  Rocky bodies of the inner solar system display a systematic depletion of the
"Moderately Volatile Elements" (MVEs) that correlates with the expected
condensation temperature of their likely host materials under protoplanetary
nebula conditions. In this paper, we present and test a new hypothesis in which
open system loss processes irreversibly remove vaporized MVEs from high nebula
altitudes, leaving behind the more refractory solids residing much closer to
the midplane. The MVEs irreversibly lost from the nebula through these open
system loss processes are then simply unavailable for condensation onto
planetesimals forming even much later, after the nebula cooled, overcoming a
critical difficulty encountered by previous models of this type. We model open
system loss processes operating at high nebula altitudes, such as resulting
from disk winds flowing out of the system entirely, or layered accretion
directly onto the young sun. We find that mass loss rates higher than found in
typical T-Tauri disk winds, lasting short periods of time, are most
satisfactory, pointing to multiple intense early outburst stages. Using our
global nebula model, incorporating realistic particle growth and inward drift
for solids, we constrain how much the MVE depletion signature in the inner
region is diluted by the drift of undepleted material from the outer nebula. We
also find that a significant irreversible loss of the common rock-forming
elements (Fe, Mg, Si) can occur, leading to a new explanation of another
longstanding puzzle of the apparent "enhancement" in the relative abundance of
highly refractory elements in chondrites.
