Description: Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7°11°S).The results indicate that midocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100-900MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100-300MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200-900MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 0.4-1wt% H2O in the parental melts.These estimates are in agreement with direct (Fourier transform IR) measurements of H2O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 0.04-0.09 and 0.30-0.55 wt% H2O for depleted and enriched MORBs, respectively. Our results are in general agreement (within ±200MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation

Description: These basalts represent the earliest response to subduction initiation in the Izu-Bonin arc system during the early Eocene. They are highly refractory and formed by high degrees of partial melting of a previously depleted source.

Description: Whole rock geochemical analyses of nonfinite suite lavas sampled during IODP Expedition 352 at two sites: U1439C and U1442A. These sites lie in the Izu-Bonin forearc, NE of Chichijima (Bonin Island). The major elements and select trace elements analyzed the X-ray fluorescence spectrometry, other trace elements including the Rare Earth Elements (REE) analyzed by ICP-MS. CO2 was analyzed with a Costech Elemental Analyzer and used to correct major elements for secondary calcite content.