S. S. Hughes

Geochemistry, petrogenesis, and tectonic implications of central High Cascade mafic platform lavas

Petrographic and multi-element geochemical data for a 37-sample suite representing Oregon9s central High Cascade mafic platform allow the distinction of basalts ( 2 ) from two types of basaltic andesites with associated high-silica basaltic andesites (53–62 wt % SiO 2 ). All units yield fractionated REE and incompatible element-enriched patterns characteristic of continental tholeiitic magmas. Mount Washington (MW)-type basaltic andesites are distinguished from North Sister (NS) types by relatively higher total REE and other LILE abundances, although lithologic similarities prevail. Trace-element signatures of both types do not support their derivation from magmas represented by basalts, but represeint near-primary melts from regions apart from the sources of primary basalt magmas. Normal basalts are modeled with 14% incongruent partial melting and concomitant olivine fractionation of spinel Iherzolite upper mantle in the presence of water-rich fluids derived from a subducted oceanic slab. Basaltic andesites are modeled as 6% to 12% partial melting of fluid-enriched spinel Iherzolite accompanied by olivine (± clinopyroxene ± plagioclase) fractionation. Unusual units, including one with ground-mass biotite and rare quartz xenocrysts, probably resulted from divergent processes of crustal assimilation, crystal fractionation, or melting from alternate sources. Early Pleistocene eruptions were largely diktytaxitic basalt outpourings in response to crustal readjustments and subsidence which occurred prior to 4.5 m.y. ago. The transformation to extensional tectonics abruptly reduced the amount of andesitic and silicic volcanism and allowed tapping of deeper, mafic magma source regions. Chemical and petrographic data support the concept of juxtaposed tectonomagmatic types such that, extension-related basalts erupted within the realm of calc-alkaline basaltic andesites and their silicic derivatives.

Apollo 15 yellow-brown volcanic glass: Chemistry and petrogenetic relations to green volcanic glass and olivine-normative mare basalts

Abstract Apollo 15 yellow-brown glass is one of twenty-five, high-Mg, primary magmas emplaced on the lunar surface in pyroclastic eruptions. Forty spherules of this glass were individually analyzed by electron microprobe and INAA for major- and trace-elements ( e.g. Sc, V, Cr, Co, La, Ce, Nd, Sm, Eu, Ho, Tm, Yb, Lu, Hf, Ta, and U). The abundances demonstrate that this primary magma was produced by partial melting of differentiated cumulates in the lunar mantle. Models are developed to explain the possible source-regions of several Apollo 15 and Apollo 12 low-Ti mare magmas as being products of hybridization involving three ancient differentiated components of a primordial lunar magma ocean: (a) early olivine ± orthopyroxene cumulates; (b) late-stage clinopyroxene + pigeonite + ilmenite + plagioclase cumulates; and (c) late-stage inter-cumulus liquid. Isotopic constraints on Sm-Nd fractionation and least-squares tests of potential mixing models require models of Apollo 15 yellow-brown glass, and Apollo 12 and 15 olivine mare basalts, involving low F ~3% cumulate remelting of sources in which olivine and orthopyroxene are residual phases. Models of the Apollo 15 green glass composition indicate higher (~4–7%) degrees of melting, but with only a slight preference for orthopyroxene-rich relative to orthopyroxene -free sources. The modeled dependence on residual orthopyroxene in mare source regions places source depths at ~400 km or greater in order to maintain olivine and orthopyroxene on the liquidus. Hybridization at such depths would likely result from convective overturn of the primordial magma ocean and suggests that separate fractionating masses were positioned at various depths. Our models for developing hybridized source regions of primary low-Ti mare magmas do not require selective contamination of primary liquids. The preferred model for Apollo 15 yellow-brown glass is presented as the commingling of 95% early cumulate olivine + orthopyroxene, 1.9% clinopyroxene, 0.27% pigeonite, 1.7% plagioclase, 0.02% ilmenite, and 0.56% trapped liquid. Respective proportions for the Apollo 15 olivine mare basalt source model are 98.5%, 0.37%, 0%, 0.79%, 0.16%, and 0.13%; and the proportions used for the preferred Apollo 12 olivine mare basalt model are 97.6%, 0.73%, 0.24%, 0.77%, 0.42%, and 0.21%. Our preferred model for the green glass source composition ( F = 0.07) requires the commingling of 97.4% early olivine + orthopyroxene, 0.18% clinopyroxene, 1.32% pigeonite, 0.85% plagioclase, and 0.28% trapped liquid.

