The Four-Phase AFM Assemblage Staurolite--Aluminum Silicate--Biotite--Garnet: Extra Components and Implications for Staurolite--Out Isograds

The prograde disappearance of staurolite can be described inthe model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) by thereaction: staurolite + muscovite + quartz = biotite + aluminumsilicate + garnet + water. The common occurrence and world—widedistribution of the assemblage staurolite-biotite-aluminum silicate-garnet(SBAG) in quartz-mica-schist suggest that the model reactionmay be over-simplified. Previous workers have suggested thatthe SBAG assemblage (1) is a strictly divariant assemblage thatbuffered water activity, (2) is stabilized by non-KFMASH components,and (3) did not attain equilibrium. We used least-squares regression to show that balanced reactionsdo not exist among the minerals in samples of SBAG assemblagesfrom Califonia and New England. The absence of reaction relationshipscan be explained by imbalances in two or three of the minorelements Zn, Mn, and either Ca or Na. The assemblage is apparentlystabilized by non-KFMASH components. Criteria for mapping staurolite-out isograds that representthe conditions of the KFMASH staurolite-out reaction dependon which of the four phases is the ‘extra’ phase,and require an understanding of the thermodynamic effects ofall the ‘extra’ components. Our results suggestthat transition zones of SBAG assemblages near staurolite-outisograds are the result of ‘extra’ components. However,it is uncertain whether µH²O of fluids in equilibriumwith SBAG assemblages varied across such zones.     

Buffering in the assemblage staurolite-aluminium silicate-biotite-garnet-chlorite

Abstract The assemblage muscovite-quartz-staurolite-aluminium silicate-biotite-garnet-chlorite with H₂O (SABGC assemblage) is invariant in the K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) system. Such five-phase AFM assemblages should be absent in nature, but reported occurrences from at least ten localities suggest that either the assemblage internally controls μH₂O or non-KFMASH components stabilize one or more of the phases. Least-squares regression analysis of minerals from South Royalton and Gassetts, Vermont, USA, demonstrates that subsets of the minerals in single SABGC assemblages from both localities are related by balanced reactions involving water. These results are consistent with the interpretation that the subassemblages fixed μH₂O at an arbitrary, specified pressure and temperature. Balanced dehydration reactions also may be written between minerals in the SABGC assemblages and four-phase assemblages from the same outcrops interpreted to have equilibrated at the same pressures and temperatures as the five-phase assemblages. These results suggest that different specimens from the same outcrops equilibrated at different values of μH₂O, supporting the conclusion that μH₂O was not controlled externally. We could not demonstrate internal control of fo₂ using measured mineral compositions because oxygen balance occurred in all reactions derived by regression. Regression analysis of published mineral compositions from New Mexico failed to identify balanced reactions involving water or oxygen either among the phases in a single SABGC assemblage or between SABGC and nearby four-phase assemblages. These results demonstrate that neither μH₂O nor fo₂ were fixed by the SABGC assemblages at these localities, and permit the interpretations that the assemblages were stabilized by the non-KFMASH components Na or Ca and that μH₂O and fo₂ were controlled externally.     

Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California

Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs.     

Petrogenesis of lunar mare basalt meteorite Miller Range 05035

Abstract— Miller Range (MIL) 05035 is a low‐Ti mare basalt that consists predominantly of pyroxene (62.3 vol%) and plagioclase (26.4 vol%). Pyroxenes are strongly shocked and complexly zoned from augite (Wo₃₃) and pigeonite (Wo₁₇) cores with Mg# = 50–54 to hedenbergite rims. Coexisting pyroxene core compositions reflect crystallization temperatures of 1000 to 1100 °C. Plagioclase has been completely converted to maskelynite with signs of recrystallization. Maskelynite is relatively uniform in composition (An₉₄Ab₆–An₉₁Ab₉), except at contacts with late‐stage mesostasis areas (elevated K contents, An₈₂Ab₁₅Or₃). Symplectites (intergrowth of Fe‐augite, fayalite, and silica) of different textures and bulk compositions in MIL 05035 suggest formation by decomposition of ferro‐pyroxene during shock‐induced heating, which is supported by the total maskelynitization of plagioclase, melt pockets, and the presence of a relict pyroxferroite grain. Petrography and mineral chemistry imply that crystallization of MIL 05035 occurred in the sequence of Fe‐poor pyroxenes (Mg# = 50–54), followed by plagioclase and Fe‐rich pyroxenes (Mg# = 20–50), and finally hedenbergite, Fe‐Ti oxides, and minor late‐stage phases. Petrography, bulk chemistry, mineral compositions, and the age of MIL 05035 suggest it is possibly source crater‐paired with Asuka (A‐) 881757 and Yamato (Y‐) 793169, and may also be launch‐paired with Meteorite Hills (MET) 01210. MIL 05035 represents an old (?3.8–3.9 Ga), incompatible element‐depleted low‐Ti basalt that was not sampled during the Apollo or Luna missions. The light‐REE depleted nature and lack of Eu anomalies for this meteorite are consistent with an origin distant from the Procellarum KREEP Terrane, and genesis from an early cumulate mantle‐source region generated by extensive differentiation of the Moon.     

