Lithium isotope systematics of the Sumdo Eclogite, Tibet: Tracing fluid/rock interaction of subducted low-T altered oceanic crust

Trace Elements and Li Isotope Compositions Across the Kamchatka Arc: Constraints on Slab‐Derived Fluid Sources

“…However, Li isotopic fractionation during metamorphic dehydration and melting of subducted basaltic crust remains a subject of debate. Variable parameters of modeling, the composition of dehydrated slab residuals, show a large range in δ 7 Li values (δ 7 Li = −1 in Simons et al, 2010; δ 7 Li = +8‰ in Marschall, Pogge von Strandmann, et al, 2007; δ 7 Li = +4.5‰ in Liu, Sun, et al, 2019). Significant δ 7 Li fractionation also appears to be absent in published δ 7 Li data for adakites (e.g., Panama adakites +1.4 to +4.6‰, Tomascak et al, 2000), which is consistent with our data for NCKD lavas.…”

“…Significant δ 7 Li fractionation also appears to be absent in published δ 7 Li data for adakites (e.g., Panama adakites +1.4 to +4.6‰, Tomascak et al, 2000), which is consistent with our data for NCKD lavas. Two possible explanations for the MORB‐like δ 7 Li values in "adakitic" arc magmas can be considered: (1) The MORB‐like Li isotopic compositions are preserved during slab dehydration (Section 5.6 will discuss this in more details), which is supported by the limited Li isotope fractionation during prograde metamorphism (e.g., Marschall, Altherr, & Rüpke, 2007; Liu, Sun, et al, 2019; Simons et al, 2010; Teng et al, 2007); (2) Alternatively, slab dehydration did produce an anomalous "heavy" Li isotopic signature, but this has been obliterated by the dominance of mantle‐Li in the mantle‐wedge magmas source and mixing by vigorous mantle convection under the NCKD (Ferlito, 2011; G. Yogodzinski et al, 1995), as proposed by Tomascak et al (2000). From this follows that subduction dehydration, slab melting and mixing with Li from the mantle wedge will only have minor effects on δ 7 Li values, and therefore, arc magmas do not deviate significantly from MORB‐type mantle.…”

“…Therefore, rather similar range in Li isotopes from arc front to back arc cannot be simply explained by the decreasing amount of slab‐derived fluid added to the source region with increasing slab depth (Moriguti & Nakamura, 1998; Tomascak et al, 2002). Moreover, such a process also fails to take into account the isotopic fractionation expected during progressive dehydration of the subducted oceanic crust (Benton et al, 2004; Liu, Sun, et al, 2019; Marschall, Pogge von Strandmann, et al, 2007; Marschall, Altherr, & Rüpke, 2007; Simons et al, 2010; Walker et al, 2009). Such a continuous decrease of δ 7 Li values in slab fluids with increasing slab depth was demonstrated in the southern Cascades (Magna et al, 2006) and high‐pressure metamorphic rocks (Halama et al, 2011; Liu, Sun, et al, 2019; Marschall, Pogge von Standmann, et al, 2007; Simons et al, 2010).…”

“…Consequently, the potential of mass‐dependent isotopic fractionation is large (Penniston‐Dorland et al, 2017; Sun et al, 2016; Tang et al, 2010; Tomascak et al, 2016). (2) The heavy isotope, 7 Li, preferentially partitions into hydrous fluids during fluid‐rock interaction (Liu, Sun, et al, 2019; Marschall, Pogge von Strandmann, et al, 2007; Moriguti & Nakamura, 1998; Penniston‐Dorland et al, 2012; Penniston‐Dorland et al, 2010; Tian et al, 2019; Xiao et al, 2011; Zhou et al, 2018). (3) Li can trace distinct slab fluids because it is relatively enriched in, and easily mobilized from, AOC and/or marine sediments, yet is depleted in the oceanic upper mantle (Marschall et al, 2017; Penniston‐Dorland et al, 2017; Tomascak et al, 2016).…”

Magnesium and Lithium Isotopic Evidence for a Remnant Oceanic Slab Beneath Central Tibet

“…Alternatively, the dehydration of altered oceanic basalts may be more plausible for generating the observed isotopic features. Prior studies have demonstrated that the fluids derived from average altered oceanic crust can have δ 7 Li as high as ~9.6‰ (Liu et al, ) and δ 26 Mg > +0.06‰ (Li et al, ; Teng et al, ). Moreover, fluids derived from slab dehydration have ocean island basalt (OIB) or MORB‐like Sr‐Nd isotopes.…”