Late Paleozoic Rifting in northern Pakistan

Metasedimentary rocks exposed in the eastern Peshawar basin and the southern Swat region of northern Pakistan provide evidence for late Paleozoic continental rifting. The onset of extensional tectonics in the Early Carboniferous is indicated by north derived clasts in the Jafar Kandao Formation eroded from thermally induced uplifts of parts of the formerly passive margin of Gondwana. Rift highlands were eroded until they were inundated during the Middle Carboniferous. Renewed uplift accompanied the eruption of basaltic lava flows during the Early Permian. Uplift along south dipping, northeast striking normal faults during the Carboniferous was accompanied by alkaline magmatism represented by the Shewa-Shahbazgarhi and Warsak porphyries and Koga syenite. Geochemistry of basaltic flows (now amphibolites) and intrusions associated with Permian uplift is similar to the coeval Panjal volcanics of northwestern India and indicates rift zone magmatism. Postrifting thermal subsidence led to the deposition of Upper Triassic marine carbonate rocks which unconformably overlie the rift basalts. A similar tectonic history in central Afghanistan suggests continuity between the two regions prior to the opening of the Neo-Tethys.

The significance of fractional crystallization in the petrogenesis of Apollo 17 Type A and B high-Ti basalts

Abstract Whole-rock and mineral analyses of 26 “new” Type A and B Apollo 17 basalts are reported. The petrography and mineral chemistry of these basalts are similar to previously reported Apollo 17 basalts. However, these “new” whole-rock data extend the compositional ranges of previously reported Type A and B basalts and require the division of the Type B basalts into Type B1 and B2 varieties. These three types display similar trends when both major and trace elements are plotted against a fractionation index of Cr/La ratio. Major element compositions of basalts from all three types fall on olivine + Ti oxide control lines. The compositional ranges of all three groups can be effectively generated by 40–50% fractional crystallization of observed phenocryst phases. The compositions of three Type A basalts are only generated after 80% fractional crystallization. Previous models of different degrees of partial melting and KREEP assimilation were developed in order to explain the range in La/Sm ratio of the Type B basalts. With the recognition of two types of “B” basalts, these models are no longer valid. This study demonstrates that Apollo 17 Type A, B1, and B2 basalts have a relatively simple petrogenesis, with the only post-magma-generation process being fractional crystallization.

Apollo 16 regolith breccias and soils: recorders of exotic component addition to the Descartes region of the moon

Abstract The subdivision of Apollo 16 regolith breccias into ancient ( ∼ 4 b.y.) and relatively younger samples on the basis of trapped 40 Ar/ 36 Ar ratios [1] makes possible, with the present-day soils as a third sample suite, a petrologic and chemical determination of regolith evolution and exotic component addition at the A-16 site. Petrologic data for the ancient breccias show that the early regolith was composed of fragments of plutonic rocks and impact melt rocks, and minerals and impact glasses derived from these lithologies. The presence of KREEPy glasses in the ancient breccias demonstrates that KREEPy lithologies and impact melts formed early in lunar history. Comparison of the ancient breccias with the young breccias and the soils shows that the mare components, mainly in the form of orange high-TiO 2 glass and green low-TiO 2 glass, were mostly added to the site after formation of the ancient breccias and prior to formation of the young breccias. The young breccias are petrologically similar to the soils. The major change in the regolith since the formation of the young breccias is an increase in maturity—the formation of fused soil particles with prolonged exposure to micrometeorite impacts. By contrast, the breccias formed at times when large impacts and soil mixing were dominant over small impacts and maturation.

