ELEMENT PARTITIONING BETWEEN ALTERED BASALTIC GLASS AND SECONDARY PHILLIPSITE: PRELIMINARY RESULTS FROM SITE U1557, SOUTH ATLANTIC TRANSECT, EXPEDITIONS 390/393

Water-rock interactions at mid-oceanic ridges play a significant role in global geochemical cycling via the chemical exchange between oceanic crust and seawater. Previous studies propose hydrothermal alteration of basaltic glass results in the complete loss of Si and alkali elements to seawater [1]. This has significant implications for the composition of subducted crust and, therefore, global tectonic and magmatic processes. However, secondary zeolite crystallization may retain some elements (Si, Na, K, and some incompatible trace elements) released during alteration and/or sequester them within the altered ocean crust. Zeolites, identified as phillipsite through petrography and Raman spectroscopy, occur within and rimming the hydrothermally altered basalt clasts recovered from Hole U1557D of the South Atlantic Transect, IODP Expeditions 390/393. We present results from electron microprobe analysis of major and select trace elements of phillipsite and altered basaltic glass phases (ABG) on selected samples from Hole U1557D to better constrain the geochemical relationship between ABG and secondary phillipsite formation, as well as the elemental partitioning between the two.

Our results demonstrate a strong association between phillipsite and ABG occurrence and the relative geochemical enrichments and depletions observed between them. Electron microprobe transects through ABG reveal trends in the enrichment and depletion of select elements. Although heterogeneity is observed along the transects, Ti, Cr, Fe, Mn, and Ca concentrations decrease, while Al concentrations increase rim-ward. Notably, Mg tends to increase toward the rims of ABG fragments at depths ≥ 613 CSF-A (m) but decreases in shallower samples. A trend of depletion of Na and K occurs from the ABG core to the rims at depths of 576 and 680 CSF-A (m), while variable enrichment of K and Na from the ABG core to the rim occurs at depths between 600-641 CSF-A (m). The range in K₂O within both the phillipsite (6.33-9.42 wt.%) and ABG (0.04-11.11 wt.%) is higher than that of the mean MORB and MORB glass (0.190 wt.%) [2], suggesting K addition to the oceanic crust derived from the seawater.

[1] Kruber et al., 2008, G3, v. 9, doi:10.1029/2008GC002119

[2] White & Klein, 2014, Treatise on Geochem., 2E. Elsevier, 4