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Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.


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TL;DR: In this paper, the authors report field, petrographic, mineralogical, and geochemical data for two P-rich rock types of the anorthosite suite (Fe-Ti oxide-apatite rock (or nelsonite) and Fe-Ti o- oxide-obatite gabbronorite (or oxide-abatitic gabbricite) from the Château-Richer, Labrieville, and St. Urbain massifs in Quebec; from the Roseland massif in Virginia; and from
Abstract: We report field, petrographic, mineralogical, and geochemical data for two P-rich rock types of the anorthosite suite—Fe-Ti oxide-apatite rock (or nelsonite) and Fe-Ti oxide-apatite gabbronorite (or oxide-apatite gabbronorite). These rocks are take from the Château-Richer, Labrieville, and St. Urbain massifs in Quebec; from the Roseland massif in Virginia; and from the Carthage massif in New York. Our data set includes X-ray fluorescence (XRF) and instrumental neutron activation (INA) analyses of whole-rock samples, as well as electron-microprobe (EMP) and INA analyses of separated apatites. At St. Urbain and Labrieville, nelsonite forms a small capping on massive hemoilmenite deposits fully contained within anorthosite, whereas at Roseland, nelsonite occurs as dikelike bodies within country rock. All three nelsonite occurrences consist almost entirely of ilmenite + apatite (but do not have a 2:1 oxide/apatite ratio), with only trace amounts of other minerals. All oxide-apatite gabbronorites form sill-like layers within or near the margins of anorthosite massifs and contain ilmenite ± magnetite, apatite, orthopyroxene + clinopyroxene, and plagioclase. All apatites are F rich and Cl poor, consistent with high-temperature magmatic crystallization. Concentrations of apatite rare earth elements (REE) are highly variable (e.g., LaN ≈ 700–2,000× chondrites), but patterns show a consistent fractionation (e.g., LaN/LuN ≈ 10–20) with small to moderate negative Eu anomalies. Interestingly, the Labrieville nelsonite and its apatite lack an Eu anomaly, consistent with crystallization in a highly oxidizing environment. Apatites in both rock types are enriched in REE relative to whole-rock samples (by factors of 3 to 10), but REE patterns of apatites and whole rocks are parallel in each case. Mass-balance calculations indicate that for nelsonite, 100 percent of the REE is contained in apatite; for oxide-apatite gabbronorites, ~100 percent of the light REE, ~90 percent of the middle to heavy REE, and ~75 percent of Eu are in apatite. These results can be explained satisfactorily by assuming that the rocks are cumulate in origin. If, on the other hand, nelsonites and oxide-apatite gabbronorites represent liquids, apatite DREE values must vary by more than a factor of 3. Our field, mineralogical, and geochemical observations indicate that most nelsonites and oxide-apatite gabbronorites are not related directly. Indeed, apatites from oxide-apatite gabbronorites contain lower Sr and MgO but higher REE than those in nelsonites, indicating that nelsonites are the more primitive rock type. Nelsonites probably formed as cumulates primarily within larger deposits of Fe-Ti oxide (locally mobilized as dikelike bodies). oxide-apatite gabbronorites also represent cumulates, but these are related spatially and genetically to associated jotunites and mangerites and appear to form at several stages in the evolution of a particular anorthosite complex.

82 citations

Journal ArticleDOI
TL;DR: Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits.
Abstract: Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits. The chemical remanence in the sediments is carried predominantly by three authigenic minerals — hematite, goethite, and a Mn · Ba compound, herein called “hydropsilomelane” — that occur as pigments and as concretions. Remanence directions generally associated with each mineral are goethite, reverse; hydropsilomelane, normal; and hematite, normal or reverse. Some samples possess a remanence that is strong and normal or strong and reverse; these generally contain only one of the authigenic minerals in abundance. Many samples, however, are weak in intensity and random in direction. When such samples are split into parts and measured, it is generally found that each part is strongly magnetized but that some parts are normal and others are reverse in direction. In such cases, the magnetization of the whole sample is resultant of the multiple components that are generally carried by two or more of the authigenic minerals. The following conclusions can be drawn concerning the acquisition of CRM in the Baja California deposits: (1) The sediments contain chemically unstable iron- and manganese-bearing minerals, such as hornblende and biotite, that are susceptible to postdepositional alteration, and they have provided the parent material for the authigenic magnetic minerals. (2) Authigenic magnetic minerals, growing from crystallites, generally acquired a remanence that was parallel to the Earth9s field when they surpassed the critical grain size. (3) The rate of CRM acquisition has not been uniform in these sediments, probably because the processes of alteration and formation of authigenic magnetic minerals depend on an interplay of highly variable factors such as the chemistry of the interstitial water, hydraulic gradients, and mineralogy of the sediments. (4) Complex variability in acquisition of CRM has led to a remanence stratigraphy that bears little discernible correlation with the geomagnetic field at the time of deposition.

82 citations

Journal ArticleDOI
TL;DR: In this article, the authors used maceral and lithotype variations to define compositional characteristics of the various lithotypes and to assess coal facies changes within the seams based on maceral analysis.

82 citations

Journal ArticleDOI
TL;DR: In this article, the lower Cretaceous strandplain coals formed behind an active shoreline in areas undergoing subsidence due to shale compaction and dewatering, and the zone of peat accumulation was remote from the shoreline and storm/tidal inundations and generally protected from fluvial flooding.

82 citations

Journal ArticleDOI
TL;DR: Partitioning behavior of Sc, Ti, V, Mn, Sr, Y, Zr, Nb, Ba, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb, Hf, and Pb between dacitic silicate melt and clinopyroxene, orthopyroxenes, and plagioclase has been determined based on laser ablation-inductively coupled plasma mass spectrometric (LA-ICPMS) analysis of melt inclusions and the immediately adjacent host mineral as mentioned in this paper.

81 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023551
20221,098
2021370
2020344
2019310
2018291