High-sensitivity Nb analysis by spark-source mass spectrometry (SSMS) and calibration of XRF Nb and Zr

Statistical analysis of X-ray fluorescence data acquired during Leg 135 indicates that this instrument produces data of comparable precision to good land-based laboratories. We also examined contamination of certain elements caused by crushing during the use of the tungsten carbide apparatus. Although the concentrations of most elements are not altered during crushing, the powders prepared on the ship should not be used in subsequent studies where key elements of the investigation include W, Co, Ta, Pb, and low levels of Nb.

Section: Accuracymentioning

confidence: 98%

Section: Accuracymentioning

confidence: 99%

See 1 more Smart Citation

Data Report: Assessment of the Precision of Leg 135 Shipboard XRF Analyses and the Contamination Introduced by Crushing in Tungsten Carbide

Hergt¹,

Sims²

1994

“…Precision is generally better than ±5% (2 sigma), although elements at very low or very high concentrations will have poorer precision because of the fewer number of data points that are pooled-ideally, a minimum of 20 separate determinations are used in the final results. Recent improvements to the MS7 have resulted in resolution of an interference on 93 Nb (because of 29 Si 16 O 4 ;Jochum et al, 1990), which, if left unresolved, would most greatly affect samples with low Nb concentrations (i.e., <l ppm).…”

Section: Analytical Techniquesmentioning

confidence: 99%

Regional Geochemistry of the Lau-Tonga Arc and Backarc Systems

Ewart¹,

Bryan²,

Chappell³

et al. 1994

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo 90 _9 2 ) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9.The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophüe elements, caused by a subduction contribution. Ageneral trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.

“…Our average of two analyses of the standard PCC-1 is also listed with a "recommended" analysis compiled from the major element and first-series transition element data of Govindaruju (1989), REE values of Stosch and Seek (1980), and Zr and Nb values of Jochum et al (1990). Major element concentrations of the analyzed peridotites are an average of three XRF runs.…”

Section: Bulk-rock Analysesmentioning

confidence: 99%

Trace Element Geochemistry of Peridotites from the Izu-Bonin-Mariana Forearc, Leg 125

Parkinson¹,

Pearce²,

Thirlwall³

et al. 1992

Self Cite

Trace element analyses (first-series transition elements, Ti, Rb, Sr, Zr, Y, Nb, and REE) were carried out on whole rocks and minerals from 10 peridotite samples from both Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc using a combination of XRF, ID-MS, ICP-MS, and ion microprobe. The concentrations of incompatible trace elements are generally low, reflecting the highly residual nature of the peridotites and their low clinopyroxene content (<2%). Chondrite-normalized REE patterns show extreme U shapes with (La/Sm) n ratios in the range of 5.03-250.0 and (Sm/Yb) n ratios in the range of 0.05-0.25; several samples show possible small positive Eu anomalies. LREE enrichment is common to both seamounts, although the peridotites from Conical Seamount have higher (La/Ce) n ratios on extended chondrite-normalized plots, in which both REEs and other trace elements are organized according to their incompatibility with respect to a harzburgitic mantle. Comparison with abyssal peridotite patterns suggests that the LREEs, Rb, Nb, Sr, Sm, and Eu are all enriched in the Leg 125 peridotites, but Ti and the HREEs exhibit no obvious enrichment. The peridotites also give positive anomalies for Zr and Sr relative to their neighboring REEs. Covariation diagrams based on clinopyroxene data show that Ti and the HREEs plot on an extension of an abyssal peridotite trend to more residual compositions. However, the LREEs, Rb, Sr, Sm, and Eu are displaced off this trend toward higher values, suggesting that these elements were introduced during an enrichment event. The axis of dispersion on these plots further suggests that enrichment took place during or after melting and thus was not a characteristic of the lithosphere before subduction.Compared with boninites sampled from the Izu-Bonin-Mariana forearc, the peridotites are significantly more enriched in LREEs. Modeling of the melting process indicates that if they represent the most depleted residues of the melting events that generated forearc boninites they must have experienced subsolidus enrichment in these elements, as well as in Rb, Sr, Zr, Nb, Sm, and Eu. The lack of any correlation with the degree of serpentinization suggests that low-temperature fluids were not the prime cause of enrichment. The enrichment in the high-field-strength elements also suggests that at least some of this enrichment may have involved melts rather than aqueous fluids. Moreover, the presence of the hydrous minerals magnesio-hornblende and tremolite and the common resorption of orthopyroxene indicate that this high-temperature peridotite-fluid interaction may have taken place in a water-rich environment in the forearc following the melting event that produced the boninites. The peridotites from Leg 125 may therefore contain a record of an important flux of elements into the mantle wedge during the initial formation of forearc lithosphere. Ophiolitic peridotites with these characteristics have not yet been reported, perhaps because the precise equiva...