Ge/Si Partitioning in Igneous Systems: Constraints From Laser Ablation ICP‐MS Measurements on Natural Samples

Abstract

Mineral/melt and intermineral Ge/Si exchange coefficients for nine common rock‐forming silicate minerals were determined by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS). Ge/Si mineral/melt exchange coefficients were found to vary by up to a factor of 10. In mafic and ultramafic systems, Ge/Si mineral/melt exchange coefficients are less than 1 for plagioclase (0.48) and olivine (0.72), close to 1 for clinopyroxene (1.17) and orthopyroxene (1.07), and greater than 1 for garnet (2.69). In felsic and silicic systems, the Ge/Si mineral/melt exchange coefficient is less than 1 for quartz (0.23), plagioclase (0.67), and potassium feldspar (0.67) but much greater than 1 for biotite (4.80) and hornblende (3.95). We show that early, olivine‐dominated fractionation of primitive basalts does not fractionate Ge/Si significantly, but subsequent cotectic crystallization of plagioclase and pyroxene can increase the Ge/Si ratio from 6 × 10−6 to 7 × 10−6. We show that the only way to decrease Ge/Si during magmatic differentiation is by crystallization of hornblende or biotite (though biotite is typically a late crystallizing phase), consistent with hornblende being a major fractionating phase in hydrous intermediate magmas. The high compatibility of Ge in hornblende makes this element, in conjunction with Si, a potentially useful approach for distinguishing between hornblende and garnet in the source regions of intermediate magmas. The high compatibility of Ge in micas suggests that Ge/Si systematics may also be useful in understanding the origin of ultrapotassic magmas, which are often thought to derive from phlogopite‐rich sources.

Elemental constraints on the amount of recycled crust in the generation of mid-oceanic ridge basalts (MORBs)

“…S6F). Similar results have been obtained on natural peridotites (16). Germanium partitions onto the tetrahedral site of pyroxenes so that D Ge in clinopyroxene and orthopyroxene is essentially identical ( Fig.…”

“…There are enough available partitioning studies with which to model peridotite melting, although we noted contradictions between the two studies (15,21), but there are no available partitioning experiments for pyroxenite melt compositions. Delaminated lower arc crust might also involve a role for hornblende (50), a mineral with Ge-Si exchange partition coefficients similar to that of garnet (16). Thus, future experimental studies of ScGe partitioning between various pyroxenite compositions for recycled crust, arclogites, Archean crustal analogs, etc., and their partial melts are going to be essential in reliably interpreting the constraints that ScGe abundances provide on the amount of recycled crust in the mantle beneath ridges.…”

Episodes of fast crystal growth in pegmatites

“…Li and Al are incompatible in quartz 25,26 . Ge is similar to Si [27][28][29] and should thus be compatible in quartz, as confirmed by He et al 30 . Li and Al should thus be systematically anticorrelated with Ge, if their concentrations are controlled by a fluid or magma undergoing crystal fractionation.…”

“…The following masses were measured: Li 7 , Na 23 , Mg 25 , Al 27 , Si 30 , P 31 , K 39 , Ti 49 , Zn 66 , Ga 69 , Ge 73 , and Ge 74 . Signals were background-corrected, normalized to Si 30 , and converted to concentrations using synthetic glass NIST 612 13 and two natural rhyolitic obsidians M3-33 and M3-86 14 as external standards and forcing the sum of all measured metals (in oxide form) to equal 100%.…”

“…We assume that k Ge for quartz is of similar magnitude. Recent work on natural quartz-silicate melt partitioning by He et al 30 confirms that Ge is compatible in quartz and only slightly less compatible than Si.…”