Preliminary results of trace element contents in mantle derived specimens from Kakanui, New Zealand, of the Jarosewich's Microbeam Reference Samples Collection: The first step of a new program
Abstract
Over the last two decades, microbeam instrumentation has been the subject of a great technological development—ever more sophisticated analytical capabilities including lower detection limits and better mass- and spectral resolution. Nevertheless, the accuracy of microananalysis is limited because of the lack of reliable reference materials for trace elements (<0.1 wt. %) with a similar composition to the unknown. The program started at the Department of Mineral Sciences, NMNH, Smithsonian Institution (Logan, 2009) has the long-term objective of establishing working values for the trace element contents of the 59 specimens (including silicates, oxides, carbonates, REE orthoposphates, and volcanic and synthetic glasses) from the Jarosewich Microbeam Reference Samples Collection. During the 1970s and 1980s Eugene Jarosewich characterized these specimens for major and minor elements by wet chemistry to be used as RM for microanalysis, Jarosewich et al., (1980). Currently, more than 600 reference samples from this collection are yearly requested by laboratories all over the world. Suitable homogeneous crystals for reference samples can only grow in an environment with constant P-T conditions for a long time such as the mantle. However, during the ascent of mantle nodules to the surface of the earth, their original composition could be affected by metasomatism. Therefore, this program focuses on a rigorous analysis of the degree and scale of homogeneity of trace element contents in mantle derived specimens, as they are the most likely to satisfy the requirements to become RMs. Because the most frequently requested specimens of this collection are from the well know locality Kakanui, New Zealand, a statistical analysis was performed based on 640 electron microprobe analyses (P, Sc, V, Cr, Ni and Zn) on the tschermakitic augite (NMNH 122142), kaersutitic hornblende (NMNH 143965) and a pyrope garnet,(NMNH 143968) in traverses across three randomly selected grains. In general, the t-test and the Fisher test (variance analysis) indicate that similar means and compositional ranges should be expected in different grains from the sampled master vials. Further micronanalytical investigation of the P distribution in the hornblende is necessary. Correlations (Cr vs. V, Ca vs. V, and Cr vs. Ti, among others) show trends that appear to represent pristine fractionations. The EPMA means are in good agreement with XRF analysis of bulk samples (1.6 g), except for Zn in the Kakanui augite. The results obtained for the Ni and V contents indicate that the Kakanui suite covers a fairly broad compositional range. The slightly high t- and low F- values obtained for Cr in the pyrope and the augite indicate a mild zonation that appears to represent a pristine fractionation. These results indicate that the Kakanui suite has not been affected by substantial post-depositional metasomatism and are therefore; good candidates for further characterization by LA-ICP-MS and secondary ion mass spectrometry in an inter-laboratory comparison effort.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V33E..01L
- Keywords:
-
- 1094 GEOCHEMISTRY / Instruments and techniques