Stanley A. Mertzman

Mineralogy, composition, and alteration of Mars Pathfinder rocks and soils: Evidence from multispectral, elemental, and magnetic data on terrestrial analogue, SNC meteorite, and Pathfinder samples

Major element, multispectral, and magnetic properties data were obtained at Ares Vallis during the Mars Pathfinder mission. To understand the compositional, mineralogical, and process implications of these data, we obtained major element, mineralogical, and magnetic data for well-crystalline and nanophase ferric minerals, terrestrial analogue samples with known geologic context, and SNC meteorites. Analogue samples include unaltered, palagonitic, and sulfatetic tephra from Mauna Kea Volcano (hydrolytic and acid-sulfate alteration), steam vent material from Kilauea Volcano (hydrolytic alteration), and impactites from Meteor Crater (relithification). Salient results for Mars Pathfinder include: (1) Band depths BD530b and BD600 and the reflectivity ratio R800/R750 are consistent with the dominant ferric mineral being nanophase ferric oxide associated with an unknown amount of H2O and occurring in composite particles along with subordinate amounts of other ferric minerals. Hematite and hematite plus nanophase goethite are most consistent with the data, but maghemite, akaganeite, schwertmannite, and nanophase lepidocrocite are also possible interpretations. Ferric oxides that are consistently not favored by the data as sole alteration products are jarosites and well-crystalline goethite and lepidocrocite. (2) The strength of the ferric adsorption edge (R750/R445) implies the Fe3+/Fe2+ values for Pathfinder rock and soil are within the ranges 0.7–3 and 3–20, respectively. (3) Ferrous silicates are indicated for subsets of Pathfinder rocks and soils. One subset has a band minimum near 930 nm that can attributed to low-Ca pyroxene. Alternatively, the band could be a second manifestation of certain ferric oxides, including nanophase goethite, maghemite, akaganeite, and schwertmannite. Another subset has a negative spectral slope from ∼800 to 1005 nm which could result from the high-energy wing of a high-Ca pyroxene and/or olivine band, a mixture of bright and dark materials, and, for rocks, thin coatings of bright dust on dark rocks. (4) Chemical data on Pathfinder rocks and soils are consistent with two-component mixtures between an “andesitic” rock with low MgO and SO3 concentrations (soil-free rock) and a global, basaltic soil with high MgO and SO3 concentrations (rock-free soil). Pathfinder rock-free soil can be modeled as a chemical mixture of SNC meteorites and the Pathfinder soil-free rock. (5) Pathfinder soil cannot be obtained by chemical alteration of Pathfinder rocks by any of the hydrolytic and acid-sulfate alteration processes we studied. Presumably, global mixing has obscured and possibly erased the elemental signatures of chemical alteration. (6) The strongly magnetic phase in palagonitic and sulfatetic tephra is titanomagnetite and possibly its oxidation product titanomaghemite (Fe-Ti spinels). The saturation magnetization of the tephra samples (0.5–2.0 Am2/kg) is at or below the low end of the range inferred for Martian dust (4±2 Am2/kg), implying that lithogenic Fe-Ti spinels are a possible candidate for the Martian strongly magnetic phase. (7) The predominantly palagonitic spectral signature and magnetic nature of Martian soil and dust are consistent with glassy precursors with imbedded Fe-Ti spinel particles. Comparison with lunar glass production rates suggests that production of sufficient quantities of glassy materials on Mars by volcanic and impact processes is sufficient to account for these observations.

