Magma Differentiation processes and volatile contents in Shatsky Rise oceanic Plateau basalts: constraints from mineral, glass and melt inclusion compositions combined with experimental and thermodynamic modeling
Final Report Abstract
The analysis of glasses and minerals of basalts from Tamu Massif recovered during the Shatsky Rise Integrated Ocean Drilling Program (IODP) Expedition 324 show that the samples from Site U1347 are evolved tholeiitic basalts containing 5.2 to 6.8 wt% MgO. The glasses have systematically higher FeO, lower Al2O3, SiO2, and Na2O concentrations than typical MORBs, and the CaO/Al2O3 ratios are amongst the highest known for MORBs. Similar to Ontong Java Plateau (OJP) the basalt composition indicate that they may be produced by more than 20 % of partial melting of the mantle. The phase equilibria were determined for three tholeiitic basalts representing different evolutionary stages (compositions AH6, AH3, and AH5 with 8.6, 8.0, and 6.4 wt% MgO, respectively). The effects of pressure (at 100, 200, 400, and 700 MPa) and small amounts of H2O on liquid lines of descent were investigated in detail. Under dry conditions, at lower pressures (≤ 400 MPa), the crystallization in the Mg-rich AH6 and intermediate AH3 basalts follows the typical sequence of tholeiitic differentiation with olivine crystallization at the liquidus followed by olivine+plagioclase and olivine+plagioclase+clinopyroxene. At high pressure (700 MPa) the crystallization sequence is reversed starting with clinopyroxene at the liquidus. Under low H2O conditions (~ 1 wt% H2O in the melt) the reversed crystallization sequence (clinopyroxene first) is observed already at 400 MPa. In contrast to the two more MgO-rich basalts, in the most evolved AH5 basalt, clinopyroxene is the liquidus phase at all investigated pressures and under both dry and low H2O conditions. The application of our mineralogical and experimental data to natural Shatsky Rise basalts implies that the magmas record partial crystallization processes occurring mainly at low pressure (100 MPa), corresponding to depths of ~3 km beneath the former spreading center, although the more primitive lavas give evidence of differentiation in a deeper reservoir at ~14 km depth (400 MPa). Our experiments emphasize the role of low melt H2O contents on stabilizing clinopyroxene and provide some new insights into the problem of “pyroxene paradox”. The apparent mantle pressures obtained for some mid-ocean ridge basalts using “dry” geobarometric approaches may have to be reconsidered if small amounts of H2O are present in silicate melts.
Publications
- (2013) Geothermobarometry of Basaltic Glasses from Tamu Massif, Shatsky Rise Oceanic Plateau. Geochemistry Geophysics Geosystems, 14, 3908- 3928
Husen A., Almeev R.R., Holtz F., Koepke J., Sano T., Mengel K.
(See online at https://doi.org/10.1002/ggge.20231) - (2016) The Role of H2O and Pressure on Multiple Saturation and Liquid Lines of Descent in Basalts from the Shatsky Rise, Journal of Petrology
Husen A., Almeev R.R., Holtz F.
(See online at https://doi.org/10.1093/petrology/egw008) - (2016). Test of the Ballhaus- Berry-Green Ol-Opx-Sp Oxybarometer and Calibration of a New Equation for Estimating the Redox State of Melts Saturated with Olivine and Spinel. Geochemistry International 54 (4), 301-320
Nikolaev, G. S., Ariskin, A. A., Barmina, G. S., Nazarov, M. A. & Almeev, R. R.
(See online at https://doi.org/10.1134/S0016702916040078)
