Abstract
Abstract. Recent U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has demonstrated that reasonable precision (3 %–10 %, 2σ) can be achieved for high-resolution dating of texturally distinct calcite phases. Absolute dating of dolomite, for which biostratigraphy and traditional dating techniques are very limited, remains challenging, although it may resolve many fundamental questions related to the timing of mineral-rock formation by syngenetic, diagenesis, hydrothermal, and epigenetic processes. In this study we explore the possibility of dating dolomitic rocks via recent LA-ICP-MS dating techniques developed for calcite. The in situ U–Pb dating was tested on a range of dolomitic rocks of various origins from the Cambrian to Pliocene age – all of which are from well-constrained stratigraphic sections in Israel. We present imaging and chemical characterization techniques that provide useful information on interpreting the resulting U–Pb ages and discuss the complexity of in situ dolomite dating in terms of textural features that may affect the results. Textural examinations indicate zonation and mixing of different phases at the sub-millimeter scale (< 1 µm), and thus Tera–Wasserburg ages represent mixed dates of early diagenesis and some later epigenetic dolomitization event(s). We conclude that age mixing at the sub-millimeter scale is a major challenge in dolomite dating that needs to be further studied and note the importance of matrix-matched standards for reducing uncertainties of the dated material.
Highlights
Recent developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have opened a new avenue for measuring absolute ages of carbonates, improving the understanding of many fundamental geological processes, such as fossilization (Li et al, 2014), tectonic faulting (Ring and Gerdes, 2016; Roberts and Walker, 2016; Nuriel et al, 2017; Parrish et al, 2018), duration of sedimentation, and diagenesis (Hodson et al, 2016; Godeau et al, 2018)
We present U–Pb ages of eight dolomite samples (Table 1) and Tera–Wasserburg inverse concordia diagrams as 207Pb / 206Pb and 238U / 206Pb linear regression isochrons of these samples (Fig. 3; data are available in the Supplement)
U–Pb isotopic ratio analyses of the syngenetic Cambrian dolomite Tm-MU-2 indicate an isochron that intercepts at 277 ± 59 Ma (MSWD = 0.53; n = 80; Fig. 3a), with an initial Pb value of 0.8664 ± 0.006
Summary
Recent developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) have opened a new avenue for measuring absolute ages of carbonates, improving the understanding of many fundamental geological processes, such as fossilization (Li et al, 2014), tectonic faulting (Ring and Gerdes, 2016; Roberts and Walker, 2016; Nuriel et al, 2017; Parrish et al, 2018), duration of sedimentation, and diagenesis (Hodson et al, 2016; Godeau et al, 2018). Despite the low concentrations of U and radiogenic Pb in carbonates (< 10 and < 2 ppm, respectively) as well as the considerable amounts of initial Pb (up to 100 ppm), a reliable age determination of calcite is obtained via isochron regression on a Tera–Wasserburg inverse concordia diagram (Tera and Wasserburg, 1972) With this method, the initial Pb composition and the age are determined by the upper and lower intercept of the regression isochron with the concordia curve. Dolomite is vastly abundant in exposed stratigraphic sequences, and its manifestation in the geological record increases towards older sedimentary strata (Warren, 2000). It is very rare in modern environments and has seldom been successfully grown in laboratory experiments at near-surface conditions (Machel, 2004, and references therein). Previous studies suggested that dolomite is formed either by diagenetic re-
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