The geochemical alteration of ocean crust, as it ages and moves from mid-ocean ridge spreading centers across the surface of the earth due to plate tectonics, and then subducts down back into the mantle, helps determine the geochemical mass balance of the crust and overlying ocean. This research is potentially transformative in that it will work to establish boron isotopes in melt inclusions in seafloor basalts as a geochemical proxy that can indicate the extent and depth at which hydrothermally altered ocean crust interacts with new rising lavas erupting on the seafloor. Melt inclusions from volcanic rocks taken from the seafloor near the East Pacific Rise in the eastern Pacific Ocean will be examined and the isotopes of boron and oxygen in them will be analyzed by the ion microprobe at the Woods Hole Institution of Oceanography. These data will be combined other geochemical analyses of the major and trace elements and volatile species in the melt inclusions and their associated volcanic glasses. Project goals will include determination of whether assimilation of seawater-altered ocean crust in shallow crustal magma chambers is a common process in mid-ocean ridge volcanic processes and whether more evolved, silicic, volcanic rocks have higher boron isotope ratios that can be unequivocally tied to increasing assimilation processes. An additional goal is to determine to what depth hydrothermal circulation penetrates down into ocean crust at fast spreading centers. Broader impacts of the work include support of two early career researchers with no prior NSF support, one who is of a gender under-represented in the sciences, and undergraduate student training.
Boron is a moderately volatile, lithophile non-metal with a low atomic mass and two stable isotopes and a 11B/10B variation of several tens of per mil in Earth’s surface environments. The strong enrichment of B in seawater and the crust and the significant difference in B isotopic compositions between modern seawater, altered oceanic crust and the depleted mantle make B a potentially powerful geochemical tracer for the secular evolution of the ocean-crust-mantle system. The very low abundance of B in mantle rocks and primitive volcanic rocks, however, has confronted us with a major analytical hurdle. Boron isotope analyses of silicate materials at the trace abundance level are highly challenging. The B isotopic compositions of the Earth’s major reservoirs (i.e., continental crust, primitive and depleted mantle) are still poorly constrained, despite a several decades-long history of research into B isotope geochemistry. Boron abundances in mantle rocks are generally extremely low and are currently not analytically accessible for B isotope analyses at a geologically relevant level of precision. Efforts to estimate the isotopic composition of the mantle have, therefore, been focused on fresh, primitive basalts that can be expected to faithfully reflect the B isotopic composition of their mantle sources. Boron isotope fractionation be- tween melt and restitic mantle is limited at the high temperatures of MORB melting, and the highly incompatible behavior of B during mantle melting leads to a near quantitative extraction of B during melting, so that the extracted melt carries the B isotopic signature of the mantle source. The number of published B isotope analyses of basalt samples and the number of the sampled MOR localities prior to this study was very limited, and the available data set was partly contradictory. Two major obstacles had to be overcome in order to determine the B isotopic composition of the mantle: (i) analytical uncertainties had to be reduced, and (ii) the evaluation of assimilation of seawater-altered materials into the basaltic magmas or their contamination with brines had to be evaluated. The analytical obstacles have been addressed in the course of this study. An improved SIMS method was developed for the determination of the boron isotopic composition of volcanic glasses with boron concentrations of as low as 0.4 − 2.5 μg/g, as is typical for MORB glasses. The analyses are completed using the WHOI Cameca 1280 large-radius ion microprobe. The newly developed setup is capable of determining the B isotopic composition of basaltic glass with 1 μg/g B with a precision and accuracy of ±1.5 per mil (2RSE) by completing 4 − 5 consecutive spot analy- ses with a spatial resolution of 50×50μm2. Samples with slightly higher concentrations (≥ 2.5μg/g) can be analysed with a precision of better than ±2 per mil (internal 2RSE) with a single spot analysis, which takes 32 minutes. The funds were used to developed the analytical techniques described above and published in Marschall & Monteleone (2014). It also provided analytical time for training of a postdoctoral fellow by an early career geoscientist.
