130 research outputs found
Expanding the application of the Eu-oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior
Experimentally rehomogenized melt inclusions from the nakhlite Miller Range 03346 (MIL 03346) and the lherzolitic shergottite Allan Hills 77005 (ALH 77005) have been analyzed for their rare earth element (REE) concentrations in order to characterize the early melt compositions of these Martian meteorites and to calculate the oxygen fugacity conditions they crystallized under. D(Eu/Sm)pyroxene/melt values were measured at 0.77 and 1.05 for ALH 77005 and MIL 03346,
respectively. These melts and their associated whole rock compositions have similar REE patterns, suggesting that whole rock REE values are representative of those of the early melts and can be used as input into the pyroxene Eu-oxybarometer for the nakhlites and lherzolitic shergottites. Crystallization fO_2 values of IW + 1.1 (ALH 77005) and IW + 3.2 (MIL 03346) were calculated. Whole rock data from other nakhlites and lherzolitic shergottites was input into the Eu-oxybarometer to determine their crystallization fO_2 values. The lherzolitic shergottites and nakhlites have fO_2 values that range from IW + 0.4 to 1.6 and from IW + 1.1 to 3.2, respectively. These values are consistent
with some previously determined fO_2 estimates and expand the known range of fO_2 values of the Martian interior to four orders of magnitude. The origins of this range are not well constrained. Possible mechanisms for producing this spread in fO_2 values include mineral/melt fractionation, assimilation, shock effects, and magma ocean crystallization processes. Mineral/melt partitioning can result in changes in fO_2 from the start to the finish of crystallization of 2 orders of magnitude. In addition, crystallization of a Martian magma ocean with reasonable initial water content results in oxidized, water-rich, late-stage cumulates. Sampling of these oxidized cumulates or interactions between reduced melts and the oxidized material can potentially account for the range of fO_2 values observed in the Martian meteorites
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Implantation of a leadless pacemaker in a pediatric patient with congenital heart disease.
Correlations and zoning patterns of phosphorus and chromium in olivine from H chondrites and the LL chondrite Semarkona
Phosphorus zoning is observed in olivines in high-FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr^(3+) charge-balancing P^(5+) substituting for Si^(4+). Normal igneous zonation involving the dominant chrome species Cr^(2+) was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P-correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P-enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P-zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules
Intra- and Intercrystalline Oxygen Isotope Variations in Minerals from Basalts and Peridotites
Igneous phenocrysts commonly exhibit zoning in major and trace element composition, reflecting (and potentially constraining) the differentiation and/or mixing histories of their parent melts. To date, little work has been done characterizing zonation of oxygen isotopes in minerals from mafic and ultramafic rocks. We present 259 ion probe (CAMECA ims-1280) measurements of δ^(18)O in 34 natural magmatic and mantle olivines and pyroxenes from five hand samples from diverse igneous environments. We compare δ^(18)O variations with zonation in other elements [especially P; analyzed by electron microprobe analysis (EMPA) and nano-secondary ionization mass spectrometry (nanoSIMS)]. There is generally a close (average within ~0·1–0·2 ‰) agreement between average δ^(18)O values of olivines measured by SIMS (standardized against San Carlos olivine) and independently known values for bulk separates from the same samples measured by laser fluorination. These data demonstrate that current ion microprobe techniques are not only precise but also accurate enough for study of sub-per-mil oxygen isotope variations in silicates (within ~0·2 ‰), provided samples are prepared and analyzed following strict guidelines. All but one of the 34 studied grains are homogeneous in δ^(18)O within a small multiple of analytical precision [estimated ±0·2‰, 1σ for most data; poorer for a subset of measurements made on small (~5 µm) spots]. This population of isotopically homogeneous grains includes some with oscillatory micrometer-scale P banding. The lack of δ^(18)O variations suggests that whatever factors lead to this common mode of trace element zonation have no detectable effect on melt–crystal partitioning of oxygen isotopes. Large (2‰) oxygen isotope variations are observed in one olivine glomerocryst from Mauna Kea, Hawaii. This glomerocryst contains P-rich domains that are either equant or skeletal or feathery in outline, and these P-rich domains are systematically low in δ^(18)O compared with adjacent, later-grown, P-poor olivine. This unusual oxygen isotope zonation pattern might reflect a kinetic fractionation during nucleation and growth of the cores of some olivine phenocrysts. We tested this hypothesis through measurements of δ^(18)O distributions in synthetic olivines grown at a range of rates and exhibiting diverse patterns of P zoning. These synthetic olivines are homogeneous in δ^(18)O, within the limits of our analyses (± 0·3–0·4‰ in this case) and show no connection between P zonation and oxygen isotope heterogeneity. We therefore think it more plausible that unusual O isotope zonation in the Mauna Kea glomerocryst reflects addition of a low-δ^(18)O component to some Hawaiian magmas just before nucleation of olivine. More generally, this study demonstrates the utility of modern SIMS techniques for in situ study of the subtle (~1‰ range) oxygen isotope variations characteristic of common mafic and ultramafic rocks