Areal distribution and age of low-K, high-alumina olivine tholeiite magmatism in the northwestern Great Basin

Field, petrographic, chemical, and geochronologic information has led to the identification and characterization of a widespread low-K, high-alumina olivine tholeiite (HAOT) magma type in the northwestern Great Basin. This basalt covers at least 22,000 km 2 and is estimated to represent a total volume of at least 650 km 3 . The time period over which HAOT lavas were erupted extends from late Miocene to Holocene (10.5–0 m.y. B.P.). This interval overlaps with the timing of Snake River, Cascade, and northwestern Basin and Range volcanism but distinguishes HAOT from the main pulse of Columbia River volcanism (∼15 m.y. B.P.). Furthermore, three major pulses of HAOT magmatism are suggested from the geochronology of this study: 0 to 2.5 m.y. B.P., 3.5 to 6 m.y. B.P., and 7 to 10 m.y. B.P. The distinctive holocrystalline, nonporphyritic, and diktytaxitic texture, the low incompatible-element concentrations, and the high MgO/FeO* of HAOT serve to distinguish this basalt from other basalts of the northwestern United States. The low incompatible-element signature accentuates the similarities between HAOT, mid-ocean–ridge basalts, and back-arc–basin basalts. These similarities, combined with the HAOT chronology, support the idea that the processes giving rise to extensional tectonism and HAOT magmatism in the northwestern Great Basin are similar to those acting in active back-arc–spreading regions.

Low-calcium garnet harzburgites from southern Africa: their relations to craton structure and diamond crystallization

Low-Ca garnet harzburgite xenoliths contain garnets that are deficient in Ca relative to those that have equilibrated with diopside in the iherzolite assemblage. Minor proportions of these harzburgites are of wide-spread occurrence in xenolith suites from the Kaapvaal craton and are of particular interest because of their relation to diamond host rocks. The harzburgite xenoliths are predominantly coarse but one specimen from Jagersfontein and another from Premier have deformed textures similar to those of high-temperature peridotites. Analyses for many elements in the harzburgites and associated iherzolites form concordant overlapping trends. On the average, however, the harzburgites are deficient in Si, Ca, Al and Fe but enriched in Mg and Ni relative to the lherzolites. Both the harzburgites and lherzolites are enstatite-rich with mg numbers [100.Mg/(Mg+Fetotal)] greater than 92 and in these respects differ markedly from residues generated by extraction of MORB. Equilibration temperatures and depths calculated for the harzburgites have the ranges 600–1,400°C and 50–200 km. Those of deepest origin overlap the interval between low-and high-temperature lherzolites that commonly is observed in temperature-depth plots for the Kaapvaal craton, suggesting that some harzburgites may be concentrated relative to lherzolites at the base of the lithosphere. The low-Ca harzburgites and lherzolite xenoliths have overlapping depths of origin, gradational bulk chemical characteristics and similar textures, and therefore both are believed to have formed as residues of Archaen melting events. The harzburgites differ from the lherzolites only in that they are more depleted. Garnets and associated minerals in harzburgite xenoliths differ from minerals of the same assemblage that are included in diamonds in that the latter are more Cr-rich, Mg-rich and Ca-poor. Coarse crystals of low-Ca garnet with the compositional characteristics of diamond inclusions commonly occur as disaggregated grains in diamondiferous kimberlites. Their host rocks are presumed to have been harzburgites and dunites. The differences in composition between the disaggregated grains that are similar to diamond inclusions and those comprising xenoliths imply some differences in origin. Possibly the disaggregated harzburgites with diamond-inclusion mineralogy have undergone repeated partial melting and depletion near the base of the lithosphere subsequent to their primary depletion and aggregation in the craton. Equilibration with magnesite may have reduced the Ca contents of their garnets and decomposition of the magnesite during eruption may have caused their disaggregation.

WATER-ROCK INTERACTIONS, ORTHOPYROXENE GROWTH, AND SI-ENRICHMENT IN THE MANTLE : EVIDENCE IN XENOLITHS FROM THE COLORADO PLATEAU, SOUTHWESTERN UNITED STATES

