Description: Lithium concentration and isotope data (δ7Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ7Li (δ7Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ7Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite–illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a−1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure.

Description: Many studies dealing with the synthesis of tourmaline report a sharp intragranular chemical gradient and extensive porosity in the core zones of the crystals, both of which still lack a reliable explanation. Using the example of olenitic tourmaline, we show that these features are likely explained by the occurrence of a precursor phase during tourmaline formation. Time-dependent piston–cylinder synthesis experiments were performed in the system SiO 2 –Al 2 O 3 –B 2 O 3 –NaCl–H 2 O at 700 °C/40 kbar with run durations of 0.5 h, 2.5 h and 216 h, starting from quartz–Al 2 O 3 –H 3 BO 3 solid mixtures and NaCl solutions. Sharply zoned olenitic tourmaline ( [4] B-rich cores, [4] B-poor rims) formed in all experiments, with its abundance increasing with increasing run duration. The amount of the additional solid product phases coesite and jeremejevite decreased with time. Extensive porosity is recognized in jeremejevite and in the cores of early grown acicular tourmaline. Textural relationships indicate that olenitic tourmaline grows at the expense of jeremejevite which acts as a crystalline precursor in this system. A possible reaction is: \[ 3{\mathrm{Al}}_{6}{\left({\mathrm{BO}}_{3}\right)}_{5}{\left(\mathrm{OH}\right)}_{3}+(12-2x){\mathrm{SiO}}_{2(\mathrm{aq})}+2y{\mathrm{NaCl}}_{(\mathrm{aq})}\to 2{\mathrm{Na}}_{y}{\mathrm{Al}}_{3}{\mathrm{Al}}_{6}({\mathrm{Si}}_{6-x}{\mathrm{B}}_{x}){\mathrm{O}}_{18}{\left({\mathrm{BO}}_{3}\right)}_{3}({\mathrm{O}}_{3-x},{\mathrm{OH}}_{1+x})+(4.5-x){\mathrm{B}}_{2}{\mathrm{O}}_{3(\mathrm{aq})}+(2.5-x){\mathrm{H}}_{2}\mathrm{O}+2y{\mathrm{HCl}}_{(\mathrm{aq})}. \] The transformation likely proceeds via a dissolution/re-precipitation mechanism, which triggers the sharp chemical zonation in olenite. Based on Rayleigh fractionation modelling, we estimate a minimum B isotope fractionation between jeremejevite and fluid with 11 B jer-fluid –2.8 at 700 °C/40 kbar. Due to the progressive dissolution of jeremejevite, the fluids 11 B values continuously decrease with increasing run duration. Hence, olenite growth concomitant with the dissolution of jeremejevite will produce scattered or inverse boron isotope patterns (heavy cores, light rims) in tourmaline, which cannot result from simple Rayleigh fractionation. Similar reactions involving jeremejevite or other precursor phases might explain chemical zonation and porous textures in tourmaline core zones reported in many experimental studies. The occurrence of natural tourmaline overgrowing jeremejevite in pegmatites of the Erongo Mountains, Namibia, gives rise to the assumption that jeremejevite might also act as a precursor for tourmaline formation in natural systems.

Description: Lithium elemental and isotopic compositions of 33 glass and whole-rock samples from nine oceanic island regions were determined to characterize the Li inventory of the deep mantle. The Li contents of the investigated lavas range from 1·5 to 13·3 μg g – 1 , whereas 7 Li ranges from 2·4 to 4·8. There are weak co-variations between the Li/Y, 7 Li, and Sr–Nd–Pb isotope compositions of the lavas, indicating that the Li elemental and isotopic characteristics of ocean island basalt to some extent reflect mantle source heterogeneity. In detail, HIMU-type lavas are characterized by 7 Li values (up to 4·8) slightly heavier than those for average normal mid-ocean ridge basalt (3·4 ± 1·4) and by comparatively low Li contents; EM1-type lavas are characterized by isotopically light Li (average 3·2) and relative Li enrichment, whereas EM2-type lavas tend to heavier 7 Li values (up to 4·4) with high Li concentrations. The Li contents and isotope characteristics of HIMU-type lavas are consistent with recycling of altered and dehydrated oceanic crust, whereas those of the EM1-type lavas can be attributed to sediment recycling. The Li characteristics of EM2-type lavas may reflect reworking of mantle wedge material that has been infiltrated by fluids derived from the subducting plate.