Low‐pressure phase equilibria of anhydrous anorthite‐bearing mafic magmas

Abstract

[1] One of the most persistent questions regarding the phase equilibria of mid-ocean ridge basalts (MORB) pertains to the petrogenesis of the anorthitic plagioclase phenocrysts (>An 90 ) that are characteristic of the more primitive members of such suites. Anorthitic phenocrysts are present in many if not most MORB suites in spite of the fact that no naturally occurring MORB glasses have ever been discovered to be in equilibrium with plagioclase more calcic than An 85 . We have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions: an N-MORB, an E-MORB, and a continental highalumina basalt. To ensure duplication of the olivine and anorthite saturation observed in natural anorthitebearing basalt, the experiments were run in An 93-6 capsules with Fo 92 olivine added to the starting glass. The compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses. Significantly, aluminous spinel (Al 2 O 3 contents of 61-68 wt%) was produced at 1290°C in all compositions and chromites (Al 2 O 3 contents of 33-42 wt%) at lower temperatures in N-MORB-derived liquids despite no spinel having been added to the starting mixture. In addition, the experiments produced basaltic liquid in equilibrium with both >Fo 89 olivine and >An 85 feldspar at temperatures of 1230°and 1210°. These liquids have compositions with Mg# (at% Mg/Mg + Fe T *100) that range from 63 to >85. The TiO 2 -MgO correlation indicates large ($16-23%) amounts of crystallization for each percent decrease in MgO. These results suggest the possibility that dry, anorthite-bearing basaltic magmas are the product of the interaction between primary melt and Al-spinel-bearing upper mantle. In addition, the results indicate that MORB magmas can undergo a large amount (>50%) of crystallization prior to reaching 8% MgO. Further, although anorthitebearing magmas have characteristics consistent with their being a significant volumetric component of MORB ''parent'' magmas, the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas.

Trace Element and Isotopic Characteristics of Plagioclase Megacrysts in Plagioclase Ultraphyric Basalts (PUB)

“…However, new information based on CO 2 contents of MIs in PUB (Drignon et al, ; Helo et al, ) indicate pressure of entrapment of 4–6 kbar (12–18 km) and therefore below the crust. This is consistent with the inference of Nielsen et al () and Kohut and Nielsen () that the high anorthite megacrysts within the upper mantle crystallize from a melt influenced by reaction with a depleted harzburgite.…”

Reequilibration Processes Occurring in Plagioclase‐Hosted Melt Inclusions From Plagioclase Ultraphyric Basalts

“…Alkali (Na 2 O + K 2 O; wt.%) versus SiO 2 (wt.%) plot for MI heated for 30 min (open circles) and 4 days (gray diamonds), compared to 1‐atm experimental data collected from MORB glasses published in the LEPR database (black triangles; Bartels et al, ; Grove & Bryan, ; Johnston & Draper, ; Kinzler & Grove, ; Kohut & Nielsen, ; Thy et al, ; Tormey et al, ; Yang et al, ), and A91‐R PUB Glass (black stars; Nielsen, ). A91‐R samples come from various places along the Blanco Transform Fault to the east of the Cascadia Channel.…”

The effects of re-homogenization on plagioclase hosted melt inclusions

“…Model parameters used were, 0.1 GPa pressure, 0% water. Plagioclase and olivine liquidus temperatures were adjusted to match the observed phase equilibria in the rehomogenization experiments and those of Kohut and Nielsen [2003].…”