Water–rock interactions and consequent orthopyroxene growth are documented by mantle xenoliths from opposite margins of the Colorado Plateau province. The interactions are inferred from a distinctive texture plus composition of orthopyroxene in spinel peridotite, in which porphyroblasts of orthopyroxene with inclusions of resorbed olivine are zoned to interiors exceptionally low in Al2O3 (<0.20 wt%). This texture plus composition has been found in xenoliths from Bandera Crater, New Mexico, in the southeastern margin of the Colorado Plateau, extending the known distribution from the northwestern margin (Grand Canyon field). Evidence that Si-enrichment locally accompanied movement of aqueous fluid is provided by an orthopyroxenite xenolith that is about 95% enstatite. The enstatite occurs in curved laths to 25 mm long, and the texture and composition (Al2O3 1 to 2 wt%, Mg/(Mg + Fe) 0.92) are attributed to growth during subsolidus interaction between peridotite and hydrous fluid. Modal orthopyroxene calculated from 4 bulk rock analyses of peridotite xenoliths from the Grand Canyon field ranges from 26 to 29%, more than in comparably depleted oceanic mantle. The mantle root of the Colorado Plateau may have formed from accreted ocean lithosphere and subsequently been enriched in Si by aqueous metasomatism at widely distributed sites. Similar fluid–rock interaction may have contributed to the orthopyroxene-enrichment characteristic of some mantle xenoliths from roots of Archaean cratons.

Phyllosilicate-poor palagonitic dust from Mauna Kea Volcano (Hawaii): A mineralogical analogue for magnetic Martian dust?

The mineralogical and elemental composition of dust size fractions (<2 and <5 μm) of eight samples of phyllosilicate-poor palagonitic tephra from the upper slopes of Mauna Kea Volcano (Hawaii) were studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), visible and near-IR reflectance spectroscopy, Mossbauer spectroscopy, magnetic properties methods, and transmission electron microscopy (TEM). The palagonitic dust samples are spectral analogues of Martian bright regions at visible and near-IR wavelengths. The crystalline phases in the palagonitic dust are, in variable proportions, plagioclase feldspar, Ti-containing magnetite, minor pyroxene, and trace hematite. No basal reflections resulting from crystalline phyllosilicates were detected in XRD data. Weak, broad XRD peaks corresponding to X-ray amorphous phases (allophane, nanophase ferric oxide (possibly ferrihydrite), and, for two samples, hisingerite) were detected as oxidative alteration products of the glass; residual unaltered glass was also present. Mossbauer spectroscopy showed that the iron-bearing phases are nanophase ferric oxide, magnetite/titanomagnetite, hematite, and minor glass and ferrous silicates. Direct observation by TEM showed that the crystalline and X-ray amorphous phases observed by XRD and Mossbauer are normally present together in composite particles and not normally present as discrete single-phase particles. Ti-bearing magnetite occurs predominantly as 5–150 nm particles embedded in noncrystalline matrix material and most likely formed by crystallization from silicate liquids under conditions of rapid cooling during eruption and deposition of glassy tephra and prior to palagonitization of glass. Rare spheroidal halloysite was observed in the two samples that also had XRD evidence for hisingerite. The saturation magnetization Js and low-field magnetic susceptibility for bulk dust range from 0.19 to 0.68 Am2/kg and 3.4×10−6 to 15.5×10−6 m3/kg at 293 K, respectively. Simulation of the Mars Pathfinder Magnet Array (MA) experiment was performed on Mauna Kea Volcano in areas with phyllosilicate-poor palagonitic dust and with copies of the Pathfinder MA. On the basis of the magnetic properties of dust collected by all five MA magnets and the observation that the Pathfinder MAs collected dust on the four strongest magnets, the value for the saturation magnetization of Martian dust collected in the MA experiments is revised downward from 4±2 Am2/kg to 2.5±1.5 Am2/kg. The revised value corresponds to 2.7±1.6 wt % magnetite if the magnetic mineral is magnetite (using Js = 92 Am2/kg for pure magnetite, Fe3O4) or to 5.0±3.0 to 3.4±2.0 wt % maghemite if the magnetic mineral is pure maghemite (using Js = 50 to 74 Am2/kg for pure maghemite, γ-Fe2O3). Comparison of the magnetic properties of bulk Mauna Kea palagonitic dust to those for dust collected by MA magnets shows that the MA magnets extracted (culled) a subset (25–34 wt %) of composite magnetic particles from bulk dust. The extent of culling of Martian dust is not well constrained. Because the Mauna Kea palagonitic dust satisfies the essential constraints of the Pathfinder magnetic properties experiment (composite and magnetic particles capable of being collected by five MA magnets), a working hypothesis for the strongly magnetic mineral present in Martian dust and soil is magnetite (possibly Ti-bearing) formed by rapid crystallization from silicate liquids having volcanic and/or impact origins. Subsequent palagonitization of the glass produces the nanophase ferric oxide phases that dominate the spectral properties of Martian bright regions at visible and near-IR wavelengths. Magnetic and phyllosilicate-poor palagonitic dust from Mauna Kea Volcano is thus a spectral and magnetic analogue for magnetic Martian dust.

