twinned orientation exsolved on (100). These lamellae have a composition close to the diopside-hedenbergite join. Asymmetric diffuse streaks parallel to a∗ indicate stacking faults parallel to (100) and possibly very thin (10–20 A) lamellae of clinobronzite parallel to (100). Pigeonite crystals are very complex crystallographically and chemically, with optically visible (001) augite exsolution lamellae and two sets of chromite exsolution lamellae. In addition, there are submicroscopic (100) augite lamellae and a second generation of clinohypersthene lamellae which appear to have exsolved from the (001) augite lamellae. The clinohypersthene host, which has a large number of stacking faults parallel to (100), has partially inverted to hypersthene of the same composition. The hypersthene occurs as very fine lamellae (less than 1000 A) parallel to the (100) plane of the clinohypersthene. XDFe-Mg values for five host-lamellae pairs in pigeonite K-4 indicate a significant amount of subsolidus readjustment. We tentatively conclude that many of the bronzite and pigeonite crystals were derived from rocks crystallized from a high level magma chamber in the lunar highland crust.

Coexisting pseudobrookite, ilmenite, and titanomagnetite in hornblende andesite of the Coleman Pinnacle flow, Mount Baker, Washington: Evidence for a highly oxidized arc magma

Abstract Pseudobrookite microphenocrysts occur in cognate inclusions in the ~305 ka Coleman Pinnacle hornblende andesite flow from the Mount Baker volcanic field, Washington. Pseudobrookites are associated with hornblende phenocrysts and glomerophyric clusters of orthopyroxene, clinopyroxene, plagioclase, ilmenite, titanomagnetite, apatite, and zircon in a matrix of fresh rhyolitic glass. Grains of pseudobrookite are rimmed by or intergrown with ilmenite. These textures are analogous to those observed between armalcolite and ilmenite in high-Ti lunar basalts. In a unique occurrence, pseudobrookite, and titanomagnetite form a symplectitic intergrowth surrounding a core of ilmenite. Mass balance calculations show that the pseudobrookite + titanomagnetite assemblage is not an isochemical decomposition of ilmenite. In the TiO2-FeO-Fe2O3 system (Mg-free), pseudobrookite and titanomagnetite solid solutions do not coexist. However, all three Fe-Ti oxides in the symplectitic assemblage contain significant amounts of Mg. In the TiO2-MgO-FeO-FeO1.5 system at high oxygen fugacities, the Mg-rich pseudobrookite + titanomagnetite assemblage is stable relative to the conjugate pair of Mg-bearing ilmenite solid solutions. At lower fO2, Fe2+ increases, Mg/(Mg+Fe2+) (Mg no.) decreases and the conjugate ilmenite pair becomes the stable assemblage at Mg no. < ~0.6. The compositions of coexisting ilmenite + titanomagnetite pairs in the Coleman Pinnacle andesite yield T = 900-1000 °C and fO2 = NNO + 1.5 to + 1.75, one of the highest redox states on record for arc magmas. The calculated fO2 range is consistent with the composition of the ilmenite in equilibrium with pseudobrookite ± rutile and with Fe3+-rich cores in hornblende phenocrysts.