Controls on the first appearance of jadeitic pyroxene, northern Diablo Range, California

A reaction producing jadeitic pyroxene in metagreywackes of the northern Diablo Range has been identified on the basis of mineral distribution, isograd patterns and composition of coexisting minerals. The appearance of jadeitic pyroxene (∼Jd₈₀) is closely followed by the disappearance of pumpellyite, which indicates that pumpellyite plays a major role in the pyroxene-producing reaction. A new projection from hematite, lawsonite, chlorite, quartz and H₂O on to the NaAlO₂-FeO-MgO ternary confirms the role of pumpellyite in pyroxene production and suggests a reaction of the form: 1.00 pumpellyite + 0.31 chlorite + 8.71 albite + 0.70 hematite + 2.00 H₂O = 8.54 jadeite + 0.57 glaucophane + 3.09 lawsonite + 5.26 quartz. Metagreywackes of the northern Diablo Range were metamorphosed under conditions of P _H2O= P _total at 200-300 °C and 7.5-10.0 kbar. Despite the low temperatures attained during metamorphism, the assumption of equilibrium yields results consistent with field observations and phase relations.     

The Origin and Significance of Garnet Phenocrysts and Garnet-Bearing Xenoliths in Miocene Calc-alkaline Volcanics from Northland, New Zealand

Xenoliths, considered to be of igneous origin and consistingof hornblendegarnetplagioclase clinopyroxene, occur in associationwith high-pressure phenocrysts in early Miocene high-silicaandesites and dacites, Northland, New Zealand. Microstructuresof these xenoliths range from coarse, even-grained sub-ophitictypes to others with coarse glomerocrysts set in a finer-grainedmesostasis. The xenoliths are commonly flow-banded and are arguedto represent direct crystallization products and crystal aggregationsfrom the calc-alkaline host or related magmas at depth. Manygarnets within these high-pressure aggregates and also discretegarnet phenocrysts are rimmed by medium—coarse-grained,interlocking hornblendeplagioclase, representing partial adjustmentto an assemblage stable at shallower levels. The garnets aretypically pyrope—almandine with 17–28 mol.% grossularand show normal, reverse and oscillatory zoning; the associatedamphibole is pargasite trending to hornblende in phenocrystrims and reaction rims. Metamorphic xenoliths with plagioclase-hornblende-quartzassemblages are also found in the rocks and are characterizedby fine-grained granoblastic mosaic microstructures with well-developedfoliation defined by preferred orientation of elongate grainsand a mineral layering. These metamorphic xenoliths are interpretedas fragments of lower-crustal country rocks accidentally incorporatedinto rising andesitic magma. Application of established experimental high-pressure phasediagrams for andesites indicates crystallization of these assemblagesat depths corresponding to 10–20-kb pressure, and appropriategeothermometers indicates the following temperatures for equilibrationof assemblages at a nominal pressure of 12 kb: garnet-augite980C; garnet-augite-hornblende 920–1020 C. Geobarometryon a single garnet—orthopyroxene-bearing xenolith indicatesa pressure of 10–12 kb for a likely temperature rangeof 950–1000C. Thus the xenoliths point to the generationof host andesite-dacite magmas at suberustal depths of 35–45km, from fractional crystallization of more mafic mantlederivedmagmas, and demonstrate that relatively silicic calc-alkalinemagmas may evolve in the mantle. The rarity of evidence forsuch a process may be linked with the obduction-related tectonicevents operative in Northland just before the magmatic episode,and to the unusually high water content in the magma.     

Geochemistry of intermediate olivine‐phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine‐phyric shergottite Northwest Africa 2990

Abstract— The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)‐phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk‐rock major‐ and trace‐element abundances (including Li), abundances of highly siderophile elements, Re‐Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and δ⁷Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La‐Nd) compared with the heavy REE (Sm‐Lu), but not as extreme as the known “depleted” shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically “intermediate” in incompatible elements. The only other basaltic or ol‐phyric shergottite with a similar “intermediate” character is the basaltic shergottite NWA 480. Rhenium‐osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.     

The karstlands of Antigua, their land use and conservation

The northeastern 110 km², or nearly 40%, of Antigua is underlain by impure limestones of the Oligocene-aged Antigua Formation, on which has developed a subdued karst landscape consisting essentially of shallow enclosed depressions (dolines), intermittently active stream valleys and widely scattered residual hills. The dispersed dolines are broad, shallow and clustered, especially in the central and southeastern sections of the limestone belt, where they attain densities of 7/km². The widely spaced residual hills attain heights of up to 40 m and localized densities of over 4/km². Five main valley systems up to 6 km in length traverse the limestone in a broadly northeast direction, carrying both autogenic drainage from within the karst area and allogenic drainage from the non-carbonate Central Plain. Karst and cave development has been constrained by the low purity of the limestones. Of the four types of carbonate islands identified within the Carbonate Island Karst Model, Antigua most closely resembles the Composite Island type. The karst has been much influenced by human activities, particularly agriculture and quarrying, and is now a focus of the burgeoning tourism industry. Virtually none of the karst is designated as protected areas, but several sites warrant protection, and several conservation strategies have been suggested.