Experimental and Natural Studies of Mn and the Isotopes of B in Granite-Pegmatite Systems

Maner, James

Experimental and Natural Studies of Mn and the Isotopes of B in Granite-Pegmatite Systems

dc.contributor.advisor	London, David
dc.contributor.author	Maner, James
dc.contributor.committeeMember	Rossman, George
dc.contributor.committeeMember	Weaver, Barry
dc.contributor.committeeMember	Elwood-Madden, Andrew
dc.contributor.committeeMember	Thomson, Robert
dc.date.accessioned	2017-09-03T20:02:59Z
dc.date.available	2017-09-03T20:02:59Z
dc.date.issued	2017-12
dc.date.manuscript	2017-08
dc.description.abstract	Partition coefficients (D) and exchange coefficients (KD) for Fe, Mn, and Mg between tourmaline (Tur), garnet (Grt), cordierite (Crd) and granitic melt were measured between 650°C and 850°C at 200 MPaH2O. Manganese is incompatible in tourmaline (DMn ≈ 0.3 to 0.9) at temperatures greater than 650°C and is highly compatible in garnet (DMn ≈ 15 to 39) and cordierite (DMn ≈ 2 to 7) at all temperatures. Tourmaline is not stable at temperatures greater than 750°C. The partition coefficients are used in a Rayleigh fractional crystallization model to test their ability to predict the point at which Mn-rich garnet (spessartine) would become saturated in an S-type granitic melt. As expected, approximately 90% of crystallization is necessary to bring the Mn content of an anatectic S-type granitic liquid to spessartine saturation at ~ 665°C and 200 MPa. The boron isotopic composition, δ11B, of borate and borosilicate minerals (including tourmaline (Tur)) from three dikes at the Little Three (LT3) pegmatite mine, Ramona, CA, were measured using secondary ion mass spectrometry (SIMS). The tourmaline at the LT3 were found to be strikingly heavy compared to tourmaline from other pegmatites. At the LT3, average values of δ11BTur are constant across magmatic portions of each dike. Values of δ11BTur only increase from massive pegmatite into miarolitic cavity where other B-bearing minerals, whose B is in 4-fold coordination, occur with tourmaline. A thermal cooling model shows that the pegmatites would have crystallized in a matter of days, which is much faster than the diffusivity of B in hydrous granitic melt. Therefore, it is unlikely that the δ11B of tourmaline records an equilibrium composition with respect to melt (or aqueous solution). Altered oceanic crust is a likely source of the pegmatite-forming melt, and B, based on similarities of boron isotopic composition and geochemistry of potential source plutons. Dozens of experiments were conducted to measure the diffusivity of B in hydrous granitic melt at 800°C at 200 MPa, and to assess the diffusive separation of 10B and 11B. In a different series of experiments, the fractionation of boron isotopes between granitic melt and aqueous solution was measured at 700°C and 800°C at 200 MPaH2O. Boron diffuses at a rate similar to Al (10-13 m2/s) in hydrous granitic melt, and 10B diffuses faster than 11B. Fractionation of boron isotopes between granitic melt and aqueous solution is shown to be significant and corroborates the results of Hervig et al. (2002). However, in one experiment (MAC230) the δ11B of the final glass is identical to the starting glass (Macusani obsidian). The rapid crystallization of the melt in MAC230 must have exceeded the diffusivity of B through melt at 500°C such that the aqueous solution (i.e. miarolitic cavity) and melt could not equilibrate; a result that corroborates the findings at the LT3 pegmatite mine.	en_US
dc.identifier.uri	http://hdl.handle.net/11244/52242
dc.language	en_US	en_US
dc.subject	Geology	en_US
dc.thesis.degree	Ph.D.	en_US
dc.title	Experimental and Natural Studies of Mn and the Isotopes of B in Granite-Pegmatite Systems	en_US
ou.group	Mewbourne College of Earth and Energy::Conoco Phillips School of Geology and Geophysics	en_US
shareok.orcid	0000-0002-6617-4084	en_US