Geochemical and Sr-Nd isotopic characteristics of granitic rocks from northern Vietnam

Abstract Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [eNd(T)=+0.7 to +1.5] and older Nd isotopic model ages (TDM∼1.7 Ga) than the latter [eNd(T)=−4.7 to −9.7; TDM∼1.3–1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan–Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate eNd(T) values (+0.6 to −2.8) and younger TDM ages (0.6–1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan–Red River shear zone related to the India–Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks.

Laboratory Simulated Acid-Sulfate Weathering of Basaltic Materials: Implications for Formation of Sulfates at Meridiani Planum and Gusev Crater, Mars

[1] Acid-sulfate weathering of basaltic materials is a candidate formation process for the sulfate-rich outcrops and rocks at the MER rover Opportunity and Spirit landing sites. To determine the style of acid-sulfate weathering on Mars, we weathered basaltic materials (olivine-rich glassy basaltic sand and plagioclase feldspar-rich basaltic tephra) in the laboratory under different oxidative, acid-sulfate conditions and characterized the alteration products. We investigated alteration by (1) sulfuric-acid vapor (acid fog), (2) three-step hydrothermal leaching treatment approximating an open system, and (3) single-step hydrothermal batch treatment approximating a closed system. In acid fog experiments, A1-, Fe-, and Ca-sulfates and amorphous silica formed from plagioclase-rich tephra, and Mg- and Ca-sulfates and amorphous silica formed from the olivine-rich sands. In three-step leaching experiments, only amorphous Si formed from the plagioclase-rich basaltic tephra, and jarosite, Mg-, and Ca-sulfates and amorphous silica formed from olivine-rich basaltic sand. Amorphous silica formed under single-step experiments for both starting materials. On the basis of our experiments, jarosite formation in Meridiani outcrop is potential evidence for an open system acid-sulfate weathering regime. Waters rich in sulfuric acid percolated through basaltic sediment, dissolving basaltic phases (e.g., olivine) and forming jarosite, other sulfates, and iron oxides. Aqueous alteration of outcrops and rocks on the West Spur of the Columbia Hills may have occurred when vapors rich in SO 2 from volcanic sources reacted with basaltic materials. Soluble ions from the host rock (e.g., olivine) reacted with S to form Ca-, Mg-, and other sulfates along with iron oxides and oxyhydroxides.

Major element geochemistry and geomorphic relationships in Brazilian Cerrado soils

