The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica

Ahmad, Qasid; Wille, Martin; König, Stephan; Rosca, Carolina; Hensel, Angela; Pettke, Thomas; Hermann, Jörg

doi:10.1016/j.chemgeo.2021.120231

Cited by 41 publications

(57 citation statements)

References 86 publications

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“…Mobility of Mo during subduction: the role of fO 2 . Recycled Proterozoic sediments that enriched the S-MAR mantle source with isotopically heavy Mo are in stark contrast to inferred, isotopically light Mo in dehydration residues of subducted Phanerozoic metasediments (Ahmad et al, 2021). The trend towards higher observed (co-) variations of δ 98/95 Mo with increasing degrees of mantle source enrichment within the S-MAR suite therefore implies recycling of a sedimentary δ 98/95 Mo or even a total Mo budget unaffected by dehydration and melting during subduction.…”

Section: Discussionmentioning

confidence: 87%

“…The mobility of Mo from the subducted material is controlled by the redox state of slab-derived aqueous fluids and hydrous melts (Bali et al, 2012;Skora et al, 2017). Significant Mo mobilisation and isotope fractionation has been observed during subduction of oxidised lithologies, leaving behind a Mo depleted and isotopically light residual slab (e.g., Freymuth et al, 2015;König et al, 2016;Chen et al, 2019;Ahmad et al, 2021). Under reducing conditions, only limited Mo mobility is expected to occur in subducted lithologies (Bali et al, 2012;Skora et al, 2017) thus preserving pre-subducted Mo signatures of the surface.…”

Section: Introductionmentioning

confidence: 99%

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Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Ahmad¹,

Wille²,

Rosca³

et al. 2022

Geochem. Persp. Let.

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Under modern oxidising Earth surface conditions, dehydrated subducted slabs show Mo isotope compositions as low as δ 98/95 Mo = −1.5 ‰, compared to the depleted mantle δ 98/95 Mo = −0.2 ‰. Such light Mo isotope compositions reflect the redoxdependent aqueous mobility of isotopically heavy Mo associated with slab dehydration. Here we analysed basaltic glasses from the South-Mid Atlantic Ridge, whose parental melts are influenced by the enriched Discovery and Shona mantle plumes. We report increasingly higher δ 98/95 Mo of up to −0.1 ‰ from the most depleted samples towards those tapping more enriched mantle sources. δ 98/95 Mo values correlate with radiogenic Sr and Nd isotopes, which indicates the recycling of Proterozoic sediments with a Mo isotopic composition that was not affected by subduction-related, oxic dehydration. We propose that the Mo isotope signatures were retained during subduction and reflect anoxic conditions during deep sea sedimentation in the mid-Proterozoic. Finally, Mo isotope fractionation between different terrestrial reservoirs likely depends on the slab redox budget, and therefore on the timing of subduction with regard to Earth's surface oxygenation.

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Section: Discussionmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 99%

Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Ahmad¹,

Wille²,

Rosca³

et al. 2022

Geochem. Persp. Let.

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“…The samples used for Nd isotope data are Om 32 (Tonalite), Om 2a (Gabbroic-Diorite), Om 2b (Trondhjemite) and Om 12 (Trondhjemite) from the Singhbhum Craton, India, which are published in Pandey et al (2019). The Mo isotope data of NIST SRM 3134 Mo RM and USGS RMs AGV-2 (Andesite), BHVO-2 (Basalt) and W-2a (Diabase) used here are published in Ahmad et al (2021). Table 2 summarises a comparison between the data processed using Parmanu-Gunak and an in-house Excel TM spreadsheet based on published methods by Siebert et al (2001) and Stracke et al (2014).…”

Section: Resultsmentioning

confidence: 99%

“…The 143 Nd/ 144 Nd ratios are presented as such, while variations in 98 Mo/ 95 Mo of samples are shown in δ notation (in 10 -3 units or parts per thousand, ‰), against RMs JMC Bern (the same 98 Mo/ 95 Mo value used in double spike deconvolution) and NIST SRM 3134, according to Equation ( 35). JMC Bern is simply termed as J&M in Ahmad et al (2021), which are essentially the same. The mean values together with the sample standard deviation (2s), standard error of mean (in 2SE) and 95% confidence intervals (95% ci) of the number of cycles measured were calculated separately using Excel TM .…”

Section: Resultsmentioning

confidence: 99%

Parmanu‐Gunak: Data Reduction Software for Isotope Dilution Analysis

Pathak

2023

Geostandard Geoanalytic Res

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Isotope dilution (ID) is a widely used analytical technique to determine elemental abundance to a high degree of accuracy and precision, using a spiked isotope tracer. This technique also enables efficient correction of the inevitable phenomenon of analytical mass fractionation during mass spectrometric analysis. It is also used to determine stable isotope variations of an element in a sample relative to a reference material, using tracers enriched in two isotopes (popularly known as the double spike method). Isotope dilution data reduction can be performed in Microsoft ® Excel TM using different algorithmic approaches. Additional software such as iolite and MATLAB ® offer algorithm implementations to perform these calculations. These are however limited to use within particular laboratories/research groups, or either require additional cost-bound software or some degree of knowledge in computer programming languages for use, or all of these. To ease this situation, a graphical user interface-based software is proposed (here named Parmanu-Gunak, meaning Atom-Calculator in Sanskrit text) to invert both single and double spiked isotope data, with the aim of making it a standard tool for ID data reduction. Examples of Nd and Mo isotopes are used to demonstrate the robustness of the program.

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“…Trace element and Mo isotopic compositions of eroded fore-arc crust melt are calculated at modal melting degree of F=20% (4 GPa and 800 o C), using weighted means of the Luzon lower crustal xenoliths (Ce=5.4 ppm, Mo=0.26 ppm, δ 98/95 Mo =-0.09 ‰; Tables S3 and S4). Same strategy is applied to calculate the trace element and Mo isotope compositions of remnant slab melt using weighted means MORB-type eclogites and metasediments from Ahmad et al (2021) and Chen et al (2019) (Tables S3 and S4). The results show that melt derived from the eroded sub-arc crust has an elevated δ 98/95 Mo (+0.01‰) (Fig.…”

Section: S4 Quantitative Calculationsmentioning

confidence: 99%

Supplemental Material: Heavy Mo isotope composition of northern Bataan adakites, Philippines: Evidence for fore-arc subduction erosion?

Liu¹,

al.²

2022

Preprint

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The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica

Cited by 41 publications

References 86 publications

Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Parmanu‐Gunak: Data Reduction Software for Isotope Dilution Analysis

Supplemental Material: Heavy Mo isotope composition of northern Bataan adakites, Philippines: Evidence for fore-arc subduction erosion?

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