No description available.

Cameron's Line in western Connecticut and its equivalents in western Massachusetts and Vermont mark a major tectonic boundary in the New England Appalachians and are considered by many to delineate the trace of the Taconian suture zone. The Cambro-Ordovician Ratlum Mountain and Rowe Formations lie to the east of Cameron's Line in western Connecticut. Mineral equilibria and compositional zoning studies for pelitic units of the Ratlum Mountain and Rowe Formations indicate that garnet porphyroblast rims equilibrated with matrix minerals at P-T conditions that are consistent with an Acadian (Devonian) metamorphic field gradient for this area. P-T conditions were calculated by simultaneously solving for a geothermometer and a geothermobarometer in low-variance assemblages. All of the possible equilibria for each sample were then calculated using an internally consistent thermodynamic database with the software TWEEQU (thermobarometry with estimations of equilibria). Both of these methods produced consistent results with peak P-T conditions of 575-650 °C and 6-9 kbars. Wavelength dispersive X-ray compositional images (WDS images) along with quantitative traverses for major (Fe, Mg, Mn and Ca) elements were collected for garnet porphyroblasts in each of the samples. Trace (Y, P, Sc, Ti and Cr) element WDS images and quantitative traverses were then collected for representative samples. Petrographic observations coupled with WDS compositional imaging show that early garnet porphyroblasts have been modified either by overgrowths of biotite and chlorite or by a second phase of garnet growth. For those garnets that show two phases of growth, compositional images reveal patchy Ca content in rounded overgrowths surrounding more homogeneous euhedral cores. This is consistent with a second phase of growth of garnet at higher pressure accompanied by anorthite breakdown. Yttrium compositional images for these sample show a very narrow band of Y enrichment that lies just inside these high-Ca overgrowths. Garnets that have been overgrown by biotite have compositional images and quantitative traverses indicative of continuous prograde growth with minor resetting of chemistry along garnet rims. These overgrowths are thought to be the result of the compositionally complex continuous reaction, Grt + Ms -> An + Bt. Although Y images for these samples show a similar narrow band of Y enrichment, in these samples this band of enrichment lies within a few microns of the garnet rim. Biotite overgrowths truncate compositional zoning for both major and trace elements and therefore postdates porphyroblast growth. Compositional images for these samples generally show euhedral zoning patterns suggesting that diffusion was very limited both during and after growth. Peak P-T conditions are consistent with previously documented conditions for this region in that they suggest a predominantly Acadian metamorphic signature. There is strong evidence that garnet porphyroblast cores predate these overgrowth textures and thus may represent a resetting of Taconian garnet cores with matrix minerals during the Acadian. Maximum P-T conditions (600 ° C and 9 kbars) were obtained from a sample collected in close proximity to Cameron's Line. If these P-T conditions represent the thermal maximum accompanying Acadian metamorphism then there is likely also some resetting of mineral assemblages to the west of Cameron's Line. / Master of Science

Thesis (M.S.)--University of Montana, 2008. / Title from title screen. Description based on contents viewed Aug. 28, 2008. Includes bibliographical references (p. 95-100).

Thesis (Ph.D.) - James Cook University, 2004. / Typescript (photocopy) Bibliography: leaves 89-97 (volume 1)

Thesis (M.S.)--Western Washington University, 2010. / Includes bibliographical references (leaves 38-40). Also issued online.

No description available.