Abstract A thorough understanding of major and trace element geochemistry is important in assessing the impacts of rapid agriculturalization and increased human occupation of the Cerrado region of central Brazil, a region with some of the oldest soils on Earth. The objectives of this paper are to summarize the major element composition of Cerrado soils in the context of their morphology and geomorphology, to describe impacts of biogeochemical cycling on the chemical properties of Cerrado soils, and to use elemental data to provide insights on the possible origin of soil parent materials on the South American erosion surface and on the Serra Geral Basalts. We sampled three major subregions selected to be representative of the main areas of the Cerrado, namely: (i) eastern Goias, (ii) northwestern Minas Gerais, and (iii) the Triângulo Mineiro area of western Minas Gerais. Five soils were sampled in each subregion, each represented by three pedons sampled at 0–0.2 and 0.8–1.0 m depths. Geomorphologically, the soils were on the South American, Velhas I, and Velhas II erosion surfaces. All sites were carefully selected to minimize potential anthropogenic contamination. General chemical characterization was by standard procedures and total major and trace elemental contents were quantified using wavelength-dispersive X-ray fluorescence spectroscopy. The soils ranged from loamy sands to clays, with Si (142–947 g kg −1 SiO 2 ), Al (14–439 g kg −1 Al 2 O 3 ), Fe (10–331 g kg −1 Fe 2 O 3 ), and Ti (3–101 g kg −1 TiO 2 ) contents closely following the clay contents. Organic C averaged 17 g kg −1 for the surface, and 7 g kg −1 for the subsurface horizons. The Na, Ca, Mg, and K contents were very low, tended to be higher in surface than in subsurface horizons because of recycling by vegetation and addition due to atmospheric deposition, and were higher in soils containing hard plinthite nodules. The Th/Zr ratios suggest that the parent materials of the clayey soils on the South American erosion surface have a common origin in sediments from rocks in the Central Plateau of Brazil that were transported eastwards to cover the underlying sandstone. Trace element signatures indicate that basalt-derived soils occurring at different elevations in the Triângulo Mineiro area formed from distinctly different magmas that may have been deposited in different basalt flow events.

Subduction-related granitic rocks of Taiwan

Abstract Late Cretaceous granitic rocks constitute an essential part of the pre-Tertiary Tananao metamorphic basement complex of Taiwan. They are dominantly of granodiorite to quartz monzonite composition. Most granitic rocks are peraluminous (A/CNK > 1.0 and normative corundum > 1%) and display moderately fractionated LREE and relatively unfractionated HREE patterns with negative Eu anomalies. On a primitive mantle-normalized trace-element diagram, they show a significant Nb depletion which is typical of the calc-alkaline magmatism from the subduction-zone environment. They fall within the volcanic arc field on the discrimination diagram of Pearce, Harris and Tindle (1984). The lack of systematic inter-element relationships suggests that the role of fractional crystallization is not significant and that these granitic rocks were derived from heterogeneous protoliths. Geochemical data suggest Taiwan granitic rocks are contaminated I-type and I-type granites related to the subduction of the Paleo-Pacific plate beneath the eastern margin of the Eurasia plate during late Mesozoic time.

Nd-Sr-O isotopic evidence for source contamination and an unusual mantle component under Luzon Arc

Abstract The Nd, Sr, and O isotopic compositions of 27 volcanic rocks from the northern Luzon Arc were determined. Their ϵNd and 87 Sr 86 Sr displayed large correlated variations (−6.1 to +9.5, 0.70335 to 0.70622). Except for three abnormal samples, the δ 18O (6.0 to 8.5%.) show an excellent correlation (r = 0.9) with the volatile contents. After correcting for the alteration effect using the correlation with volatile contents, the δ 18O range from 5.2 to 6.4%. and are close to values shown by primary mantle-derived igneous rocks. The extreme deviation of the Sr and Nd isotopic ratios away from those of the depleted mantle coupled with a mantle oxygen isotopic ratio demonstrates unequivocally that the magma of Luzon Arc was contaminated not in the crust but in its source zone by an enriched mantle component. This enriched mantle component cannot be entirely ascribed to subducted sediment if these sediments have compositions similar to that of the nearby continental crust in Taiwan or to the Pleistocene sediment being subducted presently at the Manila Trench. Additional contribution from an enriched mantle component with isotopic composition similar to that of the EMI component proposed by Zindler and Hart (1986) is required. Published Pb isotopic results (Sun, 1980) are consistent with the involvement of EMI. Such an EMI component has never been found before in an island arc setting. Ultramafic xenolith results ( Richard , 1986a,b) suggest that this enriched mantle was formed in the mantle wedge via metasomatism by fluids possibly derived from the downgoing slab. However, the involvement of the lithosphere of the approaching Eurasia continent cannot be excluded.