Olivine-normative dolerite dikes from western South Carolina: Mineralogy, chemical composition and petrogenesis

This investigation describes five Mesozoic dolerite dikes which intrude Paleozoic metamorphic and igneous rocks of the Inner Piedmont of western South Carolina. The dikes are vertical or nearly so and strike approximately N40° W. Three major northeast-trending faults also occur in the study area. Left lateral displacement of one dolerite is documented at a locality near Cleveland, South Carolina. Elsewhere, several of the dolerite dikes appear to terminate at or near the faults. — The dolerite dikes have subophitic to microporphyritic textures and consist principally of plagioclase (generally An70–80), olivine (dominantly Fo80–90) and augite with subordinate pigeonite, titanomagnetite, chromite and brown, partly glassy mesostasis. In one dike pyroxene compositions trend from augite to ferroaugite in contrast to an augite→subcalcic augite→pigeonite trend characteristic of the other dolerites. The contrasting trends primarily result from differences in SiO2 abundance in the dolerite magmas. — Major and trace element analyses indicate the presence of two different olivine-normative dolerite magma types. The two magma types are not related by near surface crystal fractionation. Models for genesis of the olivine-normative dolerite magmas by partial melting of a plagioclase peridotite upper mantle source region are presented. The models require that the source region be enriched in LREE and incompatible elements such as Rb, Ba, Hf and Th relative to Cl chondritic abundances. One magma type appears to represent a primary dolerite magma that ascended from the source region with little subsequent compositional change. The second magma type most likely experienced assimilation of clinopyroxene-garnet (eclogite) during ascent, thereby acquiring a REE pattern with a less steep negative slope for the LREE and a slight positive slope in the HREE.

Chemical diversity and origin of Precambrian charnockitic rocks of the central Pedlar massif, Grenvillian Blue Ridge Terrane, Virginia

Abstract Middle Proterozoic rocks of the Pedlar massif are comprised of varied charnockitic and granulitic lithologies representing ∼ 1150-1000 Ma Grenvillian metamorphic and igneous events in the central Appalachians of Virginia. Chemically and mineralogically diverse units include metaluminous to peraluminous charnockites, enderbites, jotunites and granulite gneisses ranging in SiO2 from 47 to 79 wt.%. Relative to bulk continental crust, typical charnockites are enriched by ∼ 3–6 × in Rb, K, Ba, LREE, Zr and Hf, less enriched in Ta, Sr, P and HREE, and depleted in Cs, Th, Ti and Sc, although low-SiO2 members have relatively high Ti and P and some high-SiO2 units are depleted in K, Rb and Ba. The Pedlar River Charnockite Suite (PRCS), Vesuvius megacrystic charnockite (VMC), Nellysford Granulite Gneiss (NGG) and the Lady Slipper Granulite Gneiss (LSGG) exhibit common chemical signatures and trends, notably evident in uniform LREE-enriched patterns with variable Eu anomalies, which reflect their derivation from a mixed protolith of volcanics and reworked volcaniclastic sediments dated at ∼ 1130–1150 Ma. Pedlar River charnockites represent portions of deep-seated granulite gneisses that were mobilized by granulite facies dehydration melting and emplaced en masse as plutons into overlying and surrounding gneisses during the Grenville episode. Divergent REE patterns in the PRCS are attributed to crystallization of quartz, feldspar ± garnet, or HREE-compatible mafic and accessory phases leading to complementary chemical signatures in mafic and felsic layers. Depletion of Cs throughout the Pedlar massif is attributed to its incompatibility with major mineral phases during either protolith evolution or granulite facies dehydration, while dispersions in K, Rb, Sr, Ba and Eu are due to the relative proportions of the major phases quartz, K-feldspar, plagioclase and biotite. Mobility of insoluble trace elements Zr, Hf, Ta and Th require partial melting and crystallization of accessory mineral phases, whereas Sc, Cr and REE mobility depends on the proportions of pyroxene and garnet. Several leucocharnockites and other felsic rocks of the PRCS show markedly elevated Th values ranging from ∼ 10 to 83 ppm which complement depleted Th in typical PRCS units. These charnockitic rocks represent low-degree partial melts (∼ 1–5%) from normal PRCS or NGG protoliths to produce high-silica magmas which separated from the main charnockitic body.