No description available.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Insets of large anhedral minerals in kimberlites are proposed to mainly have their origin from the disaggregation of mantle-derived xenoliths through mechanical abrasion by the ascending kimberlite magma. Garnet, despite being an important constituent of both mantle-derived peridotite and eclogite, forms only a minor constituent of kimberlites. This suggests that a large proportion of garnet liberated into the kimberlite through the disaggregation of such xenoliths, is consumed before the emplacement of kimberlite. This study records the breakdown mechanism of garnet by the ascending kimberlite magma by conducting experiments between pressures of 2 and 4 GPa and temperatures between 1100 and 1300°C. The starting material used in the experiments was prepared from a natural hypabyssal kimberlite that closely resembles the composition of a proposed primary Group I kimberlite magma. To the kimberlite material 5 wt % garnet, sourced from a natural peridotite, was added. It was found that garnet is not a stable equilibrium phase within the kimberlite magma under the investigated temperature and pressure conditions. Based on large volumes of phlogopite in the experiments it is concluded that garnet melts incongruently in the kimberlite magma to form phlogopite as a peritectic product and melts transitional in composition between silicate and carbonate melts. This is in contrast to more SiO2 -rich melt compositions produced as a consequence of the incongruent breakdown of both orthopyroxene and omphacite within kimberlite magmas. The consequence of the melt compositions produced in the experiments is increased solubility of CO2 in the form of carbonate (CO32-). This finding has implications for both kimberlite ascent mechanisms and the solubility of diamond transported within kimberlite magmas. / AFRIKAANSE OPSOMMING: Dit word voorgestel dat groot oneenvormige mineraalinsluitsels in kimberliet hoofsaaklik afkomstig is van xenolitiese mantelmateriaal wat verbrokkel weens die meganiese werking van die stygende kimberliet magma. Ten spyte daarvan dat granaat ‘n belangrike komponent is van peridotiet en eklogiet xenoliete afkomstig uit die mantel, vorm granaat slegs ‘n geringe deel van die kimberliet mineraalsamestelling. Dit dui daarop dat ‘n beduidende gedeelte van die granaat wat vrygstel word in die kimberliet magma deur die verbrokkeling van xenolitiese materiaal, afgebreek word deur die stygende kimberliet magma voordat dit stol. Hierdie studie ondersoek die afbreekmeganisme van granaat afkomstig van die mantel in die stygende kimberliet magma deur eksperimente uit te voer by drukke tussen 2 en 4GPa en temperature tussen 1100 en 1300°C. Die materiaal met die aanvang van die eksperimente was voorberei uit ‘n natuurlike hipabissale kimberliet wat soortgelyk is in samestelling as ‘n voorgestelde primêre Groep I kimberliet magma se samesteling. Vyf gewigspresent (5 wt %) granaat, verkry van n natuurlike peridotiet, is bygevoeg tot die kimberlietmateriaal. Daar is gevind dat granaat nie ‘n stabiele ewewigsfase is in die kimberliet magma binne die temperatuur-en drukstoestande ondersoek nie. Groot volumes flogopiet teenwoordig in die eksperimente dui daarop dat granaat inkongruent smelt in die kimberliet magma om flogopiet as ‘n peritektiese produk te vorm te same met ‘n smelt wat neig na ‘n karbonatitese smeltsamestelling. Die bevinding is in teenstelling met meer SiO2-ryke smeltsamestellings as gevolg van die inkongruente afbreek van beide ortopirokseen en omfasiet in die kimberliet magma. Die gevolg van die meer karbonatitese smeltsamestellings, is ‘n toename in die oplosbaarheid van CO2 in die smelt in die vorm van karbonaat (CO32-). Die bevinding het nagevolge vir beide kimberliet stygingsmeganismes asook die oplosbaarheid van diamant tydens die vervoer van diamant deur kimberliet magmas.

