Browsing by Author "Harris Chris"

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    Oxygen isotope composition of megacrysts from the Monastery kimberlite
    (2024) Van Blerk, Joshua; Howarth, Geoffrey; Harris Chris; Janney, Phil
    Kimberlite megacrysts are large (>1 cm diameter) crystals that are thought to have crystallized from sub lithospheric proto-kimberlite melts near the base of the subcontinental lithospheric mantle (SCLM) during complex melt-SCLM interactions. Thus, these megacrysts represent an excellent opportunity to constrain the effects of SCLM assimilation on the δ18O values of primary mantle-derived melts. Laser fluorination δ 18O values for a well-characterized suite of megacrysts from the Monastery kimberlite, South Africa, are presented to: (1) constrain the δ18O value of the mantle source and (2) evaluate the effects of melt-SCLM interactions on the δ18O value of mantle-derived magmas. The Monastery kimberlite megacryst assemblages are as follows (in order of crystallization): (1) gt + cpx + opx + Fe-poor ol; (2) gt + cpx + opx + Group 1 ilm (Cr-, Mg-poor); (3) Group 2 ilm (Cr-rich, Mg-poor) + phlog; (4) Group 2 ilm + phlog + zir; (5) Group 2 ilm + phlog + zir + Fe-rich ol; and (6) Group 3 ilm (Cr and Mg-rich) + calcic cpx. The δ18O values of the megacrysts from the initial assemblage are: δ18Ogt = 5.12 and 5.25‰ (n = 2); δ18Ocpx = 4.72 and 5.02‰ (n = 2); δ18Oopx = 5.20 and 5.55‰ (n = 2); δ18OFe-poor ol = 5.43- 5.84‰ (x̄ = 5.23‰, σ = 0.10, n = 10), all showing no correlation between δ18O values and major element compositions. This implies that these megacrysts were in equilibrium with a melt of mantle-like δ18O values. The Group 1 ilmenites have δ18O values expected for mantle ilmenites, ranging from 3.88-4.35‰ (x̄ = 4.10‰, σ = 0.15, n = 8) (also in equilibrium with a melt of mantle-like δ18O values). They show a weak positive correlation between the δ18O value and Cr#, and no correlation with Mg#. In contrast, the δ18O values of the megacrysts from assemblages three to five implies that they were in equilibrium with a melt of δ18O values below the expected mantle-like range: δ18OGroup 2 ilm = 2.74-4.46‰ (x̄ = 3.56‰, σ = 0.45, n = 11); δ18Ophlog = 4.25-5.73‰ (x̄ = 5.08‰, σ = 0.48, n = 8); δ18Ozir = 4.87-5.09‰ (x̄ = 4.98‰, σ = 0.08, n = 6); and δ18OFe rich ol = 4.53-4.94‰ (x̄ = 4.75‰, σ = 0.15, n = 5). The Group 2 ilmenites show no correlations between the δ 18O value and Mg# and Cr#. The Fe-rich olivines show positive correlations with Mg# and Ni. The phlogopites show no correlations with the major element compositions. The Group 3 ilmenites have δ18O values ranging from 2.93-4.05‰ (x̄ = 3.59‰, σ = 0.36, n = 7). These are below the δ18O values expected for mantle ilmenites, but are slightly higher than the Group 2 ilmenites. The Group 3 ilmenites show a positive correlation between the δ18O value and Mg# and no correlation of δ18O with Cr#. The δ18O values of primary mantle-derived magmas are ~5.7‰ (Eiler, 2001) unless recycled crust is present in the source or if the magma assimilates crustal material en route to the surface. The variations in the δ18O values throughout the megacryst suite suggest that the parent melt underwent open system evolution in the SCLM. It is proposed that the Monastery proto-kimberlite originated from a convecting mantle source with a mantle-like δ18O value of ~5.33‰, which experienced two stages of evolution involving fractional crystallization and assimilation/melt-wall rock interaction. The low δ18O values in the phlogopites, zircons, Fe-rich olivines, and especially the Group 2 ilmenites (as low as ~4.25‰) can be explained by the assimilation most likely of low-δ18O eclogite (first stage evolution). The increased Cr# in the melt marked by the Group 2 ilmenites suggests the assimilation of Cr-rich pyroxenite veins. The Group 3 ilmenites give increased equilibrium melt-δ18O values of up to ~5.56‰ and give increased Mg# relative to the Group 2 iii ilmenites, which may be explained by the assimilation of Mg-rich metasomatized peridotite (second stage evolution).
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    Triple oxygen isotope measurements by tunable infrared laser direct absorption spectroscopy, and applications to ratite eggshell
    (2023) Yarian, Drake; Hare, Vincent; Harris Chris
    This project introduces a new sample preparation technique and analytical methodology for highprecision measurement of the three oxygen isotopes directly in CO2 samples using Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS). After the introduction and optimization of both the sample preparation and analytical systems, it is applied to fossil ratite eggshell (RES) samples spanning the mid-Miocene through Quaternary Namib environments. A data correction scheme for relating TILDAS measurements to the VSMOW2-SLAP2 scale is also introduced. Applying this correction, analyses of the international CaCO3 standards IAEA603, NBS18, and NBS19 are shown to agree well with previously reported values. In addition, Δ'17O 1σ reproducibilities of 10, 19, and 60 per meg (1 per meg = 0.001‰) of these standards, respectively, are achieved. Analysis of RES suggests notable environmental differences between the central and southern Namib through time. RES D' 17O signal preservation in the context of global climatic and local environmental influences is investigated by relating obtained data to current triple oxygen enabled atmospheric and animal body water models. This project concludes by recommending 3 main points of focus for future research directed towards improving RES data interpretation within the context of (paleo-)environmental reconstruction: (1) adoption of international standards with composition representative of RES, (2) dedicated research into the influence vegetation type has on leaf water D' 17 O, and (3) a community-wide increase in analyses of modern RES D' 17 O data to provide a better foundation from which paleoclimatic and paleoenvironmental interpretations may be made.