Determining fluid source and possible pathways during burial dolomitization of Maryville Limestone (Cambrian), Southern Appalachians, USA

Detailed petrographic analyses along a depositional transect from a carbonate platform to shale basin reveals that dolomite is the principal burial diagenctic mineral in the Maryville Limestone. This study examines the role of burial dolomitization of subtidal carbonates. Dolomite occurs as a replacement of precursor carbonate and as inter- and intraparticle cements. Four different types of dolomite are identified based on detailed petrographic and gcochemical analyses. Type I dolomite occurs as small, irregular disseminations typically within mud-rich facies.Type II dolomite typically occurs as inclusions of planar euhedral rhombs (ferroan), 5–300 μm in size, in blocky clear ferroan calcite (meteoric) spar. Type II dolomite is non-luminescent. Type I and II dolomite formed during shallow to intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals 10–150 μm in size occurring as thin seams along stylolites and as thick bands a few millimetres in width. This dolomite consists of dominantly non-luminescent rhombs and, less commonly, orange luminescent and zoned rhombs. Type IV dolomite consists of baroque or saddle-shaped, 100–1500 μm crystals, and is non-luminescent. Type IV dolomite formed during the period of maximum burial. Types III and IV dolomite increase in abundance downslope. Type III dolomite contains 1.2–2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends, decreasing from 3.5–4.5 wl% Fe and 0.1–0.3 wt% Mn in the west (slope/basin) to 1.5–2.5 wt% Fe and less than 600 ppm Mn in the east (shelf margin), a distance of approximately 60 km. Spatial trends in Fe and Mn distributions in Type IV saddle dolomite, suggest a west-east fluid flow during late burial diagenesis. Types III and IV dolomite have a mean δ¹⁸O value of - 7.8%₀₀ and a mean δ¹³C value of + 1.1%₀₀ (relative to the PDB standard). Based on a range of assumed basinal water composition of 2.8%₀₀ SMOW, temperatures calculated from δ¹⁸O values of Types III and IV dolomite range between 75 and 160°C. ⁸⁷Sr/⁸⁶Sr data for Types III and IV dolomite range from 0.7111 to 0.7139. These values are radiogenic when compared to Cambrian marine values and are consistent with the presence of a diagenetic fluid that interacted with siliciclastic sediments. The distribution of Palaeozoic facies in the southern Appalachians indicates a Cambrian shale source for the fluids, whilst burial curves suggest a Middle Ordovician age for burial fluid movement.     

Variation of Flow Field Around Twin Cylinders with and Without the Outer Perforated Cylinder ? Numerical Study

Presence of the outer perforated cylinder reduces the direct wave impact on the inner cylinder, which has been testified by many researchers. However, the force reduction mechanism, which is complicated due to the wave-porous structure interaction, needs to be addressed in detail. The present study explains the mechanism with the aid of the computational fluid dynamics (CFD) tool STAR CCM+. This package is chosen for its capabilities to simulate viscous and turbulence effects caused by passage of waves. For the present study, flow fields around the twin cylinders with different orientations are examined with and without the outer perforated cover. Mechanism contributing to the reduction of force on the existing structure is explained in physical terms, and force reduction is quantified. The present study has direct application in the retrofitting application of offshore members.     

Devgaon (H3) chondrite: Classification and complex cosmic ray exposure history

Abstract— The Devgaon meteorite fell in India on February 12, 2001 and was immediately collected. It is an ordinary chondrite having a number of SiO₂‐rich objects and some Ca, Al‐rich inclusions. Olivines (Fa_17–19) are fairly equilibrated, while pyroxenes (Fs_4–20) are unequilibrated. Occasionally, shock veins are visible, but the bulk rock sample is very weakly shocked (S2). Chondrules and chondrule fragments are abundant. Based on chemical and petrological features, Devgaon is classified as an H3.8 group chondrite. Several cosmogenic radionuclides ranging in half‐lives from 5.6 d (⁵²Mn) to 7.3 times 10⁵ yr (²⁶Al), noble gases (He, Ne, Ar, Kr, and Xe), and particle track density have been measured. The track density in olivines from five spot samples varies between (4.6 to 9) × 10⁶ cm^?2 showing a small gradient within the meteorite. The light noble gases are dominated by cosmogenic and radiogenic components. Large amounts of trapped gases (Ar, Kr, and Xe) are present. In addition, (n, γ) products from Br and I are found in Kr and Xe, respectively. The average cosmic ray exposure age of 101 ± 8 Ma is derived based on cosmogenic ³⁸Ar, ⁸³Kr, and ¹²⁶Xe. The track production rates correspond to shielding depths of about 4.9 to 7.8 cm, indicating that the stone suffered type IV ablation. Low ⁶⁰Co, high (²²Ne/²¹Ne)_c, and large neutron produced excesses at ⁸⁰Kr, ⁸²Kr, and ¹²⁸Xe indicate a complex exposure history of the meteoroid. In the first stage, a meter‐sized body was exposed for nearly 10⁸ yr in the interplanetary space that broke up in ?50 cm‐sized fragments about a million years ago (stage 2), before it was captured by the Earth.     

