Porosity, permeability, and basalt metamorphism

Abstract

The effects of primary porosity on fluid flow during contact metamorphism were studied in basalts from central East Greenland. The gabbroic Skaergaard magma intruded interbedded massive and aa basalts with mean macroscopic primary porosities of 4% and 11%, respectively. Heat transport from the cooling gabbros led to three metamorphic mineral zones within 1 km of the contact: the actinolite + chlorite zone beyond 250 m, where the mineral assemblage records peak temperatures (7) of <550 °C; the pyroxene zone (T = 700-850 °C); and the olivine zone, within 10 m (T > 850 °C). In the actinolite + chlorite zone, aa clasts record more extensive mineralogic alteration of igneous minerals than do massive samples. Extents of prograde recrystallization in the olivine and pyroxene zones are 100% in both flow morphologies, but modal volumes of retrograde minerals in the pyroxene and olivine zones are higher in aa units. Extents of prograde reactions do not correlate with primary porosity because they were solid-solid reactions that occurred at high temperatures, whereas retrograde alteration involved low-temperature hydration reactions in which the availability of H 2 0 as a reactant, as controlled by porosity, probably influenced reaction extent. In the pyroxene zone, where mineralogic and textural evidence suggests oxygen isotope exchange equilibrium, whole-rock 5 18 0 compositions are 1.7%e to 3.0%c and are similar or lower in aa units than in massive units at any given distance from the contact. The isotopic ratios suggest average time-integrated fluid fluxes of 3.6 and 4.0 x 10 3 mol cm -2 in massive and aa units, respectively, if fluid infiltration occurred during prograde metamorphism. Similar values were computed assuming that part of the isotopic exchange was retrograde. These differences imply that time-averaged matrix permeability was ~10% higher in aa flow breccias.

Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion

“…The variability of temperatures derived from twopyroxene thermometry suggests that chemical equilibrium was attained only within 100 m of the contact during the metamorphic peak. The sharply defined transition between the pyroxene and actinolite þ chlorite zones (with no intervening hornblende zone) represents a significant apparent step-wise temperature change and is consistent with non-progressive metamorphism (Manning et al, 1993;Manning & Bird, 1995). Taylor & Forester (1979) present a detailed oxygen and hydrogen isotope study of the hydrothermal system associated with the Skaergaard intrusion.…”

“…Contact metamorphism of the basaltic rocks that form the upper walls and roof of the magma chamber resulted in the development of three zones, defined by the presence of actinolite þ chlorite (4250 m from the contact), clinopyroxene þ orthopyroxene (10^250 m from the contact), and olivine (5 10 m from the contact) (Manning & Bird, 1991, 1995Manning et al, 1993). Rocks in the pyroxene and olivine zones have granoblastic polygonal microstructures whereas basalts from the actinolite þ chlorite zone retain relict igneous microstructures (Manning & Bird, 1991).…”

“…Despite the excellent exposure of the gneissic and basaltic country rocks, previous work on the contact metamorphism associated with the intrusion is scarce and entirely concentrated on the basalts (Manning & Bird, 1991, 1995Manning et al, 1993). In this contribution, we focus on the contact metamorphism of the Precambrian amphibolitefacies gneisses that form the lower walls and floor of the intrusion.…”

Formation of wehrlites through dehydration of metabasalt xenoliths in layered gabbros of the Noe-Nygaard Intrusion, Southeast Greenland

“…Typical basalts from the Scoresby Sund region that have undergone zeolite facies metamorphism contain 1-2 wt % H 2 O . In the Miki Fjord region, where actinolite + chlorite, prehnite + epidote and prehnite + calcite mineral assemblages fill primary pore spaces in the basalts (Bird et al 1985;Manning & Bird, 1995), and where extreme Ca metasomatism of dolerite dykes produces lithologies with modes of prehnite of 40-100 % (Rose & Bird, 1994), the bulk water content of the metabasalts is likely to be double that in the Scoresby Sund region. Assuming that all H 2 O is lost from the metabasaltic xenoliths to the enclosing magma during dehydration suggests that metasomatic wehrlites and metabasaltic xenoliths should be present in volume ratios between 1:1 and 2:1.…”

Petrological, Mineralogical and Geochemical Data From the Eohellenic Ophiolitic Nappe in the Island of Skyros, Greece

“…Manning and Bird, 1995;Magganas and Koutsovitis, 2015 and references therein). During the exhumation stage circulating subduction related fluids and/or oceanic thermal waters may alter the ophiolitic rocks.…”