Code

private void diagram_PreviewMouseWheel(object sender, MouseWheelEventArgs e)
{
    // this is your current viewport - the bounding rectangle of what you can see on the screen
    var zoomRect = diagram.Viewport;

    // if that value is negative, it indicates a Zoom Out, thus increasing the current viewport bounds, will effectively zoom the document out; Else, it is a Zoom In operation, so the opposite is true.
    if (e.Delta < 0)
        zoomRect.Inflate(100, 100);
    else
        zoomRect.Inflate(-100, -100);

    // This condition checks if the new resulting bounding rectangle will exceed the bounds of the whole document, and if not - it zooms the view to it, with pivot at the center
    if (diagram.Bounds.Width >= zoomRect.Width && diagram.Bounds.Height >= zoomRect.Height)
        diagram.ZoomToRect(zoomRect, true);

    // stop the scrollviewer from scrolling
    e.Handled = true;
} 

Code

private void OnDiagramMouseWheel(object sender, MouseWheelEventArgs e)
{
	var imageBounds = imageNode.Bounds;
	var zoomRect = diagram.Viewport;
	var zoomStep = 10;

	if (e.Delta < 0)
	{
		zoomRect = new Rect
		(
			Math.Max(imageBounds.X, zoomRect.X - zoomStep / 2),
			Math.Max(imageBounds.Y, zoomRect.Y - zoomStep / 2),
			Math.Min(imageBounds.Width, zoomRect.Width + zoomStep),
			Math.Min(imageBounds.Height, zoomRect.Height + zoomStep)
		);
	}
	else
	{
		zoomRect = new Rect
		(
			Math.Min(redRect.X, zoomRect.X + zoomStep / 2),
			Math.Min(redRect.Y, zoomRect.Y + zoomStep / 2),
			Math.Max(redRect.Width, zoomRect.Width - zoomStep),
			Math.Max(redRect.Height, zoomRect.Height - zoomStep)
		);
	}

	diagram.ZoomToRect(zoomRect, true);

	// stop the scrollviewer from scrolling
	e.Handled = true;
}