Pb–Pb zircon ages of Archaean metasediments and gneisses from the Dharwar craton,southern India: Implications for the antiquity of the eastern Dharwar craton

²⁰⁷Pb–²⁰⁶Pb ages of zircons in samples of metasediments as well as ortho- and para-gneisses from both the western and the eastern parts of the Dharwar craton have been determined using an ion microprobe. Detrital zircons in metasedimentary rocks from both yielded ages ranging from 3.2 to 3.5 Ga. Zircons from orthogneisses from the two parts also yielded similar ages. Imprints of younger events have been discerned in the ages of overgrowths on older zircon cores in samples collected throughout the craton. Our data show that the evolution of the southwestern part of eastern Dharwar craton involved a significant amount of older crust (>3.0 Ga). This would suggest that crust formation in both the western and eastern parts of the Dharwar craton took place over similar time interval starting in the Mesoarchaean at ca. 3.5 Ga and continuing until 2.5 Ga. Our data coupled with geological features and geodynamic setting of the Dharwar craton tend to suggest that the eastern Dharwar craton and the western Dharwar craton formed part of a single terrane.     

Aluminum‐26 in calcium‐aluminum‐rich inclusions and chondrules from unequilibrated ordinary chondrites

Abstract— In order to investigate the distribution of ²⁶A1 in chondrites, we measured aluminum‐magnesium systematics in four calcium‐aluminum‐rich inclusions (CAIs) and eleven aluminum‐rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic ²⁶Mg (²⁶Mg*) from the decay of ²⁶A1. The inferred initial ²⁶Al/²⁷Al ratios for these objects ((²⁶Al/²⁷Al)₀ ? 5 × 10^?5) are indistinguishable from the (²⁶Al/²⁷Al)₀ ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found ²⁶Mg* in two of eleven aluminum‐rich chondrules. The (²⁶Al/²⁷Al)₀ ratio inferred for both of these chondrules is ～1 × 10^?5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0–3.1 UOCs and for aluminum‐rich chondrules from lightly metamorphosed carbonaceous chondrites (～0.5 × 10^?5 to ～2 × 10^?5). The consistency of the (²⁶Al/²⁷Al)₀ ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad‐scale nebular homogeneity with respect to ²⁶Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on ²⁶Al indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with ²⁶Al as a principal heat source for melting and metamorphism.     

A petrographic,chemical, and isotopic study of calcium‐aluminum‐rich inclusions and aluminum‐rich chondrules from the Axtell (CV3) chondrite

Abstract— Petrographic, compositional, and isotopic characteristics were studied for three calcium‐aluminum‐rich inclusions (CAIs) and four plagioclase‐bearing chondrules (three of them Al‐rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial ²⁶Al/²⁷Al ratios ((²⁶Al/²⁷Al)₀) ranging from ～5 × 10^?5 to <1.1 × 10^?5, and plagioclase‐bearing chondrules have (²⁶Al/²⁷Al)₀ ratios of ～3 × 10^?6 and lower. One type‐A CAI has the characteristics of a FUN inclusion. The Al‐Mg data imply that the plagioclase‐bearing chondrules began to form >2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al‐Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion. We also examined the compositional relationships among the four plagioclase‐bearing chondrules (two with large anorthite laths and two barred‐olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH‐1471, but neither process explains the other compositions. Our study indicates that plagioclase‐bearing objects originated through a variety of processes.