Formation constraints on Martian north polar volcanic edifices

[1] This study focuses on probable volcanic features in the north polar region of Mars in an effort to constrain their formation mechanisms and to estimate the thickness of a possible former ice sheet. Using Mars Orbiter Laser Altimeter (MOLA), High Resolution Imaging Science Experiment (HiRISE), Thermal Emission Imaging System (THEMIS), and Context Camera (CTX) data, we conduct topographic analyses of 108 putative volcanic edifices in the Borealis Volcanic Field within 69°N-81°N and 197°E-330°E. As it is rarely possible to identify rock types on Mars (such as hyaloclastite or palagonite), we are limited to the use of topographic data and images alone in our study. Therefore, we use available data for volcanic features in Iceland to predict the formational mechanisms of similar features on Mars. We examined 21 Icelandic volcanoes of subglacial and postglacial origin using Digital Elevation Models (DEMs) as well as limited differential GPS data and suggest by analogy that many of the features in the Borealis Volcanic Field were likely formed subglacially. We use these data to estimate a minimum ice sheet thickness for the area and suggest a range of minimum ice sheet thickness that varies between 57 and 610m with an average value of 283 m.

Geochemistry and petrology of Emeishan basalts and subcontinental mantle evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation (Emeishan basalts) were sampled during a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China. Two of the three sections—Emei and Tangfang are composed of continental flood basalts (CFB) while the third—Ertan is an alkalic center. Multi-element chemical analyses indicate a predominance of low-MgO transitional quartz tholeiites at Emei and Tangfang, whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites. Consanguineity of the rocks from the three sections is suggested by consistently high TiC2, K2O, incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions, but antypical of CFB.Sr isotope data for ten Emei basalt samples (87Sr/86Sr=0.7066–0.7082) which show no correlation with Rb/Sr ratios (0.02–0.12) and Nd isotopes for two of the samples (l43Nd/144Nd= 0.51171–0.51174) are interpreted as being related to the mantle evolution. The primary magmas responsible for all the three sequences have been modeled in terms of a uniformly metasomatized mantle source. Trace element models support the derivation of the Emei and Tangfang primary magmas from 10–15 percent partial melting of spinel lherzolite. followed by fractional crystallization of olivine and clinopyroxene. The primary alkaline olivine basalts at Ertan are generated by 7–10 percent partial melting of a chemically equivalent source in the garnet-peridodite stability region. The assumed mantle composition is characterized by Rb=3.8–5.5 ppm. Sr=62–83 ppm, Ba=45–64 ppm, La=3.8–5.6 ppm. and Yb=0.46–0.57 ppm. The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par tial melting. Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diapir acts as the agent for upward enrichment as well as uplift and extension of the crust.

Extravehicular activity operations concepts under communication latency and bandwidth constraints

The Biologic Analog Science Associated with Lava Terrains (BASALT) project is a multi-year program dedicated to iteratively develop, implement, and evaluate concepts of operations (ConOps) and supporting capabilities intended to enable and enhance human scientific exploration of Mars. This paper describes the planning, execution, and initial results from the first field deployment, referred to as BASALT-1, which consisted of a series of ten simulated extravehicular activities on volcanic flows in Idahos Craters of the Moon National Monument and Preserve. The ConOps and capabilities deployed and tested during BASALT-1 were based on previous NASA trade studies and analog testing. Our primary research question was whether those ConOps and capabilities work acceptably when performing real (non-simulated) biological and geological scientific exploration under four different Mars-to-Earth communication conditions: 5 and 15 min one-way light time communication latencies and low (0.512 Mb/s uplink, 1.54 Mb/s downlink) and high (5.0 Mb/s uplink, 10.0 Mb/s downlink) bandwidth conditions, which represent two alternative technical communication capabilities currently proposed for future human exploration missions. The synthesized results, based on objective and subjective measures, from BASALT-1 established preliminary findings that the baseline ConOp, software systems, and communication protocols were scientifically and operationally acceptable with minor improvements desired by the “Mars” extravehicular and intravehicular crewmembers. However, unacceptable components of the ConOps and required improvements were identified by the “Earth” Mission Support Center. These data provide a basis for guiding and prioritizing capability development for future BASALT deployments and, ultimately, future human exploration missions.