Earth and Planetary Sciences ETDs

Publication Date

6-25-2010

Abstract

Depleted mantle peridotites from the Finero body in the Ivrea Zone were infiltrated by metasomatizing melts/fluids prior to their incorporation into the lower continental crust. Metasomatism resulted in pervasive development of phlogopite and amphibole throughout much of the body, as well as the formation of phlogopite-rich segregations. Previous trace element and isotopic data have been interpreted by different authors to reflect metasomatism induced by slab-derived, rift-related (continental), or plume-related melts/fluids. Several of these geochemical studies conclude that the rocks experienced two discrete metasomatic events. Here we combine chlorine and hydrogen stable isotopic data with field and petrologic data to better constrain the number of alteration events, the source(s) of the metasomatizing fluid/melt, and the migration mechanism(s) of the fluid/melt. Four types of samples were collected from two localities: (1) foliated phlogopite + amphibole peridotite, (2) crosscutting phlogopite + amphibole segregations, (3) a single crosscutting phlogopite + orthopyroxene segregation, and (4) cumulate amphibole peridotites. Group 1 peridotites range in textures from massive, showing few deformation features, to samples with well-developed olivine + orthopyroxene ± phlogopite ± amphibole shape-preferred orientation and evidence for subgrain rotation recrystallization of olivine and orthopyroxene. Thermodynamic modeling using Perple_X indicates that Group 1 samples equilibrated at temperatures of ~800-900°C, which is consistent with deformation microstructures seen in samples collected for this study. Microprobe data from Group 1 samples show large variations within and between samples. Three distinct populations of amphiboles are defined on the basis of Na, K, Al, and Cr concentrations. Group 1 samples show a weak correlation in Cl vs. Na concentrations in amphibole, with an R² value of 0.414. Large variations in chlorine and hydrogen isotopic values occur both within and between groups, and do not obviously correlate with major cation or whole-rock concentrations: Group 1: δ37Cl = -1.3 to +3.3° (whole rock, n=8), δD = -48 to -36° (phlogopite, n=2); Group 2: δ37Cl = -2.1 and -1.7° (WR, n=2), δD = -49 and -40° (phlogopite, n=2); Group 3, one sample: δ37Cl = -0.1° (WR, n=1), δD = -80° (phlogopite, n=1); Group 4: δ37Cl = +0.8 to +1.9 (WR, n=3), δD not yet available. There is no correlation between δ37Cl values and sample location, chlorine concentration, major element composition, or phlogopite and amphibole abundances. The isotopic and compositional heterogeneity within and between groups could reflect interaction between peridotite and (a) multiple fluids from different sources, or (b) a single fluid that evolved chemically. However, hypothesis (b) would require large chlorine and hydrogen isotopic fractionations to have occurred under mantle conditions, a conclusion that is inconsistent with experimental and theoretical studies. The lack of correlation between the isotope and petrologic data is more consistent with multiple pulses of chemically and isotopically distinct fluids, with evidence for meter-scale or smaller equilibration distances. It is unlikely that the large range of δ37Cl values could have been produced solely by mantle melting in a rift or plume setting. Instead, the heterogeneity likely reflects relatively small-scale pulses of fluid/melt derived from different slab components and mantle melts in a subduction setting. Some studies argue that large-scale breakdown of serpentine at depths of ~200 km in the subducting lithosphere plays the major role in hydrating the mantle wedge and triggering formation of arc magmas. However, the Finero mantle peridotite shows that repeated smaller scale episodes of hydration from shallower, isotopically distinct slab sources can also cause significant modification of the mantle wedge. Similarly heterogeneous rocks in the upper levels of modern mantle wedges may contribute to the arc signature of melts either by interaction with magmas that pass through them or by downward entrainment into the zone of partial melting.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Sharp, Zach

Second Committee Member

Roy, Mousumi

Project Sponsors

Funding for this research was provided by the Wengerd Travel funds donated to the Earth and Planetary Sciences Department at the Univ. of New Mexico, the Gorham Research Assistant award, E&PS Dept., Univ. of New Mexico, Graduate and Professional Student Association (GPSA) research grant, Univ. of New Mexico, Geological Society of America research grant (2008) and NSF grant #EAR-0911669 to J. Selverstone and Z.D. Sharp.

Language

English

Keywords

Chlorine isotopes, mantle metasomatism

Document Type

Thesis

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