This study utilizes recent advances in the analysis of neodymium (Nd) and samarium (Sm) by thermal ionization mass spectrometry (TIMS) to constrain timescales of heating and fluid flow within the lithosphere. Garnet grows during metamorphism and can be linked to its pressure and temperature of growth, while carbonate mineralization ages can be linked to hydrothermal fluid flow. The ability to date these common minerals makes it possible to unravel the timing and duration of tectonometamorphic processes. Garnet from the Eoarchean Nuvvuagittuq Supracrustal Belt (NSB) in Québec, Canada yields an age of 2574.66 ± 0.72 Ma. This is the most precise Neoarchean age reported for this terrane and was achieved through a modified partial dissolution procedure designed to remove the effects of much older (up to 3.8 Ga) inherited mineral growth. An accurate age for the most recent metamorphic garnet in the NSB is critical, as the (controversial) Hadean Nd model age calculations for this terrane depend on the time at which the 147Sm/144Nd systematics were last altered. Carbonate mineralization ages are explored for a diverse group of thirty samples using a novel sequential acid extraction procedure. Through this procedure it is possible to constrain carbonate mineralization in a variety of geologic settings (metamorphic fluid flow, hydrothermal and ore-forming systems) to ± 20 Myr. Zoned and bulk garnet geochronology at ten sites within a unusually localized area (~5 km2) of high temperature granulite-facies metamorphism in Bristol, New Hampshire reveals multiple brief (<1 Myr) periods of garnet growth focused between 393 and 386 Ma, at peak temperatures of 730-850°C. Comparison with garnet growth ages and temperatures , in central Massachusetts (364 Ma at 950°C) and Connecticut (341 Ma at 1000°C) reveal a regional trend of pulsed high temperature garnet growth spanning ~60 Myr from north to south, the same time span bracketed by associated igneous rocks dated via zircon geochronology. Ultrahigh metamorphic temperatures were achieved during the Acadian Orogeny in New England in localized, short pulses, likely related to igneous heating and/or associated fluid flow above and beyond the regional heating due to tectonic overthickening.

This thesis explored, as its major aim, the crystallographic and compositional characteristics of a particular type of peridotitic garnet associated with kimberlite. This garnet has a highly sinusoidal rare-earth element (REE) pattern as its distinguishing feature. Before the main research question could be addressed, a technique had to be developed that enabled the rapid and straight-forward acquisition of a full profile digital X-ray diffractogram from a single sub-milligram crystal fragment. After extensive experimentation and testing, successful development and realization of a method that is capable of producing such data was achieved. The next step of this research project was to empirically investigate and assess the crystal lattice strain model (CLSM) of Blundy and Wood (1994). Careful analyses of the REE present in a suite of clinopyroxenes were compared to the crystal structure data given from the XRD technique outlined above. Clinopyroxene is particularly useful for such an assessment because the radius of the M2 site in this mineral is between the largest and smallest REE, making the distribution of REE particularly sensitive to variation of the M2 site, which in turn is a direct consequence of the overall pyroxene structure. Subsequent to illustrating that XRD data could be collected on such small material and the XRD data and structural data given from the CLSM correlate strongly, peridotitic garnets with sinusoidal REE patters were investigated. The conclusions drawn in the first two contributions – namely that it was possible to collect accurate and precise XRD data from sub-milligram specimens and that the crystal structure and REE distribution were directly related – were imperative for the deduction of conclusions in the final, major research question. The XRD analysis of many garnets with and without sinusoidal REE patterns showed the presence of a small amount of an additional phase in some of these garnets. While this phase (2 out of 3 peaks indexed as possibly orthorhombic perovskite) is not present in sufficient quantities to give rise to such a strong sinusoidal segment in the garnet REE pattern, it prompted the critical step forward in formulating a working hypothesis for the otherwise inexplicable REE patterns. Specifically, I posit that many of the sinusoidal garnets may originally have precipitated as a very high pressure phase (in the mantle transition zone or deeper) that subsequently underwent a subsolidus isochemical transformation to garnet. Possible original precursor mineralogy is a combination of two perovskites or a perovskite + iv garnet assemblage. Theoretical calculation using experimental partition coefficients demonstrated that a mixture of Ca-perovskite (CaPv) and Mg-perovskite (MgPv) REE patterns in the approximate proportions of 10% CaPv and 90% MgPv produce a REE diagram that is strikingly similar to those observed in sinusoidal single phase garnet. It has been shown experimentally that with increasing depth in the mantle, garnet plus a progressively increasing CaPv component is the stable mineral assemblage. Initial precipitation of two perovskites or CaPv + garnet as cumulates from a deep magma ocean would preserve the REE distribution of these minerals. Subsequent exhumation of such an assemblage would result in the retrogressive subsolidus phase transformation to a mineral stable at conditions of T and P of the shallow mantle environment, i.e. garnet, while retaining the REE pattern of the initial precipitate mineral assemblage. While this working hypothesis will require many more tests, its proposal may have significant implications for the mantle structure.