Abstract
The strontium isotope signature (87Sr/86Sr) of calcite precipitated in rock fractures and faults is a frequently used tool to trace paleofluid flow. However, bedrock fracture networks, such as in Precambrian cratons, have often undergone multiple fracture reactivations resulting in complex sequences of fracture mineral infillings. This includes numerous discrete calcite crystal overgrowths. Conventional 87Sr/86Sr analysis of dissolved bulk samples of such crystals is not feasible as they will result in mixed signatures of several growth zonations. In addition, the zonations are too fine-grained for sub-sampling using micro-drilling. Here, we apply high spatial resolution 87Sr/86Sr spot analysis (80 µm) in transects through zoned calcite crystals in deep Paleoproterozoic granitoid fractures using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to trace discrete signs of paleofluid flow events. We compare the outermost calcite growth zone with 87Sr/86Sr values of the present-day groundwater sampled in the same boreholes to distinguish potential modern precipitates. We then connect our results to previously reported radiometric dating and C and O isotope signatures to understand the temporal history and physicochemical evolution of fluid flow within the fractures. Comparisons of modern calcite precipitated in a borehole over a period of 17 years with modern waters prove the concept of using 87Sr/86Sr as a marker for fluid origin in this environment and for how 87Sr/86Sr changed during marine water infiltration. Intermittent calcite precipitation over very long time spans is indicated in calcite of the currently open fractures, showing an evolution of 87Sr/86Sr from ~0.705–0.707—a population dated to ~1.43 billion years—to crystal overgrowth values at ~0.715–0.717 that overlap with the present-day groundwater values. This shows that high spatial resolution Sr isotope analysis of fine-scaled growth zonation within single calcite crystals is applicable for tracing episodic fluid flow in fracture networks.
Highlights
This shows that high spatial resolution Sr isotope analysis of fine-scaled growth zonation within single calcite crystals is applicable for tracing episodic fluid flow in fracture networks
We present a comprehensive dataset of in situ Sr-isotope analysis transects derived from laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) within ancient calcite crystals from two areas in Sweden (Figure 1, revisiting crystals previously analyzed for δ13 C and δ18 O [34]) to trace fluid flow events in the mineral record
We investigate the 87 Sr/86 Sr variability of modern calcite precipitated from fracture water in a borehole at 415 m depth over the course of 17 years, and compare these 87 Sr/86 Sr values with the modern source water
Summary
87 Sr/86 Sr values have been used for sedimentary carbonates and carbonate shells to understand ocean water evolution, such as salinity variations and freshwater input from glaciations [9,10,11,12,13,14,15] In bedrocks such as crystalline rock settings, the bulk rock 87 Sr/86 Sr signature evolves over time due to the decay of 87 Rb in K-bearing minerals like biotite and K-feldspar [16]. Ca-carbonate minerals, such as calcite, precipitated in the fractures at fracture reactivation events that induce fluid flow and mixing, can preserve this 87 Sr/86 Sr signature over geological timescales if the mineral is not dissolved This is, a commonly used tool to distinguish paleofluid origin and fluctuations, in addition to other isotope diagnostics, including stable C and O isotopes [16,21,22,23,24,25,26,27]. Recent applications of microscale stable C (δ13 C) and O (δ18 O) isotope determination using Secondary Ion Mass Spectrometry (SIMS)
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