James Crosby - My Geochemical Life (so far...)

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The University of Cambridge

Noble Gas Isotope, Volatile Element Isotope and Volatile Element Geochemistry:

At the University of Cambridge, as a member of St. Catherine's College I will be part of the University of Cambridge Earth System Science NERC Doctoral Training Partnership to work on the project "The source and flux of volatiles in Earth's lithospheric mantle". 

The reason this research is important:
The present-day volatile content of the lithospheric mantle results from potentially billions of years of metasomatic enrichment but a key outstanding issue concerns the relative input of volatiles ‘leaking’ more or less continuously from the MORB source or downgoing slabs, compared with episodic fluxes of primordial volatiles from the deep mantle transported by plumes. Constraints on these fluxes are vital to understanding which branches of Earth’s chemical cycle are losing volatiles to long-term storage in the lithosphere. There is compelling evidence that a substantial volatile flux into the lithospheric mantle occurs more or less contemporaneously with mantle plume activity. More continuous metasomatism may be linked to alkali-carbonate fluids released from downgoing slabs in the convecting mantle.

To summarise:
The project involves the novel analysis of volatile elements and noble gas isotopic ratios in both nominally volatile-free minerals (apatite, clinopyroxene, olivine) and volatile phases (phlogopite, amphibole) present in mantle xenoliths in order to constrain the volatile flux in the mantle. New analyses of Cl, F, H and C together with 3He/4He and 20Ne/22Ne for samples from a variety of tectonic settings to place important constraints on the nature and extent of volatile flux from subducting slabs and the convecting mantle to the overlying lithosphere.

What I am doing: 
Micro-analytical techniques will be used to determine trace-element and volatile contents of mantle phases. This data together with noble gas isotope measurements will place important constraints on the provenance of volatiles stored in the lithospheric mantle. We anticipate that elemental ratios that remain relatively constant during melting and crystallisation will be novel diagnostic tracers of volatile provenance in the convecting mantle. An example is H2O/Ce as melt inclusions in arc lavas have much higher ratios than those of MORB and OIB. Another potential provenance tracer is the Cl content of apatite.

The University of St Andrews

Noble Gas and Volatile Element Isotope Geochemistry:

At the University of St. Andrews I have chosen to specialise in noble gas and volatile element isotope geochemistry. The work I undertake is supervised by Dr Sami Mikhail and has significant input from Prof Fin Stuart of the Scottish Universities Environmental Research Centre and Dr Feargus Abernethy of the Open University. Due to this being an ongoing research project I cannot reveal significant information until submission and publication. However, I am to use Helium, Carbon and Nitrogen isotope composition of polycrystalline diamonds from Botswana and South Africa to constrain the source of carbon and nitrogen which form diamond samples. 

For more information regarding my research, see my published abstract for the upcoming Goldschmidt and DINGUE conference in Paris during mid-August. 

The University of Manchester

Field Mapping, Organic, Isotope and Analytical Geochemistry:

During my time at the University of Manchester, the independent research I undertook could be split into two different areas but were beautifully linked. 

a) Field Mapping of the Sant Corneli anticline within the Southern Central Pyrenees of the North-Eastern Iberian Peninsula of Spain. 
b) Sample collection from the mapped area and subsequent organic geochemical analysis (n-alkane, n-alcohol and n-fatty acid) using GC-MS technology coupled with 𝛿13C and 𝛿18O isotopic analysis and X-Ray Diffraction to make palaeoenvironment deductions. 

Supervised by Dr Stefan Schröder, the field mapping involved a six-week mapping trip where structural and lithological observations were made to fully understand the geological setting and basic depositional environment of the area. The geological environment of the can be summarised into three main stratigraphic units (see Simó, 1986; Cuevas, 1992; García-Senz, 2002 and Beamud et al., 2003 references in the document if required)
1. Post-folding conglomeratic deposits from the Oligocene and Quaternary.
2. Syn-folding quartzitic and calcareous sandstones, marls, clays and turbidites from the Upper Santonian to Maastrichtian.
3. Pre-folding and post-rift calcarenites, limestones and marls from the Upper Cenomanian to Lower Santonian.
The most interesting observation was the structural dip decreasing from 40⁰ to 12⁰ through the succession. This likely represented tectonic synsedimentary growth with decreasing tectonic activity and relaxation of crustal shortening of Sant Corneli Anticline.

Supervised by Dr Bart Van dongen, the organic geochemical analysis was undertaken on six samples taken from various outcrops within the mapped area. In addition to the organic geochemical analysis; stable isotope analysis, X-Ray diffraction and loss on ignition experimentation were also undertaken. A sample of my dissertation can be observed below. As expected, the different samples had highly varying results with palaeoenvironmental deductions made and a vague attempt to constrain source rock suitability as an oil reservoir. (Admittedly, this is not anywhere near the standard I would expect of myself now but we all have to start somewhere).