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Curie: Constraining Solar System Bombardment Using In Situ Radiometric Dating
The Curie mission would constrain the existence of the putative cataclysm by determining the age of samples directly sourced from the impact melt sheet of a major pre-Imbrium lunar basin. The measurements would also enable further understanding of lunar evolution by characterizing new lunar lithologies far from the Apollo and Luna landing sites, including the very low-Ti basalts in Mare Crisium and potential olivine rich lithologies in the margins of both Mare Nectaris and Mars Crisium. Equipped with a mass spectrometer and a LIBS, Curie would also be well-placed to survey volatile components of the lunar regolith, including surface-bound hydrogen
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Identifying cryptotephra units using correlated rapid, nondestructive methods: VSWIR spectroscopy, X-ray fluorescence, and magnetic susceptibility
Understanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and incomplete due to subsequent erosion and burial processes. In contrast, the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time-consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, nondestructive, and quantitative core scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near-infrared band in tephra. Using predetermined mixtures, BDI1000VIS can accurately identify tephra layers in concentrations >15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, 29 potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near-real-time recognition of tephra units, both visible and cryptotephra.Keywords: spectroscopy, tephrochronology, magnetic susceptibility, identification, cryptotephraKeywords: spectroscopy, tephrochronology, magnetic susceptibility, identification, cryptotephr
Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust
The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification
Constraining Solar System Bombardment Using In Situ Radiometric Dating
The leading, but contentious, model for lunar impact history includes a pronounced increase in impact events at around 3.9 Ga. This late heavy bombardment would have scarred Mars and the terrestrial planets, influenced the course of biologic evolution on the early Earth, and rearranged the very architecture of our Solar System. But what if it's not true? In the last decade, new observations and sample analyses have reinterpreted basin ages and "pulled the pin" on the cataclysm - we may only have the age of one large basin (Imbrium). The Curie mission would constrain the onset of the cataclysm by determining the age of a major pre-Imbrium lunar basin (Nectaris or Crisium), characterize new lunar lithologies far from the Apollo and Luna landing sites, including the basalts in the basin-filling maria and olivine-rich lithologies in the basin margins, and provide a unique vantage point to assess volatiles in the lunar regolith from dawn to dusk
Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera
Caldera-forming eruptions of silicic volcanic systems are among the most
devastating events on Earth. By contrast, post-collapse volcanic activity
initiating new caldera cycles is generally considered less hazardous.
Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the
Kameni Volcano in the southern Aegean Sea enables the eruptive evolution
of a recharging multi-cyclic caldera to be reconstructed. Santorini’s
eruptive record has been documented by onshore products and historical
descriptions of mainly effusive eruptions dating back to 197 bce. Here we
combine high-resolution seismic reflection data with cored lithologies
from International Ocean Discovery Program Expedition 398 at four sites to
determine the submarine architecture and volcanic history of intra-caldera
deposits from Kameni. Our shore-crossing analysis reveals the deposits
of a submarine explosive eruption that produced up to 3.1 km3
of pumice
and ash, which we relate to a historical eruption in 726 ce. The estimated
volcanic explosivity index of magnitude 5 exceeds previously considered
worst-case eruptive scenarios for Santorini. Our finding that the Santorini
caldera is capable of producing large explosive eruptions at an early stage
in the caldera cycle implies an elevated hazard potential for the eastern
Mediterranean region, and potentially for other recharging silicic calderas
Rapid onset of mafic magmatism facilitated by volcanic edifice collapse: MAFIC MAGMATISM FACILITATED BY VOLCANIC EDIFICE COLLAPSE
Volcanic edifice collapses generate some of Earth's largest landslides. How such unloading affects the magma storage systems is important for both hazard assessment and for determining long-term controls on volcano growth and decay. Here we present a detailed stratigraphic and petrological analyses of volcanic landslide and eruption deposits offshore Montserrat, in a subduction zone setting, sampled during Integrated Ocean Drilling Program Expedition 340. A large (6–10 km3) collapse of the Soufrière Hills Volcano at ~130 ka was followed by explosive basaltic volcanism and the formation of a new basaltic volcanic center, the South Soufrière Hills, estimated to have initiated <100 years after collapse. This basaltic volcanism was a sharp departure from the andesitic volcanism that characterized Soufrière Hills' activity before the collapse. Mineral-melt thermobarometry demonstrates that the basaltic magma's transit through the crust was rapid and from midcrustal depths. We suggest that this rapid ascent was promoted by unloading following collapse
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